Structure, spectra, and intramolecular motions of 1,4-bis(trimethylsilyl)-1,3-butadiyne: An electron diffraction study using the results of spectral analysis based on scaled quantum-chemical force field for calculating vibrational effects

The molecular structure of 1,4-bis(trimethylsilyl)-1,3-butadiyne has been studied by the gas electron diffraction method and by ab initio calculations at the RHF and MP2 levels. A scaled quantum-chemical force field was used to interpret the vibrational spectra. The root-mean-square vibrational amplitudes and harmonic shrinkage corrections were calculated taking into account non-linear relations between Cartesian and internal vibrational co-ordinates at the level of first-order perturbation theory (h1) and with the use of the traditional scheme (h0). The total energies of the D_3d and D_3h conformers differ by about 1 cal mol^-1 (0.4 cm^-1) which implies virtually free internal rotation. It has been shown that the flexibility of the linear fragment in (CH₃)₃Si-C≡C-C≡C-Si (CH₃)₃ has increased considerably compared to that in (CH₃)₃Si-C≡C-Si(CH₃)₃.