Quantitative conformation

The ability of the MM method to calculate accurately potential energies and geometries of molecules is best suited for quantitative conformational analysis (44). Numerical values can readily be given to such terms as strain and stability (45). The topic is extensively covered in Allinger s review (lOd), therefore only some recent developments are mentioned. 

From a study of a range of 2-substituted tetrahydropyrans it was concluded that the effects of an electronegative substituent were to deshield C-2 by about 30 p.p.m. and C-3 to a much reduced extent (ca. 4 p.p.m.), but to cause small upheld shifts in the signals from C-4, C-5 and C-6 (740MR(6)233). The influence of a 2-substituent on the chemical shift of C-4 can be used for quantitative conformational analysis provided suitable reference compounds are available. A comparatively large shielding of the C-4 signal indicates a preference of the 2-substituent for the axial position. 

The anomeric effect dominates the conformational preference, but quantitative conformational predictions by presently described methods, based on additive treatments of steric effects taken in conjunction with the anomeric effect, do not accord with the observed data. 

When the number of torsional degrees of freedom is increased, the intramolecular motion in gaseous molecules is increased as well. At the same time the theoretical treatment of the motion becomes more complex, and the problems that the electron-diffraction method has to face are more difficult to handle. The molecules of this category that have been subject to quantitative conformational analysis by electron diffraction so far, are limited to cases with two or a few degrees of freedom, though qualitative observations about large amplitude motion have been made also for considerably larger molecules. 

Because there are several excellent sources (Hanack, 1965 Eliel et al., 1965) which deal with steric effects and conformational analysis, we can limit our discussion to two topics related to SS, namely the qualitative and semiquantitative evaluations of steric effects and quantitative conformational analysis. In some cases, the origins of SS can be assigned unequivocally. 

Eiji Osawa, Introduction to Quantitative Conformational Analysis Use of Computer, Plirokawa, Tokyo, 1991. 

J. E. Williams, P. J. Stang, and P. v. R. Schleyer, Annu. Rev. Phys. Chem., 19, 531 (1968). Physical Organic Chemistry Quantitative Conformational Analysis Calculation Methods. 

If to a dry cellulose fibre water is gradually added, hydrate formation will take place as a topo-chemical reaction. The water molecules will be bound at distinct places along the cellulose chains, mainly in the amorphous regions. This should give rise to a behaviour according to the Langmuir equation for the case of 3-dimensional sorption (p. 520). As is demonstrated by Fig. 33, this is in qualitative agreement with the experiments. In various ways it has been shown that the isotherms are also in quantitative conformity with the formation of hydrates. 

Winstein S, Holness NJ (1955) NeighbOTing carbon and hydrogen. Xix. Tert-butylcyclohexyl derivatives. Quantitative conformational analysis. J Am Chem Soc 77 5562-5578... 

The strength of the interaction between two biomolecular entities is closely related to the efficiency of their biological activities. If this strength is known for similar complexes composed of different molecules, it is possible to appreciate whether a molecule prefers to bind to a specific molecular motif or to another. The compared motifs can be either different molecules or different conformations of the same molecule. The determination of binding energy can then be a way to address quantitatively conformational selectivity and recognition, and complement the structural (qualitative) information derived from vibrational spectroscopic data. 

Let us note in conclusion, that the adduced above results have shown that the fractal Griffith crack model application can not only improve quantitative conformity of theoretical and experimental data, but also obtain qualitatively new picture of fracture processes. 

Thus, the cluster model allows the alternative treatment of plastic deformation mechanisms and controlling them parameters changes at testing temperature variation for amorphous glassy polymers. Both qualitative and quantitative conformity of the offered treatment to the obtained earlier experimental data is shown [16]. 

The shown in Fig. 15.31 dependences (p j( g) have the character expected fi om the adduced above description and are its quantitative conformation. The adduced in Fig. 15.32 dependence of density p of REP extruded specimens on is similar to the dependence (p. i(eg), that was to be expected, since densely packed segments fraction decrease must be reflected in p reduction. 

Winstein, S. Holness, N.J. Neighboring Carbon and Hydrogen. XIX. t-Butylcyclohexyl Derivatives. Quantitative Conformational Analysis . J. Am. Chem. Soc. 1955, 77, 5562-5578. 

Seeman, J. I. The Curtin-Hammett Principle and the Winstein-Holness Equation. New Definition and Recent Extensions to Classical Concepts /. Chem. Educ. 1986, 63, 42-48. Winstein, S. Holness, N. J. Neighboring Carbon and Hydrogen. XIX. rerf-Butylcyclohexyl Derivatives. Quantitative Conformational Analysis /. Awj. Chem. Soc. 1955, 77, 5562-5578. Curtin, D. Y. Stereochemical Control of Organic Reactions. Differences in Behavior of Diastereoisomers. I. Ethane Derivatives. The Cis Effect Rec. Chem. Pro ir. 1954, 15, 111-128. An alternative to the Curtin-Hammett argument presented on page 295 is that the epoxide reacts with thiophenoxide from conformation 64b such that the product is bom in the B ring boat (or twist-boat) conformation of 65b. 

Quantitative conformational analysis of frans -clerodanes (256) and 5a,10a-cw-clerodanes (378) has been conducted on X-ray diffraction results. 

Disubstituted cyclohexanes and, in particular, the dimethyl cyclohexanes, were among the earliest to be studied from the energetic point of view, thanks to the pioneering thermochemical work of F. D. Rossini and co-workers. We shall discuss the dimethyl compounds in some detail, because the calculations relating to these species are representative of others likely to be encountered in quantitative conformational studies. In Table 3 are listed the experimental and calculated values of AAG (difference between cis and trans isomers) for the 1,2-, 1,3-, and 1,4-dimethylcyclohexanes. ... 

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