A simplistic computational procedure for tunneling splittings caused by proton transfer

Denis S Tikhonov

doi:10.1007/s11224-021-01845-4

Abstract

In this manuscript, we present an approach for computing tunneling splittings for large amplitude motions. The core of the approach is a solution of an effective one-dimensional Schrödinger equation with an effective mass and an effective potential energy surface composed of electronic and harmonic zero-point vibrational energies of small amplitude motions in the molecule. The method has been shown to work in cases of three model motions: nitrogen inversion in ammonia, single proton transfer in malonaldehyde, and double proton transfer in the formic acid dimer. In the current work, we also investigate the performance of different DFT and post-Hartree–Fock methods for prediction of the proton transfer tunneling splittings, quality of the effective Schrödinger equation parameters upon the isotopic substitution, and possibility of a complete basis set (CBS) extrapolation for the resulting tunneling splittings.

Highlights

Large amplitude motions (LAM) are ubiquitous in most molecular systems, they are responsible for conformational interconversion and even for chemical reactions, such as in the cases of tautomerization [1,2,3,4,5]
We have introduced, implemented, and tested an approach for computing the tunneling splittings for one-dimensional large amplitude motions (LAMs)
The computational scheme is based on the adiabatic separation of LAM from the small amplitude motions, which are treated as a vibrational bath

Summary

Introduction

Large amplitude motions (LAM) are ubiquitous in most molecular systems, they are responsible for conformational interconversion and even for chemical reactions, such as in the cases of tautomerization [1,2,3,4,5]. One of the most prominent LAM manifestation is the tunneling splitting of the ground vibrational state, whenever a motion between equivalent minima is involved. Structural Chemistry of calculations to be done: one-dimensional (1D) relaxed potential energy surface (PES) scans and vibrational frequency calculations in the harmonic approximation Both computational procedures are known by users of quantumchemical packages, and the presented procedure can become an interesting one for members of the highresolution molecular spectroscopy community. We apply the proposed approach to the nitrogen inversion in ammonia to prove the workability of the method, and we use it to calculate tunneling splittings for the proton transfer motion in MA and FAD at various quantum-chemical approximations employing the def2-TZVPP basis set

Computational procedure

Using the fact that q2

Results and discussion

Method

Conclusion