Ab Initio Path Integral Simulations of Floppy Molecular Systems

Protonated methane, $\mbox{CH}_{5}^{+}$ , is one of the smallest representatives of the so–called floppy molecules whose treatment challenges both experiment and theory for decades. Recently, we succeeded in understanding the IR spectrum of the per–protonated parent system, i.e. isolated $\mbox{CH}_{5}^{+}$ itself. More recently, the IR spectra of all its H/D isotopologues, i.e. $\mbox{CD}_{5}^{+}$ , $\mbox{CHD}_{4}^{+}$ , $\mbox{CH}_{2}^{}\mbox{D}_{3}^{+}$ , $\mbox{CH}_{3}^{}\mbox{D}_{2}^{+}$ , $\mbox{CH}_{4}^{}\mbox{D}^{+}$ and $\mbox{CH}_{5}^{+}$ have been measured in a tour de force experiment by our collaborators and now wait for interpretation. It has been shown both computationally and experimentally that nuclear quantum effects are crucial, which implies that they cannot be neglected when computing infrared spectra subject to H/D isotopic substitution. Thus, our investigations are carried out in the framework of ab initio path integral simulations together with the adiabatic centroid molecular dynamics extension which readily allow for nuclear quantum effects and yield access to the quasi–classical dynamics.