Isotope effects hybridization changes

These are reasonable KIEs for a secondary isotope effect in a 1,2-C migration, and originate from the hybridization changes at the migrant carbon (sp2,6 -> sp24) QMT is therefore not invoked.77... 

Inverse isotope effects such as those found in Table IV are associated with a change in hybridization at the site of deuterium substitutions (60). Hence, it is clear from these data that the transition state is symmetrical with respect to hydridization changes at both olefinic carbons when the reaction is carried out in aqueous solutions. However the last four entries in Table IV indicate that for the oxidation of methyl cinnamate in methylene chloride solutions only the iS-carbon has undergone a hybridization change (from sp ... 

A corollary of this argument is that a change from sp to sp hybridization should result in an inverse secondary isotope effect (i.e., C/ko < 1). 

These result from bond cleavage between atoms adjacent to the isotopically substituted atom. Secondary isotope effects are caused by a change in the electronic hybridization of the bond linking the isotope, rather than by cleavage of the bond. One example of this in enzymatic reactions is the substitution... 

Any vibration for which the frequency decreases on going to the transition state contributes a factor greater than 1 to (cH/A D, and any vibration for which the frequency increases contributes a factor less than 1. A commonly observed secondary isotope effect occurs when deuterium substitution is made at a carbon that changes hybridization, as in Equations 2.77 and 2.78. 

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

For a vertical mechanism, the hybridization of the /3-carbon atom changes from sp3 to sp2, for which the a-hydrogen/deuterium kinetic isotope effect is normally in the range 1.15-1.25. 

Experimental and computational studies of the pericyclic Meisenheimer rearrangement and a competitive rearrangement of A-propargyl morphol i nc N-oxide revealed a novel inverse secondary kinetic isotope effect (kn/kD 0.8) for the rate-determining cyclization step, probably occurring because of a C(sp) to C(sp2) change in hybridization at the reaction center (Scheme 3).5... 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

Probably the strongest support in favor of the diradical mechanism is the lack of a deuterium isotope effect in the thermal decomposition of franx-3,4-diphenyl-1,2-dioxetane. In the concerted mechanism, the ring carbon of the dioxetane changes its hybridization state from sp to sp in the activated complex (23) and an inverse secondary isotope effect k lkp) would be expected. Consequently, a diradical mechanism was argued to accommodate these results. Similarly, in the thermal decarboxylation of the dimethyl a-peroxylactone, a negligible (A ///A ) = 1.06 0.04) secondary isotope effect was observed. Presumably, in the a-peroxylactone decomposition, a diradical mechanism similar to that of dioxetanes (Eq. 66) upholds. 

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