Conformational analysis, rigid

This explosion in steroid chemistry both stimulated and was aided by the development of conformational analysis (10). Many basic, physical organic chemistry principles were estabUshed as a result of the study of the logically predictable chemistry of the rigid perhydro-l,2-cyclopentenophenanthrene, steroid skeleton. 

The size of the atoms and the rigidity of the bonds, bond angles, torsions, etc. are determined empirically, that is, they are chosen to reproduce experimental data. Electrons are not part of the MM description, and as a result, several key chemical phenomena cannot be reproduced by this method. Nevertheless, MM methods are orders of magnitude cheaper from a computational point of view than quantum mechanical (QM) methods, and because of this, they have found a preferential position in a number of areas of computational chemistry, like conformational analysis of organic compounds or molecular dynamics. 

Much work has been done to determine sets of substituent effects in specific conformationally mobile or rigid frameworks, since a knowledge of such effects often furnishes valuable conformational information. For instance, it was concluded from yg- and 8-SCS values that substituents (X = OH, I) on the C(19) methyl group in some cholest-S-enes prefer an antiperiplanar orientation [X-C(19)H2-C(10)-C(1)] with respect to C(l) (81). In the following, SCS information for various classes of cyclic systems is discussed, with particular emphasis on configurational and conformational analysis. 

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

Numerous articles relevant to polymer stereochemistry have appeared since this chapter was completed, most of them dealing with conformational analysis, spectroscopy, and chirality. In this addendum I shall discuss only a few items pertaining to the optical activity of rigid polymers. This matter has recently received a lot of attention and merits a more detailed discussion than was presented earlier. 

The fluorescence In dilute solution is measured for five polyesters with terephthalate as the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer, A conformational analysis concludes that the spacers most conducive to excimer formation are the 1,3-c/s-cyclohexanediol and 1,4-e/s-eyclohexanedimethanol. This result from calculations is compatible with experimental results. 

The diagnostic coupling constant /P c for conformational analysis of phosphorinanes (P-CN = 3) is lost with phosphorinanes where P-CN = 4. Shifts and /P c values of two non-rigid and two cis-trans rigid phosphorinane 1-oxides and 1-sulfides should be studied for comparison (Tables 2 and 3). The large differences between the two classes are then evident. 

The ramifications of conformational analysis of flexible and rigid ring systems are of considerable importance to the understanding of stability and reactivity in polycyclic systems. This will become increasingly evident in later discussions. 

After criticism [142] of the interglycosidic NOEs observed in the previously described study a very precise approach towards the conformational analysis of thea-(l-3) bond was performed by NOE measurements of specifically deuterated compounds [143], The conformation was determined earlier by several interglycosidic NOEs to protons that have their resonances in an area of high spectral overlap. The synthesis of C-deuterated di- and trisaccharides made an unequivocal assignment of the enhanced signals possible. Thus, the enhancements of H2, H3, and H4 of the P-D-mannose upon irradiation of Hl. as well as the enhancement of HS. upon irradiation of HS. man are indicative of the rigid conformation at the glycosidic bond and confirmed the previous study. 

The computational study of the osmium dihydroxylation of aliphatic al-kenes is much more complicated than the case of aromatic alkenes due to the large number of conformations that the former could adopt. To overcome this issue, we considered the system to be composed of two different parts the catalyst and the olefin. For the catalyst, the conformation considered is that from the X-ray structure. As already shown in the study of styrene [95], and in some experimental works [98], the catalyst is a fairly rigid molecule. For the aliphatic alkenes under study, there is a large number of possible conformations in addition, the stability of an olefin conformation is also affected by the interactions between the olefin substituent and the catalyst. Therefore, the catalyst must be included in the conformational search. The conformational analysis was done using a scheme based on the systematic search approach [99]. The strategy consisted of two parts first we developed a method to identify all of the possible conformations afterwards, we screened all of the possible conformations at MM level to select the most stable. Finally, we only carried out the relatively expensive QM/MM calculations on these selected conformations. 

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