Nonbonded repulsive interactions

Good quality ab initio calculations have not yet been carried out in order to test whether lone pair nonbonded attraction obtains in 1,2-difluoroethane. Results of INDO calculations shown below indicate the presence of a nonbonded attractive interaction on the basis of the partial bond order p (F2px, F2px). By contrast, the presence of a nonbonded repulsive interaction favoring the anti conformation is indicated on the basis of the bond order p(F2px, F2px). 

Since there are two covalent bonds (each accounting for 2(35) and one nonbonded repulsive interaction (—(35) in this VB structure, its energy simply expresses a balance between the two corresponding energy quantities, namely ... 

The relative stabilities of the four diastereomers have been extensively investigated. First, it can easily be shown that the diastereomers must, in principle, differ in stability because there are different nonbonded (repulsive) interactions between the rings in each case. Figure 1-20 shows these differences for any two rings in the complex. When any reasonable potential function is used to estimate the magnitudes of the repulsive energies, it is concluded that the order of decreasing stability is... 

Co(sar)(en)2f For the bis(ethylenediamine)sarcosinatocobalt(III) complex, there are four possible isomers due to the asymmetric configurations about both the Co(III) ion and the sarcosinato N atom. Figure 4.5 shows two of the isomers, A-S and A-R the latter appears to be less stable than the former due to nonbonded repulsive interaction between the hydrogen atoms on the CH3 group and those on the adjacent en ring. 

Parallel molecular dynamics codes are distinguished by their methods of dividing the force evaluation workload among the processors (or nodes). The force evaluation is naturally divided into bonded terms, approximating the effects of covalent bonds and involving up to four nearby atoms, and pairwise nonbonded terms, which account for the electrostatic, dispersive, and electronic repulsion interactions between atoms that are not covalently bonded. The nonbonded forces involve interactions between all pairs of particles in the system and hence require time proportional to the square of the number of atoms. Even when neglected outside of a cutoff, nonbonded force evaluations represent the vast majority of work involved in a molecular dynamics simulation. 

Further substitution can introduce additional factors, especially nonbonded repulsions, which influence conformational equilibria. For example, methyl substitution at C—2, as in 2-methyl-l-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-l-butene the two eclipsed conformations are of approximately equal energy. Increasing the si2e of the group at... 

In cyclic systems such as 1, the dominant conformation is the one with the maximum anomeric effect. In the case of 1, only conformation lA provides the preferred antiperiplanar geometry for both oxygens. Antiperiplanar relationships are indicated by including lone pairs in the oxygen orbitals. Other effects, such as torsional strain and nonbonded repulsion, contribute to the conformational equilibrium, of course. Normally, a value of about 1.5 kcal/mol is assigned to the stabilization due to an optimum anomeric interaction in an acetal. 

The enamines derived from cyclohexanones are of particular interest. The pyrrolidine enamine is most frequently used for synthetic applications. The enamine mixture formed from pyrrolidine and 2-methylcyclohexanone is predominantly isomer 17.106 A steric effect is responsible for this preference. Conjugation between the nitrogen atom and the tt orbitals of the double bond favors coplanarity of the bonds that are darkened in the structures. In isomer 17 the methyl group adopts a quasi-axial conformation to avoid steric interaction with the amine substituents.107 A serious nonbonded repulsion (A1,3 strain) in 18 destabilizes this isomer. 

In the previous sections, we saw that, in most cases, a nonbonded attractive or repulsive interaction is enforced by both four electron destabilization and two electron stabilization. Hence, in order to simplify subsequent discussions, we shall adopt the OEMO model with neglect of overlap. Consequently, in the remainder of this work... 

The results of INDO calculations of planar and distorted difluorobenzene in which the C—F bonds have been displaced a few degrees above and below the mean plane, are shown above. As can be seen, the planar geometry maintains pi nonbonded attractive interaction. Sigma lone pair interactions will be similar to those which obtain in 1,2-difluoroethane and appear to be repulsive in nature. 

The formation of the sterically hindered 14 shows evidence for deep effects of the nonbonded repulsions of the 4 and 5 methyl groups This DHP which is extremely unstable at ambient temperatures is nevertheless formed at low temperatures at which the formation of 1 does not take place. In this case the steric interactions in the educt obviously exert a strong influence on a temperature dependent process. 

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