4778
J. Phys. Chem. 1992,96, 4118-4181
(9) Hager, W.; Demmer, D. R.; Wallace, S. C. J . Phys. Chem. 1987, 91,
1375.
(IO) Demmer, D. R.; Leach, G. W.; Outhouse, E. A.; Hager, J. W.;
Wallace, S.C. J . Phys. Chem. 1990, 94, 582.
L. H.; Callis, P. R. J . Phys.
(11) Sammeth, D. M.; Yan, S.;Spangler,
. .
Chem. 1990, 94, 7340.
(12) (a) Rizzo. J. R.: Park. Y. D.; Levv. D. H. J . Chem. Phvs. 1986.85.
6945. (b) Cable, J. R.; Tubergen, M. J.; Le*, D. H. J . Am. Chem. Soc. 1989;
1 1 1 , 9032. (c) Tubergen, M. J.; Cable, J. R.; Levy, D. H. J . Chem. Phys.
1990, 92, 5 1.
(1 3) Tubergen, M. J.; Levy, D. H. J . Phys. Chem. 1991, 95, 2 175.
(14) Sipior, J.; Sulkes, M. J . Chem. Phys. 1988, 88, 6146.
(15) Teh, C. K.; Sipior, J.; Sulkes, M. J . Phys. Chem. 1989, 93, 5393.
(16) Teh, C. K.; Sulkes, M. J . Phys. Chem. 1991, 94, 5826.
(17) Eftink, M. F.; Selvidge, L. A.; Callis, P. R.; Rehms, A. A. J . Phys.
Chem. 1990, 94, 3469.
(18) Bickel, G. A,; Leach, G.W.; Demmer, D. R.; Hager, J. W.; Wallace,
S. C. J . Chem. Phys. 1988,88, 1.
(19) Hager, J.; Ivanco, M.; Smith, M. A,; Wallace, S.C. Chem. Phys.
1986, 105, 397.
(20) Hager, J.; Wallace, S.C. J. Phys. Chem. 1984.88, 5513.
(21) Callis, P. R. J. Chem. Phys. 1991, 95, 4230.
(22) Callis, P. R., private communication.
(23) Hager, J.; Wallace, S.C. Can. J. Chem. 1985, 63, 1502.
(24) Rayner, D. M.; Szabo, A. G.J. Am. Chem. Soc. 1980, 102,6271.
(25) Chang, M. C.; Petrich, J. W.; McDonald, D. B.; Fleming, G. R.J .
Am. Chem. Soc. 1983,105,3819. Petrich, J. W.; Chang. M. C.; McDonald,
D. B.; Fleming, G. R. J. Am. Chem. Soc. 1983, 105, 3824.
(26) Philips, L. A.; Webb, S.P.; Martinez, S.J.; Fleming, G. R.; Levy,
D. H. J. Am. Chem. Soc. 1988, 110, 1352.
(27) Teh, C. K.; Gharavi, A.; Sulkes, M. Chem. Phys. Lett. 1990,165,460.
Teh, C. K.; Gharavi, A.; Sulkes, M. SPIE Proc. 1990, 1204, 820.
Jet Emission Spectra of CdI and HgI and Near-Dissociation Theory Analyses for CdI
and ZnIt
K. J. Jordan, R. H. Lipson,**’N. A. McDonald,
Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7,Canada
and R. J. LeRoy
Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo,
Waterloo, Ontario N2L 3G1, Canada (Received: October 3, 1991; In Final Form: February 14, 1992)
Emissions from CdI and HgI produced in a corona-excited supersonic jet discharge were photographed in high resolution.
A study of the vibrationally and isotopically resolved spectra for the CdI ultraviolet D2113,2-X22+transition near 338.4nm
and visible B2Z+-X2Z+transition near 646.5nm has yielded vibrational constants for the X, B, and D electronic states and
estimates of their dissociation energies. Use of neardissociation expansion analyses has yielded much more reliable extrapolation
properties for this system, and for the analogous states of ZnI, than use of conventional techniques. The analysis of the
HgI B-X spectrum near 420 nm was found to be in excellent agreement with previous studies. The trends in the X- and
B-state constants for the MI series (M = Zn,Cd, Hg) including dissociation energies derived from near-dissociation theory
are rationalized in terms of the mixed ionic-covalent bonding expected for group IIB metal halide radicals.
I. Introduction
Emissions from cadmium monoiodide, CdI, mercury monoiodide, HgI, and the other group IIB metal halide radicals were
first reported over 60 years ago by Wieland.’ Many features of
their spectra, however, are still only qualitatively understood. Four
electronic transitions of CdI are known (see Figure 1). They are,
in order of increasing energy, B2Z+-X2Z+ (660.0-560.0
C2111,2-XzZ+(360.0-354.011m),4.~D2113,2-X2Z+(351.7-328.8
nm),1,5and E2Z+-X2Z+ (255.0-235.0I I ~ ) .Electronic-state
~
vibrational constants had previously been determined from the
analysis of the C-X, E X , and E-X transitions, but B-X had not
been analyzed on the basis of well-resolved spectra.
The reasons for this are manyfold. B2Z+is an ion-pair state
which dissociates to Cd+(2SI2) + I-(lS0), This type of electronic
state is characterized by a deep potential energy well (resulting
from the strong Coulombic interaction between the ions), large
equilibrium bond lengths, and small vibrational and rotational
energy intervals. Transitions from low v’levels of B are expected
to probe high, closely spaced vibrational levels of the ground state.
A large degree of rotational excitation is expekted from conventional discharges used to produce the free radical. Finally, CdI
has a significant number of isotopomers, each with an appreciable
natural abundance. The confluence of these effects results in a
continuum-like emission spectrum which has thus far defied
analysis.
*Towhom correspondence should be addressed.
Publication 444 from the Photochemistry Unit, University of Western
Ontario.
‘NSERC University Research Fellow.
Tellinghuisen and co-workers were able to minimize many of
the problems listed above for HgI by using mild Tesla excitation
and single isotopomer
In this way, vibrational constants for seven electronic states of HgI, including the B ion-pair
state, were determined.
The free radicals MX (M = Zn,Cd, Hg; X = CI, Br, I) have
attained some prominence as gain media in the visible or nearinfrared region.*-15 Lasing on the MX B2Z+-X2Z+ transition
can be achieved by photodissociating an MX2triatomic precursor
or by discharging MX2 electrically. The very properties of the
B and X states that inhibit study of CdI spectroscopy favor
electronic transition lasing. It has also been determined that the
MX2 precursor is cyclically produced during MX B-X lasing,
resulting in long-term laser operation.
Improved MX lasing efficiency can be realized by dissociative
excitation-transfer reactions of MX, with metastable N2(A3Z:),
yielding MX*(B2Z+). Analysis of the kinetics of these reactions
depends on a good understanding of the B-X transition. However,
even the most recent work of Sadeghi et a1.I6 has relied instead
on model potential energy curves for the interpretation of their
results.
There is also current interest in the laser fluorescence excitation
spectra of group IIB metal-monocyclopentadienyl (MCp) complexes formed in supersonic jets.” The electronic structure of
these organometallic species is expected to be similar to the group
IIB metal monohalides. Spedically, identification of the vibronic
structure of those transitions involving the first ionic state (correlating to M+(2SI2 ) + Cp-(XIA,’)) would be facilitated by
comparison with a definitive vibrational analysis of the analogue
B-X transition in the corresponding metal monohalide.
QQ22-3654/9212Q96-4778%Q3.OQ/O0 1992 American Chemical Societv
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4779
Jet Emission Spectra of CdI and HgI
w
c
0
x
AV
mD-X
- 2 -1
0
t t A
1
2 AV
I
w
z
P
Q
15
350
3 b
3bO
Ahm)
Figure 2. A low-resolution ultraviolet spectrum of CdI. Sequence
structure for the C-X and D-X transitions is indicated by Au = v f - v".
The three intense features labeled A, B, and C are assigned to the 5d
'D3-5p 3@, 5d 3D2-5p 3fl,
and 5d 3DI-5p
transitions of atomic Cd,
respectively.
(Hamamatsu R636) and a lock-in amplifier.
*
Bondlength, r
Figure 1. A schematic representation of the potential energy curves for
a group IIB metal halide, MX, and their dissociation products. Arrows
indicate the three lowest energy allowed electronic transitions.
This paper presents well-resolved emission spectra of CdI and
HgI formed in corona-excited supersonic expansions (CESE). As
demonstrated for ZnI,I8 the rotational cooling in the CESE has
allowed an unambiguous vibrational analysis of the B-X transition
of CdI to be performed for the first time.
11. Experimental Section
CdI emission spectra were recorded using the experimental
arrangement that was successful for ZnI.18 Briefly, CdI was
generated by discharging Cd12 (Aldrich, 99%) in argon (Canox,
commercial grade). The Cd12 (melting point 388 "C) was heated
in a bulb nozzle within a vacuum chamber maintained at <SO
mTorr by a Roots blower (Edwards EH1200)/mechanical pump
(Edwards E2M80) combination. The resultant Cd12vapor was
entrained in argon, and the mixture was expanded through an
-0.5-mm pinhole at a stagnation pressure of -1 atm. A corona
discharge was excited by applying a positive dc voltage (500-800
V, -10-15 mA) to a stainless steel electrode running concentrically
down the axis of the nozzle.
The strongest CdI emission, obtained when the color of the jet
was yellow-white, was imaged onto the slit of a 3.4-m Jarrell-Ash
spectrograph (70-000). The B-X transition was photographed
with either Kodak Technical Pan 2415 film (for X > 640 nm) or
Kodak TMAX 400 film (for X < 640 nm), in eighth or ninth order
(reciprocal dispersion -0.55 A/",
50-pm slits). Coming glass
cutoff filters were used for order separation. Exposure times of
-6 h were necessary for the slower Technical Pan film, compared
with -2.5 h using the faster TMAX emulsion.
The ultraviolet D-X spectrum was photographed in 17th order
(reciprocal dispersion -0.32 A/",
40-pm slits) on Kodak
TMAX 400 film in -0.5 h. The band system was isolated using
a quartz Pellin-Broca prism predisperser.
HgI was formed by discharging Hg12 (Aldrich, 99%; melting
point 259 "C) in an argon jet at a stagnation pressure of -2 atm.
For currents between -8 and 15 mA, the jet had a bright bluegreen color indicative of excited HgI and Hg atoms. The B-X
transition was photographed in 13th order (reciprocal dispersion
-0.42 A/",
40-pm slits) on the TMAX 400 film. While strong
portions of the spectrum could be photographed in 10 min, exposure times were typically -1 h. Order separation was achieved
using Corning filters.
Fe and Ne hollow cathode lines in overlapping orders provided
calibration, and films were measured on a Nikon profile projector,
Model 6C.
Low-resolution spectra could be obtained by dispersing the jet
emission in a 0.35-m monochromator (Heath EU-700) and then
detecting the light using a UV-visible photomultiplier tube
III. Observed Spectra and Assignments of CdI
The potential energy curves and transitions which are important
for this work are presented schematically in Figure 1, Only the
B-X and D-X transitions of CdI were observed with any appreciable intensity. As shown in the low-resolution overview UV
spectrum of CdI (Figure 2), C-X appears to be weaker than D-X
even though the C state lies below D in energy. While observed
B-X transitions are expected to originate from low vibrational
levels of B and terminate on high vibrational levels of X, the D-X
band system involves the lowest ground-state vibrational levels.
Consequently, the latter analysis will be presented first.
IV. Analysis of the I k X Band System
Wieland's study of the CdI D-X spectrum is still the most
comprehensive to date.l He measured 90 vibrational sequence
bands of the system having Au = u'- u" ranging from -8 to +4.
In the present study, only the Au = -2, -1 ,0, +1, and +2 sequence
bands were observed, which suggests that the corona-excited
supersonic expansion is characterized by a lower vibrational
temperature than Wieland's high current dc discharge.
A reproduction of a portion of the D-X spectrum is shown in
Figure 3. The most prominent feature is the (uf,u") = (0,O)
vibrational band near 338.5 nm. The unresolved rotational envelope is degraded to the blue which indicates that the rotational
B value for the excited state, B', is greater than B Nfor the ground
state. Both (0,O)and (1,l) are doubled due to bandhead formation
in the Qzzbranch (higher frequency component) and in the P22
branch (lower frequency component). The Qu rotational envelope
is more intense than that of the PZ2branch, which is in agreement
with theoretical expectation^,'^ although it should be noted that
the former also overlaps the relatively strong RZ2branch.
The Au = -2, -1, +1, and +2 sequence bands are composed
of a series of lines that change their relative spacing as a function
of ALAThis structure is due to low-J transitions of the Q22 branch
for the different isotopomers of CdI having a common Au. While
there is only one naturally occurring isotope of iodine (12'1), there
are eight stable isotopes of Cd, with natural abundances ranging
from 0.7% for losCd to 28.9% for 114Cd.20Thus, the intensity
of the isotopic structure within a vibrational band should reflect
the normalized natural abundance of the particular Cd isotope
involved (lo6Cd:108Cd:110Cd:l 11Cd:112Cd:1l3Cd: I14Cd:116Cd I
0.043:0.03 1:0.435:0.446:0.840:0.425:1.OO:0.26 1). However, small
splittings and overlapping rotational envelopes somewhat obscured
the expected intensity pattern. Nevertheless, it was possible to
assign at least one transition in the observed spectrum for every
isotopomer, except IMCdI.
The X- and D-state frequencies are sufficiently similar (w/
178.5 cm-I, w,'
196.6 cm-I) to produce linelike sequence
bands. Given the low values of Au observed, the dominant contribution to the vibrational isotope shift will be determined by the
excited-state term values if Av 1 0 or by the ground-state term
values if Au < 0. Consequently, the isotopic features within a
-
-
4180 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Jordan et al.
TABLE I: Isotopic Transition Wavenumbers, Y (mi1), and Vibrational Assignments for the LPII~,~(V')-X*E+(V'')Transition of CdI
(V',V'?b
u("OCd1)
u( I T d I )
u( Il2CdI)
u( I "CdI)
u(i14~d~)
u(Ii6CdI)
(090)
29541.33 (5)'
(0,l)
29 362.89 (-2)
29 363.41 (10)'
29 363.74 (3)"
29 364.16 (6)
29 364.47 (-2)
(072)
29 187.38 (9)'
29 188.86 (-3)
(1,O)
29737.84 (-1)
29737.40 (4)
29 736.85 (-3)
29 736.42 (2)
29 735.91 (-2)
29 734.95 (-6)
(191)
29 559.12 (-3)
(1,2)
29382.16 (2)
29382.51 (1)
29 382.89 (4)
29 383.51 (-3)
29 384.19 (-2)
(~3)
29 207.58 (-4)
29 209.09 (-2)
(2,O)
29933.02 (3)
29931.13 (4)
29931.13 (4)
29 929.22 (-3)
(2,l)
29754.53 (-1)
29754.05 (4)
29 753.50 (1)
29 752.95 (-2)
29 752.46 (0)
29 75 1.45 (-2)
a31
29401.89 ( 5 )
(~4)
29 227.75 (-4)
29229.19 (2)
(391)
29 946.32 (-3)
29 944.44 (0)
(3,2)
29771.04 (-1)
29770.51 (2)
29769.94 ( 1 )
29 769.47 (9)
29 768.85 (1)
29767.79 (1)
(3,4)
29963.41 (-5)
29421.26
(42)
29961.41 (-3)
29959.48 (0)
29 957.70 ( 13)'
(4,3)
29786.22 (1)
29785.07 (2)
(533)
29976.37 (-1)
29 974.32 (-4)
(594)
29802.36 (3)
29 800.90 (-20)'
(6,4)
29993.31 (4)
29991.00 (-14)'
29989.13 (6)
(795)
30005.75 (0)
30003.58 (-4)
30017.99 (-2)
(876)
30020.19 (1)
'Values in parentheses are observed - calculated residuals in units of
(-2) cm-I. 'Assigned, but omitted from least-squares analysis.
cm". bThe frequency of the (0,l) bandhead of 10*CdIwas 29 362.05
TABLE I 1 Molecular Constanb (cm-') for the @I1312-X2Z+
Transition of 11'Cd1271
parameter
D2&,2
X2Z+
Te
29532.276 (14)'
0.0
we
195.992 (9)
177.957 (1 1)
0.6687 (1 4)
0.5878 (24)
Be
0.0663b
0.0716 (4)d
104~~
2.9c
3 (4)d
where a, is the vibration-rotation interaction constant. Unfortunately, unlike the ZnI analysis where many isotopic bandheads
were observed, only the (0,O)and (1,l) l14CdI bandhead separations could be used in this work with any confidence. Some
weaker separations for other isotopomers are evident in Figure
3, especially for the Au = +1 sequences, but the quality of the
films precluded accurate measurements. Consequently, there was
insufficient data to determine the ground-state a,. Combining
the Pekeris formula2' with the vibrational constants in Table I1
and the r / of 2.058 A suggested by Sadeghi et a1.I6 from model
potentials used to simulate low-resolution CdI B-X visible emission
spectra, this parameter was estimated to be 2.9 X lo4 cm-I.
The (0,O)and (1,l) bandhead separations yield E,,' = 0.0714
(2) cm-l and Bl' = 0.0710 (2) cm-I. The errors reflect the uncertainty in determining the position of the bandheads by eye on
the profile projector. These B values lead to B,' = 0.0716 (4) cm-'
and the highly uncertain estimate of a,' = 3 (4) X lo4 cm-l.
These constants imply that Are = r,' - r," = -0.078 (8) A, a result
in fair agreement with the Are = -0.055 A value determined by
Kasatani et al. from Franck-Condon factor calculations.22
'Quoted errors are l u standard errors. Constrained. CConstrained
to value determined from the Pekeris formula. dFrom head analysis
described in text.
sequence band will be ordered v(lo8CdI)> u("OCd1) > v("'Cd1)
> u(II2CdI) > ~(~l'Cd1)
> v('14CdI) > v(lWd1) for Au 1 0 and
will be in the reverse order for Au < 0.
The isotopic Q22 bandheads were fitted to a conventional
mass-reduced expression of the form (in cm-l)
u, =
T,' + w,'?); - w&,'v;
- w;v/
+ W&X,"V/
(1)
where T,' is the electronic origin of the D state, and w, and w g X ,
are the frequency and anharmonicity, respectively, for each
electronic state. vi is defined as pi(u +
for each electronic
and pi is the reduced mass
state where pi = [p(1'4Cd1271)/pi]1/2
of a particular CdI isotopomer. The observed isotopic transition
frequencies and their vibrational assignments are listed in Table
I. If the least-squares residual for one of the bandhead frequencies
was >0.09 cm-I, then that point was given zero weight in the
analysis. This left a total of 54 frequencies which were used to
determine the molecular constants given in Table 11. The standard
deviation of the fit was -0.03 cm-I.
Rotational constants for the D state were estimated from the
Q22-P22bandhead frequency separations. The calculation was
done in the manner which proved successful for the C-X spectrum
of ZnI. Standard branch formulas for the P22and Q22branches
were differentiated to find the value of J at their respective
bandheads, Jh.21 The formulas neglected centrifugal distortion,
spin doubling in the 2Z+state, and A-doubling in the 2113/2state.
These expressions for J h were substituted back into their respective
branch formulas to find the frequencies of two bandheads,
and Y ~ ( Q ~ ~The
) . resultant equation for the bandhead separation
is given by
dQ22) - Yh(P22) = ZB,"B,'/(B,'
- B,")
(2)
Each B, can be defined in terms of equilibrium parameters as
B, = p;B, - p ; a , ( ~
+ !/2)
(3)
V. Analysis of the CdI B-X Band System
The visible CdI B-X spectrum was recorded in the red between
-592 and 664 nm. The overall appearance of the low-resolution
spectrum is similar to that observed for ZnI.16J8 The strongest
emission intensity is found at the red end of the spectrum and
decreases monotonically toward the blue.
In high resolution, the vibrational bands of the B-X system are
red-shaded, c o n t i i g that the bond length of the B ion-pair state
is longer than that of the ground state. The strong intensity of
the red end of the spectrum arises from overlapping vibrational
sequence bands. As shown in Figure 4, the central portion of the
spectrum between ~ 6 5 and
2 625 nm is dominated by one vibrational progression. Below 625 nm, sequence structure reemerges although the overall emission intensity is decreasing.
The bands in Figure 4 are assigned to a ground-state progression
because the vibrational intervals (AG = 110 cm-I) increase with
increasing frequency. Each vibrational band is composed of five
or six heads that are assigned to the various isotopomers of CdI.
The largest contribution to the vibrational isotope shift arises in
the ground state since Au is expected to be large (and negative).
Consequently, the heavier isotopomer heads within a band lie a t
higher frequencies. In the spectral region defined by Figure 4,
the head splittings within a vibrational band are -10 cm-l per
1 amu change in isotopomer mass. These transitions were assumed
to originate from u' = 0 for two reasons: the overall intensity
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4781
Jet Emission Spectra of CdI and HgI
z
2
c
c
IN
Figure 3. (a) The Av = + I sequence of the D-X transition of CdI near 336.3 nm, (b) the Av = 0 sequence of C-X near 338.5 nm, and (c) the Av
= -1 sequence of C-X near 340.5 nm. For all three photographs, frequency is increasing from left to right. Isotopic assignments are indicated for
the even isotopomers of CdI (IoRCd,“OCd, ‘I2Cd, ‘I4Cd, and Il6Cd). The unlabeled bands correspond to odd isotopomers of CdI (IIICd and I13Cd).
In the Au = 0 sequence the higher frequency Q22bandheads are connected to their respective lower frequency P2, bandheads.
profile of the progression had a smooth Gaussian appearance, and
the collisions between excited CdI and argon in the jet were
expected to populate only the very lowest vibrational levels in B.
Vibrational numbering for the progression was established by
fitting the observed isotopic transition frequencies of the bands
shown in Figure 4 to eq 1, with the first three terms of eq 1
replaced by an effective upper state term value, T+ The set of
integer lower state vibrational quantum numbers was varied in
a systematic way until the standard deviation of the fit was
minimized. In this way the three bands in Figure 4 were unambiguously assigned to (0,39), (0,40), and (0,41). Changing the
best set of quantum numbers by f l increased the standard deviation of the fit by a factor >2.
The B-X analysis was continued by refitting the (O,u’? progression to an extended mass-reduced Dunham expansion of the
form (in cm-l)
vi = T,‘
+ CYn,([Pi(u’
+ Y2)In - CYm./[Pi(u”
+ Y2)Im
n
m
(4)
where Yn,( and Y,gl are the B-state and the X-state Dunham
parameters, respectively. As in the case of ZnI, the local
ground-state vibrational interval in the center of the (0,u”) progression was close to the expected AG1/2/ of the B state (=l 10
cm-I). Ground-state vibrational intervals at the red end of the
spectrum were smaller than AG,,,’ (larger 0”). Sequence structure
[( l,u”+l), (2p”+2), .I was therefore found to emerge to higher
energies than the main (0,~”)
bands. Similarly, at the blue end
of the spectrum, the local ground-state vibrational spacing was
greater than AG,,,l (smaller 0’9, and sequence structure emerged
to the red of the main progression.
The remaining assignments were made by using eq 4 in an
iterative process of extrapolation, prediction, and refitting. Only
bands having 4 2 u’ 1 0 and 29 1 u” 1 47 were observed, and
these were used to determine molecular constants. The resulting
isotopic transition frequencies and vibrational assignments are
listed in Table 111. All bands with observed - calculated residuals
from the least-squares fit 10.5 cm-I were omitted from the
4782 The Journal of Physicat Chemistry, Vol. 96, No. 12. 1992
Jordan et al.
C
Figure 4. A portion of the high-resolution spectrum of the B-X band system of CdI. The observed isotopically resolved transitions form part of the
(0.0") progression where u" = 39,40,and 41.
TABLE III: Isotopic Transition Wavenumbers, P (cm-I), a d Vibrational Assignments for tbe B*E+(v')-PE+( r") Transition of CdI
( 0 'vu'
9
v( "OCd I )
v( I 'CdI)
v( "'Cd I )
v( I I 'Cd I)
v( Il'Cd I)
v(Il6CdI)
16 194.00 (-3)"
16070.24 (-5)
16049.93 (-1 9)
15 948.27 (-60)'
15 829.91 (7)
15 809.95 (60)b
15713.30 (2)
15 693.03 (33)
15 599.22 (-5)
15 557.45 (-32)
15568.14 (1)
15 578.66 (3)
15 588.84 (-19)
15487.90 (1)
1 5 509.81 (1 80)'
15467.28 (3)
15 446.09 (3)
15 456.96 (23)
15 477.93 (28)
15 399.40 (5)
15348.45 (31)
15 336.82 (-66)'
15 358.29 (-34)
15 368.39 (-63)'
15 379.25 (1)
15 293.42 (-5)
15 273.52 (10)
15 128.91 (-26)
15070.75 (11)
16939.97 (-1 00)'
16813.58 (-110)'
16795.32 (-100)'
16552.17 (6)
16423.93 (4)
16 382.68 (-224)'
16404.78 (6)
16297.86 (-1 3)
16 258.38 (-9)
16278.63 (-13)
16 174.06 (-19)
16 134.34 (1)
16 144.64 (1 1)
16 154.53 (-3)
16 164.40 (-9)
16052.75 (-8)
15274.49 (1)
1 5 077.68 (-1 7)
16785.98 (0)
16759.05 (55)b
16 767.92 (1 4)
16 776.37 (-59)'
16655.71 (-1)
16637.13 (-9)
16527.53 (-5)
16 508.74 (-7)
16489.76 (24)
16363.15 (9)
16401.65 (6)
16382.68 (9)
15278.31 (11)
15 239.33 (14)
15 181.64 (18)
15087.96 (0)
16889.20 (-4)
16853.44 (1 5)
16862.49 (1)
16871.62 (11)
16722.48 (4)
16741.06 (1 1)
16 630.85 (3)
16612.88 (34)
16466.77 (-52)
16 504.40 (-45)
15 191.07 (-14)
15 100.88 (-19)
15 074.05 (5)
15 092.07 (-8)
15 084.45 ( 1 34)'
15 186.73 (24)
15 195.21 (-8)
'
"Values in parentheses are observed - calculated residuals in units of 10-* cm-'. 'Assigned, but omitted from the least-squares analysis.
analysis. This left a total of 69 bandhead frequenciesto determine
molecular constants. For the final fit, the ground-state frequency
( Yl,o)and anharmonicity ( Yz,o)were constrained to the values
determined from the D-X spectrum reported above. The best
set of ground-state Dunham constants excluded
but included
Ys,o( a = 0.17 cm-I). Ground- and excited-state vibrational
constantsare presented in Table IV. Note that the value of T,'(B)
given there implicitly defines an interpolated estimate of the
separation of the low-u levels u" = 0-6 observed in the D-X
spectrum from the high-u levels'u = 3 1-47 observed in the B-X
h b m coecfcieints
TABLE
Transition of rl%d'nI
Dunham parameter
T,' + yo,
YI.0 (=we)
Y30 (-A
lo y3.0
10' y5.0
(em-') for tbe B ~ E + - ~ E +
B2Z+
21464.63 (39)"
104.308 (81)
-0.176 (18)
X2Z+
0.0
1 77.957'
-0.5878'
-1.8699 (1400)
-8.265 (51)
Quoted errors are 1 u standard errors. Constrained to values determined from D-X analysis and listed in Table 11.
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4703
Jet Emission Spectra of CdI and HgI
22844.588 cm-'
2 2678.715
202
(1,121
spectrum. The uncertainty in T,'(B) (and in this spacing) implied
by this fit is unrealistically small, both because of the constraints
on the fit and because of neglect of the effect of model dependence.
However, improved estimates of its value and uncertainty are
obtained in section VII. In order to reproduce the calculated
frequencies to better than 0.01 cm-I, the ground-state constants
in Table IV were systematically rounded off in the manner suggested by L ~ R and
o ~Tellingh~isen.~~
~ ~
VI. Tbe HgI B-X Spectrum
Considerableprogress has been made in the analysis of the B-X
spectrum of Hgl. Specifically, Tellinghuisen and co-workers have
determined a B-state vibrational frequency for HgI of -1 10
Surprisingly, this value is higher than the CdI B-state
vibrational frequency of -104 cm-'established in the present work.
While it seemed very unlikely that Tellinghuisen's analysis was
in error, it was considered prudent to record the CESE spectrum
of HgI in order to confirm his result.
A small portion of the HgI B-X jet emission spectrum is
presented in Figure 5. Measured bandhead frequencies were
found to be in excellent agreement with reported values. Interestingly, the HgI jet spectrum does not appear to be particularly
cold. In fact, most of the bands involving high vibrational levels
in B which were populated by Tesla excitation were also observed
in this work. HgCl was also found to exhibit this effect.25 At
this time it is unclear why CESEs are ineffective at cooling
mercuric halide samples.
The vibrational assignments for the bands in Figure 5 were
taken from the work of Viswanathan and Tellinghuisen,6 who
arrived at their numbering by measuring the relatively large
vibrational isotope shifts (>5 cm-' for the (0,~")sequence) from
emission spectra of separate *O0HgxIsamples, where x = 127 or
129. Our experiment allowed a number of bandheads for the
stronger isotopomers of xHg1271(primarily x = 200 and 202) to
be resolved (Figure 5). The observed shifts (-1 cm-I per unit
Hg mass change) are in excellent agreement with those calculated
using the literature values of the vibrational constants. Consequently, we conclude that the HgI B-state vibrational frequency
of =1 10 cm-' is correct.
Vn. Ground-State Vibrational Analyses and Dissociation
Energies for CdI and ZnI
This section has two main objectives: (i) to determine reliable
estimates of molecular dissociation energies and (ii) to determine
compact expressions for representing and extrapolating beyond
the range of the observed vibrational energies. Both these aims
.
have been achieved by fitting the experimental energies to"neardissociation expansions" or NDE's. Since the present work
involves a more sophisticated application of that approach than
was applied in our recent study of ZnI,'* the data for that system
are reanalyzed here to yield improved estimates of its molecular
parameters.
It has long been known26that the limiting near-dissociation
behavior of vibrational spacings (and other proper tie^^'-^^) of a
diatomic molecule has a functional form determined by the nature
of the asymptotically dominant inverse-power term in the corresponding potential energy curve. The first applications of this
result involved direct least-squares fits of the limiting near-dissociation theory expressions to the energies of sets of levels lying
very near d i s s o ~ i a t i o n . ~However,
~.~~
the implications of these
results for the more usual case in which no data are available in
that limiting region were soon realized and gave rise to two other
applications. On the one hand, Tellinghuisen and collaborators3'
and Wilcomb and Bernstein3*showed that graphical extrapolations
based on the howledge of the correct limiting behavior could yield
substantially better dissociation energies than traditional methods.
On the other hand, Beckel and c o - ~ o r k e r sand
~ ~ Tromp and
L ~ R showed
o ~ ~that~ energy level expressions based on rational
polynomial expansions about the theoretically predicted limiting
behavior could provide compact and accurate representations of
even very large sets of experimental energies. In the present
section, we combine these themes with the concept of "averaging
over models" introduced by LeRoy and Lam34to obtain both
accurate estimates of the dissociation energies and compact expressions with robust extrapolation abilities for the vibrational
energies of the ground electronic states of CdI and ZnI.
For the ground states of CdI and ZnI, the asymptotically
dominant inverse-power term in the long-range intermolecular
potential is
where D is the dissociation limit and r is the internuclear distance.
For these cases, therefore, the limiting near-dissociation theory
expression for the vibrational energies is26
G(U) = D - X(~)(UD
- U)3
(6)
where uD is the effective (in general noninteger) vibrational index
at dissociation, X(6)= 7932.0/[&c6)2]''4, p is the diatom reduced
mass (in amu), and the potential coefficient c6 has units of cm-'
Ab.
In both Tellinghuisen's earlier work on Hg16 and our previous
work on ZnI,'* the recommended vibrational energy extrapolation
4784 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Jordan et al.
TABLE V Near-Dissociation Expansion Parameters for the X2Z+ Ground State of MI (M = Cd, Zn)
molecule
u” range
D-G ( u H ) O t b
CdI
04/31-47
1139
ZnI
0-41 28-4 3
1376
O u H = highest ground-state vibrational level used
C6/106c
1.384
1.275
in the fit.
X(6)b
0.0145
0.0253
where
and the powers is (in the present work) set at either s = 1 (yielding
what are called “outer” expansions) or s = 2n/(n - 2) = 3 (yielding
“inner” expansions). Following ref 36, the exponential expansions
have the form
L
D
- X(6)(0~- U ) 3 eXp[Cpj(UD
- U)’]
i= 1
D,b
ODd
T m b
21
21
7905 (179)
8854 (144)
84.0 (5.2)
73.0 (2.9)
21 462.4 (28.0)
23 720.0 (54.0)
cm-I units. CIncm-I A6units. dunitless.
was governed by an effort to find the best single-term correction
to eq 6. The expressions considered there were of the form
G(U) = D - X(6)(U,- U ) 3 [ 1 + ~ ( U D
- U)’]
(7)
where the parameters a and p were determined empirically as those
which yielded the best fits to the experimental data. The values
of D and uD yielded by these preferred fits were then taken as the
recommended values of those quantities.6,’8
While the above approach will certainly tend to yield more
meaningful results than traditional extrapolation methods, it was
pointed out in ref 34 that the largest source of uncertainty in such
fits usually arises from the “model dependence”. For example,
fits of roughly equivalent quality to forms of eq 7 defined by
different choices of the integer p yield estimates of the physically
significant parameters De and uD that differ by much more than
the uncertainties implied by the individual fits. This problem of
model dependence was also noted by Salter et ale7and was
qualitatively taken into account in their recommendation of an
improved dissociation energy for HgI. In other
it has
been found that this problem of model dependence may be addressed by performing least-squares fits to a wide range of different
types of expansions that incorporate the limiting behavior of eq
6 and averaging the results. This is the approach used here.
In the present study, the experimental energies were fitted to
families of NDEs in which the deviation from the limiting
near-dissociation behavior was represented either by a rational
polynomial (or Pad5 approximant) expansion in the variable (uD
- u ) or by an exponent expansion of the type introduced in ref
36. The former have the general form
G(U) = D - X(6)(U, - U ) 3 [ L / k f l s
(8)
G(U)
no. of cases
(10)
Since the second longest-range term in the inverse-power potential
for the molecular states of the type considered here varies as l/+,
the leading deviation from limiting behavior should be linearly
dependent on (uD - u ) . ~ ’ As a result, our recommended results
were based on fits to versions of eqs 9 and 10 in which the leading
term was linear (i.e., pI and/or q1 # 0). However, the results
obtained here are not strongly dependent on this condition.
Since the experimental data do not include levels lying in the
limiting region where eq 6 is quantitatively valid, the limiting
behavior coefficient X(6) could not be determined from such fits.
On the other hand, fixing X(6) a t a value defined by a realistic
estimate of the c6 coefficient can substantially improve the reliability of extrapolations based on fits of this type.34935As a result,
Slater-Kirkwood values of these c
6 constants were generated38,39
and used to define values of X ( 6 ) which were then held fixed in
the fits to obtain our recommended values of the molecular
constants, D and uD. These values of C, and the associated X ( 6 )
constants are listed in the first columns of Table V, which also
identifies the level energies being fitted.
Following the approach outlined above, the experimental vibrational energies were fitted to eqs 8-10 for a range of L and
M values, and the results were averaged together to obtain our
recommended values of D and uD. In this averaging, the weight
given to a parameter value yielded by a particular fit was the
inverse of the product of the square of the parameter uncertainty
implied by that fit times the overall standard error of the fit. In
particular, if a fit to model i (defined by a particular choice of
L and M> yielded a parameter value Pi with a standard 95%
confidence limit uncertainty u(Pi)(where, P = D or uD), then the
weight, wi, used in the averaging is
wj = 1 /(.jU(Pi)2)
(1 1 )
and the resulting best estimate of this parameter is given by
P = E(WiPj)/CWi
i
(12)
I
The parameter uncertainties, u ( P j ) ,were averaged in the same
way and combined with the variance in the parameter estimates
yielded by the fits to different models, and our best estimates of
the overall uncertainty in the parameter values obtained from eq
12 were generated from the expression
[ [ Cwj(pj -
1 1 Ctwiu(pi))1 1
2
112
As discussed in sections I and 11, the experimental information
on the ground state of CdI comes from two distinct sources, the
D-X band system which yields the relative energies of vibrational
levels u”= 0-6 and the D-state Tovalue, and the E X band system
which yields the relative spacings of levels u” = 3 1-47 and their
position relative to B-state levels u’= 0-4.Unfortunately, although
the absolute numbering of the X-state levels is unambiguously
determined by the isotope shifts (see section 11), the absence of
independent information regarding the value of To for the B state
means that the absolute energies of the observed X-state levels
with u ” 1 31 (relative to U” = 0) are not directly determined. For
each of the NDE functions considered, an estimate for T,(B) was
determined by fitting that function to both sets of vibrational
energies (e.g., u” = 0-6 and u” = 3 1-47 for CdI). As with the
other fitted parameters, our final estimates of the value and
uncertainty in To(B)were generated from eqs 12 and 13. Precisely
the same situation occurs for ZnI, except that there it is the C-X
(rather than the D-X) band system which determines the spacings
of the low-u” vibrational levels.
The above discussion shows that the value of and uncertainty
in the ground-state dissociation energy will depend on two separate
components. The first is the extrapolated distance from the higher
observed vibrational levels (u” = 3 1-47 for CdI) to dissociation,
and the second is the value of To(B),which determines the energies
of those levels relative to u” = 0. For both CdI and ZnI the overall
uncertainty in the latter (28 and 54 cm-I, respectively) is much
smaller than that in the former (1 77 and 134 cm-’, respectively),
so the overall uncertainty in the ground-state dissociation energy
is only weakly dependent on the uncertainty in To(B).
The approach described above yielded the recommended values
of Te(B) and of the ground-state dissociation energies and uD for
CdI and ZnI listed in columns 7-9 of Table V, where the integer
in column 6 indicates the number of individual cases (that is, the
number of models based on eqs 8-10) used in the averaging. In
both cases, the averages only included models for which the total
number of expansion parameters, L + M in eqs 8-10, was 24.
For the cases included in the averaging, the standard deviation
of the fits averaged to 0.10 and 0.11 cm-I for CdI and ZnI,
respectively.
Once uD values were obtained from the averaging-over-models
procedure described above, recommended overall vibrational energy expansions were determined. This was done in two stages,
since in spite of the large physical uncertainty in uD, predictions
Jet Emission Spectra of CdI and HgI
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4185
TABLE VI: Parameters Defining Recommended 0/4 Inner NDE’s for
AU Vibrational Levels of the Ground States of ll‘Cdlz’I and UZn1271
Darameter
114Cd1271(X2Z+1MZn1271(X2Z+)
W,j C i - 1
X(6)/cm-’
D,/cm-’
VD
lo2%
104q2
106q3
1 08q,
177.80
0.0145
7905.48
84.0
-1.2267455
3.0683057
-4.3694902
1.5814147
225.33
0.0253
8854.30
73.0
-1.5219633
4.5075140
-2.50861 77
0.77218831
generated using NDE models are very sensitive to small changes
in this constant. As a result, the fits to eqs 8-10 with (L+ M)
2 4 were repeated with uD held fmed at the rounded-off value listed
in Table V, and the recommended expansion chosen from those
results using the criterion that both the standard error of fit and
the number of expansion parameters be as small as possible. This
approach led to our selection of 0/4 “inner” expansions of the form
to represent the vibrational energies for the ground states of both
CdI and ZnI. The resulting parameter values are summarized
in Table VI. Since conventional spectroscopic parameters such
as o,and
are simply defined in terms of the derivatives of
G(v) at u = -I/*, they may also be readily determined for the NDE
models; the associated values of oeare listed in Table VI. Note
that the individual expansion parameters (qiJ have no physical
significance, so their statistical uncertainties are not reported.
However, the seven or eight digits quoted for them must be retained if predictions generated from the resulting expression are
to reproduce the input data to within experimental precision.
There is a fundamental difference between use of the traditional
Dunham expansions and of the NDE expressions of eqs 8-10. The
former are simply power series expansions about the point u =
-I/>
As such, they may be expected to behave reasonably across
the range of data being fitted and hence should yield reasonable
estimates for the displacement of the upper vibrational levels (u”
= 31-47 for CdI) relative to those near the potential minimum
(u” = 0-6) as any one of the NDE functions of eqs 8-10. Indeed,
fits to Dunham expansions yield displacements which differ from
those yielding the recommended To(B) values of Table V by much
less thanjtheir uncertainties (discrepancies of only 2.2 cm-I for
CdI and 16.4 cm-I for ZnI1*). However, use of a Dunham expansion alone provides no warning of the dominant contribution
of model dependence to those uncertainties (in T,(B)). Even more
seriously, Dunham expansions are completely unreliable for extrapolating beyond the range of the input data to estimate the
dissociation energy and the number of energies of missing levels.
At this point some comment is appropriate on the effect of the
uncertainties in the assumed values of c6 (and hence of X(6)) on
the results of the NDE fitting procedure. In the application of
eq 7 (with p = 3) to data for ZnI,I* it was suggested that the
uncertainty in c6 (that is, in X(6)) was the major source of
uncertainty in the resulting dissociation energies. However, our
present experience suggests that this apparent sensitivity is an
artifact associated with use of a single-model NDE function. We
agree with ref 18 that fits to eq 7 with p = 3 are quite sensitive
to the value of C,. In particular, repeating such fits with c6 scaled
up and down by a factor of 1.5 changes the apparent Do value
by -200 cm-l and the corresponding uD by five levels. However,
on repeating our averaging-over-models fits with c
6 scaled up and
down by a factor of 1.5, the resulting Do values change only 12
cm-I and uD changes by less than one level. Thus, model dependence is the major source of uncertainty in such fits, and
uncertainty in the assumed-known value of the long-range potential
constant c6 is not. A copy of the averaging-over-models computer
program for fitting NDE’s to experimental vibrational energies
used herein may be obtained from one of the authors on request.“O
The near-dissociationexpansion fits described above were only
applied to the data for the most abundant isotopomers 114Cd1271
TABLE MI: Excited-State Dissociation Energies ( D l , cm-I) for tbe
MI Series
molecule
CdP
ZnP
B22+
34310.1
36 230.5
c2n1,2
D2U3,2
10200.2
1 1 233.4
‘Dissociation energy errors are *179 and f144 cm-l for CdI and
ZnI, respectively.
and 64Zn1271.While it would be both desirable and feasible to
perform a simultaneous fit to the data for all isotopomers using
mass-reduced quantum numbers, that capability is not yet incorporated into our NDE fitting program.40 In any case, NDE
expressions for any other isotopomer can be generated by replacing
(uD - u) by (qD- q ) = pr(uD - u), where pi for CdI has been defined
below eq 1, and an analogous definition holds for ZnI.
Excited-state dissociation energies can be determined using the
ground-state dissociation energies, D,(X), listed in Table V. In
particular, the dissociation energy of the B2Z+ion-pair state, D,(B),
of an MI radical can be found using
D,(B) = D,(X) + IP(M) - EA(1) - Te(B)
(15)
where IP is the ionization potential of the metal M,4I EA(1) is
the electron affinity of atomic iodine$* and T,(B) is the electronic
origin of the B state. Similarly, the dissociation energy for the
C and/or D state, D,(C/D), can be calculated from
D,(C/D) = D,(X) + E(M*) - T,(C/D)
(16)
where E(M*) is either the energy of the lowest metal
(for the
C state) or the energy of the lowest M
term (for the D
The resultant values are presented in Table VII. In all cases the
largest source of error in the excited state De(s are the Uncertainties
in the values of D,(X).
3e
3e
VIII. Discussion and Conclusions
The results of the CdI B-X analysis presented here complete
a delineation of the spectroscopic properties of the group IIB metal
iodides, MI (M = Zn, Cd, and Hg). While there have been
attempts at interpreting the CdI B-X transition,16the molecular
constants obtained in this work are the first ones based on
well-resolved spectra. Our B-state vibrational frequency, Ylo=
104.3 cm-’, is in good agreement with the estimate of -1 10 cm-l
suggested by Greene and Eden,44who anticipated a value similar
to that for the analogous ion-pair state of HgI. However, it is
considerably larger than the value of 74 cm-I reported by Pate1
et ale3Their vibrational analysis was based on emission spectra
of CdI near 440 nm excited in a radio-frequency discharge in the
absence of a buffer gas. Little relaxation within the B-state
vibrational manifold is expected under those conditions. Consequently, if the emission they observed is from the B state, then
their observed transitions almost certainly originate from its high
vibrational levels and terminate on low vibrational levels in the
ground state. Unfortunately, since our B-state constants were
derived from a small range of excited-state vibrational levels (uB
< 5), assigning the high excited-state quantum numbers (uB
90) and isotopomers for the Patel bands by extrapolation was not
found to be feasible. That task must await high-resolution absorption data.
Our ground-state CdI dissociation energy, D,” = 7905 (179)
cm-I, determined using near-dissociation theory, is smaller than
estimates based on either thermodynamic data45,46or a linear
Birge-Sponer extrapolation from low-lying ground-state vibrational term values (D,” between 5 1 1292 and 12501 cm-1).16 The
analogous results for ZnI are also distinctly more reliable than
those obtained earlier. In the future, a combined-isotopes version
of the present analysis which involves simultaneous direct fits to
the data for both the B-X and C-X (or D-X) transitions should
yield much more sharply defined parameters for these systems.
The magnitude of the CdI dissociation energy and the trend in
D F s for the MI series can be understood by considering the nature
of the B and X electronic states.
Although the electronic structure of CdI has not been the
subject of theoretical scrutiny, ab initio calculations have been
4786 The Journal of Physical Chemistry, Vol. 96,No. 12, 1992
done for the low-lying states of HgCL4’ At large internuclear
separations, the ground state is purely covalent and B2Z+is 90%
ionic with a small 10%admixture of excited-state character. At
shorter internuclear separations,the B state becomes increasingly
covalent and the ground state acquires an attractive ionic component. At the equilibrium internuclear separation of the B state,
both X2Z+and B2Z+are -50% ionic and 50% covalent. This ratio
changes further at the equilibrium internuclear separation of the
ground state where XzZ+becomes -80% ionic. A similar scenario
is expected to be valid for the MI free radicals as well.
Since little is known about CdI, we have used the ionimvalent
resonance energy, A, as a measure of the ionic contribution to the
ground-state binding energy.4s This parameter, which has been
used successfully to predict the dissociation energies of Z n P and
can be calculated for MI (in
the mercurous halide
electronvolts) by the relation
where X,is the electronegativity of atom i of the diatomic. In
this way, the dissociation energies of ZnI, CdI, and HgI were
predicted to be 8227, 7581, and 3549 cm-I, respectively. When
compared to the experimentally determined values of 8854,7905,
and 2800 cm-’,6 the agreement is striking and shows that the
binding energies do reflect the amount of ionic character mixed
into the ground state. Furthermore, the experimentally derived
dissociation energies for CdI and ZnI illustrate that a comprehensive and systematic application of the “averaging-over-models”
NDE procedure provides a more realistic estimate of physically
interesting parameters than are obtained from extrapolationsusing
conventional Dunham expansions or graphical Birge-Sponer plots.
The magnitude of the B-state vibrational frequencies for the
MI series is more problematic, since w,(CdI) is < w,(ZnI) but
> w,(HgI). This differs from the situation for the halogen IX
series (X = Br, C1, F) where the ion-pair state frequencies appear
to scale inversely as the square root of the reduced mass of the
molecule.50 If a similar relationship was valid for the group IIB
metal halides, then one might expect an we = 95 cm-l for HgI.
The observed trend for the MI B-state frequencies can be rationalized qualitatively by examining the differences in the amount
of ionimvalent mixing of the electronic wave functions. Recall
that the ground-state A(Zn1) A(Cd1) > A(Hg1). By inference,
the B state of HgI must be the “purest” ion-pair state of the MI
series; that is, the B state of HgI is the MI ion-pair state with
the minimum amount of repulsive covalent character. As such,
one might expect HgI(B*Z+)to be more strongly bound than ZnI
and CdI and, as a result, to have a higher vibrational frequency
than might otherwise be expected. It suggests, from one point
of view, that HgI is the anomalous member of the series, since
the degree of ionic-covalent mixing is similar for ZnI and CdI.
However, it could also be argued that HgI is the well-behaved
member and ZnI and CdI are anomalous, since the B states of
the two latter radicals are subject to stronger covalent contamination.
Klemperer has noted a similar trend in the ground-state
stretching force constants of the group IIB metal dihalides, as well
as those for the monochlorides of Cu,Ag, and A u . ~This
~ behavior
appears to correlate with the minima in the ionization potentials
for Cd and Ag metal atoms in their respective series and was taken
as a confirmation of the ionic nature of the states under consideration.
The ground-state frequenciesappear to be well-behaved in that
they are ordered o/(ZnI) > w,”(CdI) > w,”(HgI). This is not
necessarily inconsistent with the trend in the B-state frequencies,
since the degree of mixing at r,(X) is expected to be different than
that at r,(B)!7 The ground states should be essentially ionic near
their minima, and therefore, the behavior of the ground-state
frequencies probably reflects this approximately “pure” ionic
description. The smaller ground-state frequency and dissociation
energy for HgI compared to those for ZnI and CdI reflects a more
covalent picture of HgI bonding. This appears to hold for HgI,
as well. The mercury diiodide ground-state bending force constant
is found to be higher than those for the analogous Zn and Cd
-
Jordan et al.
compo~nds.~’This was taken as an indication that HgI, bonding
is more covalent in nature. A more quantitative picture of MI
bonding will require additional theoretical guidance.
The D 2113/2state constants of this work are in very good
agreement with earlier resu1ts.I The observation of individual
isotopomer peaks, however, is new. As noted previously, the C-X
transition is weaker than D-X, even though C lies below D in
energy. This behavior of CdI has been noted in other studies and
is opposite to that observed for ZnI. At present, the reason for
this intensity anomaly is not understood, but it may be a result
of predissociation by the uncharacterized repulsive A 211 state,
which dissociates to ground-state atoms.47J3*54
Closely related to CdI is cadmium monocyclopentadienide,
whose electronic spectrum has been investigated by Ellis and
co-workers.I7 The structure of this radical is thought to be a
“half-sandwich” of C, symmetry. A single vibrational progression
with a spacing of 219 cm-’ was assigned to the Cd-ring stretching
mode. The electronic transition was tentatively assigned to the
lowest A-X band system, which has an electronic origin located
at 22261 cm-l.
Despite the analogy that has been drawn between this organometallic complex and cadmium halides, some uncertainty
exists as to whether the lowest excited state is actually ionic or
covalent. Several considerations, however, support a covalent
excited-state assignment. Firstly, the electron affinity (EA) of
the Cp radical is smaller (1.786 f 0.020 eV)55than that for atomic
iodine (3.059 eV),“, resulting in a reduced tendency to form ionic
states in CdCp. The dissociation limit of the first CdCp ionic state
(relative to the top of the ground-state potential curve) is given
simply by IP(Cd) - EA(Cp) and lies >10000 cm-l higher than
the analogous limit for CdI. secondly, an ionic CdCp excited state
implies a metal-to-ligand charge transfer. Transitions of this kind
involve a substantial charge redistribution within the molecule
and would probably result in relatively large geometrical changes.
In that case, based on Franck-Condon considerations, the strong
(0,O)transition observed in the stretching vibration would be less
likely. Finally, by approximating the Cd-Cp stretching vibration
as a local mode, its frequency should be reasonably well predicted
by scaling the CdI B-state frequency by the factor [m(I)/m(Cp)]I/’, where m is the mass of the “ligand” (Cp or I) bonded
to cadmium. An w,(B) = 104 cm-I for CdI leads to a prediction
of -125 cm-’ for CdCp, which is in poor agreement with the
observed frequency of 219 cm-I. However, scaling the covalent
CdI D-state frequency of 199 cm-’ yields a CdCp frequency of
=235 cm-I, which is much closer to the observed value.
CdI lases on the B-X transition with an output peaking near
657 nm.15 Gain exceeding 1% cm-l has been measured between
653 and 662 nmaS6Using a naturally abundant sample of Cd12
as the parent molecule, oscillation occurs on over 30 distinct lines
with the strongest transitions at 657.1 and 657.4 nm. In discharges
containing an isotopically enriched sample of Il4CdI2(98.6%) the
laser spectrum is dominated by the line at 657.1 nm, with additional weaker lines evident at 656.8, 657.3, and 659.3 nm1.57
The laser output does not correspond directly to any of the
observed transitions listed in Table 111. Therefore, the constants
in Table VI were used to predict IWdI E X bandhead frequencies
for 10 1 uB 1 0 and 60 1 ux 1 30. While a reasonably good
agreement (within 3 an-’)was found for the primary peak at 657.1
nm and the bandhead frequencies for (UB,UX) = (5,48) and (8,52),
this may be fortuitous since the assignments for the other peaks
were inconclusive. This lack of correlation between the laser output
and speclfic bandhead frequencies has also been noted for HgBr.5s
It was attributed to lasing action occumng on a nearly continuous
rotational envelope, which in turn resulted from overlapping vibrational sequence bands. Presumably, the same mechanism is
operative for CdI lasing as well.
Acknowledgment. Funding for this research has been provided
by the Network of Centres of Excellence in Molecular and Interfacial Dynamics, one of the 15 Networks of Centres of Excellence supported by the Government of Canada. The support
of the Natural Sciences and Engineering Research Council
J . Phys. Chem. 1992, 96, 4181-4194
(NSERC) and the Ontario Laser and Lightwave Research Center
(OLLRC) is also gratefully acknowledged. We thank Drs. D.
S.Yang, P.Bemath, and M. Moskovits for helpful discussions.
Registry No. Cadmium monoiodide, 14184-47-5; mercury monoiodide, 7783-30-4; zinc monoiodide, 31 246-29-4.
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ESR and ENDOR of Pentacoordinated Cobalt( I I ) Porphyrins
G.P.Diiges and J. Huttermam*
Fachrichtung Biophysik und Phys. Grundlagen der Medizin, Universitdt des Saarlandes, 6650 HomburglSaar,
Germany (Received: November 18, 1991; In Final Form: February 5, 1992)
The coordination properties of pentacoordiiated cobalt(I1) octaethylporphyrin and cobalt(I1) tetraphenylporphyrin with imidazole
and, less extensively, pyridine were studied by electron spin resonance (ESR) and electron nuclear double resonance (ENDOR)
spectroscopy as models for the heme group in deoxygenated myoglobin and hemoglobin. A range of methyl-substituted imidazoles
(at positions 1, 2, 4, and 5) a s well as perdeuterated and ISN-substituted imidazole were used a s axial ligands in order t o
aid the assignment of 'H and I4N E N D O R lines. E S R measurements were performed a t 77 K a t X-band (9.5 G H z ) and
a t 100 K at Q-band (34.5 GHz) frequencies using chloroform, toluene, and benzene a s glass-forming agents. A detailed
comparison reveals that a combination of solvent and methyl substitution affect the resolution in the g,,part and extra absorption
in the g, part of the X-band spectra. ENDOR measurements taken a t about 5 K emphasized IH and I4N interactions in
toluene. The assignment of spectra was supported by "powder"-type spectra simulation using first-order perturbation treatment
together with variation of methyl substitution. The results are interpreted in terms of a coordination geometry which is compared
with low-temperature crystal structure data.
Iatroduction
&balt(II) porphyrin complexes are, among others (for a survey
see, e.g., ref l), of interest as model systems for Co-substituted
hemo- and myoglobin which in turn are electron spin resonance
(ESR)active models for the native iron-containing proteins in
both the pentacoordinated and the oxygenated form. There have
&n quite a few ESR studies performed in this antext since the
first iron replacement in hemoglobin was carried out by Hoffman?
specifically by the Yonetani g r o ~ p . ~The
. ~ 6-fold coordinated,
oxygenated Proteins exhibit Slightly rhombic ESR Spectra with
g factors of typically 2.08, 2.00, 1.9K5 Chien and D i ~ k i n s o n ~ . ~
have Shown that the "omeric
myoglobin SuPFQm two different
stereochemical configurations of oxygen binding, whereas hemoglobin and its a- and @-subunits do not appear to exhibit such
0022-3654/92/2096-4787$03.00/00 1992 American Chemical Society