Examination of the applicability of multicomponent quantum mechanics to infrared spectroscopy calculations

Quantum mechanics (QM) calculations of infrared spectra based on the Born–Oppenheimer and harmonic oscillator approximations cause errors depending on the level of the electronic structure calculation and the anharmonicity of the potential energy surface. Errors owing to anharmonicity are particularly strong for vibrations involving hydrogen nuclei with large quantum fluctuations. A possible solution to this issue is to apply multicomponent QM (MC_QM), which extends conventional QM calculations to quantum MC systems to enable the analysis of H/D isotope effects by treating nuclei as quantum particles, such as electrons. To investigate the suitability of MC_QM for infrared spectroscopy, this study calculated the vibrational frequencies of several H/D substituents using conventional QM and MC_QM methods and compared them with the corresponding experimental values. Results demonstrate that the MC_QM values exhibit improved reproducibility compared to the experimental values by incorporating nuclear quantum effects.