Controlling achiral and chiral conformations of benzyl alcohol by ortho-halogenation, collisional relaxation and dimerisation

Abstract

Ortho-Halogenated benzyl alcohol can exist in two different low energy chiral conformations, only one of them with an OH–X contact (X = Cl, Br, I). A third, achiral conformation is enabled by the halogen substitution. We show by IR spectroscopy in supersonic jets that the achiral monomer is less stable than the chiral conformation with OH–X contact, but both can be produced in similar amounts using helium as a carrier gas. The robust OH transition observed for the achiral monomer is a sensitive benchmark for the conformational energy sequence, since the chiral conformation without OH–X contact is at least an order of magnitude less abundant, although it is often predicted by DFT to be slightly more stable and separated from the achiral conformation by a low barrier. This competing chiral conformation must be energetically higher by at least a few tenths of a kJ mol⁻¹ to be consistent with experimental observations. That is indeed predicted by high-level energy calculations at the DFT-optimised structures. The most stable dimers of ortho-halogenated benzyl alcohols involve torsionally homo- and heterochiral pairings of the two OH groups, where the hydrogen bond-accepting OH group forms a cooperative intramolecular OH–X contact. The achiral monomer conformation is suppressed in these dimers. A homochiral dimer is formed almost exclusively for Cl, whereas its heterochiral variant is progressively co-stabilised with increasing halogen size. The stretching wavenumber of the donor OH in the dimers depends on the relative chirality of the donor and acceptor conformations. The consistent picture that emerges for Cl, Br and I substitution in ortho-position of benzyl alcohol is discussed in the context of interconversion barriers, heavy atom tunneling, π–π stacking, suppression of OH–π bonding, chirality synchronisation, and shortcomings of DFT approaches in reproducing the observations. The homochiral aggregation preference observed for simple benzyl alcohol is conserved and even enhanced upon ortho-halogenation, albeit partly by different interactions.