DEUTERIUM KINETIC ISOTOPE EFFECT AND TEMPERATURE-DEPENDENCE IN THE REACTIONS OF CH[(2)PI] WITH METHANE AND ACETYLENE

The rate coefficients of the reactions of CH and CD with CH4, CD4, C2H 2, and C2D2 have been measured in the temperature range 290 < T < 700 K. All the rate constants show a slightly negative temperature depende nce. They can be fitted in the observed temperature range by the follo wing expressions: k(CH+CH4) = (6.7+/-0.3)(T/293 K)(-(0.4+/-0.1)) x 10( -11) cm(3) molecule(-1) s(-1); k(CH+CD4) = (4.2+/-0.2)(T/293 K)(-(0.3/-0.1)) x 10(-11) cm(3) molecule(-1) s(-1); k(CD+CH4) = (5.7+/-0.2)(T/ 293 K)(-(0.32+/-0.04)) x 10(-11) cm(3) molecule(-1) s(-1); k(CD+CD4) = (3.6+/-0.1)(T/293 K)(-(0.27+/-0.08)) x 10(-11) cm(3) molecule(-1) s(- 1); k(CH+C2H2) = (3.2+/-0.2)(T/293 K)(-(0.2+/-0.1)) x 10(-10) cm(3) mo lecule(-1) s(-1); k(CD+C2H2) = (2.8+/-0.1)(T/293 K)(-(0.13+/-0.06)) x 10(-10) cm(3) molecule(-1) s(-1); k(CH+C2D2) = (3.2+/-0.2)(T/293 K)(-( 0.2+/-0.1)) x 10(-10) cm(3) molecule(-1) s(-1); and k(CD+C2D2) = (2.8/-0.2)(T/293 K)(-(0.2+/-0.1)) x 10(-10) cm(3) molecule(-1) s(-1), wher e all error estimates are +/-2 sigma and represent the precision of th e fit. The observed temperature and pressure dependences are in accord ance with a barrierless addition of the methylidyne radical to methane or acetylene with rapid decomposition of the adduct. The deuterium is otope effects on the reactions are not well-described by loose-transit ion state models and may reflect changes in the vibrational frequencie s in the transition state for initial complex formation.