Theoretical and General Considerations

spectroscopy and its application in carbohydrate chemistry and on 2D n.m.r. techniques for structure analysis of carbohydrates have appeared. 

Empirical force field calculations on the simplest acyclic diols show that there is a tendency for 1,2-diols to adopt the gauche O-C-C-0 conformations. In protic media, these conformations are stabilized by specific activation. The stereochemical properties of the glycosidic linkages in flexible pyranose rings have been studied by PCILO quantum chemical methods. The energies of thirty six conformers were calculated, as were the influences of solvent on the equilibrium and hence the magnitude of the exo-anomeric effect. 

The calculations showed that the exo-anomeric effect is in the range -1 

Configuration-dependent conformational transmission in trigonal bipyramidal phosphorus(V) compounds enhances the gauche (-) conformation population around the 6 linkage in phosphorylated 

Solid state C-n.m.r. by the c.p.-m.a.s. technique has been used to study the metal-sugar complexes (1) and (2). Related cop er(II) complexes of Schiff s bases from chitosan were also studied. 

High-resolution n.m.r. spectroscopy of carbohydrates and glycoproteins has been reviewed.A new coupling-constant-torsion-angle relationship for three-bond spin-spin coupling has been derived. It is of the form of a Karplus 

Spin-lattice values (Tj) for o -(2 3)- and a-(2 6)-isomers of iV-acetyl-neuraminyl-lactose have been shown to be similar, suggesting that the internal motions are also similar.  

Triflu or oacetic anhydride has been recommended as a solvent for H n.m.r. of carbohydrates since triflu oroacetylation of the hydroxy-groups, which occurs on dissolution, results in downfield shifts pf the ot-protons, giving well resolved spectra. The method was applied to 270 MHz studies of 2-acetamido-2-deoxy-hexoses.  

Empirical shift parameters have been used to calculate the chemical shifts induced at any other carbon atom in the C n.m.r. spectrum of D-xylose derivatives agreement was poor unless the compounds are very similar. A rapid micro-method ( jUg) for determining C-labelling patterns in carbohydrates based on n.m.r. has been developed. The results were useful for calculating the specific activity using field-desorption m.s. and the method avoids the use of chemical degradation.  

The use of n.ra.r. spectroscopy in the study of mono- and oligosaccharides, covering both H and C data, as well as data from 

Ab initio quantum mechanical methods have been used to compare the anomeric effects of thiosugars with their oxygen counterparts. The lower anomeric effect in sulphur systems was attributed to lower dipolar contributions to the total energy when compared with the 

AS is -2.7 calK mol for the conversion of the axial (oC) conformer to the equatorial (/9) conformer in an 85s 15 mixture of trichlorofluoromethane-trichlorodeuteromethane. Conformational energies of C-2 carbon-substituted tetrahydropyrans form the basis of an important paper on the conformational analysis of such compounds. The values obtained will be of use in the field of -glycosides. An exception to the Imbach criteria for determining the anomeric configuration of D-ribofuranosyl nucleosides has 

An analysis based on the approach to the catalyst surface of various configurational isomers in the Koch and Stuart deuterium exchange for hydrogen at carbon atoms using Raney nickel and heavy water (see Vol. 12, ch. 2, p. 4-, ref. 8) has elicited the criteria for the reaction these include the requirement that at least two hydroxy groups must be present, that equatorial hydrogens are exchanged more rapidly than axial ones, and that the isomerization is kinetically controlled. 

The H-bonding system of Sheldrick (1976) could not be reproduced with a reasonable energy. 

Isbell has reviewed the historical development of concepts of ring-conformation and neighbouring-group effects in carbohydrates prior to 1940.  

From a model based on two methanol molecules, it has been shown that the maximum probability for the O-H -O hydrogen bond angle is 163°. From eighteen neutron diffraction studies of carbohydrates the mean angle was found 

A stereographic spherical representation of the conformational itinerary of carbohydrates using Cremer and Pople puckering parameters has been published.  

An improvement to two-dimensional J spectroscopy on nulling of the residual solvent resonance has been demonstrated the technique was applied to the analysis of the spectrum of uridine. Complex H n.m.r. spectra have been resolved by three-dimensional plotting of peaks, using computer graphics to tilt the picture through 45 projections were used to obtain effectively decoupled spectra. 

In situ reactions of hydroxylic compounds with trichloroacetylisocyanate have been studied by H n.m.r. Although no carbohydrate examples were given, the reaction, which can be carried out in the n.m.r. tube, has obvious application in this area since it causes shifts in the 0 -protons.  

Ab initio SCFMO calculations at the ST0-3G level have been carried ---------1- 2- 

Detailed analysis of H n.m.r. spectra of methyl-epoxytetra-hydropyrans has shown the steric requirement of a methyl group the molecules represent simple models of anhydropyranoses. Analysis of J data for trans-2-methyl-3 A-epoxytetrahydropyran (1) shows that it exists entirely in the 0-opposed conformation A whereas for the cis- 

2-methyl compound (2) the equilibrium consists of 4-0% A and 60% B. The results confirm the previously assumed repulsive effect between epoxide and tetrahydropj ran ring oxygen making (1) A and (2) A unexpectedly more stable. Partial deuteration of exchangeable 

Most signals, seen as characteristic multiplets (doublets to octets), 

Anomeric effects in 2-chloro-, 2-methoxy-, 2-hydroxy-, and 2-methyl- 

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N.M.R. Spectroscopy and Conformational Features Theoretical and General Considerations Acyclic Systems Furanose Systems Pyranose Systems Oligosaccharides Nucleosides Other Molecules... 

The properties desired of an - ideal liquid phase arsj contradictory and a compromise must be reached between theoretical] and practical considerations (8,9). It is generally desirable for the liquid phase to have a wide temperature operating range. Ideally, this range would include all temperatures from the lowest to the highest used in 6LC, approximately -60 C to 400 C. Ho phasel... 

W. Odling, A Manual of Chemistry Descriptive and Theoretical, Part 1, London, 1861, Advertisement and General Considerations . 

CE is now recognized as a mature technique alongside HPLC and other modern analytical techniques. A harmonized general monograph that presents both theoretical and practical considerations of the technique has been recently adopted by the Pharmacopoeial Discussion Group (PDG) for implementation into the USP, the... 

No extensive comparison with experiment to test the values in Table IV will be made. The close agreement between the purely theoretical and the experimental results in the case of helium and neon allows one to place confidence in the R values for ions with these structures and the same remark applies with less force in the case of the argon structure, where only a small empirical correction was introduced. It is interesting to note that the theoretical values 3-57 and 6-15 for the rubidium and the caesium ion agree very well with the experimental ones, 3-56 and 6-17 (Table III), which were not used at all in the evaluation of the empirical corrections for these structures. Finally, we may mention that our values agree in general with those of Fajans and WulfE.i obtained by them from the experimental R values for salt solutions by the application of only the simplest theoretical considerations. 

Sections II to IV have been mainly about various aspects of chemical bonding on the theoretical level. Bonding considerations of this kind easily acquire an abstract, speculative character if not related to concrete experimental data. It seems somewhat artificial to extend this paper by a comprehensive discussion of particular examples, which in any case will be the subject of a series of forthcoming papers. However, this article cannot be terminated without a brief survey of the relation between the different bonding parameters and experiments in general. This task can unfortunately only be undertaken on a rather superficial level. 

In solution, although solute contributions can generally be singled out, difficulties arise sometimes solvent-solute interactions may induce a shift of the solute absorption and consequently of its susceptibility or hydrogen bonded molecular complexes may modify the liquid structure. This situation has been studied both theoretically and experimentally by Zyss and Berthier (10) and by Ledoux and Zyss (13) in the case of urea derivatives in various solvents and in crystal showing the importance of environment considerations and thus the limitations of an oriented gas model for crystals. 

The same general considerations apply to nonazo pigments. Attempts have been made to theoretically interpret absorption spectra of polycylic compounds by quantum mechanics. These studies compared the absorption of systems such as indigo [1] with known spectra of polymethine dyes. However, the investigation of solid state spectra is aggravated by interactions within the crystal lattice and has rarely been studied. 

In the 45 years since its proposal, Frank s autocatalytic mechanism (Section 11.3, above) has spawned numerous theoretical refinements including consideration of such factors as reversibility, racemization, environmental noise, and parity-violating energy differences. [100,101] In contrast to the above examples of stereospecific autocatalysis by the SRURC, however, none of these theoretical refinements is supported by experimental evidence. While earlier attempts to validate the Frank mechanism for the autocatalytic amplification of small e.e.s in other experimental systems have generally been unsuccessful, several recent attempts have shown more promising results. [102,104]... 

The following protocol and discussion contains fundamental principles and as much detail as possible but remains a general outline of the procedures and practical considerations. Each individual experiment must be well planned, with sufficient theoretic contemplation given to the unique characteristics of the study target and experimental materials. Whenever possible, practical procedural bps have been included in an attempt to save time, trouble, and materials. 

D = D° exp(-ac ), where D is the diffusion, D represents the zero-concentration limit, c is the concentration, a and v are parameters, fits the data from a wide variety of probes and matrix polymers ( ). Several theoretical justifications for this behavior have been presented (97-1011. but it is not possible to tell yet which, if any, is uniquely correct. The treatments range from simple physical considerations (98) to treatments of hydrodynsumical interaction of probe and matrix (97,991. Other more complex and general treatments (1001 do not explicitly arrive at the stretched exponential form, but do closely fit the available data. Much more work needs to be done on probe diffusion in such transient networks. Beyond enhancing the arsenal of gel characterization, the problem is quite fundamental to a number of other important processes. 

There has been considerable interest in theoretical and quantum chemical calculations applied to the bipyridines over the past 25 years. 7i-Electron distributions, electron densities, and molecular orbital calculations on all the bipyridines have been determined, and the results are generally in accord with the known chemical reactions of the molecules.Calculations applied to 2,2 -, 3,3 -, and 4,4 -bipyridines have been correlated with ionization potentials,and reduction potentials ° "and electrical susceptibilities of most of the bipyridines have been determined.The ability of 3,3 - and 4,4 -bipyridines to act as electron-transfer bridges has been calculated. ... 

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