Rates and equilibrium constants have been determined for the proton-transfer reaction of 4nitrophenylnitromethane, NOzC6H4CH2N02, and its cta-deuterated analogue NO2C6H4CD2NO2, with the strong base tetramethylguanidine [HN=C(NMe2)2], at temperatures between -60°C and + 65°C in a range of aprotic solvents. Spectrophotometry and the stopped-flow technique were used.The reaction is a simple proton-transfer process leading to an ion-pair. The kinetic isotope effects are correlated with the polarity of the solvents, as measured by the dielectric constant or by the empirical parameter ET. In the less polar solvents they are exceptionally large. In toluene, for example, at 25°C the rate ratio kH/kD = 45 k 2, the activation energy difference E p -EF = 4.3 kO.3 kcal mol-l (16 kJ mol-l),$ and the ratio of the pre-exponential factors loglo(AD/AH) = 1.5k0.2 ; and even larger values of loglo(AD/AH) are found for mesitylene (1.94k 0.06) and cyclohexene (2.4rf: 0.2). Positive deviations from linear Arrhenius plots are found for these solvents. Tunnelling is the only interpretation that can account for these results. For the more polar solvents (dielectric constant 7 to 37), the isotope effects are closer to the range predicted by semi-classical theory.The isotope effects in all solvents have been fitted to Bell's equation for a parabolic barrier, and the barrier dimensions calculated for each solvent, in two ways. (a) It is first assumed that only the proton moves, so that the effective mass LVH = 1 a.m.u. ; the best values of the barrier height (EH> and width at the base (2b) are determined by trial and error. The tunnelling corrections so calculated depend on the solvent polarity ; for the less polar solvents they are large, and the values also explain quantitatively the deviations of the Arrhenius plots from linearity. The calculated barrier dimensions are also solvent-dependent ; EH is lower in solvents of higher polarity by a factor of about 2 compared with those of low polarity, and 2b is greater by 0.1 -0.2 A. (b) It is assumed, alternatively, that the effective mass may be increased above 1 by reason of solvent or other motions coupled to that of the proton ; the values of E H derived in (a) are assumed, and the best values of LVH and 2b' determined by trial and error. The calculated values of 2b' are then all about equal, those of the less polar solvents being unchanged from (a) ; the values of m& in the less polar solvents are 1.00 a.m.u. as before, but in the more polar solvents they are 1.17-1.27 a.m.u., increasing with polarity.The suggested interpretation is that the solvent-solute interactions affect the height of the barrier and that motions of solvent molecules are coupled with the motion of the proton in the more polar solvents but not in the less polar ones ; reorganization of solvent molecules accompanies the protontransfer in the more polar solvents, but only electron-polarization in the less polar. Tunnelling has large effects in the less polar solvents, where the proton is the only atom that moves in the...
Kinetic isotope effects for the proton-transfer reaction between 4-nitrophenylnitromethane and the tertiary amine bases tri-n-butylamine, triethylamine and quinuclidine, in toluene, have been determined at temperatures between -15°C and + 35°C by the stopped-flow technique. The rate ratioskH/kDforthese basesat 25"Carerespectively 14+ 1,11.050.7and 15.6k0.6. Theratiosofthe pre-exponential factors AD/& are respectively 1, 4 and 9. These values are interpreted in terms of tunnelling. Bell's equations for an unsymmetrical parabolic barrier give almost the same barrier width for all three bases. Earlier results for such reactions in acetonitrile are interpreted in terms of coupling of the motions of solvent molecules with that of the proton.
h/*D tetramethylguanidine 25.0 ± 0.6 4.75 ± 0.34 45.8 ± 1.1 14.2 ± 0.2 45 ±2b quinuclidine 63.8 ± 1.5 21.8 ±0.9 17.2 ±0.4 7.20 ± 0.12 15.6 ± 0.6C triethylamine 2.71 ± 0.04 2.08 ± 0.13 24.3 ± 0.5 9.14 ± 0.13 11.0 ± 0.7C tri-n-butylamine 0.675 ± 0.008 0.480 ± 0.032 31.7 ± 0.5 11.0 ± 0.1 14 ± 1°d iethyl-n-butylamidine 12.6 ± 0.2 9.93 ± 0.81 27.1 ± 1.4 9.84 ± 0.36 11.7 ± 1.0d diethyl-zz-nonylamidine 34.9 ± 0.1 19.6 ± 1.2 11.4 ±0.6 5.39 ±0.19 8.0 ± 0.3d pentamethylguanidine 499 ±5 518 ± 13 44.1 ± 1.1 13.8 ± 0.2 13.7 ± 0.4e 0 Estimated by using = &H/2(A:H/fcD)1,442 and published1•10 values of and b Reference lb. c Reference lc. d Reference 10a. e Reference 1 Ob.It was recently pointed out, however, that these large isotope effects could be experimental artifacts if isotopic exchange, and consequent loss of deuterium from the deuterated substrate, occurred under the ionization-reaction conditions.3 Detailed kinetic analysis showed that values of kD would then be erroneously low, and treatment of some data under this assumption reduced very large isotope effects to values of the order of kK/kD = 10-15.4 This criticism was countered by the argument that the rate of hydrogen exchange in this substrate was not known and that internal return5 could easily make exchange much slower than ionization;6 in that eventuality, deuterium would not be lost during
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