The initial steps involved in the thermal decomposition of formic acid in the temperature range of 1370–2000 K have been investigated by monitoring the IR emission intensities at 3.4 and 4.6 μm, corresponding to the reactant and carbon monoxide, respectively, behind reflected shock waves in mixtures diluted in Ar (0.1–1.5 mol%, total densities 5.3×10−6–3.2×10−5 mol cm−3). It was found that the decomposition proceeded via channel (1) HCOOH+Ar→CO+H2O+Ar dominantly and the contribution of channel (2) HCOOH+Ar→CO2+H2+Ar was very small. An Arrhenius expression of the second order rate constant was obtained as k1,0 =1014.32  exp(−40.4 kcal mol−1/RT) cm3 mol−1 s−1. Ab initio calculations were performed for probable transition states (TS) of channels (1) and (2). The results showed that the potential energy for a TS of channel (1) was lower than that of channel (2) by 20–30 kcal mol−1. On the basis of a RRKM weak collision, k2,0 values were estimated which was smaller than k1,0 by about two orders, being consistent with the experimental results.

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Ab initio MO calculations have been carried out for the unimolecular decomposition of oxalic acid. We used the Hartree-Fock (HF) method with LCAO approximation mainly using the 3-21G basis set with standard parameters to optimize the geometries for the three conformers of oxalic acid and eight probable transition states. The energy gradient technique was employed. Normal modes and vibrational frequencies were calculated by using the 3-21G basis set. It was found that the lowest energy path was (COOH)2 -* C02 + CO + H20 (2), having a five-center transition state. From the results of ab initio calculations, the first-order rate constant for channel 2 was evaluated as k2 = 1014•9 exp(-29.8 kcal mol™1/7?7) s™1, over the temperature range 300-1300 K, in terms of transition-state theory. The thermal decomposition of oxalic acid vapor diluted in Ar has been also briefly investigated behind reflected shock waves over the temperature range 850-1300 K. The decomposition was monitored by IR emission and vacuum-UV absorption from products. The decomposition product analysis was also done by gas chromatography. Although the rate constant could not be evaluated because of the very low reactant concentration and the too fast decomposition, major products observed were C02, CO, and H20, being consistent with the results of the ab initio calculations and the previous infrared multiphoton study by Yamamoto and Back.

The thermal decomposition of formaldehyde was investigated behind reflected shock waves by monitoring time-dependent CHjO and CO concentrations by IR emission and H-atom concentration by the ARAS method. From the IR emission experiment, it was found that the CH20 decay and the CO formation rates were the same and showed a strong dependence on the reactant mole fraction. An Arrhenius expression for the second-order rate constant of CH20 + Ar -CHO + H + Ar was obtained as k\ = 1015•50 exp(-75.0 kcal mor'/RT) cm1 23 mol"1 s"1 over the temperature range 2200-2650 K. Rate constants determined from the rate of H-atom production agreed with this expression. The alternative channel CH20 + Ar -CO + H2 + Ar appeared to have a smaller rate under the experimental conditions studied. Relative rates of the competing channels are discussed on the basis of recent ab initio calculations for their threshold energies.