An analysis of electron diffraction data on bis(trimethylsilyl)acetylene taking into account nonlinear relations between Cartesian and internal vibrational coordinates

Abstract The electron diffraction data on bis(trimethylsilyl)acetylene were analyzed in terms of the one-dimensional dynamic model of free Si(CH 3 ) 3 group rotations about the Si–C C–Si axis. The root-mean-square amplitudes and harmonic shrinkage corrections were calculated taking into account nonlinear relations between Cartesian and internal vibrational coordinates at the level of first-order perturbation theory ( h 1) and with the use of the traditional scheme ( h 0). The experimental r α distances were virtually independent of the approximation used to calculate vibrational effects. The r h 1 parameter values much better approximated the equilibrium geometry than the familiar r α r h 0 parameters. The r h 1 -structure of bis(trimethylsilyl)acetylene refined to Si–C(H 3 ) 1.877(4), Si–C 1.841(4), C C 1.239(3), C–H(av.) 1.108(3) A, (H 3 )C–Si–C 109.2(2)° and Si–C–H(av.) 111.3(2)°. Electron diffraction data on silylacetylenes were systematized in terms of r g parameters equal to r h 1 for bonded distances.