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[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] 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
[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] Loss of Cl from CHCl3+ with little excess energy is found to proceed via two distinct pathways, the reaction channel which is being followed is determined by a symmetry-induced kinetic isotope effect. Computational investigations suggest competition between a statistical adiabatic process and a process of electronic predissociation. For CHCl4− and CH2Cl3−, rotational predissociation contributes to the kinetic isotope effects for all isotopomers, whereas the particularly large effect observed for CH35Cl337Cl− also originates from symmetry-induced degeneracy of vibrational modes in the C3V point.
[en] In order to study the diastereotopically secondary deuterium isotope effects on the thermal isomerization of cyclobutane, the compounds cyclobutene, [1-2H]cyclobutene, and [3-2H]cyclobutene were prepared, and their gas-phases isomerizations were followed. From the data, correlations between hybridization changes and secondary hydrogen/deuterium isotope effects and the observed diastereotopically distinct hydrogen/deuterium effects suggest that two cyclobutene C3-H bonds may have substantially different hybridizations at the transition state. 16 refs