Knowledge Bank

Both protonated methanol(CH$3$OH$\mathrm{2+}$) and methylamine(CH$3$NH$2$) show two interesting large amplitude motion, namely, internal rotation and inversion. The internal rotation spectrum results from the rotation of the CH$3$ moiety against the OH$2$ group in case of protonated methanol and of NH$2$ group in the case of methylamine. The other interesting phenomenon is inversion involving the OH$2$ group and NH$2$ group in the case of protonated methanol and methylamine, respectively. The influence of inversion in methylamine, on bands in the near and middle infrared has been investigated. Microwave and far-IR spectrum of methylamine has been measured and spectral frequency calculations derived from rotation-internal rotation-inversion analysis has been reported. However, in the past, detection of protonated methanol (CH$3$OH$\mathrm{2+}$), in interstellar clouds has been impossible due to the lack of both laboratory spectra and calculation.\newline In this contribution {\it{ab initio}} based potential energy surface (PES) and dipole moment surface (DMS) is presented for both protonated methanol and methylamine. The PES and DMS are developed by least squares fitting of {\it{ab initio}} energy values computed at CCSD(T)/AVTZ level of theory and dipole moments at MP2/AVTZ level of theory. Internal rotation and inversion transition states and normal-mode frequencies will be reported. One-dimensional tunneling splitting calculations will be reported. Tunneling splittings at higher dimensionality, ro-vibrational states and transition intensities are calculated using reaction path Hamiltonian(RPH) as implemented in MULTIMODE(MM) code. MM treats polyatomic molecules with large-amplitude motion as and one special coordinate which is the large-amplitude vibrational coordinate. Complete integration is performed over reaction path coordinate, and the N-mode MULTIMODE coupling approximation for the evaluation of the matrix elements applies only to the 3N - 7 normal coordinates.