Correlation of an Alcohol's αC−D Stretch with Hydrogen Bond Strength in Complexes with Amines

Abstract

The enthalpies of H-bond formation between 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as donor and tertiary amines of varying pKa's as acceptors were determined by isothermal calorimetry. The stretching frequencies of the αC−D of HFIP-d2 in dilute solution and in different H-bonded amine solvents were measured by Raman spectroscopy. Dilute HFIP-d2 in chloroform showed two C−D stretching features at 2195 and 2168 cm-1 which are assigned to the anti and gauche conformations of the molecule on the basis of precedent and theoretical calculations. The molecular structure, relative energies, and C−D stretching frequency (νC-D) of the anti and gauche conformations of HFIP and HFIP-d2 and their H-bonded complexes with trimethylamine were calculated by density functional theory using a B3LYP/6-31+G(d,p) basis set. In the monomeric state, the calculated νC-D for the gauche conformation was 52 cm-1 lower than the anti conformations. Both νC-D bands were red-shifted when the O−H(D) group functioned as a H-bond donor with amine receptors. Among the different tertiary amines, the alcohol forms the weakest H bond with N,N-dimethylaniline and the strongest bond with quinuclidine. The enthalpy of H-bond formation and the gauche and anti αC−D, stretching frequencies were correlated, having slopes of −2 ± 1 and −4 ± 1.5 cm-1/(kcal mol-1), respectively. The results indicate increasing H-bond strength results in an increase in negative hyperconjugation of the vicinal C−D bond. Observation of νC-D of specifically labeled (HO)C−D groups can be used to provide a spectroscopic characterization of a specific H bond present in macromolecular complexes when the OH group is bound to a single H-bond acceptor.