Ab Initio and Kinetic Calculations for the Reactions of NH(X3Σ-) 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 Σ - ) with CH 4, CH 3 F, CH 2 F 2, and CHF 3 have been studied systematically using ab initio molecular orbital theory. The G2(MP2) calculations reveal that all reactions involve significant energy barriers. The effect of fluorine substitution was examined. The NH + CHF 3 reaction was found to possess the highest barrier among the four reactions. The barriers for both NH + CH 3 F and NH + CH 2 F 2 reactions are about 2 kcal/mol lower than that for the NH + CH 4 reaction. The rate constants for the four reactions have been deduced using transition-state theory with asymmetric Eckart tunneling correction and hindered rotor approximation over the temperature range 200−3000 K. The following least-squares-fitted expressions for the rate constants were obtained: k H 1 (NH+CH 4 ) = (9.41 × 10 -18 ) T 2.28 e -10233/ T, k H 2 (NH+CH 3 F) = (1.69 × 10 -18 ) T 2.31 e -9217/ T, k H 3 (NH+CH 2 F 2 ) = (1.52 × 10 -18 ) T 2.32 e -9080/ T, k H 4 (NH+CHF 3 ) = (2.12 × 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 significant and “normal” kinetic isotope effects.