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

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ABSTRACT:We investigated structures, vibrational frequencies, and rotational Ž . Ž

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Chemical properties such as heat of formation and density of methylnitroimidazole derivatives were predicted and analyzed by using density functional theory (DFT). Successive addition of energetic nitro groups into an imidazole ring increases both the heat of formation and the density. Using the chemical property values computed by DFT, explosive performance was analyzed with the Cheetah program, and compared with those of TNT, RDX, and HMX, which are currently used widely in military systems. When both C-J pressure and detonation velocity were used as explosive performance, methyldinitroimidazole derivatives show better performance than TNT, while methyltrinitroimidzole is almost close to RDX. Since methylnitroimidazole derivatives have a good merit, i.e. low melting point for melt loading, they are forecasted to be used widely in various military and civilian application.