Abstract
We have quantum chemically investigated the rotational isomerism of 1,2‐dihaloethanes XCH2CH2X (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.
Highlights
The energy profile for rotation around the C C bond in 1,2disubstituted ethanes features four stationary points, that is, two staggered conformers connected via two eclipsed transition states, as schematically illustrated in Figure 1a.[1]
Pointed out that stabilization of the gauche conformer is caused by orbital and electrostatic interactions, Thacker and Popelier[13] attributed it solely to electrostatics, while MartínPendás and coworkers[14] explained it based on both electrostatic and exchange-correlation interactions.[15]. In view of this ongoing and highly relevant controversy, we have investigated the origin of the gauche effect within the framework of quantitative Kohn-Sham molecular orbital (KSMO) theory using the series of 1,2-dihaloethanes XH2C CH2X
The overall bond energy ΔE has been divided into two major components using the activation strain model (ASM):[16] the strain (ΔEstrain) that results from the distortion of the two CH2X radicals from their equilibrium structure to the geometry they acquire in the XH2C CH2X
Summary
The energy profile for rotation around the C C bond in 1,2disubstituted ethanes features four stationary points, that is, two staggered conformers (gauche and anti) connected via two eclipsed transition states (syn and anticlinal), as schematically illustrated in Figure 1a.[1]. Pointed out that stabilization of the gauche conformer is caused by orbital and electrostatic interactions, Thacker and Popelier[13] attributed it solely to electrostatics, while MartínPendás and coworkers[14] explained it based on both electrostatic and exchange-correlation interactions.[15] In view of this ongoing and highly relevant controversy, we have investigated the origin of the gauche effect within the framework of quantitative Kohn-Sham molecular orbital (KSMO) theory using the series of 1,2-dihaloethanes XH2C CH2X We show that, at variance to the currently prevailing model, the switch in preference from gauche for X = F to anti for X = Cl I, is caused by the increasing steric demand of the substituent X, as the latter descends down a group in the periodic table This trend does not originate from hyperconjugation, which we show to always favor gauche but only overrule steric (Pauli) X···X repulsion in the gauche conformation for the smallest substituent X, in our model systems, the second-row fluorine atom. We highlight how geometrical relaxation, in particular, the effect of the variation of the C C bond length, upon internal rotation around this bond can mask the change in the various orbital and electrostatic interactions and needs to be taken into consideration to properly identify causalities
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