Article

# HD isotope effect on the dihydrogen bond of N H4 + ...Be H2 by ab initio path integral molecular dynamics simulation

Yokohama City University, Yokohama, Kanagawa, Japan

The Journal of Chemical Physics (Impact Factor: 2.95). 12/2006; 125(20):204310. DOI: 10.1063/1.2388257 Source: PubMed

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**ABSTRACT:**We investigate isotope effects in dihydrogen systems using the nuclear orbital plus molecular orbital (NOMO) theory, which simultaneously determines nuclear and electronic wave functions without the Born-Oppenheimer approximation. In order to estimate the vibrational averaging bond distances, the analytical energy gradients are used for the NOMO theory. The bond distances of three dihydrogen-bonded systems, namely H3NX+ ··· XBeH, LiX ··· XC2H and H2BX ··· XF for X = H, D and T, are obtained. The X ··· X bond distances decrease with respect to the substitution T → D → H, which is contrary to the behaviour of typical hydrogen bonds. This indicates that isotope effects strengthen the X ··· X bond. This behaviour is analogous to the Ubbelohde effect observed in the solid phase. - [Show abstract] [Hide abstract]

**ABSTRACT:**In this paper, we describe an automated integration-free path-integral (AIF-PI) method [Wong, K.-Y.; Gao, J. J. Chem. Phys. 2007, 127, 211103], based on Kleinert’s variational perturbation (KP) theory, to treat internuclear quantum-statistical effects in molecular systems. We have developed an analytical method to obtain the centroid potential as a function of the variational parameter in the KP theory, which avoids numerical difficulties in path-integral Monte Carlo or molecular dynamics simulations, especially at the limit of zero-temperature. Consequently, the variational calculations using the KP theory can be efficiently carried out beyond the first order, i.e., the Giachetti-Tognetti-Feynman-Kleinert variational approach, for realistic chemical applications. By making use of the approximation of independent instantaneous normal modes (INM), the AIF-PI method can be applied to many-body systems, and it was shown previously that the AIF-PI method is accurate for computing the quantum effects including a water molecule and the collinear H3 reaction. In this work, the accuracy and properties of the KP theory are further investigated by using the first three-order perturbations on an asymmetric double-well potential, the bond vibrations of H2, HF, and HCl represented by the Morse potential, and a proton-transfer barrier modeled by the Eckart potential. The zero-point energy, quantum partition function, and tunneling factor for these systems have been - [Show abstract] [Hide abstract]

**ABSTRACT:**Structural and spectroscopic properties of hydrazine, H(2)N-NH(2), it being a floppy or fluxional molecule in a vacuum, are investigated by means of ab initio molecular dynamics, ab initio path integral molecular dynamics, and ab initio ring polymer molecular dynamics simulations in conjunction with "on-the-fly" MP2 and CIS(D) electronic structure calculations. Whereas the former method relies on the classical approximation of nuclear motion, quantum effects on structure and dynamics are taken into account at finite temperatures by ab initio path integral and ab initio ring polymer molecular dynamics, respectively. It is shown that quantum-mechanical fluctuation effects of the nuclei, in addition to their purely thermal activation, cause significant configurational fluctuations due to strongly anharmonic vibrations and thus increase the explored regions on the Born-Oppenheimer potential energy surface at room temperature. Including these effects, in turn, leads to significant improvements in the computed spectra compared to stick spectra obtained at the equilibrium structure by means of harmonic normal-mode analysis, as well as by classical ab initio molecular dynamics. This family of methods, combining electronic structure with path integrals, offers a powerful and general computational approach not only to molecular structure determination of floppy or fluxional molecules, but also to evaluation of their electronic and vibrational spectra.