The molecular structure and the puckering potential function of 1,1-dimethylsilacyclobutane determined by gas electron diffraction and relaxation constraints from ab initio calculations

Gas electron diffraction data are applied to determine the geometrical para meters of the 1,1-dimethylsilacyclobutane molecule using a dynamic model wh ere the ring puckering is treated as a large amplitude motion. The structur al parameters and parameters of the potential function were refined taking into account the relaxation of the molecular geometry estimated from ab ini tio calculations at the Hartree-Fock level of theory using a 6-311G** basis set. The potential function has been described as V(phi) = V-0[(phi/phi e) (2)=1](2) with V-0 = 1.3 +/- 1.2 kcal/mol and phi(e) = 29.7 +/- 4.5 degrees , where phi is the puckering angle of the ring. A more reliable estimation of the barrier height (V-0 = 0.56 kcal/mol) was obtained by solving a one-d imensional Hamiltonian and fitting the transition frequency 0 --> 2 to the experimental one. The geometric parameters at the minimum of V(phi) (r(a) i n Angstrom, angle(alpha) in degrees, and errors given as three times the st andard deviations including a scale error) are: r(Si-C-m) = 1.872(2), r(Si- C-r) = 1.885(2), r(C-C)= 1.563(4), r(C-H-av) = 1.115(3), angle CmSiCm = 109 .9(47), angle CrSiCr = 79.2(11), angle SiCHm = 113.9(17), angle SiCHeq = 11 8.2(32), angle SiCHax = 108.6(32), angle CC5Heq = 112.1(32), angle CC5Hax = 107.2(32), angle HC3 = 107.6, angle delta(CmSiCm) = 3.9, angle delta(HC3H) = 4.2, where the tilts delta and angle HC3H are estimated from ab initio c onstraints, indexes m and r refer to methyl and ring. The structural parame ters are compared with those obtained for related compounds. (C) 1999 Elsev ier Science B.V. All rights reserved.