AB-INITIO STUDY OF THE STRUCTURES, ENERGETICS, AND BONDING OF THE ISOMERS BENH AND HBEN

The structures, energetics, spectra, and bonding of the isomers BeNH a nd HBeN are described theoretically at the ab initio Hartree-Fock and configuration interaction levels. The species HBeN is still unknown ex perimentally, and the results reported in this paper represent the fir st theoretical prediction of its existence. For the molecule BeNH, vib rational frequencies with an unequivocal assignment of the NH stretchi ng are the only experimental data found in the literature. In contrast with the scarceness of experimental information, and complementing pr evious studies on BeB, BeC, BeN, and BeF, this paper reports detailed results on geometries, vibrational frequencies, relative stabilities, excited electronic states, and electronic rearrangements leading to th e bonding in the molecules BeNH and HBeN. The molecule BeNH is 10.5 kc al/mol more stable than HBeN, and the favorite route for dissociation involves breaking the Be-N bond in both BeNH and HBeN. The three lowes t electronic states in BeNH are very closely spaced with the first exc ited singlet state (1PI) only about 1500 cm-1 higher than the ground s tate (1SIGMA+); in contrast, for HBeN the first 3PI is about 8000 cm-1 higher than the 3SIGMA- ground state. Their vibrational frequencies a re quite distinct, and the predicted values for HBeN will certainly al low an unequivocal identification of this species. As to the bonding i n the molecule BeNH, an unexpected 2s2 --> 2p(x)1 2p(y)1 electronic pr omotion leads to a pi-type bond between the beryllium and nitrogen ato ms which is not found in the isomer HBeN. In both molecules a signific ant ionic character also contributes to the bonding between these two atoms.