Theoretical Studies of Inorganic Compounds. 341) Energy Decomposition Analysis of E–E Bonding in Planar and Perpendicular X2E−EX2 (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I)

Abstract

AbstractThe nature of E–E bonding in group 13 compounds X2E–EX2 (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. The calculated equilibrium geometries of all molecules B2X4−Tl2X4 have a perpendicular (D2d) geometry. The largest energy barriers for rotation about the E‐E bond are predicted for the hydrogen species B2H4−Tl2H4. The EDA shows that the rotational barriers of B2X4−Tl2X4 may not be used for an estimate of the hyperconjugative strength in the D2d structures except for the tetrahydrides. The values for the planar (D2h) transition states reveals that π conjugation of the halogen lone‐pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B2I4 is stronger than in B2H4 although the latter compound has a higher rotational barrier than the former. In B2F4, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al2X4−Tl2X4 have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al2H4−Tl2H4 compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (ΔEelstat) and molecular orbital (ΔEorb) components of the E–E bonding are similar in magnitude.Thecalculated B‐B bond dissociation energies of B2X4 (De = 93.0–108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al2X4−Tl2X4 have weaker bonds (De = 16.6–61.7 kcal/mol). In general, the X2E‐EX2 bond dissociation energies follow the trend for atoms E: B ≫ Al > Ga > In > Tl and for atoms X: H > F > Cl > I.