Electronic structure and physical properties of MiXi clusters (M = B, Al; X = N, P; i = 1, 2, 3): Ab initio study.

Ab initio calculations of structural stability and properties of selected IIIa-Va monomers (BN, BP, AlN, AlP), and dimers and trimers thereof are presented. It was demonstrated that multireference methods are needed for an appropriate description of the electronic structure of the diatomics under study. The energy differences between the quasi-degenerated states of monomers were determined: in the range of 220-257 cm(-1) for the X3Pi --> A1Sigma+ in BN, 277-311 cm(-1) for the X3Pi --> A3Sigma(-) in AlN, and 344-768 cm(-1) for the X3Sigma(-) --> A3Pi in the AlP molecule. Many stable isomers of dimers and trimers were optimized and the structural patterns of the species were established. Although the (BN)(i) isomers prefer linear and cyclic geometries with a metal-nonmetal sequence, the structure of the (AlN)(i) species is determined by strong N--N interactions; the geometric patterns of the (BP)(i) and (AlP)(i) systems are more involved. The electronic structure and physical and chemical properties of the most stable dimers and trimers were calculated. As to electronic spectra, absorption bands in the visible and infrared region were found. The calculated low-lying excited states, electron distribution and partially biradical character of structures along with the variety of stable isomers indicate high chemical reactivity of the studied IIIa-Va microclusters.