¹³C, ¹⁸O, and D Fractionation Effects in the Reactions of CH₃OH Isotopologues with Cl and OH Radicals

Abstract

A relative rate experiment is carried out for six isotopologues of methanol and their reactions with OH and Cl radicals. The reaction rates of CH2DOH, CHD2OH, CD3OH, (13)CH3OH, and CH3(18)OH with Cl and OH radicals are measured by long-path FTIR spectroscopy relative to CH3OH at 298 +/- 2 K and 1013 +/- 10 mbar. The OH source in the reaction chamber is photolysis of ozone to produce O((1)D) in the presence of a large excess of molecular hydrogen: O((1)D) + H2 --> OH + H. Cl is produced by the photolysis of Cl2. The FTIR spectra are fitted using a nonlinear least-squares spectral fitting method with measured high-resolution infrared spectra as references. The relative reaction rates defined as alpha = k(light)/k(heavy) are determined to be: k(OH + CH3OH)/k(OH + (13)CH3OH) = 1.031 +/- 0.020, k(OH + CH3OH)/k(OH + CH3(18)OH) = 1.017 +/- 0.012, k(OH + CH3OH)/k(OH + CH2DOH) = 1.119 +/- 0.045, k(OH + CH3OH)/k(OH + CHD2OH) = 1.326 +/- 0.021 and k(OH + CH3OH)/k(OH + CD3OH) = 2.566 +/- 0.042, k(Cl + CH3OH)/k(Cl + (13)CH3OH) = 1.055 +/- 0.016, k(Cl + CH3OH)/k(Cl + CH3(18)OH) = 1.025 +/- 0.022, k(Cl + CH3OH)/k(Cl + CH2DOH) = 1.162 +/- 0.022 and k(Cl + CH3OH)/k(Cl + CHD2OH) = 1.536 +/- 0.060, and k(Cl + CH3OH)/k(Cl + CD3OH) = 3.011 +/- 0.059. The errors represent 2sigma from the statistical analyses and do not include possible systematic errors. Ground-state potential energy hypersurfaces of the reactions were investigated in quantum chemistry calculations at the CCSD(T) level of theory with an extrapolated basis set. The (2)H, (13)C, and (18)O kinetic isotope effects of the OH and Cl reactions with CH3OH were further investigated using canonical variational transition state theory with small curvature tunneling and compared to experimental measurements as well as to those observed in CH4 and several other substituted methane species.

Hopping Conductance in Molecular Wires Exhibits a Large Heavy-Atom Kinetic Isotope Effect

“…These conductance KIEs are exceptionally large, ranging from 3 to 30. Primary KIEs for reactions involving 13 C-labeled molecules, and to a lesser extent 15 N-isotopologues, are documented in solution- and gas-phase studies and are generally quite small ( 13 C KIE < 1.01) near room temperature. − However, unlike typical reaction kinetics studies in which single bond breaking or formation rates are the primary focus, the conductance KIEs reported here represent an aggregate of multiple intramolecular polaron transfer events, involving many bonds in the wire backbone.…”

Comprehensive Investigations of the Cl + CH 3 OH → HCl + CH 3 O/CH 2 OH Reaction: Validation of Experiment and Dynamic Insights

“…The first term is the electronic partition function accounting for the SO splitting of Cl( 2 P): the SO states 2 P 3/2 of Cl have a degeneracy of 4 and are 882 cm −1 lower than the SO states 2 P 1/2 of Cl with a degeneracy of 2. 37 The impact parameter was chosen from a uniformly distributed random number ζ∈½0,1 according to b = b max ζ 1/2 . N r and N total denote the numbers of reactive and total trajectories, respectively, at the specified temperature.…”

“…45 The slightly higher rotational excitation in HCl in calculations might be partly attributable to the neglected SO correction from the Cl atom in the entrance channel, which lowers the reactant asymptote by 0.84 kcal/mol. 37 In other words, the available energy would be reduced by 0.84 kcal/mol, if SO correction is considered. Further investigations including SO splitting are needed to resolve this issue.…”

“…Unlike the F + CH 3 OH reaction where corresponding pathways are both barrierless, [21][22][23][24][25] the barrierless R2 channel dominates in low temperatures due to a significant barrier in R1. 26,27 Indeed, experimentally, the branching ratio of R2 has been determined to be close to unity up to 500 K. 18 Various techniques have been employed to study the kinetics of R2 over the temperature range of 200-600 K. 18,19,[27][28][29][30][31][32][33][34][35][36][37][38][39] Although there are significant inconsistencies among these measurements, it is clear that the reaction is fast and the temperature dependence of the thermal rate coefficient is not Arrhenius, signaling complex kinetics. 39 A wealth of knowledge on the dynamics of R2 has been acquired through detailed scattering experiments, 20,35,[40][41][42][43][44][45] which have been summarized in two reviews.…”

“…Information regarding stationary points, including the geometries, frequencies, and energies, was mostly determined at relatively low levels, including MP2, QCISD(T), G2, or density functional theory methods. 20,26,27,47,48 In the work of Feilberg et al, 37 the energies of the stationary points were refined at the level of CCSD(T) with extrapolated basis sets (CCSD(T)/EB) based on MP2/AVTZ calculated geometries, denoted as CCSD(T)/EB//MP2/AVTZ. There is only one dynamical investigation via trajectories launched from the transition state region of R2, with the energies and forces computed on the fly at the low theory level of HF/6-31G or MP2/6-311G(d,p).…”