Isotope effects in the electronic spectra of ammonia from ab initio semiclassical dynamics

TL;DR

This paper demonstrates that the thawed Gaussian approximation can accurately capture isotope effects in ammonia's electronic spectra, outperforming harmonic models in describing subtle spectral features.

Contribution

It shows that the semiclassical thawed Gaussian approximation effectively models isotope effects in vibrationally resolved electronic spectra of ammonia isotopologues.

Findings

Thawed Gaussian approximation captures isotope effects accurately.

Harmonic models fail to reproduce the observed spectral progressions.

Semiclassical trajectories explain differences in isotope spectra.

Abstract

Despite its simplicity, the single-trajectory thawed Gaussian approximation has proven useful for calculating vibrationally resolved electronic spectra of molecules with weakly anharmonic potential energy surfaces. Here, we show that the thawed Gaussian approximation can capture surprisingly well even more subtle observables, such as the isotope effects in the absorption spectra, and we demonstrate it on the four isotopologues of ammonia (NH$_{3}$, NDH$_{2}$, ND$_{2}$H, ND$_{3}$). The differences in their computed spectra are due to the differences in the semiclassical trajectories followed by the four isotopologues, and the isotope effects-narrowing of the transition band and reduction of the peak spacing-are accurately described by this semiclassical method. In contrast, the adiabatic harmonic model shows a double progression instead of the single progression seen in the experimental spectra. The vertical harmonic model correctly shows only a single progression but fails to describe the anharmonic peak spacing. Analysis of the normal-mode activation upon excitation provides insight into the elusiveness of the symmetric stretching progression in the spectra.