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 distributi on technique by observing H+ (D+) ions from the methyl groups. The activate d conversion of acetic acid to acetate has been witnessed by observing the formation of oriented acetate species. It has been found that the activatio n energy for acetate formation varies from 0.39+/-0.06 eV at low coverage t o 1.1+/-0.1 eV at high coverage, possibly because of acetic acid stabilizat ion at high coverage by hydrogen bonding. Methyl groups are oriented with o ne C-H bond azimuthally directed in the [<1(1)over bar>0] direction. At hig h coverage, the methyl groups are tilted slightly in the [<1(1)over bar>0] direction due to repulsive acetate-acetate interactions. The activated rota tion of the methyl groups on adsorbed acetate is clearly observed by the el ectron stimulated desorption ion angular distribution method and the barrie r 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 ca used by higher amplitude CH3 wagging motions compared to CD3, which permit slightly higher hyperconjugation for CH3 as this group interacts with the a nchoring carboxyl group. (C) 2000 American Institute of Physics. [S0021-960 6(00)70706-9].