Geometrical and Kinetic Isotope Effects on R–H(D)···R Type Intramolecular Hydrogen Bonds (R = CH2, NH, and O) Using a Multi-Component Molecular Orbital Method

Abstract

Abstract We theoretically analyzed the geometrical isotope effect and kinetic isotope effect for R–H(D)···R type proton (deuteron) transfer on the intramolecular hydrogen bond of (Z)-1,3-pentadiene (R=CH2), 3-imino-1-propen-1-amine (R=NH), and malonaldehyde (R=O) by using Hartree–Fock (HF) and second-order Møller–Plesset (MP2) levels of a multi-component molecular orbital (MC_MO) method to treat the quantum effects of nuclei. As the protonic and deuteronic basis functions, our proposed exponent values of 24.1725 and 35.6214 for the single protonic and deuteronic Gaussian-type functions were employed, respectively. In the case of R=NH and O, the calculated R5–D6 and R1···D6 bond lengths are shorter and longer than those of the R5–H6 and R1···H6 bonds due to the anharmonicity of the potential. The charge densities around the deuteron for all R=CH2, NH, and O cases are larger than those around the proton because the deuteron is more localized than the proton. The ratio of rate constant (kaH⁄kaD) obtained by MP2 level theory of the MC_MO method is estimated as 10.16, which is in reasonable agreement with the experimental result (12.2). For the systematic analysis of the intramolecular proton (deuteron)-transfer reaction, we also estimated the kaH⁄kaD of R=NH and O.