Fingerprint region of the formic acid dimer: variational vibrational computations in curvilinear coordinates

TL;DR

This study develops curvilinear kinetic energy models for variational vibrational computations of the formic acid dimer, analyzing mode coupling and comparing results with experimental spectra to improve understanding of its vibrational structure.

Contribution

It introduces a new variational vibrational approach using curvilinear coordinates for the formic acid dimer, including mode coupling effects and detailed vibrational state analysis.

Findings

Good agreement with experimental vibrational spectra for several bands

Identification of challenges in modeling closely spaced fundamental vibrations

Highlighting the need for improved potential energy surfaces

Abstract

Curvilinear kinetic energy models are developed for variational nuclear motion computations including the inter- and the low-frequency intra-molecular degrees of freedom of the formic acid dimer. The coupling of the inter- and intra-molecular modes is studied by solving the vibrational Schr\"odinger equation for a series of vibrational models, from two up to ten active vibrational degrees of freedom by selecting various combinations of active modes and constrained coordinate values. Vibrational states, nodal assignment, and infrared vibrational intensity information is computed using the the full-dimensional potential energy surface (PES) and electric dipole moment surface developed by Qu and Bowman [Phys. Chem. Chem. Phys. 18, 24835 (2016); J. Chem. Phys. 148, 241713 (2018)]. Good results are obtained for several fundamental and combination bands in comparison with with jet-cooled vibrational spectroscopy experiments, but the description of the $\nu_8$ and $\nu_9$ fundamental vibrations, which are close in energy and have the same symmetry, appears to be problematic. For further progress in comparison with experiment, the potential energy surface, and in particular, its multi-dimensional couplings representation, requires further improvement.