Fluorine Gauche Effect Explained by Electrostatic Polarization Instead of Hyperconjugation: An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Study

Joseph C R Thacker,Paul L A Popelier

doi:10.1021/acs.jpca.7b11881

Joseph C R Thacker, Paul L A Popelier

Open Access

https://doi.org/10.1021/acs.jpca.7b11881

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Abstract

We present an interacting quantum atoms (IQA) study of the gauche effect by comparing 1,2-difluoroethane, 1,2-dichloroethane, and three conformers of 1,2,3,4,5,6-hexafluorocyclohexane. In the 1,2-difluoroethane, the gauche effect is observed in that the gauche conformation is more stable than the anti, whereas in 1,2-dichloroethane the opposite is true. The analysis performed here is exhaustive and unbiased thanks to using the recently introduced relative energy gradient (REG) method [ Thacker , J. C. R. ; Popelier , P. L. A. Theor. Chem. Acc . 2017 , 136 , 86 ], as implemented in the in-house program ANANKE. We challenge the common explanation that hyperconjugation is responsible for the gauche stability in 1,2-difluoroethane and instead present electrostatics as the cause of gauche stability. Our explanation of the gauche effect is also is seen in other molecules displaying local gauche conformations, such as the recently synthesized "all-cis" hexafluorocyclohexane and its conformers where all the fluorine atoms are in the equatorial positions. Using our extension of the traditional IQA methodology that allows for the partitioning of electrostatic terms into polarization and charge transfer, we propose that the cause of gauche stability is 1,3 C···F electrostatic polarization interactions. In other words, if a number of fluorine atoms are aligned, then the stability due to polarization of nearby carbon atoms is increased.

Highlights

The gauche effect was coined by Wolfe[1] in a paper that observed the stability of the gauche conformation in X−CH2− CH2−X when the electronegativity of the substituent (X) increases
We discuss the total energy differences between the gauche and anti conformations in 1,2-difluoroethane and 1,2dichloroethane. This total energy difference is not interacting quantum atoms (IQA) based, in that all atoms are summed over to give molecular kinetic, exchange−correlation, and electrostatic energies. We start with this coarse analysis, that is, one that lacks atomic resolution but instead looks at how the various energy types act together, over the whole system to acquire chemical insight based on molecular energy to show our conclusions do not require atomic definitions to be corroborated
We partition the energy into three components: kinetic, exchange−correlation, and electrostatics, as shown in eqs [7, 8], and 9, respectively

Summary

Introduction

The gauche effect was coined by Wolfe[1] in a paper that observed the stability of the gauche conformation in X−CH2− CH2−X when the electronegativity of the substituent (X) increases Wolfe partially attributed this phenomenon to “size” or “steric” factors of the substituent atoms. Prior to Wolfe’s work, the gauche effect was already observed in 1960 by Klaboe and Nielsen[2] who studied 1,2-difluoroethane using infrared and Raman spectroscopy at variable temperatures. They concluded that the anti and gauche conformations were of comparable stability. The IQA approach calculates energetic terms by integrating the appropriate quantum mechanical densities over the topological atoms. IQA considers four components as defined in eq 1,

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