Density functional study of the adsorption of 1,4-cyclohexadiene on Pt(1 1 1): origin of the C–H stretch red shift

The adsorption of 1,4-cyclohexadiene (CHD14) on Pt(1 1 1) was analysed using first-principle density functional theoretical calculations. CHD14 adsorbs at the bridge site via quadra-σ-type bonding. The adsorption energy was calculated to be 146 kJ/mol. CHD14 can also adsorb at the hollow site via the formation of a di-σ- and a π-bond with an adsorption energy of 142 kJ/mol. Upon adsorption CHD14 adopts a boat conformation. Strong π-electron donation to the platinum surface in combination with a hyperconjugative interaction of these π-orbitals with the axial σCH orbital helps to explain the observed red shift of the axial C–H stretching frequency and the corresponding C–H bond lengthening.