NASA - National Aeronautics and Space Administration

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of atomic chlorine with pyridine (C5H5N) as a function of temperature (215–435 K) and pressure (25–250 Torr) in nitrogen bath gas. At T Z 299 K, measured rate coefficients are pressure independent and a significant H/D kinetic isotope effect is observed, suggesting that hydrogen abstraction is the dominant reaction pathway. The following Arrhenius expression adequately describes all kinetic data at 299–435 K for C5H5N: k1a = (2.08 Æ 0.47) × 10-11 exp[–(1410 Æ 80)/T] cm3 molecule-1 s-1 (uncertainties are 2s, precision only). At 216 K r T r 270 K, measured rate coefficients are pressure dependent and are much faster than computed from the above Arrhenius expression for the H-abstraction pathway, suggesting that the dominant reaction pathway at low temperature is formation of a stable adduct. Over the ranges of temperature, pressure, and pyridine concentration investigated, the adduct undergoes dissociation on the time scale of our experiments (10-5–10-2 s) and establishes an equilibrium with Cl and pyridine. Equilibrium constants for adduct formation and dissociation are determined from the forward and reverse rate coefficients. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the addition reaction: DrH1298 = -47.2 Æ 2.8 kJ mol-1, DrH10 = -46.7 Æ 3.2 kJ mol-1, and DrS1298 = -98.7 Æ 6.5 J mol-1 K-1. The enthalpy changes derived from our data are in good agreement with ab initio calculations reported in the literature (which suggest that the adduct structure is planar and involves formation of an N–Cl s-bond). In conjunction with the well-known heats of formation of atomic chlorine and pyridine, the above DrH values lead to the following heats of formation for C5H5N–Cl at 298 K and 0 K: DfH1298 = 216.0 Æ 4.1 kJ mol-1, DfH10 = 233.4 Æ 4.6 kJ mol-1. Addition of Cl to pyridine could be an important atmospheric loss process for pyridine if the C5H5N–Cl product is chemically degraded by processes that do not regenerate pyridine with high yield.