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Myth and Reality in the Attitude toward Valence‐Bond (VB) Theory: Are Its ‘Failures’ Real?
Author(s) -
Shaik Sason,
Hiberty Philippe C.
Publication year - 2003
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200390094
Subject(s) - valence bond theory , chemistry , cyclobutadiene , contradiction , aromaticity , simple (philosophy) , valence (chemistry) , computational chemistry , natural bond orbital , theoretical physics , mythology , molecule , density functional theory , molecular orbital , epistemology , physics , philosophy , organic chemistry , theology
According to common wisdom propagated in textbooks and papers, valence‐bond (VB) theory fails and makes predictions in contradiction with experiment. Four iconic ‘failures’ are: a ) the wrong prediction of the ground state of the O 2 molecule, b ) the failure to predict the properties of cyclobutadiene (CBD) viz. those of benzene, c ) the failure to predict the aromaticity/anti‐aromaticity of molecular ions like C 5 H $\rm{_{5}^{+}}$ and C 5 H $\rm{_{5}^{-}}$ , C 3 H $\rm{_{3}^{+}}$ and C 3 H $\rm{_{3}^{-}}$ , C 7 H $\rm{_{7}^{+}}$ and C 7 H $\rm{_{7}^{-}}$ , etc ; and d ) the failure to predict that, e.g. , CH 4 has two different ionization potentials. This paper analyzes the origins of these ‘failures’ and shows that two of them (stated in a and d ) are myths of unclear origins, while the other two originate in misuse of an oversimplified version of VB theory, i.e. , simple resonance theory that merely enumerate resonance structures. It is demonstrated that, in each case, a properly used VB theory at a simple and portable level leads to correct predictions, as successful as those made by use of molecular‐orbital (MO) theory. This notion of VB ‘failure’, which is traced back to the VB‐MO rivalry, in the early days of quantum chemistry, should now be considered obsolete, unwarranted, and counterproductive. A modern chemist should know that there are two ways of describing electronic structure, which are not two contrasting theories, but rather two representations or two guises of the same reality. Their capabilities and insights into chemical problems are complementary, and the exclusion of any one of them undermines the intellectual heritage of chemistry.