Single-bond conformational

Conformational analysis (Section 3 1) Study of the conforma tions available to a molecule their relative stability and the role they play in defining the properties of the molecule Conformations (Section 3 1) Nonidentical representations of a molecule generated by rotation about single bonds Conformers (Section 3 1) Different conformations of a single molecule... 

Conformation about a Csp —Csp single bond (Conformation of the ring (Nitrogen inversion... 

More than any other factors, single-bond conformation and ring conformation dictate overall molecular size and shape. Thus, proper assignment of ground-state conformation is a very important task for calculation. 

The importance of single-bond conformation is nevermore apparent than for polypeptides. Here, distinct local domains involving a-helices and P-sheets (among other structures) occur commonly, and these in turn dictate overall (tertiary) stmcture of proteins and ultimately protein function. Interestingly, proteins appear to exhibit well-defined shapes, that is, exist as a single conformer or a very few closely-related conformers. This is the reason that they can be crystallized and their structures determined, and is certainly a major factor behind the ability of proteins to direct specific chemical reactions. 

Conformation (Section 6.3) A shape that a molecule can assume by rotation about a single bond. Conformational analysis (Section 6.3) Analysis of the energies of the various conformations of a compound. 

Draw energy vs. angle of rotation graphs for single bond conformations. 

Free rotation about the carbon-carbon single bond. Conformations. 

Valence Shell Electron Pair Repulsion (VSEPR) Theory Hybridization of Atomic Orbitals, sp, sp, sp Single Bonds Conformational Isomers Pi Bonds Pi Barrier to Rotation C/s and Trans, 2p-3p Triple Bonds Cumulenes... 

This means that at room temperature, the various single-bond conformers undergo essentially unrestricted interconversion, with the all-frans conformer comprising >95% of the mixture. Of course, the equilibrium composition can be displaced in favor of the less stable conformers at higher temperatures, a fact which frequently results in temperature-dependent photochemistry because polyene photochemistry tends to be highly dependent on ground state conformational factors (vide infra). As well, it has often been exploited for the preparation of samples enriched in the less stable conformers for study by matrix isolation techniques at very low temperatures, where conformational interconversion ( con-formerization ) is slow. 

The diene component must be in the s-cis (the "s"refers to the conformation about a single bond) conformation to yield the cyclic product with the cis C C required by the six-membered ring. For this reason, cyclic dienes usually... 

Weber-Helfand model the primary parameter is the correlation time for cooperative backbone transitions, X]. At the lower temperatures studied, xq plays an increasing role in the Weber-Helfand model but xj is still the major factor. This is an interesting point in itself since cooperative transitions were also found to predominate when the Weber-Helfand model was applied to the polycarbonates. Here in the polyformal, single bond conformational transitions do play a larger role and this can be seen in the three bond jump model as well by the drop of m to 1 at lower temperatures. Since xj and x are both measures of the time scale for cooperative motions, it is interesting to note that the Arrhenius summaries of the two correlation times in Tables II and III are very similar. This similarity, taken together with the domination of cooperative transitions in the Interpretations, supports the utility of both models though the Weber-Helfand model is developed from a more detailed analysis of chain motion. 

In many cases a mononuclear structure with a terminal, doubly bonded heteroelement ligand maybe unfavorable with respect to the alternative dinuclear structure where the ligand adopts a bridging, singly bonded conformation (Scheme 37). Facile interconversion of the two forms may occur, leading to analogous synthetic strategies and reactivity. 

The first group, being the weakest indicator of crystallinity, refers to absorption bands assigned to certain single -bond conformations. For example, the ratio of the 1450 (trans) and 1435 cm (cis) bands of 1,4,-polybutadiene does indeed increase with increasing crystallinity, but it is equally clear that the trans conformer also is present in the amorphous component. 

In general, neglecting any secondary relaxation of Arrhenius type, the typical relaxation times of the cooperative single bond conformation dynamics may be taken as persistence times displaying temperature dependence. This can be described by the modified Vogel-Fulcher-Tamaiin (VFT) equation [18] ... 

Rotation about Single Bonds Conformations CHAPTER 2... 

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