科研之友
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摘要
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(-1) and the barrier to planarity 1.76 kcal/mol (617 cm(-1)). Temperature investigations of the Raman spectrum have shown that the liquid 1 solidifies into a plastic mesophase at similar to 210 K and crystallizes below similar to 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 1-4. Examination of the molecular graphs has demonstrated that the Si-Si bond paths (BPs) in 1 and 2 are significantly bent ("banana" bonds), thus decreasing the SE of these molecules. The corresponding Si-Si BP lengths are increased by Delta approximate to 0.03 angstrom in comparison to the corresponding endocyclic Si-Si interatomic distances, while the Si-C BPs in all four molecules are nearly straight lines (Delta approximate to 0.002 angstrom). 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.
- 出版日期2012-10-22
