H/D isotope effect of methyl internal rotation for acetaldehyde in ground state as calculated from a multicomponent molecular orbital method.

We have analyzed the differences in the methyl internal rotation induced by the H/D isotope effect for acetaldehyde (CH₃CHO) and deuterated acetaldehyde (CD₃CDO) in ground state by means of the multicomponent molecular orbital (MC_MO) method, which directly accounts for the quantum effects of protons and deuterons. The rotational constant of CH₃CHO was in reasonable agreement with experimental one due to the adequate treatment of the protonic quantum effect by the MC_MO method. The C–D bond distances were about 0.007 Å shorter than the C–H distances because of the effect of anharmonicity of the potential. The Mulliken population for CD₃ in CD₃CDO is larger than that for CH₃ in CH₃CHO because the distribution of wavefunctions for the deuterons was more localized than that for the protons. The barrier height obtained by the MC_MO method for CH₃CHO was estimated as 401.4 cm^-1, which was in excellent agreement with the experimentally determined barrier height. We predicted the barrier height of CD₃CDO as 392.5 cm^-1. We suggest that the internal rotation of the CD₃ group was more facile than that of the CH₃ group because the C–D bond distance was observed to be shorter than the C–H distance. Additionally the localized electrons surrounding the CD₃ group in CD₃CDO caused the extent of hyperconjugation between the CD₃ and CDO groups to be smaller than that in the case of CH₃CHO, which may have also contributed to the observed differences in methyl internal rotation. The differences in bond distances and electronic populations induced by the H/D isotope effect were controlled by the difference in the distribution of wavefunctions between the protons and deuterons. [ABSTRACT FROM AUTHOR]