Steric effects on the dialkyl substituted X2C2Si silylenes:: A theoretical study

With the aim of recognizing the steric effects on the silylenic H2C2Si structures, ab initio and DFT calculations are carried out on 24 structures of X2C2Si (where X is hydrogen (H), methyl (Me), isopropyl (i-pro), and tert-butyl (tert-Bu)). These species are at either triplet (t) or singlet (s) states. They are confined to the following three, sets of structures (1(x), 2(x) and 3(x)). Structures 1(x) include silacyclopropenylidenes (1(s-H) and 1(t-H)) and their 2,3-disubstituted derivatives (1(t)-(Me), 1(s-Me); 1(t-i-pro), 1(s-i-pro); 1(t-tert-Bu), 1(s-tert-Bu)). Structures 2(x) include vinylidenesilylenes (2(s-H) and 2(t-H)) and their 3,3-disubstituted derivatives (2(t-Me), 2(s-Me); 2(t-i-pro), 2(s-i-pro); 2(t-tert-Bu), 2(s-tert-Bu)).Structures 3(x) include ethynylsilylenes (3(s-H) and 3(t-H)) and their 1,3-disubstituted derivatives (3(t-Me), 3(s-Me); 3(t-i-pro), 3(s-i-pro); 3(t-tert-Bu), 3(s-tert-Bu)). Singlet-triplet energy separations (Delta Es-t,(x)) and relative energies for the above structures are acquired at HF/6-31G*, B1LYP/6-31G*, B3LYP/6-31G*, MP2/6-31G*, HF/6-31G**, BILYP/6-31G**, B3LP/6-31G**, and MP2/6-31G** levels of theory. The highest Delta E(s-t,)x is encountered for 1,. All singlet states Of X2C2Si, are. more stable than their corresponding triplet states. Linear correlations are found between the LUMO-HOMO energy gaps of the singlet 1(s-x) and 2(s-x) with their corresponding singlet-triplet energy separations calculated at B3LYP/6-31G**. The seven structures 2(s-Me) 2(t-Me), 3(s-Me) 1(t-Me), 1(s-Me), 1(s-tert-Bu), and 3(t-tert-Bu) do not appear to be real isomers. Different stability orders are obtained as a function of the substituents (X). The order of stability for six isomers Of H2C2Si is 1(s-H) > 2(s-H) > 3(s-H) > 2(t-H) > 3(t-H) > 1(t-H). Replacing hydrogen atoms by methyl group (X=Me) presents a new stability order: 1(s-Me) > 3(s-Me) > 2(s-Me) > 3(t-Me) > 2(t-Me), > 1(t-Me); and for (i-pro)(2)C2Si is 1(s-i-pro), > 2(s-i-pro)approximate to 3(s-i-pro) > 3(t-i-pro) approximate to 2(t-i-pro) > 1(t-i-pro). Using the larger tertbutyl group as a substituent (X), yet it offers a more different stability order for six structures of (tert-BU)(2)C2Si: 1(s-tert-Bu) > 3(s-tert-Bu) > 2(s-tert-Bu) > 3(t-tert-Bu) > 1(t-tert-Bu) > 2(t-tert-Bu)- Among eight levels employed, B3LYP16-31G** appears as the method of choice. (c) 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:619-633, 2006; Published online in Wiley InterScience (wwwinterscience.wiley.com).