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) wa s observed from the unimolecular decomposition of acetic acid in a fas t flow reactor with 0.8 Torr of Ar carrier gas. Activated CH3COOH mole cules with an excitation energy of approximately 95 kcal mol(-1) were produced via the successive reactions H+CH2ICOOH --> HI+CH2COOH and HCH2COOH --> CH3COOH. The nascent vibrational distributions for H2O an d CO2 were determined by simulation of the experimental emission spect ra. The H2O emission is mainly from the (O nu(2)1) --> (0 nu(2)0) tran sitions 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 transition s with high excitation in nu(2) (nu(2) less than or equal to 5). The e xtremely high bending excitation is explained by the release of the en ergy in changing from the bent OCO geometry of the carboxyl group to t he linear structure of the CO2 molecule. The H2O and CO2 relative emis sion intensities and RRKM calculations suggest that the unimolecular d ecomposition of CH3COOH proceeds through the two competing pathways, H 2O + CH2CO and CO2 + CH4, with approximately 2 times higher probabilit y of water formation; the threshold energies for the two unimolecular reactions must be less than 70 kcal mol(-1).