We have analyzed the differences in the methyl internal rotation induced by the HD isotope effect for acetaldehyde (CH3CHO) and deuterated acetaldehyde (CD3CDO) in ground state by means of the multicomponent molecular orbital (MC̱MO) 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 MC̱MO method. The C–D bond distances were about 0.007Å 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 MC̱MO method for CH3CHO was estimated as 401.4cm1, which was in excellent agreement with the experimentally determined barrier height. We predicted the barrier height of CD3CDO as 392.5cm1. 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 HD isotope effect were controlled by the difference in the distribution of wavefunctions between the protons and deuterons.

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