A Conformational Study on Silacyclohexane. Comparison ofab initio (HF, MP2), DFT, and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

Abstract

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI‐MP2, RI‐DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair‐to‐chair inversion. Each of them consists of two sofa‐like transition states, two twist forms with C1 symmetry (twist‐C1), two boat forms with Si in a gunnel position (C1 symmetry), and one twist form with C2 symmetry (twist‐C2). All methods calculate the relative energy to increase in the order chair < twist‐C2 < twist‐C1 < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol–1 for the twist‐C2, twist‐C1, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol–1 from MM3 and RI‐DFT calculations, respectively. The boat form with Si at the prow (Cs symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI‐DFT optimization under Cs symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair‐to‐twist‐C1 transition state, however, and it is clearly not a part of the chair‐to‐chair inversion.

Conformational energies of silacyclohexanes C5H10SiHMe, C5H10SiH(CF3) and C5H10SiCl(SiCl3) from variable temperature Raman spectra

“…In the series, the largest drop in relative energy occurs for silacyclohexane C 5 H 10 SiH 2 with the introduction of the first Si‐atom. With about 16 kJmol –1 , the relative twist energy still is large enough so that the presence of the twist form safely can be disregarded for temperatures below ≈200–250 °C . In this respect, C 6 H 12 and C 5 H 10 SiH 2 share a common trait, which greatly facilitates conformational analysis.…”

Structural Studies of 1,1-Dimethyl-2-oxy-1-silacyclohexane by Means of Matrix Isolation Infrared Absorption Spectroscopy

“…2,3 Structure of substituted cyclohexane allows existence of different conformers. [4][5][6][7] As conformational structure is important to the molecule's different physical and chemical parameters such as reactivity, hydrophobicity, adsorption properties, etc., it is important to determine the possible conformers of every new member of the family of substituted cyclohexanes.…”

Comment on “Relative Energies, Stereoelectronic Interactions, and Conformational Interconversion in Silacycloalkanes”

“…Five years ago, we published a conformational study on silacyclohexane 1. The potential (or conformational) energy surface (PES) was mapped using the MM3 force field 2 by varying two opposite dihedral angles of the ring atoms in small steps within the range from 90° to −90°.…”

“…The authors then found that intrinsic reaction path (IRC) calulations 4, 5 connected the half‐chair transition state 1g # to the chair 1a and twist 1c , and argue: “The potential energy diagram for the chair‐chair interconversion of silacyclohexane (Fig. 4) is different than the one previously proposed 1, but is consistent with one of the proposed chair‐to‐chair interconversion mechanisms for cyclohexane.” Furthermore, in their concluding remarks the authors write: “The path for the chair‐to‐chair conformational interconversion of silacyclohexane proceeds via the 2,5‐twist [ 1c , our insertion] conformer (intermediate) without going through a boat conformation.” These statements are incorrect, and in this paper we show that our previous description of the chair‐to‐chair interconversion for silacyclohexane indeed corresponds to the lowest‐energy pathway.…”