Theoretical study of protonated aluminium oxide

Abstract

Using an internally contracted multireference configuration interaction approach, threedimensional potential energy functions of the X2Σ+ (AlOH+) and X2Π (HAlO+) states have been generated. The variational calculations of the vibrational levels of AlOH+ showed that the large amplitude bending modes exhibit inverse anharmonicity and are coupled with the AlO stretching modes. The energy of the HAlO+ isomer at its equilibrium geometry and its lowest dissociation asymptote were calculated to be almost identical. A large barrier on the dissociation path prevents, however, the low lying rovibronic levels to predissociate. Using a variational Renner-Teller approach such rovibronic states for J= 1/2 and 3/2 were calculated, and strong anharmonic resonances were detected in the excited rovibronic states. The potential energy function connecting the HAlO+ and AlOH+ isomers includes a conical intersection between the 2Σ+ and the 2Π states. The barrier for isomerization relative to the AlOH+ structure was calculated to be 4·34 eV. The MRCI calculations yielded for AlO a large proton affinity of 9·35 eV. Contrary to previous theoretical studies we found that the electronic ground state of AlO+ is the 1Σ+ state and not the 3Π state. The ab initio spectroscopic constants for its three lowest electronic states are given.