Vibrational Spectra, DFT Calculations, Unusual Structure, Anomalous CH2 Wagging and Twisting Modes, and Phase-Dependent Conformation of 1,3-Disilacyclobutane
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- 作者：Mohamed Z. M. Rishard ; Richard M. Irwin ; Jaan Laane
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：February 8, 2007
- 年：2007
- 卷：111
- 期：5
- 页码：825 - 831
- 全文大小：139K
- 年卷期：v.111,no.5(February 8, 2007)
- ISSN：1520-5215
文摘
Our previously published infrared and Raman spectra of 1,3-disilacyclobutane (13DSCB) and its 1,1,3,3-d4isotopomer have been reexamined and partially reassigned on the basis of DFT and ab initio calculations.The calculations confirm previous microwave work that the CSiC angles in the ring are unexpectedly largerthan the SiCSi angles. This may arise from the partial charges on the ring atoms. The calculations are inexcellent agreement with the observed spectra in both frequency and intensity. They also demonstrate thatthis molecule has CH2 wagging and twisting vibrations with frequencies below 1000 cm-1, about 200 cm-1lower than expected. These unprecedented low values can be explained by the decreased slope in the potentialenergy curves for these vibrations as the sideways motions of the CH2 groups result in attractive forcesbetween the positively charged hydrogens on the carbon atoms and the negatively charged hydrogens on thesilicon atoms. The theoretical calculations also confirm the previous conclusions that the individual molecules(vapor) have C2v symmetry whereas in the solid the molecules become planar with D2h symmetry. Thevibrational coupling between the ring-angle bending mode and the SiH2 in-phase rocking, which is presentfor the C2v structure, is forbidden for D2h and hence disappears.