Unstable 1,1,2,2-Tetramethyl-1,2-disilacyclobutane and Its Polymerization. Vibrational Spectroscopy and Quantum-Chemistry Study
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- 作者：Vitaly G. Avakyan ; Sergey S. Bukalov ; Rinat R. Aysin ; Larissa A. Leites
- 刊名：Organometallics
- 出版年：2012
- 出版时间：October 22, 2012
- 年：2012
- 卷：31
- 期：20
- 页码：7063-7073
- 全文大小：479K
- 年卷期：v.31,no.20(October 22, 2012)
- ISSN：1520-6041
文摘
The structure of 1,1,2,2-tetramethyl-1,2-disilacyclobutane (1) has been investigated by experimental means, by Raman and IR spectra, and by quantum-chemical calculations at various levels of theory. All the data obtained show that the four-membered ring of 1 is puckered, the ring puckering mode being 80 cm鈥? and the barrier to planarity 1.76 kcal/mol (617 cm鈥?). Temperature investigations of the Raman spectrum have shown that the liquid 1 solidifies into a plastic mesophase at 210 K and crystallizes below 110 K. Strain energies (SEs) for 1, the parent molecule 1,2-disilacyclobutane (2), and, for comparison, their 1,3-isomers (3 and 4) were estimated as enthalpies of corresponding homodesmic reactions taken with a reversed sign. These values (in kcal/mol) for 1 (8.6) and 2 (12.4) appeared lower than those for 3 (18.3) and 4 (19.6). To rationalize these results, a QTAIM topological analysis of electron density distribution was carried out for 1鈥?b>4. Examination of the molecular graphs has demonstrated that the Si鈥揝i bond paths (BPs) in 1 and 2 are significantly bent (鈥渂anana鈥?bonds), thus decreasing the SE of these molecules. The corresponding Si鈥揝i BP lengths are increased by 螖 鈮?0.03 脜 in comparison to the corresponding endocyclic Si鈥揝i interatomic distances, while the Si鈥揅 BPs in all four molecules are nearly straight lines (螖 鈮?0.002 脜). Monitoring of the process of 1 polymerization on heating to room temperature has revealed that this process is not spontaneous, as was supposed previously, but is initiated by minor radical impurities. Indeed, the value of the ring-opening enthalpy for 1 was calculated as 70 kcal/mol, too high for spontaneous thermal polymerization to occur.