Experimental Reference Data for Hexafluorinated Propanol by Exploring an Unusual Intermolecular Torsional Balance.

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.