Isotope effect for hydrogen bonds on H2O and NH3 cluster

Abstract

We analyzed geometrical isotope effect (GIE) by substitutions of proton(H)/deuteron(D) in water-ammonia clusters as a model of interactions between surfaces of a protein and surrounding water molecules. Multi-component molecular orbital method with Hartree-Fock level of theory (MC_MO-HF) was applied to treat quantum effect of nuclei explicitly. Single s-type GTF was used for each proton and deuteron with 6-31G(d,p) electron basis set. Trimers composed of water and/or ammonia molecule works as an acceptor and a donor of hydrogen for each hydrogen bond were considered in this study, comparing with corresponding monomers and dimers. As for monomers, the bond lengths of X-H(D)(X=O,N) by MC_MO method were longer than those by conventional MO method because of quantum effect. The bond lengths of X-D were shorter than those of X-H due to anharmonicity of potential. Concerning dimers and trimers, the distances between O(N) showed the same order as increasing number of ammonia molecules. The bond lengths of X-H(D) derived from MC_MO method were longer than those derived from conventional MO method, while the bond lengths of H(D)...Y(Y=O,N) and the distances between O(N) derived from MC_MO method were shorter than those derived from conventional MO method. The bond lengths of X-D and D...Y were shorter and longer than those of X-H and H…Y, respectively.