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Theoretical Studies of Inorganic Compounds. 34 1) Energy Decomposition Analysis of E–E Bonding in Planar and Perpendicular X 2 E−EX 2 (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I)
Author(s) -
Szabó Andrea,
Kovács Attila,
Frenking Gernot
Publication year - 2005
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.200500183
Subject(s) - chemistry , halogen , lone pair , crystallography , hyperconjugation , hydrogen bond , perpendicular , planar , molecule , decomposition , atomic physics , physics , geometry , alkyl , mathematics , computer graphics (images) , organic chemistry , computer science
The nature of E–E bonding in group 13 compounds X 2 E–EX 2 (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 B 2 X 4 −Tl 2 X 4 have a perpendicular (D 2d ) geometry. The largest energy barriers for rotation about the E‐E bond are predicted for the hydrogen species B 2 H 4 −Tl 2 H 4 . The EDA shows that the rotational barriers of B 2 X 4 −Tl 2 X 4 may not be used for an estimate of the hyperconjugative strength in the D 2d structures except for the tetrahydrides. The values for the planar (D 2h ) transition states reveals that π conjugation of the halogen lone‐pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B 2 I 4 is stronger than in B 2 H 4 although the latter compound has a higher rotational barrier than the former. In B 2 F 4 , 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 Al 2 X 4 −Tl 2 X 4 have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al 2 H 4 −Tl 2 H 4 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 (Δ E elstat ) and molecular orbital (Δ E orb ) components of the E–E bonding are similar in magnitude.Thecalculated B‐B bond dissociation energies of B 2 X 4 (D e = 93.0–108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al 2 X 4 −Tl 2 X 4 have weaker bonds (D e = 16.6–61.7 kcal/mol). In general, the X 2 E‐EX 2 bond dissociation energies follow the trend for atoms E: B ≫ Al > Ga > In > Tl and for atoms X: H > F > Cl > I.