Thermal unimolecular decomposition of formic acid

Abstract

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 k_1,0 =10^14.32 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, k_2,0 values were estimated which was smaller than k_1,0 by about two orders, being consistent with the experimental results.