Geminal repulsion

In tetramethylethane, geminal repulsions between methyls cause expansion of the C-C-C angle, leading to enhanced gauche butane-type interactions in the anti form only. This repulsion counters the fact that there are more gauche interactions in the gauche conformer (3) than in the and (2). The net effect is that the two conformers have very nearly equal enthalpies. 

One error in the above analysis is the assumption that the steric relationships present in -butane will carry over unchanged in 2,3-dimethylbutane. It is true that there are gauche Me Me relationships in 2,3-dimethylbutane, and these are expected to be destabilizing. However, there are also other interactions, including a geminal repulsion, a steric interaction between methyls attached to the same carbon (Figure 2.8). The geminal repulsion will occur in addition to the vicinal repulsion. 

SCHEME 14. Repulsive interactions developing between two geminal diethyl groups... 

For a series of geminal dimethyl trisubstituted alkenes, the same trend of regioselectivity was earlier recognized by Thomas and Pawlak as well as Rautenstrauch and coworkers . By examining molecular models and assuming that the reaction is concerted, Thomas and coworkers proposed that the difference in regioselectivity could be due to the different conformations and steric repulsions in the transition states. 

In the group of nonfluorescing alkanes the lifetimes are very short, x < 0.3 nsec an upper limit of i < 3 x 10 sec is estimated. The absence of fluorescence for these compounds may have two causes the low R, i.e., low rate coefficient of the Si Sq radiative transition and the short lifetime, i.e., the very fast chemical decomposition. In the C5 and C7-C10 cycloalkanes the ring strain, which is mainly caused by the repulsive interaction of their unfavorably displaced H atoms, may enhance the rate coefficient of the chemical decay by C-H decompositions. In the excited sates of the geminally branched... 

The discussion of Kapral s kinetic theory analysis of chemical reaction has been considered in some detail because it provides an alternative and intrinsically more satisfactory route by which to describe molecular scale reactions in solution than using phenomenological Brownian motion equations. Detailed though this analysis is, there are still many other factors which should be incorporated. Some of the more notable are to consider the case of a reversible reaction, geminate pair recombination [286], inter-reactant pair potential [454], soft forces between solvent molecules and with the reactants, and the effect of hydrodynamic repulsion [456b, 544]. Kapral and co-workers have considered some of the points and these are discussed very briefly below [37, 285, 286, 454, 538]. 

TABLE 10. Conventional strain energies (CSE), hybridizations, s-character, overlap values, overlap repulsions and geminal delocalizations of propane, cyclobutane, cyclopropane and their heterologues with X = NH, O, SiH2, PH, S from Reference 47 ... 

Destabilizing (antibonding) overlap repulsions between geminal bonds increase (i.e. IBP becomes more negative) in the order open-chain compound < four-membered ring < three-membered ring. 

Apparently, the electric charge of the membrane can be neutralized by an increase of the inorganic salts concentration. This neutralization is believed to decrease mutual repulsion of the molecules in the membrane, and thus to increase the density of molecules packing in the membrane and its viscosity. The probability for the products of PET reaction to escape geminate recombination is expected to decrease in this case due to the cage effect. 

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