Combined ab initio computational and statistical investigation of a model C-H center dot center dot center dot O hydrogen bonded dimer as occurring in 1,4-benzoquinone

Ab initio molecular orbital calculations were performed on the C-H ... O hy drogen bonded dimer pattern of the fragment O=C-C(=C)-H as occurring in 1,4 -benzoquinone. The computations were combined with a statistical analysis o f published crystal structures containing this dimer. For computational rea sons, the dimer was approximated by dimers 1,4-benzoquinone-propenal and pr openal-propenal. The optimal geometry obtained at the SCF+MP2 level of theo ry is very close to the mean geometry observed in crystals. The total bindi ng energy Delta E at the optimal geometry is calculated as -17.9 kJ/mol. Ea ch C-H ... O hydrogen bond contributes about -6 kJ/mol, and the rest comes from stabilizing electrostatic interactions between the carbonyl groups. Th e potential energy surface has a broad shape at the minimum, allowing consi derable geometric variations with only slight energetic disadvantages. The dimer geometries observed in crystals (n = 53) are all in the low-energy re gion of the potential energy surface. Only 4 of the 53 dimers have calculat ed Delta E values more than 3 kJ/mol above the global minimum. The dimer wi th the least favorable geometry has a Delta E value about 7.0 kJ/mol above the global minimum.