The lowest singlet and triplet potential energy surfaces of H-shift isomerization for HXYH (X,Y = As, Sb, and Bi) systems have been explored through ab initio calculations. The geometries of the various isomers were determined at the QCISD/LANL2DZdp level of theory and confirmed to be minima by vibrational analysis. Our model calculations indicate that the range of π-bond strength is 12-20 kcal/mol, as indicated by the singlet-triplet gap of the HXYH molecules. The hydrogen-shifted isomers H2XY in the singlet state are considerably unstable than the singlet-state parent isomers by 24-49 kcal/mol. The triplet isomers are comparable in stability with respect to the parent singlet molecules, HXYH, within 3 kcal/mol for X lighter than Y. The gap increases to 8 kcal/mol for the homonuclear compounds with X = Y. It increases further to 17 kcal/mol for X heavier than Y.
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