Structures and stabilities of hemi-bonded vs proton-transferred isomers of dimer radical cation systems (XH3YH3)+ (X,Y = N, P, As)

Structures, bonding and relative stabilities of the radical dimer cations (XH3YH3)+ (X,Y = N, P, As) have been studied theoretically. Two kinds of structures (hemi-bonded and proton-transferred isomers) are obtained for each system. For (NH3N/P/AsH3)+ the stable conformer is the proton-transferred structure; for (PH3PH3)+ and (AsH3AsH3)+ the stable structure is the hemi-bonded one; for (PH3AsH3)+ three proton-transferred and one hemi-bonded isomers were found with the stability order: (HPH3+⋯AsH2) I > (H3PH+⋯AsH2) II > (H3P⋯AsH3+) > (H3AsH+⋯PH2) III. The hemi-bonds have large interaction energies 25.2–35.1 kcal/mol and are partially covalent in nature, while the proton-transferred structures have moderate interaction energies 6.5–22.2 kcal/mol.