Binary conformers of a flexible, long-chain fluoroalcohol: dispersion controlled selectivity and relative abundances in a jet.

ABSTRACT

The complex conformational panorama of binary 4,4,4-trifluoro-1-butanol (TFB) aggregates was investigated using chirped-pulse Fourier transform microwave spectroscopy, aided by conformational searches using CREST (Conformer-Rotamer Ensemble Sampling Tool) and quantum chemistry calculations. From nearly 1500 initial dimer geometries, 16 most stable binary candidates were obtained within a relative energy window of ∼4 kJ mol-1. Rotational spectra of five binary conformers were experimentally observed in supersonic expansion and assigned. Interestingly, three out of the five observed binary conformers are composed solely of monomer conformers, which were not observed in their isolated gas phase forms in jet expansion. In addition, an observed dimer that is made exclusively of the most stable TFB monomer subunits does not correspond to the global minimum. The intricate kinetically and thermodynamically controlled dimer formation mechanisms are discussed, and a modified kinetic-thermodynamic model was developed, providing conformational abundances that are in good agreement with the experiment. Subsequent non-covalent interaction analyses reveal that the observed conformers are held together by one primary O-H⋯O hydrogen bond and secondary intermolecular C-H⋯O, C-H⋯F, and/or O-H⋯F interactions, as well as C-H⋯H-C London dispersion interactions between the methylene groups. Further symmetry-adapted perturbation theory analyses of the TFB dimer conformers and related alcohol dimers reveal a considerable rise in dispersion contributions with increasing n-alkyl carbon chain length and highlight the role of dispersion interactions in preferentially stabilizing the global minimum of the TFB dimer.