Theoretical infrared spectroscopy of protonated methane isotopologues

The vibrational spectroscopy of protonated methane and its mixed hydrogen/deuterium isotopologues remains a challenge to both experimental and computational spectroscopy due to the iconic floppiness of CH₅⁺. Here, we compute the finite-temperature broadband infrared spectra of CH₅⁺ and all its isotopologues, i.e. CHD_5-⁺ up to CD₅⁺, from path integral molecular dynamics in conjunction with interactions and dipoles computed consistently at CCSD(T) coupled cluster accuracy. The potential energy and dipole moment surfaces have been accurately represented in full dimensionality in terms of high-dimensional neural networks. The resulting computational efficiency allows us to establish CCSD(T) accuracy at the level of converged path integral simulations. For all six isotopologues, the computed broadband spectra compare very favorably to the available experimental broadband spectra obtained from laser induced reactions action vibrational spectroscopy. The current approach is found to consistently and significantly improve on previous calculations of these broadband vibrational spectra and defines the new cutting-edge for what has been dubbed the "enfant terrible" of molecular spectroscopy in view of its pronounced large-amplitude motion that involves all intramolecular degrees of freedom.