Bond compression

These charge-transfer structures have been studied [4] in terms a very limited number of END trajectories to model vibrational induced electron tiansfer. An electronic 3-21G-1- basis for Li [53] and 3-21G for FI [54] was used. The equilibrium structure has the geometry with a long Li(2)—FI bond (3.45561 a.u.) and a short Li(l)—H bond (3.09017 a.u.). It was first established that only the Li—H bond stietching modes will promote election transfer, and then initial conditions were chosen such that the long bond was stretched and the short bond compressed by the same (%) amount. The small ensemble of six trajectories with 5.6, 10, 13, 15, 18, and 20% initial change in equilibrium bond lengths are sufficient to illustrate the approach. 

A theoretical explanation for such an anomalous phenomenon in certain nonalternant hydrocarbons has first been attempted, in case of pentalene, by Boer-Veenendaal and Boer followed by Boer-Veenen-daal et Snyder and Nakajima and Katagiri for other related nonalternant hydrocarbons. By making allowance for the effects of <7-bond compression, these authors have shown that a distorted structure resembling either of the two Kekule-type structures is actually energetically favored as compared with the apparently-full symmetrical one. 

Klein, O., Bonvehi, M. M., Aguilar-Parrilla, F., Jagerovic, N., Elguero, J., and Limbach, H.-Fl. (1999). Flydrogen bond compression during triple proton transfer in crystalline pyrazoles. A dynamic NMR study. Israel J. Chem. 39, 291-9. 

M.J.Bodnar, "Adhesives for Bonding Compressed Graphites to Steel, PATR 2604 (1959>... 

A bulky substituent close to the reaction centre may increase the non-bonded compression energy as the transition state is formed this will cause an increase in A//. It will also hinder the close approach of solvent molecules to the reaction centre, thus reducing the maximum amount of stabilization possible (steric inhibition of solvation). This will result in a further increase in AH, but since decreased solvation means less ordering of solvent molecules about the transition state, there is a compensating increase in AS. Another effect of the bulky substituent may be to block certain vibrational and rotational degrees of freedom more in the (more crowded) transition state than in the initial state, and so to reduce AS. These are the most important of the simple effects of a bulky substituent and can be used to explain most of the relationships of Table 25. 

When non-bonded compression does eventually appear, its effects on both AH and AS are unmistakeable. A sharp increase in AH, as is observed, for example, for the hydrolysis of ethyl trimethylacetate, is accompanied by a decrease in AS, as expected if steric hindrance of solvation is occurring. 

The data for acid-catalyzed ester formation in cyclohexanol are doubly interesting. The activation parameters are closely similar to those for the acid-catalyzed hydrolysis of the corresponding ethyl esters. The enthalpy of activation is considerably higher than for esterification in methanol this is probably a result of steric inhibition of solvation, as well as non-bonded compression in the transition state, as suggested by the entropies of activation, which are also significantly higher than with methanol, especially for compounds without ortho substituents which presumably have more transition state solvation to lose. 

Hughes and Volger12 explained the reactivity sequence (Table 2), Me (1) > Et(0.42) > PeBCO(0.33) > Bus(0.06), as due to steric non-bonded compressions in the transition state. For example, in the ethyl transition state (I), there will be extra interactions between the a-methyl group and the two mercury atoms, as compared with the methyl transition state. 

For the more tight clusters SiO2 C70 and CS2 C7o, the local minimum of the PES corresponds to symmetric structure D h, the M(SiO) and R CS) distances are, respectively, 0.013 and 0.045 A shorter and their vstr frequencies experience a blue shift by 80 cm-1 (Si-O) and 145-245 cm-1 (C-S). As in the above clusters, the frequency of the deformation vibration nu e ) is red shifted by 78 (SiCA) and 145 (CS2) cm-1. Like the C-O and Be-F bonds, the Si-O bonds compressed in the cage become more polar. In the CS2 C7o cluster, a noticeable charge is transferred from the cage to the sulfur atoms, each of which acquires about 0.01 e (Table 11). 

In molecules, both nonbonding pairs and multiple bonds compress bond angles. 

The bond angles in ammonia (107.3°) are slightly smaller than the ideal tetrahedral angle, 109.5°. The nonbonding electrons are spread out more than a bonding pair of electrons, so they take up more space. The lone pair repels the electrons in the N—H bonds, compressing the bond angle. 

Although biphenyl is slightly twisted, the angle of twist is small, therefore, conjugation between the rings is not affected. Biphenyl thus shows a very intense absorption band at 252 run (K-Band). Biphenyl derivatives with bulky substituents in the ortho positions are more stable in twisted conformations than in the planar conformation, which suffers serious non-bonded compressions from the juxtaposed substituents. The loss of conjugation in the twist conformation of 2,2-dimethylbiphenyl is reflected in its UV spectral data, which now structurally is like two moles of o-xylene. 

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