Dynamics of the Gas-Phase Reactions of Chloride Ion with Fluoromethane:  High Excess Translational Activation Energy for an Endothermic S_N2 Reaction

Abstract

Guided ion beam tandem mass spectrometry techniques are used to examine the competing product channels in the reaction of Cl(-) with CH(3)F in the center-of-mass collision energy range 0.05-27 eV. Four anionic reaction products are detected: F(-), CH(2)Cl(-), FCl(-), and CHCl(-). The endothermic S(N)2 reaction Cl(-) + CH(3)F --> CH(3)Cl + F(-) has an energy threshold of E(0) = 181 +/- 14 kJ/mol, exhibiting a 52 +/- 16 kJ/mol effective barrier in excess of the reaction endothermicity. The potential energy of the S(N)2 transition state is well below the energy of the products. Dynamical impedances to the activation of the S(N)2 reaction are discussed, including angular momentum constraints, orientational effects, and the inefficiency of translational energy in promoting the reaction. The fluorine abstraction reaction to form CH(3) + FCl(-) exhibits a 146 +/- 33 kJ/mol effective barrier above the reaction endothermicity. Direct proton transfer to form HCl is highly inefficient, but HF elimination is observed above 268 +/- 95 kJ/mol. Potential energy surfaces for the reactions are calculated using the CCSD(T)/aug-cc-pVDZ and HF/6-31+G(d) methods and used to interpret the dynamics.

Static and dynamic reaction pathways involved in the reaction of O− and CH3F

“…The reactions involving methyl fluoride (CH 3 F) have attracted much attention because of their important roles in atmospheric chemistry and combustion chemistry [1][2][3][4][5][6][7]. As CH 3 F is a typical polar molecule, the reaction between CH 3 F and a nucleophile probably undergoes a process of the bimolecular nucleophilic substitution (S N 2) reaction, e.g.…”

Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism

“…The bimolecular nucleophilic substitution (S N 2) reaction is the most common reaction of that type, in which a nucleophile (often negatively charged) approaches a saturated carbon from one side, displaces a leaving group on the opposite side of the carbon atom, resulting in inversion of the carbon centre. Strong solvent effects of these reactions in solution have prompted investigations of the gas-phase S N 2 reaction to probe the intrinsic reaction mechanisms without solvent123456789101112131415161718192021222324252627. Numerous experimental and theoretical studies have revealed that an inverse secondary kinetic isotope effect (KIE), that is, k H / k D <1 (where k denotes the thermal rate constant), is characteristic for a thermal S N 2 reaction when the isotopically substituted atom is not directly involved in a reaction12345678910.…”

“…In strong contrast, by using a guided ion beam technique, Ervin and co-workers discovered that the symmetric 37 Cl − +CH 3 35 Cl→ 35 Cl − +CH 3 37 Cl reaction exhibits a large normal secondary isotope effect on the cross-section, in addition to a reaction threshold versus collision energy substantially higher than the calculated barrier height1114. They also observed that the reaction threshold for the endothermic Cl − + CH 3 F→CH 3 Cl+F − reaction is considerably higher than the reaction endothermicity12. Hase and co-workers performed extensive direct molecular dynamics studies on many exothermic and a few centre barrier S N 2 reactions, and uncovered the non-statistical nature and inefficiency of energy transfer between intermolecular and intramolecular modes in these reactions131415161718.…”

Impact of Sub-Doppler Measurements on Centrifugal-Distortion Terms: Rotational Spectrum of Methyl Fluoride Revisited

“…Ground-State Spectroscopic Parameters for12 CH 3 F Values in brackets were kept fixed in the fitting procedure: A 0 to Papousek et al19 value, hyperfine parameters to computed values. Only our measurements were considered.…”