The hydrogen-bonded dimer of 1,1,1,3,3,3-hexafluoro-2-propanol forms an intermolecular -OH⋅⋅⋅O- torsional balance, in which the acceptor OH group can point away from or towards the fluorine atoms of the donor. It prefers the former arrangement in the free dimer, but dinitrogen coordination of the acceptor OH favors the latter, as FTIR spectroscopy in supersonic jets suggests. A multi-step divide-and-conquer strategy was employed to rule out density functional and other inexpensive quantum chemical methods within the harmonic approximation. Among 20 exploratory single determinant calculations, only those based on the B3LYP-D3 functional provide a satisfactory description of six carefully assessed experimental constraints for this fluorous hydrogen bond competition. Low barrier intermolecular torsion balances are proposed more generally as non-covalent conformational energy benchmarking tools.

1,1,1,3,3,3-hexafluoro-propan-2-ol aggregates preferentially into an achiral dimer of achiral monomers,b ut the trimer is found to prefer three metastable chiral monomer units arranged into astrained OH···O hydrogen-bonded ring, which is reinforced by secondary CH···FC interactions.This is shown by ac ombination of infrared, microwave,a nd Raman spectroscopyi ns upersonic jet expansions and supported by high-level quantum chemical calculations.I ti nvolves an activation of the monomers by > 15 kJ mol À1 ,c learly driven by the muchs tronger hydrogen-bond interaction available to the gauche and even more to the cis monomer units.

Molecular nitrogen as a weak hydrogen bond acceptor is added to formic and acetic acid and their monodeuterated isotopologues. FTIR spectroscopy of supersonic expansions in the O-H stretching region reveals the formation of the weakly bound N-carboxylic acid complexes. Their respective spectral downshifts from the monomer fundamental vibration are used to benchmark electronic structure calculations and vibrational perturbation theory. The small size of the investigated systems allows for a wide range of electronic structure levels to be explored. The O-H stretching vibration of an open dimer of acetic acid can be discriminated from the cyclic dimer vibrations by its higher susceptibility to coexpanded nitrogen.

The monomers and hydrogen-bonded dimers of benzyl alcohol, cyclohexylmethanol, and 2-methyl-1-propanol are investigated by jet-FTIR spectroscopy, complemented by Raman spectra and quantum chemical calulations, including CCSD(T) corrections. A large variety of London dispersion effects from the interacting carbon cycles is revealed, sometimes adding to and sometimes competing with the alcoholic hydrogen bonds. Conformational (in-)flexibility provides the key for understanding these effects, and this requires accurate predictions of monomer conformational preferences, which are shown to be subtly at variance with experiment even for some triple-ζ MP2 calculations. In some observed dimers, cooperative OH···OH···π patterns are sacrificed to optimize σ-π dispersion interactions. In other competitive dimers, dispersion interactions are far from maximized, because that would imply a substantial weakening of the hydrogen bond. In the series from methanol dimer to 1-indanol dimer, which this contribution bridges, B3LYP-D3 appears to switch from an overestimation to a slight underestimation of cohesion, but overall it provides a very useful modeling tool for vibrational spectra of systems affected by both hydrogen bonds and London dispersion.

FTIR spectroscopy of supersonic expansions is used to characterize alcohol dimers with one, two, and several nitrogen molecules attached to them. The nitrogen coating causes progressive spectral downshifts of the OH stretching fundamentals which are related to and explain matrix isolation shifts. Comparison of methanol, tert-butyl alcohol and ethanol as well as deuteration of methanol assist in the assignment. Alcohol monomers and trimers are significantly more resistant to nitrogen coating due to a lack of cooperativity and dangling bonds, respectively. In the case of ethanol, the role of conformational isomerism and combination bands is further elucidated. The experimental findings help rationalize the anomalously small OH stretching dimerization shift of methanol in the gas phase, in comparison to that of tert-butyl alcohol.