Rotational barriers of disilane,hexafluorodisilane, and hexamethyldisilane: Ab initio,density functional,and molecular mechanics (MM3) studies

We investigated structures, vibrational frequencies, and rotational barriers of disilane (Si₂H₆), hexafluorodisilane (Si₂F₆), and hexamethyldisilane (Si₂Me₆) by using ab initio molecular orbital and density functional theories. We employed four different levels of theories (i.e., HF/6–31G*, MP2/6–31G*, BLYP/6–31G*, and B3LYP/6–31G*) to optimize the structures and to calculate the vibrational frequencies (except for Si₂Me₆ at MP2/6–31G*). MP2/6–31G* calculations reproduce experimental bond lengths well, while BLYP/6–31G* calculations largely overestimate some bond lengths. Vibrational frequencies from density functional theories (BLYP/6–31G* and B3LYP/6–31G*) were in reasonably good agreement with experimental values without employing additional correction factors. We calculated the ΔG^?(298 K) values of the internal rotation by correcting zero-point vibration energies, thermal vibration energies, and entropies. We performed CISD/6–31G*//MP2/6–31G* calculations and found the ΔG^?(298 K) values for the internal rotation of Si₂H₆, Si₂F₆, and Si₂Me₆ to be 1.36, 2.06, and 2.69 kcal/mol, respectively. The performance of this level was verified by using G2 and G2(MP2) methods in Si₂H₆. According to our theoretical results, the ΔG^?(298 K) values were marginally greater than the ΔE^?(0 K) values in Si₂F₆ and Si₂Me₆ due to the contribution of the entropy. In Si₂H₆ the ΔE^?(0 K) and ΔG^?(298 K) values were coincidently similar due to a cancellation of two opposing contributions between zero-point and thermal vibrational energies, and entropies. Our calculated ΔG^?(298 K) values were in good agreement with experimental values published recently. In addition, we also performed MM3 calculations on Si₂H₆ and Si₂Me₆. MM3 calculated rotational barriers and thermodynamic properties were compared with high level ab initio results. Based on this comparison, MM3 calculations reproduced high level ab initio results in rotational barriers and thermodynamic properties of Si₂H₆ derivatives including vibrational energies and entropies, although large errors exist in some vibrational frequencies. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1523–1533, 1997