On the ⁴J_HH long‐range coupling in 2‐bromocyclohexanone: conformational insights

Abstract

2-Bromocyclohexanone is a model compound in which a 4JH2, H6 coupling constant is observed, whereas the corresponding 4JH2,H4 is absent. The observed long-range coupling is not only a result of the known W-type coupling, in the axial conformation, but also because of the less usual diaxial spin-spin coupling in the equatorial conformer. The carbonyl group plays a determining role in describing the coupling pathway, as concluded by natural bond orbital (NBO) analysis; although the sigma C2-H2-->sigma*C1(O)-C6 and sigma C6-H6 -->sigma*C1(O)-C2 interactions in the axial conformer contribute for transmitting the spin information associated with the W-type coupling, the strong sigma C2-H2-->pi*C=O and sigma C6-H6-->pi*C=O hyperconjugations in the equatorial conformer define an enhanced coupling pathway for 4JH2,H6,despite the inhibition of this coupling because of nO-->sigma*C(O)-C interaction and the large carbonyl angle. These findings provide the experimental evidence that orbital interactions contribute for the conformational isomerism of 2-bromocyclohexanone.

The case of infrared carbonyl stretching intensities of 2-bromocyclohexanone: Conformational and intermolecular interaction insights

“…The ab initio HF/6-31g(d, p) calculations indicate the axial conformer of 2-bromocyclohexanone as the most stable one in the isolated state by 2.5 kcal mol À1 , which is in reasonable agreement with the MP2/aug-cc-pVDZ result obtained elsewhere (2.1 kcal mol À1 ) [6]. It is expected that the equatorial conformation increases in population on going from the vapor and non-polar solvents to increasingly more polar solvents, since the equatorial conformer is significantly more polar than the axial form (calculated dipole moments of 4.7 D against 3.4 D, for the equatorial and axial conformers, respectively).…”

“…It is expected that the equatorial conformation increases in population on going from the vapor and non-polar solvents to increasingly more polar solvents, since the equatorial conformer is significantly more polar than the axial form (calculated dipole moments of 4.7 D against 3.4 D, for the equatorial and axial conformers, respectively). In the isolated state, the calculated conformational energy is due to intramolecular interactions operating in 2-bromocyclohexanone, namely steric, electrostatic and hyperconjugative effects [4][5][6]15].…”

“…Ab initio (MP2/aug-cc-pVDZ) calculations have shown that the axial conformer of 2-bromocyclohexanone is ca. 2.1 kcal mol À1 more stable than the equatorial one in the isolated state [6], and a similar amount must be expected in non-polar solution, such as 2-bromocyclohexanone in n-hexane, opposite to observed through an infrared study [7]. Thus, this work aims at elucidating the origin of the spectroscopic behavior of 2-bromocyclohexanone, in order to rationalize its conformational preferences in the vapor phase using infrared spectroscopy and quantum chemical calculations as tools.…”

“…However, it is well known that acetic anhydride self-associates in cyclohexane [9] and acetone dimerizes in the vapor phase [10], but there are no experimental results for 2-bromocyclohexanone in the vapor state, and the available data for the isolated molecule refer to those obtained by means of theoretical calculations [3][4][5][6]. Ab initio (MP2/aug-cc-pVDZ) calculations have shown that the axial conformer of 2-bromocyclohexanone is ca.…”

“…This compound undergoes equatorial-axial interconversion ( Fig. 1), and several studies have used theoretical and experimental techniques to define the conformer populations and intramolecular interactions for each conformer both in the isolated and condensed phases [1][2][3][4][5][6][7]. The equatorial conformer of 2-bromocyclohexanone is more polar (l = 4.7 D) than the axial one (l = 3.4 D); therefore, it is expected that the equatorial preference in the conformational equilibrium increases on going from the gas or non-polar solvents to increasingly more polar solvents.…”