No abstract available

[en] The first demonstration of a diastereotopically distinct secondary deuterium isotope effects for a prototypical concerted pericyclic reaction, the thermal isomerization of hexa-1,3Z,5-triene to 1,3-cyclohexadiene, is reported. The data were analyzed by a computer program. 12 refs., 1 fig

[en] Partly deuterated pyridine, examining the kinetic energy spectra of the ions obtained into a double focusing mass spectrometer MAT-311 by MIKE and HV methods, was studied. (authors)

No abstract available

[en] Secondary β-tritium isotope effects in E2 reactions of 2-arylethyl derivatives have been measured. The effects are often larger than expected for complete rehybridization of the C-H(T) bond from sp³ to sp². Model calculations show that a tunnel correction to the secondary isotope effect can result when the bending motions of the non-transferred β-hydrogen are coupled with the stretching motions of the transferred β-hydrogen. They also predict that tunneling can lead to an abnormally steep temperature dependence of the secondary isotope effect, and this prediction has been confirmed by experiment. Experimental tests of other predictions are under way. 6 refs.; 6 tabs

No abstract available

[en] The mechanism of the Meyer-Schuster rearrangement of tertiary arylpropargyl alcohols to α,β-unsaturated carbonyl compounds is discussed. The data, inverse solvent isotope effects, k/sub H₂O/, k/sub D₂O/ = 0.36-0.48, rho vs. sigma⁺ = -2.3 at the reaction site and -1.6 at the rearrangement terminus, (k/sub H//k/sub d/)/sub α/ at the rearrangement terminus = 0.92, and relatively large negative ΔS²⁺, all suggest an ion-dipole intermediate undergoing nucleophilic attack by H₂O as the rate-determining step. The rearrangement of eight triaryl- and diarylpropargyl alcohols is reported

[en] The EPR spectra of a series of five symmetrically deuterated triphenylmethyl radicals were investigated in order to provide a critical test of the vibrational model of the deuterium isotope effect in an odd-alternant aromatic system. The experimentally determined values of k (eq 2) are k > 1 at para and ortho positions and k < 1 at meta positions, whereas the vibrational model predicts the reverse. Spin density calculations using Coulomb and resonance integral perturbations at deuterium-substituted positions indicate that the effect cannot be explained as a result of spin density changes upon deuteration. The observed changes in hydrogen hyperfine splitting constants (a/sup H/) at undeuterated positions, however, do qualitatively agree with the calculated spin density changes. The results indicate that the deuterium isotope effect in complex radicals agree with the calculated spin density changes. The results indicate that the deuterium isotope effect in complex radicals such as triphenylmethyl cannot be interpreted simply as a result of out of plane vibrations of a -(CH)₂C-H fragment. 27 references, 5 figures, 5 tables

[en] The problem of kinetic solvent isotope effects in H₂O--D₂O mixtures on systems for which proton transfer from H₃O⁺ and some subsequent reaction step are each partly rate determining is considered, and an expression relating the isotope effect to the atom fraction of deuterium in the solvent is derived for a symmetrical isotope exchange process such as acid-catalyzed aromatic hydrogen exchange. This expression is then applied to data presented here for the detritiation of 1,3,5-trimethoxybenzene-2-t as well as to literature values for the same reaction and the detritiation of azulene-l-t. The analysis yields isotopic (Bronsted) exponents, α/sub i/ = 0.67 for trimethoxybenzene and α/sub i/ = 0.57 for azulene, which are not affected substantially by inclusion of corrections for nonadherence to the rule of the geometric mean and which are in good agreement with conventional Bronsted exponents as well as other measures of transition-state structure for these reactions

[en] Short communication