Gauche, Ortho, Transoid andAnti Conformations of the Tetrasilanes SiMe3SiX2SiX2SiMe3 (X=H, F, Cl, Br, I): A Computational and Vibrational Spectroscopic Study

Chair, Boat and Twist Conformation of Dodecamethylcyclohexasilane and Undecamethylcyclohexasilane: A Combined DFT and Raman Spectroscopic Study

The conformations of dodecamethylcyclohexasilane Si6Me12 and undecamethylcyclohexasilane Si6Me11H have been investigated by ab initio calculations employing the B3LYP density functional with a 6-31+G(d) basis set. Local minima as well as transition structures were calculated with imposed symmetry constraints. For Si6Me12, three unique minima, which correspond to the chair, twist and boat conformations were located with relative zero-point-vibration-corrected energies of 0.0, 7.8 and 11.4 kJ mol(-1). A half-chair conformation with four coplanar silicon atoms connects the chair and twisted minima via an energy barrier of 16.0 and 8.2 kJ mol(-1), respectively. A second transition structure with a barrier of 3.9/0.3 kJ mol(-1) connects the twist with the boat structure. Solution Raman spectra of Si6(CH3)12 and Si6(CD3)12 fully corroborate these results. Below -40 degrees C, the symmetric SiSi ring breathing vibration is a single line, which develops a shoulder (originating from the twist conformer) at longer wavelengths whose intensity increases with increasing temperature. From a Van't Hoff plot, the chair/twist enthalpy difference is 6.6+/-1.5 kJ mol(-1) for Si6(CH3)12 and 6.0+/-1.5 kJ mol(-1) for Si6(CD3)12, which is in reasonable agreement with the ab initio results. Due to the low barrier, the boat conformation cannot be observed, because either the lowest torsional vibration level lies above it or a rapid interconversion between the twist and boat conformations occurs, resulting in averaged Raman spectra. For Si6Me11H, six local minima were located. The chair with the hydrogen atom in the axial position (axial chair) is the global minimum, followed by the equatorial chair (+1.9 kJ mol(-1)) and the three twist conformers (+5.3, +8.0 and +8.1 kJ mol(-1)). The highest local minimum (+11.9 kJ mol(-1)) is a C(s) symmetric boat with the hydrogen atom in the equatorial position. Two possible pathways for the chair-to-chair interconversion with barriers of 13.9 and 14.5 kJ mol(-1) have been investigated. The solution Raman spectra in the SiSi ring breathing region clearly show that below -50 degrees C only the axial and equatorial chairs are present, with an experimental deltaH-value of 0.46 kJ mol(-1). With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental deltaH-value is 6.9 kJ mol(-1), in good agreement with the ab initio results. Due to the low interconversion barriers, the various twist conformers cannot be detected separately.

Identification and thermodynamic treatment of several types of large-amplitude motions

We present a partially automated method for the thermodynamic treatment of large-amplitude motions. Starting from the molecular geometry and the Hessian matrix, we evaluate anharmonic partition functions for selected vibrational degrees of freedom. Supported anharmonic vibration types are internal rotation and inversion (oscillation in a double-well potential). By heuristic algorithms, we identify internal rotations in most cases automatically from the Hessian eigenvectors, and we also estimate the parameters of anharmonic partition functions (e.g., potential barrier, periodicity, and symmetry number) with thermodynamically sufficient precision. We demonstrate the validity of our schemes by comparison to pointwise calculated ab initio potential curves.