Geminal Effects

When two functional groups are borne by the same atom, they interact to cause either stabilization or destabilization. This is the geminal effect.20 If the substituents are both strong o-acceptors and n-donors [for example a gem-dimethoxy-C(OMe)2-], a stabilization occurs.21 This is easy to understand. Oxygen, being electronegative, is a o-acceptor and o co is low in energy and is 71-donor thanks to its lone pair. Therefore, we have here a double anomeric effect ... 

Generally, geminal effects are only modeled via valence angle terms, i.e., nonbonded interactions are only considered for 1,4- and higher interactions. 

The symbiotic effect is also common in organic chemistry, but here it has been called the clustering, anomeric, or geminal effect.Clustering refers to the stabilization caused by adding several substitutents to the same carbon atom. Some extreme examples are shown by Reactions (1.20) and (1.21), in which the number of bonds of each kind is preserved. 

Geminal difluorination ot diethyl diazomalonate can be effected with molecu lar fluorine (10% in nitrogen) to give diethyl difluoromalonate m 70% yield [90] (equation 27)... 

The reaction rates and product yields of [2+2] cycloadditions are expectedly enhanced by electronic factors that favor radical formation. Olefins with geminal capto-dative substituents are especially efficient partners (equations 33 and 34) because of the synergistic effect of the electron acceptor (capto) with the electron donor (dative) substituents on radical stability [95]... 

Effects of Structure on Equilibrium Aldehydes and ketones react with water in a rapid equilibrium. The product is a geminal diol. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

The calculated values (Table 2) consistently show the triplet preference for the mono-substituted TM diradicals though the S-T gap is small and close to that of the parent species 2 (Y = H). However, the values show slight singlet preference of 27 (Y = OH) and 29 (Y = SiHj). The singlet states are stabilized by the p-Ocjj-q interaction (Fig. 17a) in 29 and probably by the p-ct -q interaction in 27, where is lowered in energy by the inductive effect by the geminal OH group (Fig. 17b). 

Keywords tr-Relaxation o-Relaxation Geminal interaction Inverted bond Lone pair effect Orbital phase continuity Ring strain... 

In triaziridine 23, the o-relaxation is appreciable due to the lone pair effect (Sect. 2.1.4) as is the case with triphosphirane 14 which has almost the same SE as tetra-phosphetane 15. Triaziridine 23 has lower SE (31.8 kcal moF ) than tetraazetidine 24 (SE = 33.4 kcal moF ) (Scheme 12) [12]. Lone pairs on nitrogen atoms have high s-character. The hybrid orbitals for the N-N bonds have lower s-character (sp ) in 23 than that (sp ) in triazane 25 [12]. The geminal a-a interaction is less antibonding in 23 (IBP, = -0.004) than in 25 (IBP. = -0.014). The geminal... 

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