A theoretical study on C2HXSi silylenes (X = H, CN, NH2, and OMe)

Relative stabilities and structural characters of 30 silylenic C2HXSi species (X=H, NH2 CN, and OMe), with singlet (s) and/or triplet (t) states: are calculated at six levels of theory: HF/6-311++G**, MP3/6-31G*, B1LYP/6-311++G**, B3LYP/6-311++G**, MP2/6-311++G**, and MP4(SDTQ)/6-311++G**. The four possible isomers considered for C2SiHX are (i) 3-X-1-silacyclopropenylidene (1(s-X) and 1(t-X)), (ii) X-vinilydensilylene (2(s-X) and 2(t-X)), (iii) ethynyl-X-silylene (3(s-X), and 3(t-X)), and (iv) (X-ethynyl)silylene (4(s-X) and 4(t-X)). The GIAO-NICS calculations show that singlet cyclic structures, 1(s-X), are considerably more aromatic than benzene. Conversely, triplet cyclic C2HCNSi breaks down through optimization, and transforms into a novel high-spin acyclic carbenosilylene minimum (1(t-CN)). Singlet 3(s-NH2) and triplet 3(t-NH2) cross at a divalent angle (LXSiC) of 152 degrees. This angle narrows to 137 degrees for crossing of singlet 3(s-CN) and triplet, 3(t-CN). The smallest LXSiC occurs at 132 degrees for crossing of 3(s-H) and 3(t-H). (c) 2007 Wiley Periodicals, Inc.