Kinetics of the reaction of atomic hydrogen with cyanoacetylene from T = 200 to 298 K

Rate coefficients have been measured for the H + HC3N (cyanoacetylene) reaction at T ) 200, 250, and 298 K and at P ) 0.5, 1.0, and 2.0 Torr He using a discharge-flow mass spectrometry apparatus. The reaction was monitored under pseudo-first-order conditions with the H atom concentration in large excess over the HC3N concentration ([H]/[HC3N] ) 100-665). H atoms were generated by the fast reaction F + H2 f H + HF or by microwave discharge in H2. Fluorine atoms were produced by microwave discharge in an approximately 5% mixture of F2 in He. Low-energy (24 eV) electron-impact mass spectrometry was used to monitor the HC3N decay kinetics to obtain the bimolecular rate coefficients. At T ) 298 K the rate coefficients were found to be pressure independent over the range of pressures studied with an average value k ) (2.1 ( 0.3) 10 -13 cm 3 molecule -1 s -1 . This implies that the high-pressure limit is reached in these experiments not only at T ) 298 K but also at the two lower temperatures. The temperature dependence of the measured high-pressure limiting rate coefficients is given by the following Arrhenius expression: k ) (1.1 ( 0.1) 10 -12 exp[-(500 ( 14)K/T ]c m 3 molecule -1 s -1 . A transition state theory model using G2M or CCSD(T) energies and Eckart tunneling corrections has been employed to calculate high-pressure limiting rate coefficients for this reaction. The reaction’s mechanism and implications to the atmospheric chemistry of Titan are discussed.