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Increased‐valence structures and hypervalent molecules
Author(s) -
Harcourt Richard D.
Publication year - 1996
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/(sici)1097-461x(1996)60:1<553::aid-qua54>3.0.co;2-8
Subject(s) - hypervalent molecule , chemistry , valence bond theory , modern valence bond theory , generalized valence bond , orbital hybridisation , valence (chemistry) , delocalized electron , molecular orbital theory , valence electron , molecular orbital , three center two electron bond , core electron , formal charge , atomic orbital , atomic physics , molecule , molecular physics , electron , physics , quantum mechanics , organic chemistry , reagent
Hypervalent molecules may involve the use of increased‐valence structures to provide valence bond descriptions of their electronic structure. For electron‐rich molecules with four electrons distributed among three overlapping (nuclear‐centered) atomic orbitals, the increased‐valence structures are Y(BOND)A·B and Y·A——B. Each structure involves a fractional electron‐pair bond and a one‐electron bond. It is deduced that the Armstrong‐Perkins‐Stewart valence of the A atom is able to exceed unity in each of these structures when the three bonding electrons occupy nonorthogonal localized molecular orbitals. It is also shown that increased valence for the A atom does not occur when the four electrons occupy localized molecular orbitals to give the valence‐bond structure Y—A—B with three overlapping atomic orbitals, and the same number of orbital variational parameters as occurs in the wave functions for either of the increased‐valence structures. The results of ab initio valence bond calculations with minimal basis sets are reported for H − 3 l, CH − 5 , HF − 2 , F − 3 , CIF 3 , and FF 3 , and the resulting wave functions for resonance between six canonical Lewis structures are related to those for resonance between the two increased‐valence structures. The use of the latter structures to indicate how electronic reorganization proceeds via one‐electron delocalizations for S N 2 reactions is redescribed, and an elementary argument is presented to deduce that this class of reactions cannot involve the delocalization of a pair of electrons in concert from the nucleophile. Increased‐valence wave functions are used to deduce an expression for the avoided crossing for the transition state of the identity S N 2 reaction. © 1996 John Wiley & Sons, Inc.