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The Gauche Effect in XCH₂ CH₂ X Revisited.

Daniela Rodrigues Silva^1,2, Lucas de Azevedo Santos^1,2, Trevor A Hamlin¹

¹Department of Theoretical Chemistry, Amsterdam Institute of Molecular and Life Sciences (AIMMS), Amsterdam Center for Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV, Amsterdam (The, Netherlands.

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry

|February 8, 2021

Summary

Quantum chemical analysis reveals hyperconjugation favors gauche conformations in 1,2-dihaloethanes. The gauche effect arises from hyperconjugation and Pauli repulsion, with steric effects determining the preferred conformation.

Keywords:

activation strain model bond theory conformational analysis energy decomposition analysis gauche effect

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Area of Science:

Quantum Chemistry
Computational Chemistry
Molecular Modeling

Background:

The gauche effect describes the preference for a gauche conformation over an anti conformation in certain molecules.
Previous models attributed the gauche effect solely to hyperconjugation, particularly for fluorine substituents.

Purpose of the Study:

To investigate the rotational isomerism of 1,2-dihaloethanes (XCH2CH2X, where X = F, Cl, Br, I) using quantum chemical methods.
To elucidate the underlying electronic and steric factors governing the gauche effect in these systems.

Main Methods:

Quantum chemical calculations using ZORA-BP86-D3(BJ)/QZ4P.
Kohn-Sham molecular orbital (KS-MO) analyses to study orbital interactions.

Main Results:

Hyperconjugative orbital interactions favor the gauche conformation for all halogens (F, Cl, Br, I).
The interplay between hyperconjugation and Pauli repulsion between halogen lone-pair orbitals is the primary cause of the gauche effect.
Steric Pauli repulsion becomes significant for larger halogens (Cl, Br, I), favoring the anti conformation over gauche.

Conclusions:

The gauche effect is a result of a balance between attractive hyperconjugation and repulsive Pauli interactions.
The observed conformational preferences in 1,2-dihaloethanes are explained by the combined influence of electronic and steric factors.
The current understanding of the gauche effect needs to incorporate the role of Pauli repulsion for heavier halogens.