15 July 1994
ELSEVIER
CHEMICAL
PHYSICS
LETTERS
Chemical Physics Letters 224 ( 1994) 38 l-390
Metal hyperfine structure in magnesium chloride.
The millimeter-wave spectrum of 25MgC1and 26MgC1
M.A. Anderson ‘, L.M. Ziurys 2
Department of Chemistry. Arizona State University, Tempe, AZ 85287-1604, USA
Received 6 April 1994
Rotational spectra of the magnesium and chlorine isotopomers of MgCl (X ‘Z’) have been measured in the frequency range
260-380 GHz using direct absorption techniques. The species studied ( 25Mg3sC1,2sMg37CI, 26Mg35Cl,and 26Mg37C1)were created in a dc glow discharge by the reaction of magnesium vapor with CCL,. Several rotational transitions were recorded for each
isotopomer, and fine structure, as well as hyperfime splittings due to the 25Mg nucleus, were observed. Rotational, spin-rotation
and hype&me constants were determined for these radicals. The average value of the Fermi contact term was !+( 25Mg) = - 3 17.8
(4.9) MHz, indicating a large electron density at the magnesium nucleus.
1. Introduction
The alkaline earth monohalide species are considered to have primarily ionic bonding, with an electronic structure chiefly represented by an M+Xconfiguration [ 11. These molecules all have *C electronic ground states, and therefore are free radicals
with a single unpaired electron. This electron is
thought to reside in a s-type orbital centered on the
metal atom, giving rise to the M+X- structure. In
fact, several ionic bonding models have been developed to predict the properties of the alkaline earth
monohalide species. Such models include a moditied-Rittner type theory which includes the effects of
polarization, developed by Toning and collaborators
[ 21. Another model by Rice, Martin and Field [ 31
treats the metal ion as being perturbed by a ligand
field due to the halide ion X-. Such models are useful
because they test semi-classical theories of bonding.
’ NASA Space Grant Fellow.
* NSF Presidential Faculty Fellow.
While the alkaline earth halide radicals are chiefly
ionic molecules, they do have some non-negligible
covalent character to their structure. This covalent
mixing is apparent because of there is often non-zero
electron density at the halide nucleus, as determined
from evaluation of molecular hypertine constants
(e.g. Refs. [4,5]). Bernath et al. [6] attributes this
covalent contribution to the bonding as arising from
polarization of the halide orbitals by the metal M+
nucleus. On the other hand, more recent calculations
by Buckingham and Olegario [ 1 ] suggest that the covalent character comes from charge transfer from the
X- to M+, as well as electron overlap.
The bonding in alkaline earth halides therefore is
not completely understood. The models, however, can
be better developed if they can be directly compared
to experimental data, in particular to hyperline measurements. Hyperfine structure is very sensitive to the
electronic properties of molecules, and thus serves as
a useful probe of chemical bonds. Examining the hyperfine structure of the alkaline earth monohalides
has been particularly instructive, because in certain
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved
SSDIOOO9-2614(94)00532-U
382
MA. Anderson,L.M. Ziurys /Chemical PhysicsLetters 224 (1994) 381-390
cases both nuclei in the molecule have nuclear spin.
Hence, comparison of the magnitudes of the relative
hype&e parameters indicates the distribution of the
unpaired electron in the species.
Extensive studies have been carried out of the hyper-fine structure for many alkaline earth monohalides, especially for the fluoride series BaF [ 7 1, SrF
[ 8 1, CaF [ 91, and MgF [ 5 1. Hyperfine parameters
have been evaluated for CaBr and CaI, as well [ 61,
although the structure in the former molecule is not
at all understood. In general, the constants for the
metal nuclei (when they have a spin) are larger than
for the halide nuclei, indicating that the unpaired
electron is primarily located in the metal nucleus, i.e.
the ionic M+X- structure. However, for the fluoride
series, the hf parameters on the metal nucleus gradually decrease from BaF to SrF, CaF, and finally MgF,
as the fluorine constants increase [ 51. Thus, some
percentage of the electron density study transfers from
the metal nucleus to that of fluorine. Consequently
the bond becomes less ionic and more covalent as the
metal nucleus becomes smaller.
For the chloride series, however, evaluation of the
hyperfine parameters has not been as complete. Although both abundant chlorine isotopes have nuclear
spins (35C1:1=3/2; “Cl: Z=3/2), chlorine hf parameters have been determined only for Ca35C1[ lo]
and Sr35C1[ 111. Moreover, the metal hyperfine constants have been measured for only one species,
“‘BaCl [ 71. Although the rotational spectrum of
24MgC1has been recorded by Bogey et al. [ 121, these
authors did not observe hyperline splittings, which
would arise from the chlorine nucleus.
In order to better evaluate the bonding in alkaline
earth monochlorides, we have measured the pure rotational spectra of the magnesium and chlorine
isotopomers zsMg35C1, 25Mg37C1, 26Mg35C1 and
26Mg37C1.
These measurements complement the past
high resolution work on magnesium chloride by Bogey
et al. [ 121. These authors recorded various rotational transitions of both 24Mg35C1and 24Mg37C1
arising from the v= 0, 1, and 2 vibrational modes, as
well as one transition of zaMg35C1( Y= 0 ) , using millimeter wave direct absorption spectroscopy in the
130-290 GHz region. We have measured seven transitions of 25Mg35C1and four transitions “Mg3’C1, respectively, as well as nine transitions each for
2aMg35C1and 2aMg37Cl,in the frequency range 260-
377 GHz. Although no chlorine hyperfine structure
was observed, we have resolved hyperfine splittings
arising from the “Mg spin ofZ= 5/2, and have determined the Fermi contact Z+ constant, as well as the
quadrupole parameter of eqQ. In this Letter we present our measurements and derived spectroscopic
constants for these isotopomers.
2. Experimental
The measurements were carried out using the ASU
millimeter/sub-millimeter
direct absorption spectrometer, which is described in detail elsewhere [ 13 1.
Briefly, the instrument consist of a coherent, tunable
source of mm and sub-mm radiation, a gas cell incorporating a Broida-type [ 14 ] oven, and a detector system. The sources for the spectrometer are Gunn oscillators, which operate in the range 65-140 GHz; to
generate the higher frequency radiation used for these
experiments, a Schottky diode tripler and quadrupler
were used. The radiation is launched from a scalar
feedhorn and coupled into the cell quasi-optically using several teflon lenses. The cell is a double-pass system. After the radiation makes its second pass through
the cell, it is reflected from a wire grid and into the
detector. The detector is an InSb bolometer, which is
cooled to 4.2 K with liquid helium. Phase sensitive
detection is achieved at 2fthrough FM modulation
of the Gunns and use of a lock-in amplifier. The spectrometer is operated under computer control through
an IEEE bus.
Magnesium chloride was generated in a dc discharge using a mixture of Mg vapor, argon, and carbon tetrachloride. The discharge current used was
~250 mA. The metal vapor was generated in the
Broida-type oven, and carried into the cell using = 25
mTorr of argon carrier gas. Approximately 7 mTorr
of CCb was added to this mixture. The spectra were
recorded by scanning both in increasing and decreasing frequency, and the resulting data averaged. Center frequencies for transitions were determined by
fitting Gaussian curves to the line profiles. Typical
linewidths were 600-800 kHz. The spectra for 26MgC1
and 25MgClwere observed with the magnesium and
chlorine isotopes in their natural abundances ( 24Mg:
78.60%; 25Mg 10.11%; 26Mg: 11.29%; 35C1:75.4%;
“Cl: 24.6%).
MA. Anderson, L.M. Ziurys / Chemicd Physics Letters 224 (1994) 381-390
383
Table 1
18+19
35/2+37/2
37/2+39/2
19+20
37/2+39/2
39/2+41/2
20+21
39/2+41/2
4112-4312
21+22
41/2+43/2
43/2+45/2
15+16
16+17
17+18
la-19
19+20
20+21
20+21
19-+20
l&+19
17+18
16-17
21+22
212024.939
272027.425
272030.015
272032.671
272035.348
272031.758
272076.938
272079.540
272082.022
272084.638
212087.2 11
272089.716
-0.070
-0.078
- 0.070
-0.083
-0.141
-0.168
0.130
0.299
0.077
0.042
0.033
0.017
22+23
16+17
17-18
18+19
19+20
20+21
21-22
21+22
20+21
19+20
la-19
17+18
22-23
286319.452
286321.821
286324.232
286326.674
286328.998
286331.676
286372.766
286374.770
286376.937
286379.432
286381.481
286384.224
-0.069
-0.047
- 0.050
- 0.075
-0.196
0.527
0.162
0.215
- 0.040
0.009
-0.355
0.016
23+24
17+18
18+19
19+20
20+21
21+22
22+23
22-23
21-22
20-+21
19+20
la+19
23-~24
300610.224
300612.471
300614.730
300617.015
300618.930
300621.123
300664.197
300666.370
300668.259
300670.488
300672.743
300674.961
- 0.070
-0.037
-0.037
-0.030
-0.307
0.341
-0.317
0.314
0.032
0.002
-0.002
-0.019
24+25
la+19
19+20
20-21
21-22
22+23
23+24
23-24
22+23
21-22
20-121
19-20
24+25
314897.121
314899.212
314901.340
314903.437
314905.591
314907.018
314952.051
314953.444
314955.531
314957.586
314959.702
314961.791
-0.022
-0.021
-0.011
-0.021
0.169
0.396
-0.316
-0.120
0.021
-0.014
-0.015
-0.036
43/2+45/2
45 /2+47/2
45/2+47/2
47/2+49/2
47/2+49/2
49/2+51/2
19+20
20+21
21-22
22+23
23+24
24-25
24+25
23+24
22-23
21+22
2o-b21
25+26
.329179.872
329181.844
329183.842
329185.779
329187.822
329188.867
329235.834
329236.104
329238.671
329240.574
329242.563
329244.526
-0.007
-0.012
-0.001
-0.016
0.264
0.399
-0.157
-0.195
0.026
-0.006
-0.005
-0.036
20+21
21+22
22+23
23+24
24-125
25+26
25+26
24+25
23-24
22-23
21+22
26-27
343458.31 a
343460.195
343462.073
343463.864
0.001
0.007
0.016
-0.002
0.192
-0.478
0.494
-0.173
0.011
-0.022
-0.025
-0.051
21-22
22-23
23+24
24-25
25-26
26+27
26+27
25+26
24-+25
23+24
22+23
27-+28
343465.641’
343515.707.
* 343517.458
343519.220
343521.085
343522.948
357732.281
357734.062
357735.823
357737.474
357739.069
357790.128.
357791.710
357793.341
357795.096
357796.813
’
0.013
0.019
0.021
-0.008
0.164
-0.306
0.270
-0.198
-0.024
-0.060
-0.063
-0.137
MA. Anderson, L.M. Ziwys / ChemicalPhysics Letters 224 (1994) 381-390
384
3. Results
The seven rotational transitions recorded for
2sMg3sC1are listed in Table 1. As the table shows,
each rotational transition consist of twelve hyperfine
components which arise because of the 25Mgspin of
512. Some of the components are blended at the
higher Ntransitions. Table 2 lists the nine transitions
recorded for 26Mg3sC1.While the magnesium-26 nucleus does not have a spin, I= 3/2 for 35C1.Hyperfine
splittings due to the chloride nucleus, however, were
not observed in these data, although spin-rotation
interactions were recorded. Hence, each rotational
transition consist of two components separated by
y= 63.5 MHz. This result is not unexpected. Bogey et
al. [ 121 did not observe any hyperfine interactions
in the N= 19+ 20 transition of 2aMg3sC1.We, in fact,
remeasured the same transition. Our frequencies
agree with those of Bogey et al. to within f 50 kHz.
Table 3 presents the four transitions recorded for
the zsMg37C1isotopomer. Again, hyperfine interactions were observed in these data arising from the
25Mgnucleus. Thus, each transition consist of twelve
hf components, some of which are blended together
Table 2
Observed transition frequencies of 26Mg35C1:
X ‘Z+ (u= 0)
N+N’
J-rJ
vob, (MHz)
v0, - VW, (MHZ)
18-19
35/2+37/2
37/2-+39/2
265952.954
266016.225
-0.004
0.007
19+20
37/2+39/2
39/2+41/2
279928.911
279992.203
-0.020
0.028
39/2-t41/2
41/2+43/2
293901.342
293964.558
-0.003
<O.OOl
21+22
41/2+43/2
43/2+45/2
301869.999
307933.193
- 0.003
0.003
22-23
43/2+45/2
45/2+47/2
321834.740
321897.889
0.011
- 0.002
23-24
4512-4712
47/2+49/2
335795.366
335858.455
0.017
-0.028
24+25
47/2-+49/2
49/2+51/2
34975 1.684
349814.781
0.001
-0.001
25-26
49/2+51/2
51/2+53/2
363703.561
363766.621
0.010
<O.OOl
26-21
51/2+53/2
53/2+55/2
377650.179
377113.830
0.003
0.009
20+21
at higher N. Hypertine interactions attributable to
chlorine were not observed in these data, although the
37C1nucleus has a spin of 3/2. As Table 4 illustrates,
chlorine hf splittings were not resolved in the
2aMg37C1data, either. In the nine transitions studies
here, only spin-rotation interactions were observed,
with an average splitting of yo= 62.1 MHz.
Figs. 1 and 2 show representative spectra of the
magnesium 25 and 26 isotopomers of MgCl measured in this work. In Fig. 1, the N=23-+24 transition of 25Mg35C1and N=22+23
transition of
2sMg37C1
are disp layed, which occur near 343 and 322
GHz, respectively. In both spectra, almost all the
twelve hyperfine components are resolved, which
arise from the 25Mgspin and are indicated by quantum number F. In Fig. 2, the N= 26-27 transition of
2aMg35C1and 26Mg37C1are shown near 378 and 369
GHz. In these data, only the spin-rotation interactions are apparent, which give rise to fine structure
doublets indicated by quantum number J. These data
appear in emission because of the phase used on the
lock-in amplifier; also, second derivative spectra are
recorded because detection is done at twice the modulation frequency. Estimated experimental errors on
the frequency measurements are f 150 kHz.
Table 5 lists the spectroscopic constants derived for
all four MgCl isotopomers. These data were analyzed
using the following ‘I; Hamiltonian, which assumes a
case b, coupling scheme:
Ei,, =B~2+y~~~+bf-~+cfz,I?z
+eqQ[3fi-Z(Z+1)]/41(2Z-1).
(1)
Implicit in the rotation and spin-rotation terms in
the above expression are centrifugal distortion corrections Do and yD. Also, the hyperfine constants b
and c are the Frosh and Foley [ 15 ] definitions, and
cr, the nuclear spin-rotation term, was set to zero.
For the magnesium 26Mg35C1and 26Mg37C1spectra, only Bo, Do, and y. were determined from the data.
No hyperline structure was observed for these two
molecules, so corresponding constants could not be
derived. Also, a better fit for these species was obtained by using a centrifugal distortion correction, ~D’D,
in the Hamiltonian. The rotational constant B. derived for 26Mg35C1is in good agreement with that estimated from the work of Bogey et al. [ 121. No other
MA. Anderson, L.M. Ziutys /Chemical Physics Letters 224 (1994) 381-390
Table 3
Observed transition frequencies of %g”C1:
X ‘Z+ (v= 0)
N-N’
J+J
F-F’
vobm
(MHz)
V.&S- UC&
(MHz)
22-23
43/2+45/2
19+20
20+21
21+22
22-23
23-24
24-25
24+25
23+24
22-23
21+22
20+21
25-26
321789.126
321791.123
321793.121
321795.082
321796.986
321798.025
321843.552
321844.617
321846.433
321848.359
321850.367
321852.325
-0.068
- 0.033
-0.017
-0.012
0.104
0.157
-0.100
-0.019
0.026
0.013
0.040
0.015
20--*21
21-b22
22+23
23+24
24-25
25+26 >
25+26
335747.777
335749.664
335751.546
335753.369
-0.049
-0.022
-0.004
0.004
0.347
-0.391
0.249
-0.488
0.022
0.030
0.062
0.040
45/2-+47/2
23-24
45/2+41/2
41/2+49/2
24+25 1
23+24
22-23
21+22
26+27
24+25
47/2+49/2
4912-5112
21-22
22+23
23+24
24-25
26+27 1
25-26
26+27
25-26 1
24+25
23+24
22-23
27-~28
25-26
49/2+51/2
51/2+53/2
’ Blended lines.
385
22+23
23+24
24-25
25+26
26+27
21+28 1
21-28
26+27 >
25-26
24+25
23+24
28+29
335755.320’
335803.322 ’
335805.424
335807.232
335809.127
335810.982
349702.135
349703.925
349705.704
349707.387
349109.070
l
349158.453 ’
349760.104
349761.763
349763.521
349765.275
363652.031
363653.734
363655.401
363656.957
363658.490’
363708.763 ’
363701.272
363711.805
363713.473
363715.156
-0.039
-0.013
0.011
0.008
0.244
-0.288
0.334
-0.196
0.022
0.009
0.012
-0.014
-0.026
0.002
0.014
0.002
0.231
-0.130
0.142
-0.217
0.002
-0.021
-0.007
-0.015
386
MA. Anderson, L..M. Ziurys /Chemical Physics Letters 224 (1994) 381-390
Table 4
Observed transition frequencies of 2sM837ck X 2P (u=O)
N-rN’
J-d’
%dMHz)
vti-
18+19
35/2+31/2
37/2+39/2
259842.714
259904.528
0.018
- 0.006
19+20
37/2+39/2
39/2+41/2
273498.112
273559.930
0.008
0.015
20+21
39/2+41/2
41/2+43/2
287150.092
287211.858
-0.011
-0.027
21-22
41/2+43/2
43/2+45/2
300798.499
300860.280
-0.024
0.00s
22+23
43/2+45/2
45/2+47/2
314443.190
314504.913
- 0.002
<O.OOl
23-24
45/2+47/2
4712-4912
328083.954
328145.641
0.014
0.013
24+25
47/2+49/2
49/2+51/2
341720.605
341782.286
0.007
0.03s
25+26
49/2+51/2
51/2+53/2
355352.976
355414.601
-0.018
-0.010
26427
51/2+53/2
53/2+55/2
368980.978
369042.532
0.02 1
- 0.006
u&MHz)
comparisons can be made because data for the other
isotopomers do not exist.
Hyperflne constants were determined for 25Mg35C1
and 25Mg37Cl,in addition to Bo, D,,, and yO.Because
only transitions with N= 18+ 19 and higher were observed for these two species, the spin-spin dipolar
term c could not be derived. Therefore, it was fmed
to zero in the tit for both molecules. Hyperfine parameter b and quadrupole term eqQ, however, were
determined, and are given in the table. Because c= 0,
the Frosh and Foley b constant is equivalent to the
Fermi contact term, i.e. b= &.
4. Discussion
The 25MgCl and 26MgC1measurements are additional evidence that the bonding in MgCl must be
chiefly ionic, with the unpaired electron density
chiefly on the metal atom. For the 26Mgisotopomer,
no hyperline structure was observed in any of the
transitions recorded, although both the 3sC1and “Cl
nuclei have spins. In fact, the data is well fit by a
Hamiltonian with just rotational and spin-rotation
terms, including centrifugal distortion corrections.
These constants reproduce the measured transition
frequencies for the magnesium-26 isotopomers to
better than pob- vcalc5 35 kHz. Therefore, there appears to be no residual effects due to unresolved hyperfine splittings; the interactions must obviously be
insignificant.
The inability to observe chlorine hyperfine splittings in MgCl is not unexpected. The 35C1hypexfme
parameters determined for CaCl [ 10 ] and SrCl [ 111
are quite small, as shown in Table 6. The Frosh and
Foley b constant is a 19 MHz for both these species,
and the c parameter is 12.5 and 7.7 MHz for CaCl
and &Cl, respectively. If the Cl hyperfine constants
are similar in magnitude for MgCl, splittings due to
hyperfme interactions would be unresolved at the high
N values measured here, given the observed linewidths. For both CaCl and SrCl studies, the measurements of the hyperfine structure were done by observing very low N transitions, or with higher
resolution spectroscopic techniques.
For 25Mg35C1and 25Mg37C1,hyperfime structure is
clearly apparent in the data, and it can be effectively
reproduced by considering interactions due to the
25Mg nucleus only. The spectroscopic constants derived for the two 25 magnesium isotopomers reproduce the measured transition frequencies to better
than vobs- vcalcS500 kHz, even for blended lines.
Hence, neglecting effects of the chlorine nuclear spin
does not appear to degrade the data analysis. The
Fermi contact terms determined for 25Mg35C1and
25Mg37C1are -319.1 (2.9) MHz and -316.4 (3.9)
MHz, respectively, indicating a relatively substantial
electron density at the magnesium nucleus. Hence,
Mg’Cl- must be the dominant configuration for the
bonding in this molecule. Moreover, the &term for
MgCl is very close in value to the Fermi contact constants for the 25Mgnucleus of other ionic magnesium
compounds, including MgF and MgOH. As shown in
Table 7, &(MgF) = - 304.1 MHz and & (MgOH) =
-304.4 MHz. In addition, the magnesium 25 quadrupole coupling constants for MgCl, MgF and
MgOH are similar. For the two halide molecules,
esQ= - 20 MHz, while the constant for MgOH is near
- 40 MHz, but with a large error so it could conceivably be near in value to - 20 MHz as well.
The hyperfine constants of 25MgC1contrast those
derived for the “‘Ba nucleus (I= 3/2) of BaCl. Be-
MA. Andetmn, L.M. Ziwys / ChemicalPhysics Letters 224 (1994) 381-390
387
25Mg35C1
N=23+24
11
I
334,440.g
I_
334,490.g
I
I
‘334.5 I.8
25Mg37C1
I
II
329,162.5.
N=22+23
J+-+
J++
F=25+26
I
I
329,212.S
I
I
329,26 .5
FREQUENCY(MHz)
Fig. 1. Spectra of the N=23+24 rotationaltransitionof 2sMg3sCland N=22+23 transitionof 2sMg37C1
observed in this work near 343
and 329 GHz, nxpectively. Twelve hyperhe components, which arise from the 2sMg nuclear spin of 5/2 and are labeled by quantum
number F, are present in &se data for each transition. These spectra represent a single, four minute scan.
MA. Anderson, L.M. Ziurys /Chemical Physics Letters 224 (1994) 381-390
388
26Mg3T1
I
N=26+27
J=
51
J=ysf
55
2
2
377,632.4
377,687.4
377,732.4
26Mg37C1
N=26_,27
J=i
J
367,961.g
.ss
2
=5’ .-m -53
2
2
368,011.g
368,061.g
FREQUENCY (MHz)
Fig. 2. Spectra of the N=26-+27 transition of 26Mg3SC1
and 26Mg37C1
near 377 and 368 GHz observed in this work. In these data, only
spin-rotation interactions are resolved, resulting in doublets labeled by quantum number J. Hypefine effects arising from the chlorine
nucleus were not observed. The weak set of lines near the J= 51/2-+53/2 spin component of 26Mg35C1
are due to vibrationally excited
2sMg3sCI.
MA. Anderson, L.M. Ziurys /Chemical Physics titters 224 (1994) 381-390
389
Table 5
Molecular constants for MgCk X ‘E+ (o= 0) ’
Millimeter-wave (MHz)
Constant
YD
7004.96789(41.)
0.00744255(36)
63.520(35)
0.000225(21)
=Mg3’C1
Bo
Do
Yo
m
6843.90817(60)
0.00710374(52)
62.069(53)
0.000219(32)
2SMgs5Cl
Bo
Do
Yo
b
26Mg”CI
BO
Do
YO
c 7005.54897
7165.0272(76)
0.0077881(74)
64.68( 18)
-319.1(2.9)
C
0.0
b
- 19.0( 1.5)
eqQ
2sMg37CI
Ref. [ 121
7003.976( 15)
0.007441( 13)
63.11(21)
-316.4(3.9)
BO
Do
Yo
b
C
0.0
b
- 17.5( 1.4)
eqQ
’ Errors quoted are 3a statistical uncertainties and apply to last quoted digit.
b Fixed to zero (see text).
Table 6
Cl hypertine constants for alkaline earth chlorides
CaSSClb
SP5Cl =
’ In MHz.
l
b
C
eqQ
19.3013(4)
18.663(89)
12.455(4)
7.72(30)
1.002(4)
3.961(84)
bRef. [lo].
‘Ref. [ll]
cause the magnetic moments of these two metal nuclei are very close in value (p1(25Mg)= -0.8547;
c(I(“‘Ba) = +0.927), comparison of the relative hyTable 7
Metal hyperline constants for alkaline earth radicals ’
b
-319.1(2.9)
-316.4(3.9)
- 309.01(26)
- 304.4(4.6)
2314(9)
’ In MHz: b This work.
dRef. [5]. “Ref. [16].
C
eqQ
0.0 c
0.0 c
14.72(22)
0.0 c
96(20)
-19.0(1.5)
-17.5(1.4)
-20.02(50)
-41(17)
- 134(42)
c Held fued.
‘Ref. [7].
pet-tine constants can be done directly. For barium
chloride, bF( 13’Ba) x 2282 MHz, almost an order of
magnitude larger than for the 25Mgnucleus in MgCl.
The quadrupole term for 13’BaC1is larger as well, with
eqQ GZ134 MHz, as opposed to eqQ x - 20 MHz for
“MgCl. The di ff er e n ce in magnitudes of the Fermi
contact term indicates that there is considerably less
electron density on the 25Mgnucleus than on the 13’Ba
nucleus. Hence, the M+Cl- structure must not be as
prominent in MgCl as in BaCl, suggesting that there
is more covalent character to the bonding in magnesium chloride. In addition, eqQ is larger in BaCl, indicating a larger field gradient across the metal nucleus than in MgCl, further evidence of enhanced
ionic character in the heavier species.
Acknowledgement
This work was supported by NSF grant AST-9253602 and NASA grant NAGW 2989. The authors
thank J.L. Destombes for helpful comments on this
project.
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