Correct structure

The synthesis of the correct structure and the optimization of parameters in the design of the reaction and separation systems are often the single most important tasks of process design. Usually there are many options, and it is impossible to fully evaluate them unless a complete design is furnished for the outer layers of the onion. For example, it is not possible to assess which is better. 

Robust implementations can currently propose correct structures from spectroscopic data, especially when the molecular formula and C NMR spectrum are available, or from 2D NMR spectra. 

If a tran sition state has notheen reach ed after a certain niiinherof steps, th e user may n eed to upgrade th e Hessian an d repeat th e calculation. It may be necessary if many calculation steps have been don e, and the curren t geo in etry differs con siderably from th e starting poin t. fh e Hessian calculated at th e starting poin t an d updated at each n e v poin t m ay n ot be appropriate at the geo in etry reach ed by th e search. fh is procedure can also h elp to get to a good startin g point where the Hessian has a correct structure with only one negative eigen value. 

The same reaction performed in ether at 0°C (336) gives the same major adduct, but the structure proposed by Acheson et al. corresponds to 86, although such a structure is hardly compatible with the presence of an isolated low-field proton. Very recently, in a reinvestigation of these cyclo-additions of DMA to azoles (338, 339), Acheson et al. were able to establish the correct structure of the adducts on the base of CNMR spectra and X-ray diffraction studies. The adduct of thiazole is represented by formula 87, and it results from the rearrangement of the... 

These interesting results have been quoted by Sokolov (79RCR289) and by Acheson and Elmore (78AHC(23)263). However, they proved to be erroneous, as to both structure (348) (a fluoroborate, not a complex) and structure (349) (for which (350) represents the correct structure established by X-ray crystallography (83T2193)). 

Restraints due to artifacts may, by chance, be completely consistent with the correct structure of the molecule. However, the majority of incorrect restraints will be inconsistent with the correct structural data (i.e., the correct restraints and information from the force field). Inconsistencies in the data produce distortions in the structure and violations in some restraints. Structural consistency is often taken as the final criterion to identify problematic restraints. It is, for example, the central idea in the bound-smoothing part of distance geometry algorithms, and it is intimately related to the way distance data are usually specified The error bounds are set wide enough that all data are geometrically consistent. 

The atoms of a protein s structure are usually defined by four parameters, three coordinates that give their position in space and one quantity, B, which is called the temperature factor. For well refined, correct structures these B-values are of the order of 20 or less. High B-values, 40 or above, in a local region can be due to flexibility or slight disorder, but also serve as a warning that the model of this region may be incorrect. 

The familiar problem of misleading stoichiometries, and the frequent impossibility of deducing the correct structural formula from the empirical composition is well illustrated by the... 

Correct structure proofs were earlier difficult to obtain, usually involving several reaction steps, and the structures were, therefore, assigned by analogy in many cases. However, through Raney nickel desulfurization (cf. Section VII,C), through NMR spectroscopy,and through polarographic analysis such problems are now easily solved. 

The correct structure of the condensation product of 2-thiocyano-thiophene and chloral in the presence of sulfuric acid is (173). ... 

The name 5-azaorotic acid should be given to allantoxanic (oxonic) acid but it is not yet commonly used. The elucidation of the correct structure of this compound was closely linked to the solution of the course of oxidation of uric acid mentioned earlier. 

Dimethylquinoxaline (303) has been reported to undergo a Diels-Alder reaction with maleic anhydride to give 304, 305 having been postulated to be the reactive form. However, attempted confirmation of this unexpected result has shown that 304 is not the correct structure of the reaction product. " In 1931, other chemical evidence was advanced in support of structure 305,but it would no longer be considered valid. 

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

The correct structure (3) for this compound was first proposed in 1922 by Pieroni and Moggi on the basis of the isolation of succinic acid by chromic acid oxidation. Full confirmation of this structure was more recently obtained by Potts and Smithby the degradation outlined in Scheme 1. The dipyrrylbutane was synthesized by the lithium aluminum hydride reduction of the known dipyrrylbutane-... 

Detailed discussions of some of the remaining peaks in Figures 7 and 8 and in the mass spectra of 10a and the D20-exchanged analogs is of more interest to the mass spectrometrist than to the carbohydrate chemist. The probable origins of these peaks will be discussed here, however, because there will be occasions when the carbohydrate chemist must dig into a spectrum in order to satisfy himself that he has interpreted the spectrum in terms of a correct structure. 

There are several ways to choose the more plausible of two structures differing in their arrangement of atoms. As pointed out in Example 7.1, the fact that carbon almost always forms four bonds leads to the correct structure for ethane. Another approach involves a concept called formal charge, which can be applied to any atom within a Lewis structure. The formal charge is the difference between the number of valence electrons in the free atom and the number assigned to that atom in the Lewis structure. The assigned electrons include—... 

Although it is unlikely that the tetrahydrofuran ring would open under these conditions of polymerization, the polymer was hydrolyzed in 0.2 M sodium hydroxide solution, in order to confirm its structure. Clear colorless liquid was obtained after acidification followed by esterification with diazomethane. Its IR and NMR data compares exactly to that obtained from 59 which was prepared from the neutral hydrolysis of 57 and esterification of the resultant acid with diazomethane. Since the apparent sole product obtained from hydrolysis of the polymer was 59, they conclude that 58 represents the correct structure for this polymer. 

The correct structure of this compound, which was predicted by Wolfrom et al.,w was subsequently confirmed by X-ray crystallography.19 Nevertheless, this mistaken identification has been quoted in the literature as late20 as 1990. A crystal structure of a-D-Fru/-l,2 2,3 -p-D-Fru/(6) has also been published.21... 

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