Weak C-H•••O and C-H•••F-C hydrogen bonds in the oxirane-trifluoromethane dimer

The oxirane-trifluoromethane dinner generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. The rotational spectra of the parent species and of its two C-13 isotopomers in combination with ab initio calculations have been used to establish a C-s geometry for the dimer with the two monomers bound by one C-(HO)-O-... and two C-(HF)-F-...-O hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-(HO)-O-... and C-(HF)-F-... hydrogen bonds, r((OH)-H-...) and r((FH)-H-...), are 2.37 and 2.68 Angstrom, respectively. The C-(HF)-F-...-C interactions give rise to the HCF3 internal rotation motion barrier of 0.55(1) kJ/mol, which causes the A-E splittings observed in the rotational spectra. The analysis of the structural and energetic features of the C-(HO)-O-... and C-(HF)-F-...-C interactions allows us to classify them as weak hydrogen bonds. Ab initio calculations predict these weak interactions to produce blue shifts in the C-H vibrational frequencies and shortenings of the C-H lengths.