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 S 4 -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 OHÁ Á ÁO 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. † Electronic supplementary information (ESI) available: Interactive and static structures, tables of experimental and computational details, energy diagrams for different levels of calculation, and additional experimental spectra. See

Understanding benzyl alcohol aggregation by chiral modification: the pairing step

“…7 provides two survey FTIR spectra of the OH-stretching region for heated B expansions in helium. The difference reflects the signal-to-noise improvements achieved since the original B study 13 , based on technical modifications which were recently described 6 . The new spectrum (top trace) was ob-Physical Chemistry Chemical Physics…”

“…Initial searches for minima and transitions states were conducted with the B3LYP [34][35][36][37] functional together with Grimme's D3 correction with two-body terms and Becke-Johnson damping 38 and the def2-TZVP basis set 39 . This choice is based on previous performance for systems with chirality recognition 6,28 . Results were reoptimized with the larger minimally augmented may-cc-pVTZ basis set 40 and complemented by B2PLYP-D3 41 (also with two-body terms and BJ-damping), ωB97X-D 42 and M06-2X 43 functional results, also using may-cc-pVTZ.…”

“…Beyond the question of whether hom or het pairings are preferred in the dimers of B and P and which hydrogen bond topology they exhibit, the lowering or raising of the racemization barrier in B by dimerization will be investigated. In the analo-gous aggregation study of transiently chiral 1,2-ethanediol and its chirality-controllable 1,2-cyclohexanediol homolog 6 , the barrier was found to be lowered significantly 30 , but not enough to prevent largely statistical hom/het dimer formation in a jet expansion. In monomeric B, the barrier is an order of magnitude lower, providing a better starting point for chirality synchronization.…”

“…As the driving force for chirality synchronization in such a cluster can be quite small, low temperatures, as they are achievable in supersonic jet expansions, are indicated. However, at low temperature, the interconversion barriers between enantiomeric conformers quickly become insurmountable and statistical dimer formation prevails 6 . The relative energy information is then hidden on the experimental side.…”

“…12 Here, we use linear FTIR spectroscopy and spontaneous Raman scattering, which offer nicely complementary viewpoints on the dimer as well as relative abundances of different isomers. Their disadvantage of reduced sensitivity is compensated by long, pulsed slit nozzles (FTIR 6,18 ) and powerful lasers (Raman 6 ). The lack of size selectivity is compensated by concentration variation and conformational discrimination is facilitated by the sensitivity of the OH chromophore to subtle interaction details.…”