Ab initio and kinetic calculations for the reactions of NH(X-3 Sigma(-)) with CHxF4-x and CDxF4-x (x = 1, 2, 3, 4)

The direct hydrogen abstraction mechanisms on (3)A" potential surfaces for the reactions of NH((3)Sigma(-)) with CH4, CH3F, CH2F2, and CHF3 have been studied systematically using ab initio molecular orbital theory. The G2(MP2 ) calculations reveal that all reactions involve significant energy barrier s. The effect of fluorine substitution was examined. The NH + CHF3 reaction was found to possess the highest barrier among the four reactions. The bar riers for both NH + CH3F and NH + CH2F2 reactions are about 2 kcal/mol lowe r than that for the NK + CH4 reaction. The rate constants for the four reac tions have been deduced using transition-state theory with asymmetric Eckar t tunneling correction and hindered rotor approximation over the temperatur e range 200-3000 K. The following least-squares-fitted expressions for the rate constants were obtained: k(1)(H)(NH+CH4) = (9.41 x 10(-18))T-2.28 e(-1 0233/T), k(2)(H)(NH+CH3F) = (1.69 x 10(-18))T-2.31 e(-9217/T), k(3)(H)(NH+C H2-F-2) = (1.52 x 10(-18))T-2.32 e(-9080/T), k(4)(H)(NH+CHF3) = (2.12 x 10( -18))T-2.29 e(-10750/T), in cm(3) molecule(-1) s(-1). The deuterium kinetic isotope effects have also been investigated. All reactions show the signif icant and "normal" kinetic isotope effects.