Pseudo-Jahn-Teller origin of geometry and pseudorotations in second row tetra-atomic clusters X4 (X=Na,Mg,Al,Si,P,S) -: art. no. 044321

Experimentally determined or ab initio calculated molecular geometries carry no information about their origin. Employing the Jahn-Teller (JT) vibronic coupling effects as the only source of instability and consequent distortions of high-symmetry molecular configurations, we have worked out a procedure that allows us to trace the origin of particular geometries and determine the detailed electronic mechanism of their formation. This procedure is illustrated by considering a series of X-4 clusters with X=Na, Mg, Al, Si, P, and S. It shows explicitly why Na-4, Si-4, and Al-4 have a rhombic geometry in the ground state, while Mg-4 and P-4 are tetrahedral, whereas S-4 is a trapezium. Even when the minimum-energy geometries are the same (as in the case of rhombic Na-4, Si-4, and Al-4), the electronic mechanism of their formation is quite different. In particular, in Na-4 and Si-4 the rhombic minima are produced by a strong pseudo JT coupling between two excited states in the square-planar configuration (different in the two cases) that stabilizes one of them and makes it the ground state by rhombic distortions. The rhombic configuration of Al-4 is due to the pseudo JT effect in its ground-state square-planar configuration, and the trapezium in S-4 is formed by two pseudo JT couplings essentially involving excited states. In several cases this analysis shows also the tunneling paths between equivalent configurations. (c) 2006 American Institute of Physics.