Interaction of alcohols with 2-fluoro- and 4-fluorophenylacetylenes: infrared-optical double resonance spectroscopic and computational investigation.

Alcohol complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Methanol forms a cyclic complex with both the fluorophenylacetylenes incorporating C-H···O and O-H···π hydrogen bonds, the structure of which is similar to that of the corresponding water complex but different from that of a phenylacetylene-methanol complex. The anti conformer of ethanol also binds in a similar fashion to both the fluorophenylacetylenes. Additionally, the gauche conformer of ethanol binds to 2-fluorophenylacetylene in a distinctly different structural motif that incorporates C-H···F and O-H···π hydrogen bonds. The OH group of trifluoroethanol interacts primarily with the π electron density of the C≡C bond. The π electron density of the C≡C bond is the principal point of interaction between the alcohols and both the fluorophenylacetylenes. The present results are indicative of the fact that fluorine substitution on the phenyl ring is sufficient to eliminate the subtle hydrogen bonding behavior of phenylacetylene.