Microwave Rotation Spectra of Hydrogen-Bonded Molecules

Abstract

The pure rotation spectra have been observed for the hydrogen-bonded bimolecules CF3COOH–HCOOH, CF3COOH–CH3COOH, and CF3COOH–CH2FCOOH. For CF3COOH–HCOOH, the O···O distance RH was found to be 2.69±0.02 Å. The O···O distance becomes 0.011 Å longer when D is substituted for H in the hydrogen bond, i.e., RD—RH=0.011 Å. The heat of formation was found to be —ΔH=15.8 kcal/mole. Similar values, RH=2.67 Å and RD—RH=0.012 Å, were obtained for CF3COOH–CH3COOH, but in this molecule the spectrum of the singly deuterated species shows that the D atom tunnels between the two partners in the bimolecule at a rate faster than the rotational frequency. For a symmetric double minimum potential function the barrier to tunneling is found to be less than 5000 cm—1. The rapid tunneling also implies that only a sixfold barrier to the internal rotation of the CF3 and CH3 groups can exist, and therefore the nearly free rotation of these groups in the bimolecules and in dimers can be expected. This free rotation would explain the failure to detect the low K lines in the pure rotation spectra. The rapid tunneling of the D (and H) atoms in the hydrogen bond also implies an inversion doubling which would contribute to the width of the v(OH) bands in the carboxylic acid dimer infrared spectra. The difference RD—RH predicts a shortening of the O···O distance of 0.073 Å when the O–H vibration is excited, which provides a mechanism to couple the low hydrogen-bond stretching frequency and the v(OH) frequency to produce the broad dimer bands.