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

Abstract

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.

The Chiral Trimer and a Metastable Chiral Dimer of Achiral Hexafluoroisopropanol: A Multi‐Messenger Study

“…We used 2.0-6.0 GHz chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy [25,26] to probe the pure rotational spectra of the HFIP clusters.T he acquired rotational spectrum is very rich in transitions and required stepwise assignments by removing the strong transitions associated with other species such as the monomer and dimer species to reduce spectral congestion. [20,22] With the high-resolution CP-FTMW experiments and the observation of the higher-energy isomer gt rather than c-tt,i tis clear that c-tt is missing, consistent with the low predicted conversion barrier from c-tt to tt. Thes pectrum we associate with the tt isomer features ad istinctive m c -type spectrum with nearly equally-spaced Qb ranches,i ndicative of an ear-prolate asymmetric rotor.…”

“…Fort he dimer, [20,21] the achiral tt-form was recently found to dominate. [22] In this work, we show that the most stable HFIP trimer consists of three chiral building blocks g and g',rather than three C s -symmetric t monomer units,despite acumulated excess energy penalty of 15 kJ mol À1 and the low abundance of the g conformation in jet-cooled monomers and dimers. This is even more surprising because the t!g conversion Figure 1.…”

“…From this data set, two sets of transitions belonging to HFIP dimers were initially assigned. [22] All obtained spectroscopic constants are summarized in Table 2. Additional weak m a -type transitions populating the spectral baseline were also detected at relative intensities consistent with the calculated dipole components of tt.…”

“…[22] It will be interesting to investigate even larger HFIP clusters, pushing the limit of the current experimental multi-messenger approach, to follow how homochiral HFIP helices in the crystal emerge at the mesoscopic level. Thes tudy provides ad etailed insight into the structural and energetic changes of the preferred structures of the HFIP subunits in their isolated form and as at rimer, where emerging bulk properties appear.…”

“…We first obtained IR and complementary Raman spectra of jet-cooled HFIP [22] in the H-bonded OH stretching region to provide as urvey of the formed HFIP clusters.T he main experimental spectra, together with dimer-scaled harmonic simulations of the trimers,are given in Figure 2, whereas the full spectra are provided in Figure S3 (Supporting Information). Thei nfrared spectrum exhibits three OH stretching peaks with ac ommon cluster origin ( Figures S4 and S5) and as houlder at the central peak.…”