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The Gauche Effect in XCH 2 CH 2 X Revisited
Author(s) -
Rodrigues Silva Daniela,
Azevedo Santos Lucas,
Hamlin Trevor A.,
Fonseca Guerra Célia,
Freitas Matheus P.,
Bickelhaupt F. Matthias
Publication year - 2021
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.202100090
Subject(s) - hyperconjugation , chemistry , lone pair , steric effects , halogen , pauli exclusion principle , gauche effect , atomic orbital , molecular orbital , crystallography , stereochemistry , computational chemistry , molecule , physics , condensed matter physics , quantum mechanics , electron , organic chemistry , medicine , alkyl , surgery
We have quantum chemically investigated the rotational isomerism of 1,2‐dihaloethanes XCH 2 CH 2 X (X = F, Cl, Br, I) at ZORA‐BP86‐D3(BJ)/QZ4P. Our Kohn‐Sham molecular orbital (KS‐MO) analyses reveal that hyperconjugative orbital interactions favor the gauche conformation in all cases (X = F−I), not only for X = F as in the current model of this so‐called gauche effect. We show that, instead, it is the interplay of hyperconjugation with Pauli repulsion between lone‐pair‐type orbitals on the halogen substituents that constitutes the causal mechanism for the gauche effect. Thus, only in the case of the relatively small fluorine atoms, steric Pauli repulsion is too weak to overrule the gauche preference of the hyperconjugative orbital interactions. For the larger halogens, X⋅⋅⋅X steric Pauli repulsion becomes sufficiently destabilizing to shift the energetic preference from gauche to anti , despite the opposite preference of hyperconjugation.