KINETICS OF THE REACTIONS OF ACETONITRILE WITH CHLORINE AND FLUORINE-ATOMS

The rate coefficients for the reactions of chlorine and fluorine atoms with acetonitrile have been measured using relative and direct method s. In the case of chlorine atoms the rate coefficient k(1) was measure d between 274 and 345 K using competitive chlorination and at 296 K us ing laser flash photolysis with atomic resonance fluorescence. The rat e coefficient measured at ambient temperature (296 +/- 2 K) is (1.15 /- 0.20) x 10(-14) cm(3) molecule(-1) s(-1), independent of pressure b etween 5 and 700 Torr (uncertainties are 2 standard deviations through out). This result is a factor of 6 higher than the currently accepted value. The results from the three independent determinations reported here yield the Arrhenius expression k(1) (1.6 +/- 0.2) x 10(-11) exp[- (2140 +/- 200)/T] cm(3) molecule(-1) s(-1). Product studies show that the reaction of Cl atoms with CH3CN proceeds predominantly, if not exc lusively, by hydrogen abstraction at 296 K. The rate coefficient for t he reaction of fluorine atoms with acetonitrile was measured using bot h the relative rate technique and pulse radiolysis with time-resolved ultraviolet absorption spectroscopy. The rate coefficient for the reac tion of F atoms with CH3CN was found to be dependent on total pressure . The observed rate data could be fitted using the Tree expression wit h F-c = 0.6, k(0) = (2.9 +/- 2.1) x 10(-28) cm(6) molecule(-2) s(-1), and k(infinity) = (5.8 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), wi th a zero pressure intercept of (0.9 +/- 0.4) x 10(-11) cm(3) molecule (-1) s(-1). The kinetic data suggest that the reaction of F atoms with CH3CN proceeds via two channels: a pressure-independent H atom abstra ction mechanism and a pressure-dependent addition mechanism. Consisten t with this hypothesis, two products were observed using pulsed radiol ysis with detection by UV absorption spectroscopy. As part of the prod uct studies, relative rate techniques were used to measure k(Cl+CH2ClC N) = (2.8 +/- 0.4) x 10(-14) and k(F+CH2FCN) (3.6 +/- 0.2) x 10(-11) c m(3) molecule(-1) s(-1).