Molecular structures of 1,1-dimethyl-1-silacyclopent-3-ene, (CH3)(2)SiC4H6, and 1,1-dimethyl-1-germacyclopent-3-ene, (CH3)(2)GeC4H6, obtained by gas-phase electron diffraction and theoretical calculations

The structures of 1,1-dimethyl-1-silacyclopent-3-ene and 1,1-dimethyl-1-ger macyclopent-3-ene have been determined by gas-phase electron diffraction (G ED), augmented by the results from ab initio calculations employing the sec ond-order Moller-Plesset (MP2) level of theory and the 6-311+G(d) basis set . All the electrons were included in the correlation calculation. Both mole cules were found to possess C-2 upsilon symmetry, thus the rings are planar . The results for the principal distances (r(g)) and angles (angle(alpha)) from the combined GED/ab initio study of 1,1-dimethyl-1-silacyclopent-3-ene (DMSiCP) (with estimated 2 sigma uncertainties) are r(Si-C) = 1.888(3) Ang strom, r(Si-C-t) = 1.867(3) Angstrom, r(C-C)= 1.503(9) Angstrom, r(C=C) = 1 .335(14) Angstrom, [r(C-H)] = 1.109(7) Angstrom, angle CrSiCr = 96.3(5)degr ees, angle CtSiCt = 110.6 degrees (ab initio), angle SiCrCr = 102.3(4)degre es, angle(CCC)(r) = 119.6(4)degrees, angle HC2H = 106.1 degrees (ab initio) , angle C4C3H = 120.7 degrees (ab initio), angle(SiCtH) = 111.4 degrees (ab initio) (C-t = ring C atom, C-t = methyl C atom). The results for the prin cipal distances (r(g)) and angles (angle(alpha)) from the combined GED/ab i nitio study of 1,1-dimethyl-1-germacyclopent-3-ene (DMGeCP) (with estimated 2 sigma uncertainties) are r(Ge-C-r) = 1.963(4) Angstrom, r(Ge-C-t)= 1.944 (4) Angstrom, r(C-C) = 1.504(12) Angstrom, r(C=C) = 1.345(12) Angstrom, [r( C-H)] = 1.096(11) Angstrom, angle CrGeCr = 93.4(9)degrees, angle CtGeCt = 1 11.0 degrees (ab initio), angle GeCrCr = 103.0(9)degrees, angle(CCC)(r) = 1 20.3(6)degrees, angle HC2H = 106.6 degrees (ab initio), angle C4C3H = 120.1 degrees (ab initio), angle(GeCtH) = 110.8 degrees (ab initio).