Thermal excitation of rotation of the methyl group in chemisorbed acetate on Cu(110)

The adsorption of acetic acid on the Cu(110) surface has been investigated by the time-of-flight electron stimulated desorption ion angular distribution technique by observing H+(D+) ions from the methyl groups. The activated conversion of acetic acid to acetate has been witnessed by observing the formation of oriented acetate species. It has been found that the activation energy for acetate formation varies from 0.39±0.06 eV at low coverage to 1.1±0.1 eV at high coverage, possibly because of acetic acid stabilization at high coverage by hydrogen bonding. Methyl groups are oriented with one C–H bond azimuthally directed in the 〈11̄0〉 direction. At high coverage, the methyl groups are tilted slightly in the 〈11̄0〉 direction due to repulsive acetate–acetate interactions. The activated rotation of the methyl groups on adsorbed acetate is clearly observed by the electron stimulated desorption ion angular distribution method and the barrier height for CH3 rotation is estimated to be 12±6 meV, and for CD3 groups to be 7±4 meV. The small inverse D isotope effect is postulated to be caused by higher amplitude CH3 wagging motions compared to CD3, which permit slightly higher hyperconjugation for CH3 as this group interacts with the anchoring carboxyl group.