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We analyzed the H/D isotope effect in the methyl torsional interactions accompanying two methyl internal rotations for acetone (CH3COCH3) and deuterated acetone (CD3COCD3 and CH3COCD3) 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 CH3COCH3 and CD3COCD3 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 CD3 group was shorter than the C–H distance in CH3 owing to the anharmonicity of the potential, the difference in potential energy surfaces of CH3COCH3, CD3COCD3, and CH3COCD3 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 CH3COCH3, which is in excellent agreement with the experimental results. For CH3COCD3, two potential energies were obtained for CH3 and CD3 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. [ABSTRACT FROM AUTHOR]

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]

We have developed a fragment molecular orbital (FMO)–multi-component MO (MC_MO) method to analyze isotope effect due to differences between the quantum effects of protons and deuterons for large molecules such as proteins and DNA. The FMO-MC_MO method enables the determination of both the electronic and the protonic (deuteronic) wave functions simultaneously, and can directly express isotope effects, including coupling effects between nuclei and electrons. In our calculations of two polyglycines, which serve as prototypes for biological molecules, by this method, we clearly observed the geometrical relaxation induced by the H/D isotope effect in the intramolecular hydrogen bonding portions of the molecules. The H/D isotope effect on the interfragment interaction energy, including that of the hydrogen bonding parts, was also demonstrated: the hydrogen bond was weakened by replacement of hydrogen with deuterium. We also developed electrostatic potential approximations for use in the FMO-MC_MO calculations, and the accuracy of the energy differences induced by the isotope effect was independent of the approximation level of the FMO-MC_MO. Our results confirmed that the FMO-MC_MO method is a powerful tool for the detailed analysis of changes in hydrogen bonding and interaction energies induced by the H/D isotope effect for large biological molecules. [ABSTRACT FROM AUTHOR]

Abstract: To estimate the kinetic isotope effect (KIE) for nucleophilic substitution (SN2) reaction, F− +CH4 →CH3F+H−, and deuterated ones, we have considered the geometrical isotope effect (GIE) induced by the difference of the protonic and deuteronic wavefunctions using the multi-component molecular orbital (MC_MO) method. By replacing protons with deuterons, the C–D bond lengths are about 0.007Å shorter than C–H. We estimated the ratio () of rate constants based on MC_MO results. The geometrical and energetic differences between H and D compounds are induced by relaxation of electronic structures due to the difference in the wavefunctions of the proton and deuteron. [ABSTRACT FROM AUTHOR]

We present the methodology and applications of multicomponent molecular orbital (MC_MO) method, which can take into account of the quantum effect of light particles, such as proton and deuteron, directly. We summarize the equations of the MC_MO method at the Hartree–Fock (HF) level and beyond the HF. The methodology of the MC_MO with fragment molecular orbital (FMO) method for large molecular systems is also described. We discuss the development of nuclear basis function based on the GTF, which is used in MC_MO and FMO-MC_MO calculations. We present the applications of MC_MO and FMO-MC_MO method for the analysis of geometrical isotope effect and kinetic isotope effect induced by H/D isotope effect. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Highlights: [•] PIMD simulations with PM6-DH+ potential are carried out for Cl−(H2O) n clusters. [•] The geometric isotope effects on the rearrangement of single and multi shell structures are presented. [•] The competition of intramolecular and intermolecular nuclear quantum effects on the cluster structures is shown. [•] The correlations between r(Cl…O) and other vibration motions are discussed. [Copyright &y& Elsevier]

Abstract: To estimate the kinetic isotope effect (KIE) for hydrogen (or deuterium) abstraction from H(D)OR (R=H, CH3, and CN) by an OH radical, we have considered the geometrical isotope effect (GIE) induced by the difference of the protonic and deuteronic wavefunctions using the multi-component MO method. The difference by the GIE of hydrogen bond was about 0.005Å. The ratio of the rate constant of the reaction for R=H, HO+HOR→HOH+OR and HO+DOR→HOD+OR, is estimated as 4.4 by our calculation, which is reasonable agreement with experimental result of 6.0±2.0. We have found that the difference of the nuclear wavefunction of the proton and deuteron affects the changes of geometry and electronic charge density, which plays an important role to theoretically determine the effective potential energy surfaces and the corresponding KIE between H and D compounds. [Copyright &y& Elsevier]