Nonplanarity at tri-coordinated aluminum and gallium: cyclic structures for X₃H_n^m (X = B, Al, Ga)

Abstract

Structures and energies of X₃H₃^2-, X₃H^4-, X₃H₅, and X₃H₆⁺ (X = B, Al and Ga) were investigated theoretically at B3LYP/6-311G(d) level. The global minimum structures of B are not found to be global minima for Al and Ga. The hydrides of the heavier elements Al and Ga have shown a total of seven, six and eight minima for X₃H₃^2-, X₃H₄^-, and X₃H₅, respectively. However, X₃H₆⁺ has three and four minima for Al and Ga, respectively. The nonplanar arrangements of hydrogens with respect to X₃ ring is found to be very common for Al and Ga species. Similarly, species with lone pairs on heavy atoms dominate the potential energy surfaces of Al and Ga three-ring systems. The first example of a structure with tri-coordinate pyramidal arrangement at Al and Ga is found in X₃H₄^- (2g), contrary to the conventional wisdom of C₃H₃⁺, B₃H₃, etc. The influence of π-delocalization in stabilizing the structures decreases from X₃H₃^2- to X₃H₆⁺ for heavier elements Al and Ga. In general, minimum energy structures of X₃H₄^-, X₃H₅, and X₃H₆⁺ may be arrived at by protonating the minimum energy structures sequentially starting from X₃H₃^2-. The resonance stabilization energy (RSE) for the global minimum structures (or nearest structures to global minimum which contains π-delocalization) is computed using isodesmic equations.