The reduced cohesion of homoconfigurational 1,2-diols

Abstract

By a combination of linear FTIR and Raman jet spectroscopy, racemic trans-1,2-cyclohexanediol is shown to form an energetically unrivalled S₄-symmetric heterochiral dimer in close analogy to 1,2-ethanediol. Analogous experiments with enantiopure trans-1,2-cyclohexanediol reveal the spectral signature of at least three unsymmetric homochiral dimers. A comparison to signal-enhanced spectra of 1,2-ethanediol and to calculations uncovers at least three transiently homochiral dimer contributions as well. In few of these dimer structures, the intramolecular OH⋯O contact present in monomeric 1,2-diols survives, despite the kinetic control in supersonic jet expansions. This provides further insights into the dimerisation mechanism of conformationally semi-flexible molecules in supersonic jets. Racemisation upon dimerisation is shown to be largely quenched under jet cooling conditions, whereas it should be strongly energy-driven at higher temperatures. The pronounced energetic preference for heterochiral aggregation of vicinal diols is also discussed in the context of chirality-induced spin selectivity.