An ab initio study of 5,6-disubstituted 1,3-cyclohexadienes

A computational study of 1,3-cyclohexadiene and some 5,6-disubstituted derivatives is presented. We examine the geometry, vibrational frequencies, and barrier to inversion of 1,3-cyclohexadiene, and discuss how these change with level of theory from HF to CCSD(T). We then examine these quantities at the HF and MP2 levels for various derivatives such as the alcohol, diol, dimethoxy, and a series of cyclic diol derivatives. The barrier to ring inversion correlated with the reported rate of Diels-Alder reaction, but not to the syn-anti ratio. Analysis of the photoelectron spectrum of the dienes offers no insight towards understanding the syn/anti ratio. The Cs-symmetric transition structures to Diels-Alder reaction are similar for the reaction of 1,3-cyclohexadiene with both ethene and ethyne.