Study on the Z/E thermal isomerization of acetaldehyde N,N-dimethylhydrazone in cyclohexane by density functional theory computations

The purpose of this study is to compare solvent effects of the polarizable continuum model (PCM) and ONIOM (our Own N-layered Integrated molecular Orbital molecular Mechanics) model. The test was performed by calculating Gibbs energies of reaction and activation for the Z/E thermal isomerization reaction of acetaldehyde N,N-dimethylhydrazone at 298.15 K in the solvent of cyclohexane. Four density functional theory typed methods, B3LYP, BB1K, MPW1B95 and MPWB1K with the 6-311++G(3df,2p)//6-31G(2df,p) basis sets were employed to give estimates of Gibbs energies. We compared the calculated results with the available experimental observation (established by Clarke et al. [Clarke et al., J. Chem. Soc. Perkin Trans II 11 (1991) 1649.]). As a result of the calculations, it is found that with the ONIOM model the four density functional theory typed computations gave estimates of Gibbs energy of activation in agreement with the experimental observation while the PCM calculated values overestimated the energy of activation by about 3 or 4 kcal/mol. For the calculated free energies of reaction, the four density functional theory typed computations with the PCM and the ONIOM models gave very close results with a range of about 1 kcal/mol.