Isotope effects, symmetry-induced

SYMMETRY-INDUCED KINETIC ISOTOPE EFFECTS IN ION-MOLECULE REACTIONS... 

Symmetry-induced kinetic isotope effects cause changes in the rate of a chemical process involving identical nuclei when the nuclear symmetry of the system is altered... 

The generally contrathermodynamic stereochemistry, as well as the disparate substituent, solvent and isotope effects, are consistent with the zwitterionic mechanism illustrated in Fig. 6.8 [56], in which the orbital symmetry analysis plays a small but essential role. It is assumed that substitutional desymmetriza-tion is insufficient to destroy the essential symmetry of the tt orbitals, which sets the reactants on a path in which the four interacting C atoms are initially coplanar, but are induced by orbital symmetry conservation to bond along the diagonal. The preferred direction of approach and the stereochemical consequences then follow directly. 

Experimental and theoretical (B3LYP/6-31G calculations) results for 2 4-2-cycloreversion of anti-o, > -benzene dimer (38) and -o,o -naphthalene dimer (39) reveal that the process, which is symmetry forbidden in the ground state, is accelerated through electron transfer by at least 10 -fold. The observed inverse secondary isotope effect of hs/ d8 = 0.91 0.02 for the PET-induced cycloreversion of (39) is due to the isotope effect on the pseudo-equilibrium between (39) and (39" "), rather than on the subsequent chemical step. 

Because of the selection rule AK = 0, the rotational constant about the symmetry axis, A (C for an oblate top), cannot be evaluated from the rotational spectrum of a symmetric top. This selection rule follows because there is no dipole moment component perpendicular to the symmetry axis, and applies rigorously for a rigid molecule. Recent theoretical developments have shown that actually a very small dipole moment perpendicular to the symmetry axis can be induced through the effects of centrifugal distortion. This lifts the A AT = 0 selection rule and allows the observation of AK = 3, 6,... transitions with a sensitive spectrometer. This has enabled the structures of pyramidal XY3 molecules, which have only two structural parameters (see Table II), to be evaluated directly from and h without the need for isotopic information. The observation of so-called forbidden transitions often results... 

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