Real or Artificial Stability of Ions and Deuterated Variants Based on Ab Initio Calculation and Rotational Spectrum

Abstract

Protonated methane, , has unusual vibrational and rotational behavior because its three non-equivalent equilibrium structures have nearly identical energies and its five protons scramble freely. Thehighly f lexible , molecular ion has been shown by ab initio calculations to have 120 symmetrically equiv-alent minima of Cs symmetry in its ground electronic state. Complete proton rearrangement, making all min-ima accessible to each other, is possible as a result of two large-amplitude internal motions: an internal rota-tion about the C3 axis with an ab initio barrier of 30 cm-1 and an internal f lip motion with an ab initio barrierof 300 cm-1 that exchanges protons between the H2 and groups. We calculate the structure of the J =21 and 1 0 rotational transitions of , and also other variants containing . Althoughmany theoretical papers have been published on the quantum mechanics of these systems, a better under-standing requires spectral and conformational analysis. Post Hartree-Fock, M & oslash;ller-Plesset and DFT calcu-lation with the correlation consistent polarized valence double and triple zeta basis sets have done for thezero-point energies of . The present results indicates the mode 8, 12, and 10 agree with qualitativeof , which is highly f luxional and has a complex spectrum while the C-X bonds which are broken andreformed all the time. The spectrum of mode 12 is highly complex with huge red-and some blue shifts. In par-ticular, they can be attributed to the rapid coupling of the original CH-stretching normal mode to motionsmore closely related to isomerization, i.e., bending or rocking. There has thus been a long debate whether has a structure at all or not and is it real rotational motions or artificial. In addition, we include the con-tribution to the torsional barrier from the zero point energies of the other (high-frequency) vibrations, theeffect of centrifugal distortion, and the effect of second-order rotation-vibration interactions