OBSERVATION OF THE UNIMOLECULAR DECOMPOSITION PATHWAYS OF CHEMICALLY ACTIVATED ACETIC-ACID BY FOURIER-TRANSFORM INFRARED-EMISSION SPECTROMETRY

The infrared chemiluminescence from vibrationally excited H2O and CO2 molecules in their respective ranges 3200-3900 and 2000-2400 cm(-1) was observed from the unimolecular decomposition of acetic acid in a fast flow reactor with 0.8 Torr of Ar carrier gas. Activated CH3COOH molecules with an excitation energy of approximately 95 kcal mol(-1) were produced via the successive reactions H+CH2ICOOH --> HI+CH2COOH and H+CH2COOH --> CH3COOH*. The nascent vibrational distributions for H2O and CO2 were determined by simulation of the experimental emission spectra. The H2O emission is mainly from the (O nu(2)1) --> (0 nu(2)0) transitions with v(2) less than or equal to 5, similar to the emission of H2O eliminated from activated ethanol, which has been observed earlier in this laboratory. The CO2 emission is from Delta v3 = -1 transitions with high excitation in nu(2) (nu(2) less than or equal to 5). The extremely high bending excitation is explained by the release of the energy in changing from the bent OCO geometry of the carboxyl group to the linear structure of the CO2 molecule. The H2O and CO2 relative emission intensities and RRKM calculations suggest that the unimolecular decomposition of CH3COOH proceeds through the two competing pathways, H2O + CH2CO and CO2 + CH4, with approximately 2 times higher probability of water formation; the threshold energies for the two unimolecular reactions must be less than 70 kcal mol(-1).