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
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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.
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(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
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(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
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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.
