H/D isotope effect in methyl torsional interaction of acetone as calculated by a multicomponent molecular orbital method.

We analyzed the H/D isotope effect in the methyl torsional interactions accompanying two methyl internal rotations for acetone (CH(3)COCH(3)) and deuterated acetone (CD(3)COCD(3) and CH(3)COCD(3)) in the ground state by means of the multicomponent molecular orbital (MC_MO) method, which directly accounts for the quantum effects of protons and deuterons. Our estimated rotational constants and moments of inertia for CH(3)COCH(3) and CD(3)COCD(3) agreed well with the experimental results because of the adequate treatment of protonic and deuteronic quantum effects afforded by the MC_MO method. Because the C-D bond distance in the CD(3) group was shorter than the C-H distance in CH(3) owing to the anharmonicity of the potential, the difference in potential energy surfaces of CH(3)COCH(3), CD(3)COCD(3), and CH(3)COCD(3) was strongly related to the differences induced in geometrical parameters by the H/D isotope effect. The potential energy obtained by the MC_MO method was estimated as 290.88 cm(-1) for CH(3)COCH(3), which is in excellent agreement with the experimental results. For CH(3)COCD(3), two potential energies were obtained for CH(3) and CD(3) internal rotations. The MC_MO method successfully elucidated the H/D isotope effect for methyl-methyl repulsive interactions by allowing the adequate treatment of protonic and deuteronic wave functions. The potential energies and bond distances associated with methyl internal rotation induced by the H/D isotope effect were also controlled by the distribution of wave functions of protons and deuterons.