Conformational landscape and intricate conformational relaxation paths of 4,4,4-trifluoro-1-butanol: Rotational spectroscopy and quantum chemical calculations

Abstract

Fluorine-containing aliphatic alcohols have attracted considerable interest in part because they are widely used as solvents for peptides and proteins and for some chemical reactions. In the present work, the rotational spectrum of 4,4,4-trifluoro-1-butanol was investigated using a chirped-pulse Fourier transform microwave spectrometer in combination with a pulsed molecular expansion. The long carbon chain and the OH group in 4,4,4-trifluoro-1-butanol afford substantial conformational flexibility. In total, 27 possible geometries were identified using a semiempirical tight-binding based quantum chemistry program and also by carrying out systematic manual conformational searches. These conformations were verified to be true minima at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6–311++G(2d,p) and MP2/6–311++G(2d,p) levels of theory. 14 conformers that are spectroscopically distinguishable were obtained within an energy window of 18 kJ mol−1. Rotational spectra of three conformers were assigned, including four 13C isotopologues and one 18O isotopologue of the most stable conformer in their natural abundances. One of the detected conformers exhibits an intramolecular hydrogen bond formed between a non-hydroxy hydrogen atom of the –CH2OH group and a fluorine atom of the –CF3 group, which was identified by a quantum theory of atoms in molecules (QTAIM) analysis. The intricate conformational relaxation pathways and the corresponding interconversion barriers were explored to rationalize the experimental relative abundances of the detected conformers.