Capturing the isotope effects in the vibrationally resolved electronic spectra with ab initio semiclassical dynamics*

The thawed Gaussian approximation [1] has been shown to accurately reproduce the vibrationally resolved electronic spectra of molecules with weakly anharmonic potential energy surfaces [2,3,4]. Here, we demonstrate that the thawed Gaussian approximation can capture even more subtle observables, such as the isotope effects on such spectra, and we use four isotopologues of ammonia as an example. As a single trajectory semiclassical method, the thawed Gaussian approximation benefits from the on-the-fly ab initio implementation [2,4], in which the energy, gradient, and Hessian of the potential energy surface is evaluated only locally, avoiding the task of precomputing global surfaces. Whereas the standard global harmonic approximations fail due to the high anharmonicity of the first excited electronic state of ammonia, the experimental spectra of ammonia isotopologues (NH₃, NDH₂, ND₂H, ND₃) are very well reproduced by the semiclassical on-the-fly calculations. Moreover, the isotope effects - narrowing of the transition band, shift in the 0-0 transition, and decreasing of the peak spacing - are all captured by this semiclassical method. The activation of different normal modes upon excitation in the case of each isotopologue sheds the light on the elusiveness of the symmetric stretch progression in the spectra.

 

[1] E. J Heller, .J. Chem. Phys. 62, 1544 (1975)

[2] M. Wehrle, M. Sulc, and J. Vanícek, J. Chem. Phys. 140, 244114 (2014)

[3] M. Wehrle, S. Oberli, and J. Vanícek, J. Phys. Chem. A 119, 5685 (2015)

[4] T. Begušic, E. Tapavicza, J. Vanícek, J. Chem. Theory Comput. 18, 3065 (2022)