Intermolecular interactions steric effects

Ibis hindered rotation of >f2-H2 is governed by various forces, which can be divided into bonded (electronic) and nonbonded interactions ( steric effects). The direct electronic interaction between M and H2 results from overlap of the appropriate molecular orbitals. Nonbonded interactions such as van der Waals forces between the q2-H2 atoms and the other atoms on the molecule may vary as 2- 2 rotates. Intermolecular interactions should not contribute much to the barrier to rotation of q2-H2, as the metals are far apart. However, they may have a minor effect on the coordination geometry about M, which could in turn affect M-H2 binding. 

In summary, steric strain from /-butyl groups correlates with increasing deviation from linearity of the Br Se Br groups and with increasing P Se distances. There is, however, no straightforward correlation of intermolecular interactions (Se -Br, Br- -Br, Se- -H, Br- -H), steric effects and non-equidistance of Se Br bonds in the series of related trialkylphosphane selenide dibromides. 

Many aspects of intramolecular nitrile oxide cycloadditions are similar to those of the intermolecular ones. Due to the proximity of the reacting groups, however, there are also several items that differ significantly. While HOMO-LUMO interactions and steric effects direct the intermolecular nitrile oxide cycloaddition to 1-alkenes to produce 5-substituted isoxazolines, the intramolecular cases often show a different behavior. With most of them, regioselectivity is determined by geometric constraints and cycloadditions occur in the exo mode to furnish the annulated bicycle (Scheme 6.42). 

A substrate binds an enzyme at the active site. Substrate-enzyme binding is based on weak intermolecular attractions contact forces, dipole forces, and hydrogen bonding. Steric effects also play an important role because the substrate must physically fit into the active site. Some enzymes have confined active sites, while others are open and accessible. A restricted active site can lead to high selectivity for a specific substrate. Low specificity can be advantageous for some enzymes, particularly metabolic and digestive enzymes that need to process a broad range of compounds with a variety of structures. Because enzymes are composed of chiral amino acids, enzymes interact differently with stereoisomers, whether diastereomers or enantiomers. 

The electrostatic part, Wg(ft), can be evaluated with the reaction field model. The short-range term, i/r(Tl), could in principle be derived from the pair interactions between molecules [21-23], This kind of approach, which can be very cumbersome, may be necessary in some cases, e.g. for a thorough analysis of the thermodynamic properties of liquid crystals. However, a lower level of detail can be sufficient to predict orientational order parameters. Very effective approaches have been developed, in the sense that they are capable of providing a good account of the anisotropy of short-range intermolecular interactions, at low computational cost [6,22], These are phenomenological models, essentially in the spirit of the popular Maier-Saupe theory [24], wherein the mean-field potential is parameterized in terms of the anisometry of the molecular surface. They rely on the physical insight that the anisotropy of steric and dispersion interactions reflects the molecular shape. 

In Section II, devoted to intermolecular processes, it appeared that most of the quantitative analysis of steric effects was made using a single parameter approach. However, analysis has shown that a correct description of the size of a substituent rests on its preferred conformational states, which are related to the interactions with both the planar heteroaromatic ring to which it is bonded and neighboring groups. This was the topic of Section III. 

In addition to the intermolecular interactions, the intramolecular interactions may also be taken into account in a similar way. This rather limited approach may nevertheless be useful for calculating molecular conformation and even molecular symmetry. Deviations from the ideal conformations and symmetries may also be estimated this way, provided they are due to steric effects. 

So far, the discussion has been confined to isolated planar n systems. However, intramolecular and intermolecular interactions, such as steric effects and solvent effects, may influence the spectra considerably. In this section, some examples are discussed as a review of some of the more important aspects of such effects. 

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