Ab initio studies on the vibrational spectra and conformational isomerism of 2,4-dimethyl-1,3-pentadiene.

Ab initio was used to study the structure of various conformational isomers and their vibrational spectra of 2,4-dimethyl-1,3-pentadiene (2,4-PD) in detail. Two stable conformations, s-trans and s-cis, were found in which s-trans is more stable. The geometry of stable conformations and charge distributions were studied, and the effect of different basis sets on geometry optimization is discussed. The results of complete optimization indicate that molecular skeleton is nearly in a plane, its largest deviation is only 0.3 degrees. Therefore, it is reasonable and available to hypothesis that the molecule has Cs symmetry. The thermal dynamics conformations were calculated and compared with experimental values. DeltaH(o) between two conformations of 2,4-PD measured from experiment is 4.36 kJ/mol, deltaS(o) is 2.56 J/mol K., calculated results are slightly different from experimental ones. Vibrational frequencies of 2,4-PD conformers have been studied by ab initio molecular orbital calculations using different basis set. The calculated vibrational frequencies are analyzed and compared with the experimental spectra.