Experimental and Theoretical Study of the Carbon-13 and Deuterium Kinetic Isotope Effects in the Cl and OH Reactions of CH₃F

Abstract

A laser flash photolysis-resonance fluorescence technique has been employed to determine absolute rate coefficients for the CH3F + Cl reaction in N2 bath gas in the temperature range of 200-700 K and pressure range of 33-133 hPa. The data were fitted to a modified Arrhenius expression k(T) = 1.14 x 10(-12) x (T/298)2.26 exp{-313/T}. The OH and Cl reaction rates of (13)CH3F and CD3F have been measured by long-path FTIR spectroscopy relative to CH3F at 298 +/- 2 K and 1013 +/- 10 hPa in purified air. The FTIR spectra were fitted using a nonlinear least-squares spectral fitting method including line data from the HITRAN database and measured infrared spectra as references. The relative reaction rates defined by alpha = k(light)/k(heavy) were determined to be k(OH+CH3F)/k(OH+CD3F) = 4.067 +/- 0.018, k(OH+CH3F)/k(OH+(13)CH3F) = 1.067 +/- 0.006, k(Cl+CH3F)/k(Cl+CD3F) = 5.11 +/- 0.07, and k(Cl+CH3F)/k(Cl+(13)CH3F) = 1.016 +/- 0.006. The carbon-13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3F have been further investigated by quantum chemistry methods and variational transition state theory.

Theoretical study of the kinetics of reactions of the monohalogenated methanes with atomic chlorine

“…The calculated value of the rate constant for the hydrogen abstraction reaction CH 3 F + Cl of 3.3×10 −13 cm 3 molecule −1 s −1 at 298 K is very close to that of 3.5×10 −13 cm 3 molecule −1 s −1 unamimously recommended by the IUPAC and NASA [12–14] evaluations of the kinetic data. Our calculated value of k (CH 3 F+Cl) at room temperature is very close to the reported results of 2.7×10 −13 derived by Hitsuda et al [19], 3.2×10 −13 of Wallington et al [18], 3.4×10 −13 of Tuazon et al [17], 3.5×10 −13 of Sarzyński et al [22], (3.5–3.9) ×10 −13 of Marinkovic et al [21], 3.6×10 −13 of Manning and Kurylo [15], and that of 3.8×10 −13 cm 3 molecule −1 s −1 of Tschuikow-Roux et al [16] after correction taking into account the current value of the rate constant for the reference reaction CH 4 + Cl [65]. Figure 3 shows a comparison of calculated values of k (CH 3 F+Cl) with the available results of experimental measurements in a wide temperature range.…”

“…The calculated values of the rate constants k (CD 3 X+Cl) are distinctly lower compared with the values of their counterparts, k (CH 3 X+Cl), especially at low temperatures. On the other hand, the values derived in this study and the reported values of KIE [21] calculated at the different levels of theory are higher than those estimated experimentally. It may suggest that the stabilization by collisions of the molecular complexes formed during the reaction should be explicitly considered in the description of the reaction kinetics.…”

“…The calculated values of KIE at room temperature are of 10.1, 8.6 and 9.6 for the CH 3 F/CD 3 F, CH 3 Cl/CD 3 Cl and CH 3 Br/CD 3 Br reaction systems, respectively. These calculated values of KIE at 298 K are distinctly higher than those obtained experimentally of 5.1–6.2 [21, 22], 4.9–5.4 [30–32] and 6.5 ± 0.3 [38] for the reaction systems ordered analogously as above. The significance of the kinetic isotope effect diminishes with rising temperature and the values of KIE at 500 K are over twice as low as those at room temperature.…”

“…This is reflected in the form of the recommended Arrhenius’ expression for k (CH 3 F+Cl)/cm 3 molecule −1 s −1 of 4.0×10 −12 exp(−730/T) preferred by IUPAC [13] and that of 1.96×10 −11 exp(−1200/T) favored by NASA [12]. On the other hand, the results of the kinetic investigations performed recently by Marinkovic et al [21], in the widest temperature range of 200–700 K suggest a non-Arrhenius behavior of the kinetics of CH 3 F+Cl, which is described by k (CH 3 F+Cl)/cm 3 molecule −1 s −1 in the form of the 1.14×10 −12 ×(T/298) 2.26 ×exp(−313/T). Unfortunately, there are no other studies on the kinetics CH 3 F+Cl conducted at sufficiently high temperatures, which could confirm this conclusion of Marinkovic et al [21].…”

Kinetic Isotope Effects in Asymmetric Reactions

“…This phenomenon results from conformational sampling due to excess (or even zero point) vibrational energy residing in the reactant that allows the traversal of the dividing surface between reactants and products at a point distant from the first‐order saddle point 19. 20 In spite of the potential complications arising from the use of transition structures or first‐order saddle points to describe the transition state, it appears to be a robust approximation in most cases.…”