Ab initio calculations on the ground and excited states of BeOH and MgOH

摘要

Ab initio configuration interaction calculations have been carried out on the ground and excited electronic states of the BeOH and MgOH molecules as well as of the cations BeOH~+ and MgOH~+, for linear and bent geometries. The excited states of the above molecules have not previously been calculated and experimental information exists only for the A-X system in MgOH and MgOD. The present results show that in the excited states the molecules MgOH and BeOH have similar M-O (M = Mg, Be) stretching and bending potentials. In general, the stretching potentials are rather complicated, showing a number of avoided crossings. Furthermore, most of the excited states show minima at R near R_(min) of the corresponding cations BeOH~+ and MgOH~+, indicating Rydberg contributions to the molecular excited states. The first excited state in both BeOH and MgOH is 2 ~2A', which along with 1 ~2A", forms the 1 ~2#PI# state of linear geometries and which in both systems has minimum energy at a bent geometry with bond angle near 115 deg. In MgOH, the 2 ~2A' state is the A state of the observed A-X spectra and the theoretical transition energy and the barrier to linearity are in good agreement with the corresponding experimental quantities. Analogous spectra for BeOH, not reported as yet, would be expected on the basis of the present calculations at higher energies than the MgOH spectra by 0.6 eV. The results on the molecular ground-state potentials are similar to those of previous calculations, showing a linear minimum geometry for MgOH but with a very shallow bending potential, and for BeOH a bent minimum but with only a 50 cm~(-1) barrier to linearity.