Molecular structures of acetylene derivatives of tin 7. Bis(trimethylstannyl) acetylene: analysis of electron diffraction data taking into account nonlinear relations between Cartesian and internal vibrational coordinates

The electron diffraction data on bis(trimethylstannyl)acetylene, Me3SnC=CSn Me3, were analyzed in the framework of the one-dimensional dynamic model of free internal rotation of the SnMe3, group about the axis of the Sn-C=C-Sn linear fragment. The root-mean-square amplitudes and harmonic shrinkage co rrections used in the analysis were calculated from the scaled quantum-chem ical force field (i) taking into account nonlinear relations between Cartes ian and internal vibrational coordinates at the first-order level of pertur bation theory (h1) and (ii) using a conventional approach (h0). The r(h1) p arameters of internuclear distances describe the equilibrium geometry of th e Me3SnC=CSnMe3 molecule much better than the commonly accepted parameters r(alpha) = r(h0). Substituent effects on the geometry of the acetylene frag ment are discussed.