Rate Constants for the Reactions of Chlorine Atoms with Deuterated Methanes: Experiment and Theory
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Long-path FTIR spectroscopy and ab initio calculations combined with conventional transition state theory were used to study the kinetics of the reactions of Cl atoms with deuterated methanes. The following experimental relative rate constants for the reaction of Cl atoms at 298 ± 5 K and 760 ± 5 Torr were determined: CH3D, (6.5 ± 0.5) × 10-14; CH2D2, (4.2 ± 0.5) × 10-14; CHD3, (1.9 ± 0.3) × 10-14; CD4, (5.4 ± 0.4) × 10-15. All experimental and theoretical rate constants are in units of cm3 molecule-1 s-1 and are relative to the 1.0 × 10-13 cm3 molecule-1 s-1 rate constant for the reaction of Cl with CH4. All experimental uncertainty limits are 2σ. The geometries, energies, and frequencies of the reactants, products, and transition states were calculated at the level of the second-order Møller−Plesset approximation using a 6-311++G(2d,2p) basis set. The following theoretical relative rate constants were calculated at 298 K using conventional transition state theory combined with an Eckart one-dimensional tunneling correction: CH3D, 6.8 × 10-14; CH2D2, 4.2 × 10-14; CHD3, 2.1 × 10-14; CD4, 4.4 × 10-15. The theoretical rate constants agree well with the experimental results. The curvature in both the experimental and theoretical rate constants as a function of deuteration is due to a secondary kinetic isotope effect, involving mainly the rate constant preexponential factors. The large decrease in Cl atom rate constant in going from CH4 to CH3D (i.e., the increase in curvature at CH3D) is due to the reduced symmetry in the transition state and a mass-dependent effect. The implications for previous studies, atmospheric chemistry, and chemical reactivity are discussed.Keywords:
Kinetic isotope effect
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By a high-field 2 Hmr study of deuteriated 1,3,3-trimethylbicyclo[2.2.1]heptan-2-ones (fenchones), we have established that the geminal deuterium isotope effects on the exo-6-, endo-6-, and methyl deuterons are 1.46 ± 0.06 Hz (0.019 ppm), 1.61 ± 0.06 Hz (0.021 ppm), and 1.46 ± 0.06 Hz (0.019 ppm), respectively. From this study it is clear that high-field 2 Hmr has the potential of providing directly all the information on the degree of deuterium substitution at carbon previously obtained indirectly by 13 Cmr, while retaining the ability to identify chemically nonequivalent deuterons.
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The rates of reaction of 2-(4-methoxyphenyl)oxirane (4-methoxystyrene oxide), trans-3deutereo-2-(4-methoxyphenyl)oxirane and 3,3-dideutereo-2-(4-methoxyphenyl)oxirane in water solutions were measured as functions of pH. Kinetic deuterium isotope effects for the reactions of the mono- and di-deuterated (4-methoxyphenyl)oxiranes were determined for both the acidcatalyzed hydrolysis to diols and the pH-independent reactions leading mostly to rearranged aldehyde and involving a 1,2-hydrogen migration. The inverse kinetic deuterium isotopes for acid-catalyzed hydrolyses of the deuterated (4-methoxyphenyl)oxiranes to diols are consistent with rate-limiting epoxide ring opening. The magnitudes of the normal kinetic deuterium isotope effects on the pH-independent reactions of deuterated 4-methoxyphenyloxiranes are significantly smaller than the deuterium isotope effect on the aldehyde-forming step, and are rationalized by a reversible epoxide ring opening step that is partially rate-limiting. The magnitude of the partitioning isotope effect on the hydrogen migration step is consistent with isotope effects determined by Professor Coxon’s laboratory on the Lewis acid-catalyzed rearrangements of deuterated phenyloxiranes in organic solvents.
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The primary deuterium and tritium isotope effects, δ(XH) − δ(XD/T), were measured for 55 compounds having one or more intramolecular hydrogen bonds. The primary isotope effects were measured at various temperatures. For compounds displaying tautomerism the primary isotope effects are found to have contributions from both intrinsic and equilibrium isotope effects. The primary tritium isotope effect, PΔ(1H,3H), and the primary deuterium isotope effect, PΔ (1H,2H), are shown to be related by This finding is valid for both tautomeric compounds and compounds with localized hydrogen bonds. Large negative primary tritium and deuterium isotope effects were observed for compounds displaying tautomerism and having sulfur as donor or acceptor. These isotope effects show a strong temperature dependence, which is related to the change in equilibrium due to isotope substitution. For the compounds with localized hydrogen bonds, the primary deuterium and tritium isotope effects correlated with the two bond deuterium isotope effect on 13C chemical shifts. The primary deuterium and tritium isotope effects are therefore a measure of the hydrogen bond strength for compounds with localized hydrogen bonds. Copyright © 2000 John Wiley & Sons, Ltd.
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Deuterium NMR
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Isotope effects have been measured for the abstraction of hydrogen from a series of organic substrates by the perfluoro radical, Na+ -O3SCF2CF2OCF2CF2*, in water. Both primary and secondary deuterium isotope effects were measured, with the primary isotope effects ranging in value from 4.5 for isopropanol to 19.6 for acetic acid. The values for the alpha- and beta-secondary deuterium isotope effects were 1.06 and 1.035, respectively. It was concluded that tunneling contributes significantly to the production of the observed, large primary kinetic isotope effects in these C-H abstraction reactions.
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The secondary α deuterium and heavy atom kinetic isotope effects found for two different SN2 reactions suggest that the magnitude of secondary α deuterium kinetic isotope effects can be determined by the length of only the shorter (stronger) reacting bond in an unsymmetrical SN2 transition state rather than by the usual nucleophile−leaving group distance. Although this means the interpretation of these isotope effects is more complex than has been recognized, the results suggest that they can be used to determine whether an SN2 transition state is symmetrical or unsymmetrical.
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Deuterium kinetic isotope effects (k2H/k2D) have been measured for the elimination of HBr from a series of substituted phenethyl bromides. No variation in the magnitude of the isotope effect was observed. The k2H values have been correlated by the Hammett equation to give ρ= 2·85. Some evidence for proton tunnelling has been obtained.
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The primary deuterium and tritium isotope effects, δ(XH) − δ(XD/T), were measured for 55 compounds having one or more intramolecular hydrogen bonds. The primary isotope effects were measured at various temperatures. For compounds displaying tautomerism the primary isotope effects are found to have contributions from both intrinsic and equilibrium isotope effects. The primary tritium isotope effect, PΔ(1H,3H), and the primary deuterium isotope effect, PΔ (1H,2H), are shown to be related by This finding is valid for both tautomeric compounds and compounds with localized hydrogen bonds. Large negative primary tritium and deuterium isotope effects were observed for compounds displaying tautomerism and having sulfur as donor or acceptor. These isotope effects show a strong temperature dependence, which is related to the change in equilibrium due to isotope substitution. For the compounds with localized hydrogen bonds, the primary deuterium and tritium isotope effects correlated with the two bond deuterium isotope effect on 13C chemical shifts. The primary deuterium and tritium isotope effects are therefore a measure of the hydrogen bond strength for compounds with localized hydrogen bonds. Copyright © 2000 John Wiley & Sons, Ltd.
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Deuterium NMR
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Nitrogen and deuterium kinetic isotope effects were measured in the Menshutkin reaction between methyl iodide and a series of para-substituted N,N-dimethylanilines in ethanol. The nitrogen kinetic isotope effect increases for the more electron-donating substituents [0·9989, 1·0032, and 1·0036 for 4-C(O)Me, H and 4-Me, respectively], in agreement with the Hammond postulate. The secondary deuterium isotope effect, however, exhibits the reverse trend (1·045, 0·989, 0·975 per deuterium, for the respective substituents). This discrepancy is rationalized in terms of solvent molecule participation in the transition state.
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This chapter contains sections titled: Introduction Hyperconjugation and Secondary β-Deuterium Isotope Effects The Conformational Dependence of Secondary β-Deuterium Isotope Effects and the Structures of Cationic Transition States More Remote Isotope Effects. Models for the Origin of γ-Deuterium Isotope Effects
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Hyperconjugation
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