Quantum Molecular Dynamics Calculations of Ultrafast Time Scales and Infrared Spectra of Protonated Methane: Quantifying Isotope-Specific Lifetimes

Understanding the quantum dynamics of molecules undergoing large-amplitude vibrational motion is challenging because of their flat and anharmonic energy landscapes. Using quasiclassical ab initio path integral dynamics, time scales of large-amplitude motion, lifetimes, and IR spectra are computed for protonated methane, CH5+, and its deuterated species, CH4D+, CH3D2+, CH2D3+, CHD4+, and CD5+. The time scales of quantum pseudorotations are shown to depend sensitively on the particular isotopic substitution pattern, which characterizes the measured infrared spectra.