J . Phys. Chem. 1986,90, 6766-6769
6766
I
1
1
I b
I
W
Y
U
a
I
i
U
4
B
1100
.
\
lo00
cm'
900
Figure 2. The infrared spectra for LiC103 vapor (700 K) isolated in
argon matrices (12 K) containing (A) -0.2% H20and (B) 1.0%H20.
The bands are labeled as b, bidentate; d, dimer; h, monohydrate; and *,
impurity.
This is reasonable when compared with the splitting value of 42
cm-l noted above for the more tightly bound monohydrate of
NaC103.
While the effect of hydration of the tridentate-bound KC103
ion pair seems to mimic the effect of hydration of the tridentate-bound NaC10, ion pair, monohydration of the LiC103 ion
pair produces a result more like that noted for the bidentate form
of NaC103. Thus the intensity of the strong bands at 1100 and
887 cm-' for LiC103 in a dry argon matrix (Figure 2, curve A)
was rapidly diminished as the matrix water content was increased
to 1.O% and then 2.0%. Further, no new bands appeared in the
immediate vicinity of these features that have been assigned to
the split components of the u3 mode of the bidentate ion pair but
a new doublet, attributed to the components of the v3 mode of
the monohydrate H20*LiC1O3,emerged at 1079 and -905 cm-I.
Not surprisingly, this result suggests that the monohydrated
lithium ion continues to bind with the chlorate ion in a bidentate
fashion so that a sizeable splitting (i.e., 174 cm-I) of the v3 degeneracy is retained. Nevertheless, the apparent reduction in the
chlorate distortion that accompanies monohydration is significantly
greater than the corresponding reduction experienced by LiNO,,
an effect that may suggest that the LiClO, ion pair moves
somewhat toward a tridentate-bound structure upon monohydration.
The conclusion reached from this study of the monohydrates
of the alkali-metal chlorate ion pairs is that, for samples prepared
by isolating salt vapors heated to 700 K, the abundance of the
bidentate form of NaC103 is comparable to that of the tridentate-bound ion pair, while, for the LiC103 ion pair, bidentate
binding is preferred. This result does not explain the apparent
incompatiblity of the published data for the vapors at 700 and
1000 K but does affirm the validity of the 700 K result. An
attempt to rationalize the difference in the results reported for
the two temperatures must be conjectural in nature, but it can
be noted that weak bands in the present LiC103 dry-matrix spectra
were found to match the values assigned to the tridentate form
of the ion pair based on the 1000 K vapor isolation5 (Le., at 972,
960,904, and 896 cm-I). However, the relative intensities of these
bands were not constant from one deposit to the next, nor were
the intensities visibly diminished by enriching the matrices with
water, an implausible result if these bands are, in fact, produced
by a tridentatebound monomer ion pair. It may also be pertinent
that extensive molecular dissociation of LiC103 has been reported
for vacuum vaporization of the anhydrous salt at 700 K so that
severe dissociation problems are a possibility at 1000 K.Io
Acknowledgment. Support of this research under N S F Grant
CHE-8420961 is appreciated.
(10) Smyrl, N.; Devlin, J. P. J . Chem. Phys. 1974, 60, 2540.
Impllcatlons of the Rotationally Resolved Spectra of the Alkoxy Radicals for Their
Electronlc Structure
Stephen C. Foster, Yen-Chu Hsu,+ Cristino P. Damo, Xianming Liu, Chung-Yi Kung,
and Terry A. Miller*
Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, Ohio 4321 0
(Received: November 3, 1986)
Laser excitation spectra of several alkoxy radicals including methoxy, ethoxy, vinoxy, and isopropoxy have been recorded
at low temperature in a supersonic free jet expansion. These spectra in all cases show well-resolved rotational structure.
These structures are used to determine the symmetry of the two electronic states involved in the transitions. The symmetries
so determined are rationalized in terms of simple molecular orbital theory.
Introduction
Alkoxy radicals, ROO,play major roles as oxidation intermediates in the combustion of hydrocarbons and in the chemistry
of the upper atmosphere. Their prominence in these processes
makes our lack of spectroscopic information about them very
surprising. Although the simplest member of this series, methoxy
(CH@?, has been the Subject of a large
ofsPtroscoPic
studies,' there remain fundamental spectroscopic and structural
Present address: Institute of Atomic and Molecular Sciences, Academia
Sinica, P.O.Box 23-166, Taipei, Taiwan, Republic of China.
(1) Brossard, S. D.; Carrick, P. G.; Chappell, E. L.; Hulegaard, S. G.;
Engelking, P. C. J . Chem. Phys. 1986, 84, 2459.
0022-3654/86/2090-6766$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6761
Letters
-
questions yet to be answered a-bout even-this radical. For example,
the electronic origin for the AZAl X2E system of CH30' has
not been definitively established. Most authors have accepted the
original assignment of Inoue et al.? but the recent work of Brossard
et al.' has proposed an alternative scheme. This latter assignment
has been questioned by Garland and C r o s l e ~ . In
~ addition, CH30'
has an orbitally degenerate ground electronic state and is expected
to show Jahn-Teller distortion effects. Yet, the Jahn-Teller effect
is not well characterized.'
The larger saturated alkoxy radicals, C2H50'and i-C3H70*,
have received much less attention. Radiative lifetimes have been
measured, and vibronic spectra recorded, for the A X systems
of both molecules.4d However, no rotational data have been
obtained which could yield detailed structural information about
these radicals. Such data are of course very difficult to obtain
from room temperature spectra of these moderately large radicals.
There are other questions to be answered about the simplest,
substituted methoxy radicals. Next to CH3Uitself, the most work
has probably been done on the simplest alkenoxy radical, CH2CHO, which we call vinoxy. It has been the study of several recent
ab initio calculation^.^^^ It has been detected by Hunziker et aL9
using kinetic absorption spectroscopy and Jacox'O observed its IR
spectrum in an Ar matrix. Its laser-induced fluorescence (LIF)
was first detected in a flow system by Inoue and Akimoto,ll and
later Kleinermanns and LuntzlZ used L I F to detect CHzCHO
produced in a crossed beam experiment. We recently pub_lishedL3
X
a detailed analysis of the rotational structure of the A
electronic transition. This experiment utilized the LIF spectrum
of CH2CH0 produced in a supersonic free jet expansion and
cooled to -3 K to reduce spectral congestion.
It is our belief that such LIF experiments on cold radicals in
a jet offer much promise toward solving many of the outstanding
problems involving these species. We are presently involved in
a series of such experiments on CH30' and its homolog CH3S'
and results will be published in the near f ~ t u r e . ' ~ . The
' ~ subjects
of this report are the jet-cooled LIF spectra of the simple alkanoxy
radicals, C 2 H 5 0 , ethoxy, and (CH3),CH0, isopropoxy. The
electronic spectra of these radicals have been previously reported,
but in no case has rotational structure been resolved. In our
present experiments, relatively well-resolved rotational spectra are
obtained.
We defer to a subsequent publication a detailed analysis of these
spectra giving molecular constants, as this work is still in progress.
However, the present data clearly reveal the nature of the rotational transition, e.g. A-B hybrid type band, C-type perpendicular
band. Even at this relatively crude level of the rotational analysis,
it appears that a significant new understanding of the nature of
the electronic structure and transition is possible.
At first thought one might expect the rotational band structure
of all these radicals to be similar. However we find this to be
4.
-
-
(2) Inoue, G.; Akimoto, H.; Okuda, M. J. Chem. Phys. 1980, 72, 1769.
Chem. Phys. Lett. 1980, 63, 213.
(3) Garland, N. L.; Crosley, D. R. Paper TC9, 41st Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 1986.
(4) Inoue, G.; Okuda, M.; Akimoto, H. J. Chem. Phys. 75, 1981,
2060-2065.
(5) Ebata, T.; Yanagishita, H.; Obi, K.; Tanaka, I. Chem. Phys. 1982, 69,
27-33.
(6) (a) Ohbayashi, K.; Akimoto, H.; Tanaka, I. J. Phys. Chem. 1977,81,
798. (b) Jeffrey Balla, R.;,Nelson, H. H.; McDonald, J. R. Chem. Phys. 1985,
99, 323.
(7) Baird, N. C.; Taylor, K. F. Can. J. Chem. 1980, 58, 733. Baird, N.
C.; Gupta, R. R.; Taylor, K. F. J. Am. Chem. Soc. 1979, 101, 4531.
(8) Dupuis, M.; Wendoloshi, J. J.; Lester, W. A. J. Chem. Phys. 1982, 76,
488.
(9) Hunziker, H. E.; Kneppe, H.; Wendt, H. R. J. Phorochem. 1981,12,
377. Hunziker, H. E.; Kneppe, H.; McLean, A. D.; Siegbahn, P.; Wendt, H.
R. Can. J. Chem. 1983, 61, 993.
(10) Jacox, M. E. Chem. Phys. 1982, 69, 407.
(11) Inoue, G.; Akimoto, H. J. Chem. Phys. 1981, 74, 425.
(12) Kleinermanns, K.; Luntz, A. C. J. Phys. Chem. 1981, 85, 1966.
(13) DiMauro, L.; Heaven, M.;Miller, T. A. J. Chem. Phys. 1984, 81,
2339.
(14) Hsu, Y.-C.; Miller, T. A,, to be submitted for publication.
(15) Foster, S. C.; Miller, T. A., to be submitted for publication.
I
CH30.
.t
Figure 1. Schematic representation of the geometries of the radicals
considered in this paper. The quasi-lone-pair p orbitals on the 0 atom
are indicated for each molecule. The indicated symmetry plane and
inertial axes are consistent with the conventions of Herzberg.I9
emphatically not the case. Methoxy itself exhibits a B,C perpendicular band structure of a prolate symmetric top. Vinoxy
exhibits a hybrid A,B-type band structure of a near prolate
symmetric top. Ethoxy, however, exhibits a perpendicular C-type
band structure of a near prolate top with no visible parallel
component, while isopropoxy shows the opposite-a parallel band
with no visible perpendicular structure for this near oblate top.
In the following we will briefly explain the experimental details
and give our results. We will then offer arguments as to how,
surprisingly enough, these diverse observations can be explained
with a consistent model of the electronic structure of the states
involved in these transitions.
Experimental Section
The alkoxy radicals were generated in situ in a Campargue
supersonic free jet expansion by UV laser photolysis of their
corresponding alkyl nitrites (RONO). The alkyl nitrites were
synthesized according to published procedures16and stored at -78
O C until ready for use.
The alkyl nitrites were entrained in a H e flow by passing
high-pressure helium (- 10 atm) over a liquid sample maintained
in a low temperature bath. The seeded flow was then expanded
through a 200-pm nozzle into the jet chamber. Ethyl nitrite and
isopropyl nitrite were photolyzed with an ArF excimer laser and
a frequency tripled Nd:YAG laser, respectively. The resultant
alkoxy radicals were excited with a Nd:YAG-pumped tunable dye
laser (frequency doubled or mixed with fundamental YAG as
appropriate), and the subsequent total fluorescence was collected
with an fl lens onto an EMI9659QB photomultiplier. Signals were
processed with a LeCroy gated integrator, digitized, and stored
in a minicomputer for subsequent processing. All spectra were
calibrated with an I2 reference spectrum. Further details of this
technique can be found in several earlier reports from this laborat~ry.~~J~J~
Results and Discussion
Figure 1 shows the structures of the four radicals under consideration utilizing the conventions of HerzbergI9 for the axes and
(16) Organic Syntheses Collective, Vol. 2, Blatt, A. H., Ed.; Wiley: New
York, 1943. Organic Syntheses Collective, Homig, E. C . , Ed.; Vol. 3; Wiley:
New York, 1955.
(17) Heaven, M.; DiMauro, L.; Miller, T. A. Chem. Phys. Lett. 1983, 95,
347
- ..
(18) Miller, T. A. Science 1984, 223, 545.
6768 The Journal of Physical Chemistry, Vol. 90,No. 26, 1986
TABLE I:
Letters
Comparison of Electronic and Vibrational Frequencies in
Alkoxv Radicals
A-R
radical
OH
CH3O
CZH30
C2H60
C3H70
C-0 vibrational freq‘
R
AIR
transition
frea.Tm.
-. cm-I
A
32 402
31 540
28 784
29 204
-27 167
3180
683
1143
606
560
3735
1022
1540
1074
0.85
0.67
0.74
0.56
’The frequency listed is obviously for the OH stretch in OH and in
the other cases for varying degrees of admixture of CO stretch and
C-C-0 asymmetric stretch.
symmetry planes. The three radicals, vinoxy, ethoxy, and isopropoxy, have only a plane of symmetry and their electronic wave
functions can be characterized as transforming according to one
of the two irreducible representations of the point group C,.
Interestingly enough, each of these radicals is a near symmetric
top 1(. 5 0.95). In the case of vinoxy and ethoxy the tops are
nearly prolate while for isopropoxy it is nearly oblate. Methoxy,
of course, has C3, symmetry and is precisely a prolate symmetric
top. The C, symmetry plane defined for the other radicals is also
indicated for CH30’ in Figure 1.
The electronic transition in CH30’ has long been known to be
A2Al R2E. In terms of a one-electron excitation, the transition
can be roughly described as the promotion of an electron from
an a, sp hybrid orbital on 0, with C O bonding character, to fill
one of the half-filled doubly degenerate e orbitals. This is
analogous to the near-UV A22-X211 transition in OH’ where a
pu electron is promoted to a half-filled p?r, or jm,,
(nearly lone-pair
px or p,, on oxygen) orbital. As one can see from Table I, the
frequencies of the electronic transitions in OH’ and CH30’ are
quite similar. Also from the above-mentioned, one-electron-jump
description of the transition, one might expect a definite weakening
of the bond to 0 and a concomitant decrease in vibrational frequency for this bond’s stretching. Table I confirms this to be the
case for both OH’ and CH30*.
As the C3, symmetry of CH30’ is lowered to C,, the excited
A, state correlates to an A’ state while the E degenerate ground
state decomposes into an A’ and an A” state. Figure 1 shows,
for example for ethoxy, that the A’ state corresponds to putting
the unpaired electron into an orbital with the symmetry axis of
the oxygen p orbital located in the ugplane, while the A” state
corresponds to that unpaired electron going into a p orbital with
its symmetry axis perpendicular to the plane.
It is not particularly obvious whether A’ or A” is the ground
state of ethoxy. However, the rotational structure of ethoxy as
revealed by Figure 2 allows us to determine the electronic symmetry of that ground state. (Herzberg19 has tabulated the correlation between band structure and types of vibronic transition
for slightly asymmetric tops and our analysis here is consistent
with that table.)
As indicated by Figure 1, the (near) top axis, a, is located for
ethoxy in the u, or x-y plane. If the transition moment lies in
this plane, it would, generally speaking, have nonvanishing components along both the a and b axes and thus give rise to a hybrid
(A,B) transition. Since the dipole components in the ( x y ) plane
are symmetric with respect to us,the transition would have to be
either A‘ A‘ or A“ A“. Making the seemingly reasonable
assumption that the upper state in the transition is derived from
the AI upper state in C H 3 0 Eequires the ground state to be A’.
Thus the tzansition would be AZA‘ X2A‘ and the ground state
would be X2A’ corresponding to the unpaired electron residing
in the in-plane p-type orbital.
However, from Figure 2 it is clear that the only observable
rotational structure is of the perpendicular type. While this could
arise from a transition moment along the b axis, the corresponding
parallel component along the a axis must be quite weak to give
this spectrum. A much more attractive explanation is that we
-
-
-
-
(19) Herzberg, G. Molecular Spectra and Structure; Van NostrandReinhold: New York, 1966.
29?80
23782
29784
29?86
29788
29790
Wavenumber (cm”)
Figure 2. Laser excitation spectrum of the ethoxy radical. The rotational
assignments are made using prolate symmetric top quantum numbers.
are looking at a purely C-type perpendicular band that could result
from the promotion of an electron to a half-filled p,-like orbital
pointing out of the usplane. Indeed, a more detailed analysis of
the rotational structure-confir? a C-type transition. This implies
that the transition is A2Af-X2A“ with the ground-state X2A”
corresponding to the unpaired electron occupying the out-of-plane
(us) p,-type orbital.
The splitting of the 2E ground state of methoxy into 2A’ and
’A” states in ethoxy is expected on symmetry grounds. Our
determination that the 2A’f state is lower in energy is not inconsistent with simple MO arguments. When the C3, symmetry of
methoxy is lowered to C, in ethoxy, the in-plane p-type orbital
localized on 0 can mix with the partially bonding, in-plane sp
hybrid orbital. This gives rise to a sp2 hybrid orbital of a’ symmetry which is lower in energy than the a” nearly pure p-type
0 orbital pointing perpendicular to the plane. Thus two of the
three available electrons completely fill the a’ orbital, leaving the
one unpaired electron for the af’ orbital and giving rise to the
ground 2A” state.
We note that the geometrically similar radical, vinoxy, has
previously had its rotational structure analyzed13as a hybrid (A,B)
type band implying (see above) either an A‘-A’ or A”“”
transition. This may sound contradictory to our analysis for ethoxy
since, if we retained the A‘ upper state, the rotational analysis
would require that as the E ground state of methoxy splits, its
lower component is A’ for vinoxy and Af’ for ethoxy. However,
the key to resolving this problem is to recognize that in vinoxy
there are two resonance structures corresponding to ethenyloxy,
CH,=CHO:, and formyl methyl, CHz-CH=O:, both of A”
symmetry. Strong repulsion caused by configuration interaction
between the resonance structures has been well demonstrated by
a b initio calculationss which indicate that the observed UV
transitio? is between these two A” states, Thus again the ground
state is X2A” corresponding to the unpaired electron being in the
out-of-plane p,-type orbital, with the lower energy in-plane sp2
hybrid orbital completely filled with two electrons.
Presumably there would be an A‘ excited state in vinoxy corresponding to the ethoxy A‘ excited state formed by promotion
of an electron from the sp2 hybrid orbital into the half-filled a”
orbital lying in the general energy ra_ngeof the_A” excited state.
One can note from Table I that the AZA; X 2 c transition of
vinoxy is quite similar in frequency to the A2A’ X2A” transition
--
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6769
Letters
ISO-PROPOXY
A+R
08
qQ-branch
h
27164
nisa
27i66
27167
T= 2K
27168
2716s
Wavenumber (cm-‘)
Figure 3. Laser excitation spectrum of the isopropoxy radical. The
rotational assignments are made by using oblate symmetric top quantum
numbers. The lines are broadened because of unresolved K components.
of ethoxy. Indeed, even the CO stretching frequency decreases
similarly; however, in the case of vinoxy the vibration is more
accurately the C-C-0 stretching motion and its lower frequency
likely results from less formyl methyl character in the excited A”
state than the ground state.
The isopropoxy radical is the final case for which we presently
have rotationally resolved spectra. Although the K-manifolds are
not resolved, it is clear from Figure 3 that the observed rotational
structure is that of a parallel type band. Herzberg’s tablesig show
that a purely parallel band is not possible, so that we must assume
it is the parallel component of a hybrid band with the weaker
perpendicular component as yet unobserved. (By way of reference,13 in the vinoxy hybrid band the parallel component was well
over an order of magnitude stronger than the perpendicular one.)
A hybrid band just as we saw above for vinoxy requires either
an A‘-A’ or A‘j-A” transition. Thus we are forced to the conclusion that in going from ethoxy to isopropoxy either the symmetry of the ground or excited state in the transition must change.
Since one does not expect replacing a H with a CH3group, well
away from a roughly localized CO excitation, would affect the
nature of the transition greatly (a fact confirmed by Table I), this
result appears rather surprising.
The only explanation that we can offer is based upon Figure
1, which shows that isopropoxy is the only one of the radicals to
be a near oblate symmetric top. Thus as Figure 1 shows the crs
plane no longer contains a C-C-0 bond but rather bisects the
(CH3)2-C dihedral angle. This causes the in-plane p orbital to
point toward the H while the methyl groups are now out-of-plane.
If we assume that part of the increased stability of the a’ sp’ orbital
results from hyperconjugation to the CH3(CH2)in ethoxy (vinoxy),
then that influence is removed in isopropoxy because the in-plane
a’ orbital does not point in the direction of the out-of-plane CH3’s.
Indeed, if anything the out-of-plane a” orbital is stabilized by such
hyperconjugation. This might be sufficient to ::verse the ordering
of the two lowest states in isopropoxy. Alternatively, a b initio
calculations2’ on the isopropyl chloride positive ion suggest that
the energy gap between the A’ excited state and an A” excited
state is lessened considerably compared to ethyl chloride positive
ion, an effect which at least qualitatively is explicable by the same
sort of hyperconjugation argument. Thus it does not seem too
unlikely that the excited state in this transition is A” preserving
2A” as the ground state. (It is, of course, impossible to completely
rule out the possibility that the observed transitions are from a
metastable A’ level above the ground A” state. However, efforts
to provide experimental evidence for such a hypothesis by observing
another transition in the vicinity in either the excitation or emission
spectrum have proved fruitless.)
It is clear that the observations of rotational structure for the
heavier alkoxy radicals yield some very interesting results concerning their electronic structure. However, they raise further
questions for study, e.g. the magnitude of the energy gap between
the A’ and A” states derived from the initially degenerate E state
in methoxy, the possibility of other electronic transitions in vinoxy
and isopropoxy in particular, etc. Finally, while the rotational
structure does much to identify the symmetry of the electronic
states, it does not precisely define their nature. Left open is the
question of how localized the p-type orbitals are on 0. In O H
they are likely well localized, but the degree of localization is less
clear in the larger alkoxy radicals. Detailed rotational constants
with the resulting precise molecular bond lengths and angles may
help to resolve this question. Future work on these radicals in
the cold supersonic jets is presently underway to answer many
of these questions.
Acknowledgment. This work was supported by the National
Science Foundation under Grant No. CHE-8507537. The authors
acknowledge very helpful discussions with Dr. R. Pitzer.
(20) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S.
Handbook of Her Photoelectron Spectra of Fundamental Organic Molecules;
Halsted: New York, 1981.