Nonplanarity at Tri-coordinated Aluminum and Gallium: Cyclic Structures for X3Hnm (X = B, Al, Ga)

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Abstract
Structures and energies of X3H32-, X3H4-, 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 X3H32-, 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 X3H32- 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 X3H32-. 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.