Temperature dependence of the rate constant for the Cl−+CH3Br reaction down to 23 K

Abstract

Rate constants measured for the Cl−+CH3Br→CH3Cl+Br− reaction increase by over two orders of magnitude when the temperature is reduced from 300 to 23 K. Calculations done with the rotating bond approximation provide a simple explanation of this unusually strong negative temperature dependence of the rate constant.

Quantum Dynamics of Gas‐Phase SN2 Reactions

“…The PESs were used in extensive classical dynamics calculations by Hase and co-workers [5, 43±56] and were also employed in many of the quantum-dynamical calculations reviewed in this work. Clary and co-workers [38,60] scaled the geometry of the saddle point in Hase's PESs without changing the barrier heights, and obtained, in their quantumdynamics calculations, a somewhat better agreement with experiment (see Section 3). Tucker and Truhlar [61] also provided a six-body PES (based on second order M˘ller±Plesset, MP2/6±31G(d,p), calculations) for reaction (2), which was later slightly modified by Billing.…”

“…A microcanonical version of the reduced dimensionality theory, [96,98] which requires the vibrational frequencies of the Cl À ¥¥¥CH 3 Br complex (not treated explicitly by RBA theory), was used. [38,60] The PES of Wang et al [48] was scaled and the rate constant agrees well with the experimental data (see Figure 7). [38] A different extension of the two-dimensional collinear model is to allow for nonzero angles of approach of the nucleophile with respect to the molecular axis of symmetry of the prolate symmetric top.…”

“…For the Cl À + CH 3 Br reaction, Le Garrec et al [38] report a three-dimensional RBA calculation with the umbrella bending angle being considered explicitly. A microcanonical version of the reduced dimensionality theory, [96,98] which requires the vibrational frequencies of the Cl À ¥¥¥CH 3 Br complex (not treated explicitly by RBA theory), was used.…”

“…[37] This rate constant is three orders of magnitude larger than that for Cl À + CH 3 Cl, but still two orders of magnitude below the classical capture theory limit. Le Garrec et al [38] measured the thermal rate constant down to 23 K and found a dramatic increase in the rate constant as the temperature is reduced. Graul and Bowers [33] found that the translational energy distribution of the products (Br À and CH 3 Cl) is smaller than the phase space theory (PST) prediction, which indicates that the product molecule is vibrationally hot.…”

Low Temperature Experiments on Gas-Phase Chemical Processes

“…reaction. The temperature dependence of this rate coefficient is shown in Figure 2 and is well represented by a SN2 mechanism (Le Garrec et al 1997b). Unfortunately this kind of behaviour cannot be generalised especially for the case of slow reactions with molecular hydrogen (e.g.…”