Hydrogen Bonding in 2,2,2-Trifluoroethanol

Abstract

Context 2,2,2-Trifluoroethanol (TFE) is known as a membrane mimetic solvent. The IR spectrum, ¹H NMR spectrum and ¹³C NMR spin‒lattice relaxation times (T₁) and nuclear Overhauser effect (NOE) data are consistent with extensive hydrogen bonding in TFE, but do not lead to structural features of the hydrogen bonding. Hence DFT computations were carried out. The results predict the existence of a set of H-bonded dimers and trimers. The bond lengths and dihedral angles in these complexes are obtained, together with their dissociation energies. Computations were also performed for the geometry of the two conformers of the isolated monomer. The structure of one of the dimers consists of a 7-member cyclic fragment with a free CF₃CH₂ side chain. One set of the trimer structure involves the OH of a third monomer H-bonding to one of the F in the CF₃ group of the side chain of this dimer; and thereby creating three trimer isomers. A fourth trimer cluster is formed from three monomers in which three OH∙∙∙O bonds create a cyclic fragment with three CF₃CH₂ side chains. The high dissociation energy (with respect to three monomers) indicate the high stability of the trimer complexes. The structural features of the trimer complexes resemble the structure of a conventional liquid crystal molecule and is postulated to resemble the latter in properties and function in solution but at a much shorter timescale because of the noncovalent bonding. This hydrogen bonding phenomenon of TFE may be related to its function as a membrane memetic solvent. Methods Initially IR and NMR spectroscopic methods were used. Standard procedures were followed. For computational calculations a hybrid meta-GGA DFT method, M06-2X/6-311 + G**, was used. The basis set, 6-311 + G**, is of triple-ζ quality, in which polarization functions were added for all atoms and a set of s and p diffuse functions was augmented for the C and F atoms. All computations were carried out with the Q-Chem 5.0 program and the (99, 590) grid was applied for the DFT numeric integrations.