CHEMICAL RELAXATION OF H-BONDS IN FORMIC-ACID VAPOR STUDIED BY RESONANT PHOTOACOUSTIC-SPECTROSCOPY

The chemical relaxation of hydrogen bonds in dimeric formic acid (meth anoic acid) was studied in the gas phase by means of precise measureme nt of the speed of sound and the-sound absorption as functions of the pressure and the temperature by exciting standing acoustic waves. The first radial acoustic resonance of a cylindrical cavity was excited by a modulated CO2 laser beam. Resonance profiles were measured and reco rded by a computer-controlled system. Due to the high information cont ent of this method, a consistent set of thermodynamic and kinetic para meters can be obtained. The equilibrium constants of the dimer/monomer and the cis-/trans-monomer equilibrium, the dissociation rate constan t of (HCOOH)(2) and the mean relaxation time of the vibrational states of the monomer-dimer mixture were determined by fitting a detailed th eoretical model of the resonator to the measured values for the resona nce frequency and the resonance broadening for total. pressures in the range from 0.3 to 50 mbar and temperatures from 290 to 325 K. We obta ined 159 +/- 2 J/(mol K) for the entropy and 61.8 +/- 0.5 kJ/mol for t he enthalpy of dissociation at 300 K. We inferred a value for the enth alpy of isomerization of 10 +/- 5 kJ/mol. For the first time the press ure dependence of the dissociation rate constant was determined. It wa s found that the unimolecular decay is in the second-order regime at t hese low pressures as expected. The mean collision efficiency for the dissociation process relative to the dimer was obtained for HCOOH, He, and Ar to be 0.5 +/- 0.2, 0.05 +/- 0.02, and 0.08 +/- 0.02, respectiv ely, independent of the temperature. We measured an average (pr) value of 10 +/- 2 ns bar for the relaxation of the vibrational degrees of f reedom. The activation energy of the dissociation of dimeric formic ac id was determined to be 33 +/- 1 kJ/mol.