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Local Modes: Their Relaxation, Polarization, and Stereoselectlve Excitation by Lasers

Perry, Joseph W. and Zewail, Ahmed H. (1982) Local Modes: Their Relaxation, Polarization, and Stereoselectlve Excitation by Lasers. Journal of Physical Chemistry, 86 (26). pp. 5197-5205. ISSN 0022-3654. doi:10.1021/j100223a027.

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In this paper, we present new results concerning the nature and relaxation of methyl and aromatic CH stretching overtones of low-temperature crystalline durene (1,2,4,5-tetramethylbenzene). The experimental polarization ratios of the bands (Δv_(cH) = 5) are used to compare three limiting models (local modes, independent methyl modes, and fully delocalized modes) in light of new information on the conformation of durene obtained by low-temperature X-ray diffraction. Armed with these results, we provide a picture for the origin of the splittings observed for the Δv_(cH) = 5 spectra at 1.7 K and discuss the nature of the vibrational energy distribution of the single-photon excited states of the methyl and aromatic subsets; the aromatics are equivalent local modes by symmetry (line width > observed splitting) whereas the methyls are inequivalent local modes which are split (line width < splitting) by the molecular and crystal fields. The dynamics of the high-energy states (Δv_(cH) = 5) are inferred from the line widths. The methyl bands, which are much narrower than the aromatic band at low temperature, are broadened as the temperature is increased by a process which results in partial coalescence at room temperature. Consequently, specific inequivalent bond modes on nonrigid groups (e.g., methyl) can be selectively excited by optical pumping at low temperatures. In addition, crystal field splittings of the CH modes, particularly at low temperature, can be large enough to allow selective excitation of bond modes which would be equivalent in the isolated molecule. Thus we propose the use of low-temperature matrices or crystals as a molecular environment for (polarized) photoselection experiments on high-energy overtones.

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Additional Information:© 1982 American Chemical Society. Received: June 28, 1982; In Final Form: September 9, 1982. This work was supported by a grant from the National Science Foundation, No. CHE-8112833. We thank Professor G. Rossman of the Department of Geology at the California Institute of Technology for the use of the Cary-17 spectrophotometer. We also thank Professor B. Henry for useful comments and suggestions.
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Arthur Amos Noyes Laboratory of Chemical Physics6665
Issue or Number:26
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Official Citation:Local modes: their relaxation, polarization, and stereoselective excitation by lasers Joseph W. Perry and Ahmed H. Zewail The Journal of Physical Chemistry 1982 86 (26), 5197-5205 DOI: 10.1021/j100223a027
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70544
Deposited By: Ruth Sustaita
Deposited On:30 Sep 2016 19:38
Last Modified:11 Nov 2021 04:31

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