Molecular structures and thermodynamic stabilities of cycloalkadienes and their methoxy derivatives: An ab initio and DFT study

The geometry-optimized molecular structures and total energies of 4- to 6-m embered cycloalkadienes, and of a number of their mono- and dimethoxy deriv atives, have been calculated by ab initio (HF/6-31G*, MP2/6-31G*//HF/6-31G* ) and DFT (B3LYP/6-31G*) methods. By comparison with available experimental data, the reliability of these computational methods for an estimation of the relative stabilities (enthalpies) of the isomeric forms of the title co mpounds was tested. The experimental enthalpies of isomerization proved to agree best with the respective theoretical data based on the mean of the HF /6-31G* and B3LYP/6-31G* energies. The theoretical calculations were then e xtended to several isomeric methoxy-substituted cycloalkadienes, for which no previous thermodynamic data exist. Some structural features of the title dienes were also discussed.