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A PCILO Study of Conformation and Internal Rotation in Mono‐substituted Benzenes
Author(s) -
Gerhards Jürgen,
Ha TaeKyu,
Perlia Xavier
Publication year - 1982
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.19820650111
Subject(s) - chemistry , delocalized electron , cndo/2 , internal rotation , molecule , computational chemistry , rotation (mathematics) , mindo , wave function , electronic structure , stereochemistry , crystallography , atomic physics , organic chemistry , physics , mechanical engineering , geometry , mathematics , engineering
The effect of Kékulé representation and hybrid function of O‐atoms in the PCILO‐CNDO framework of conformation and internal rotation in mono‐sub stituted benzenes Ph‐X (XNH 2 , OH, OCH 3 , CH 3 , CHO, NO 2 ) is studied. Three variational criteria for the choice of the appropriate third‐order energy, proposed to symmetrize the PCILO results, are critically examined in relation with the height of rotational barrier in these molecules. The study shows that, in all cases, the most stable conformation is qualitatively correct predicted by the PCILO method. Since the barrier to internal rotation in the studied aromatic systems arises predominantly from delocalization effect, it is proposed to employ the arithmetic mean of the third‐order energy of the two Kékulé structures. In molecules, in which the third‐order energy between the two Kékulé structures is larger than 2 kcal/mol, however, the lower third‐order energy representation alone seems to be appropriate. In phenol and anisole the spa‐hybridization type of the O‐atoms offers better values of rotational barrier, whereas in the sp 3 ‐type the delocalization is overestimated in the planar conformation.