Calculations On High Resolution Spectra Of Small Molecules

The topic of the thesis is on the theoretical calculations ofsmall molecules with spin-vibronic couplings or large-amplitude motion, which are heavily studied in chemical physics these days. For molecules with vibronic couplings, like Jahn-Teller molecules or Renner-Teller molecules, Born-Oppenheimer approximation is no longer valid and diabatic models are used. Vibronic energy levels of some typical Jahn-Teller and Renner-Teller cations are calculated in the thesis. Moreover, molecules with large-amplitudemotion, which can’t be described as vibration atthe equilibrium geometry, like large-amplitude umbrella vibration and internal rotation motion are studied. The internal rotation motion of CH3OH+ and the umbrella vibration of As H3+ are studied. These calculations agree well with experimental data.(1)For Jahn-Teller cation CH3F+ ?? ?2X E, diabatic potential energy surfaces were fitted using energy gradients and non-adiabatic coupling vectors calculated with MRCI method. The vibronic energy levels were calculated variationally, with a parallel computer program. The diabatic PESs of CD3F+ ?? ?2X E were obtained using a coordinatetransformation. The vibronic calculation shows the splittings of CH3F+ energy levels due to tunneling of wavefunctions in three energy minima. For Renner-Teller cations, like HCCCl+, diabatic PESs were calculated with CCSD(T) method and fitted with internal coordinats. The vibronic energy levels were calculated variationally. The calculation shows there is a strong mixing between two bending modes due to strong vibronic interactions.(2)CH3OH+ has an internal rotation mode, the relative rotation between OH and CH3. To treat this special type of motion, a symmetrized coordinate was used, corresponding kinetic energy operator was obtained, and the potential energy curve was calculated.The minimum and saddle point energy geometries of the cation correspond to the saddle and minimum energy geometries of the neutral molecule, respectively. The barrier of cation’s internal rotation is half of that neutral molecule. The torsional energy levels of CH3OH+/CH3OD+/CD3OD+were calculated.(3)As H3+ and PH3+ both have large-amplitude umbrella motion. We used a two-dimensional model to describe the umbrella vibration, obtained ab initio PES and calculated the energy levels variationally. They all show splittings of energy levels due to umbrella motion, which agrees well with experimental data.