Long-Range, Resonant Vibrational Energy Exchange in Polyatomic Molecules: The Fundamental Acetylenic CH Stretching Spectrum of CH3Si(C.tplbond.CH)3

Abstract

published in Advance ACS Abstracts, May 1, 1994. 0022-3654/94/2098-5614~04.50/0 of 1000, indicating that the molecule is in the extreme local mode limit. However, in the fundamental region there is littledifference between the symmetrized local mode and the normal mode wave functions, although they suggest different dynamics since they represent qualitatively different classical motions. In the local mode model, it is natural to interpret the splitting between equivalent bonds dynamically, as the time it takes for the excitation, initially localized on one bond, to transfer to one of the other symmetrically equivalent bonds. To better understand theobserved splitting in CH3Si(C=LH)j, we also examine the splitting of the symmetric and antisymmetric CH modes in diacetylene. For this molecule, the splitting is known from experiment and the force field from ab initio calculations. This allows us to separate out different contributions to the splitting, which is not possible for CH3Si(C=CH)3 at this time. Experimental Section Triethynylmethylsilane was prepared from CH3SiC13 [Aldrich] and H C s C M g C l [Aldrich] in tetrahydrofuran according to the procedures outlined in ref 7. The final product was 77% pure by lH NMR. The impurity, THF, caused no complications in the recording of the spectrum since its infrared absorptions occur a t a different frequency than the compound of interest. The spectrometer, which was described in detail earlier,ja consists of a continuously scanning continuous wave 3.0-pm color center laser [Burleigh] and a molecular beam machine with bolometer detection. Helium gas passed, a t various backing pressures, over a room-temperature liquid sample of CH3Si(CECH)~. The gas line leading into the machine was at 90 OC and the 50 pm diameter nozzle a t 120 OC. Above 50 psi He backing pressures, severe clustering was observed as a negative signal from the bolometer. This is caused by predissociating species leaving the molecular beam path upon absorption of radiation. Only slight clustering occurred at 30 psi of He, and the spectrum taken at this pressure was used for the analysis. We used two high-finesse etalons (7.5 GHz and 150 MHz free spectral range) to monitor and linearize the laser frequency. The instrumental line width of our spectrometer is approximately 8 MHz, due to slightly nonorthogonal crossings of the laser with the molecular beam. An absorption spectrum of acetylene in a gas cell was recorded along with the bolometer signal and was combined with published frequencies8 to establish an absolute frequency calibration of the spectrum.