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The Gauche Effect in XCH2 CH2 X Revisited.

ABSTRACT: We have quantum chemically investigated the rotational isomerism of 1,2-dihaloethanes XCH2 CH2 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.

SUBMITTER: Rodrigues Silva D 

PROVIDER: S-EPMC8048458 | biostudies-literature | 2021 Apr

REPOSITORIES: biostudies-literature

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The Gauche Effect in XCH<sub>2</sub> CH<sub>2</sub> X Revisited.

Rodrigues Silva Daniela D   de Azevedo Santos Lucas L   Hamlin Trevor A TA   Fonseca Guerra Célia C   Freitas Matheus P MP   Bickelhaupt F Matthias FM  

Chemphyschem : a European journal of chemical physics and physical chemistry 20210225 7

We have quantum chemically investigated the rotational isomerism of 1,2-dihaloethanes XCH<sub>2</sub> CH<sub>2</sub> 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  ...[more]

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