A. Winkler et al., CHEMICAL RELAXATION OF H-BONDS IN FORMIC-ACID VAPOR STUDIED BY RESONANT PHOTOACOUSTIC-SPECTROSCOPY, The Journal of chemical physics, 100(4), 1994, pp. 2717-2727
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.