Ni. Butkovskaya et al., OBSERVATION OF THE UNIMOLECULAR DECOMPOSITION PATHWAYS OF CHEMICALLY ACTIVATED ACETIC-ACID BY FOURIER-TRANSFORM INFRARED-EMISSION SPECTROMETRY, Journal of physical chemistry, 99(28), 1995, pp. 11115-11121
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).