Chemical shifts experimental

Atom Chemical shift (experimental) (02MRC743) Absolute shielding (calculated) Atom Chemical shift (experimental) (02MRC743) Absolute shielding (calculated)... 

It is also possible to measure the isotropic equivalent chemical shifts experimentally using magic angle spinning (MAS) on powder specimens. These experiments are much simpler than the single crystal measurements and require... 

A longstanding problem in the interpretation of X-ray photoelectron and absorption spectra (XPS, XAS,...) is the theoretical calculation of the core excitation and ionization energies (CIE s) and of their chemical shifts, experimentally measured in molecular systems or in condensed matter [1-3]. In computing these properties, one has to take into account various effects (i) electron correlation, particularly strong between inner-core electrons [4,5], which prevents the use of simple SCF schemes (ii) electron relaxation, which increases with the number of electrons [6-9], making the use of Koopmans theorem [10] rather inaccurate (iii) for systems containing... 

Fig. 48. Schematic diagram of the hydrogen bond characteristic, the N—H bond length, and the amide H chemical shifts experimentally determined for a-helical and /3-sheet poly(L-alanines).

Compound Calculated metal shieldings Calculated metal NMR chemical shifts Experimental... 

Chemical Shifts, Experimental and Calculated Intensities, and Results of Best-Fit Minimization for C NMR data ... 

A number of other software packages are available to predict NMR spectra. The use of large NMR spectral databases is the most popular approach it utilizes assigned chemical structures. In an advanced approach, parameters such as solvent information can be used to refine the accuracy of the prediction. A typical application works with tables of experimental chemical shifts from experimental NMR spectra. Each shift value is assigned to a specific structural fragment. The query structure is dissected into fragments that are compared with the fragments in the database. For each coincidence, the experimental chemical shift from the database is used to compose the final set of chemical shifts for the... 

Commercial implementations of this general approach are ACD/I-Lab [36], Specinfo (Chemical Concepts) [37], WINNMR (Bruker), and KnowItAll (Bio-Rad) [38]. Figure 10.2-3 shows the workspace generated by ACD/I-Lab after predicting a H NMR spectrum. ACD calculations are currently based on over 1 200 000 experimental chemical shifts and 320 000 experimental coupling constants [36]. 

Fast and accurate predictions of H NMR chemical shifts of organic compounds arc of great intcrc.st for automatic stnicturc elucidation, for the analysi.s of combinatorial libraries, and, of course, for assisting experimental chemists in the structural characterization of small data sets of compounds. 

In general, the computation of absolute chemical shifts is a very difficult task. Computing shifts relative to a standard, such as TMS, can be done more accurately. With some of the more approximate methods, it is sometimes more reliable to compare the shifts relative to the other shifts in the compound, rather than relative to a standard compound. It is always advisable to verify at least one representative compound against the experimental spectra when choosing a method. The following rules of thumb can be drawn from a review of the literature ... 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule... 

A number of MO calculations has been carried out, and these have had mixed success in predicting chemical reactivity or spectroscopic parameters such as NMR chemical shifts and coupling constants. Most early calculations did not take into account the contribution of the sulfur 3d-orbitals to the ground state, and this accounts for some of the discrepancies between calculations and experimental observations. Of the MO methods used, CNDO/2 and CNDO/S have been most successful the INDO approximation cannot be used because of the presence of the sulfur atom. 

Structure calculation algorithms in general assume that the experimental list of restraints is completely free of errors. This is usually true only in the final stages of a structure calculation, when all errors (e.g., in the assignment of chemical shifts or NOEs) have been identified, often in a laborious iterative process. Many effects can produce inconsistent or incorrect restraints, e.g., artifact peaks, imprecise peak positions, and insufficient error bounds to correct for spin diffusion. 

Figure 18.16 One-dlmenslonal NMR spectra, (a) H-NMR spectrum of ethanol. The NMR signals (chemical shifts) for all the hydrogen atoms In this small molecule are clearly separated from each other. In this spectrum the signal from the CH3 protons Is split Into three peaks and that from the CH2 protons Into four peaks close to each other, due to the experimental conditions, (b) H-NMR spectrum of a small protein, the C-terminal domain of a cellulase, comprising 36 amino acid residues. The NMR signals from many individual hydrogen atoms overlap and peaks are obtained that comprise signals from many hydrogen atoms. (Courtesy of Per Kraulis, Uppsala, from data published in Kraulis et al.. Biochemistry 28 7241-7257, 1989.)...

In solution the cis and trans isomers may co-exist, as demonstrated by N NMR and UV-visible spectra. The N NMR chemical shift of the trans isomer is shifted ca. 60 ppm downfield relative to the cis isomer." The visible absorption band of S-nitrosothiols corresponds to a weak n K transition in the 520-590 nm region. The absorption maxima of trans conformers are red-shifted by ca. 30 nm relative to those of the cis isomer. Two absorptions are observed in the 520-590 nm region in the experimental spectra of RSNO derivatives." ... 

The aforementioned exception and the rather limited experimental material available do not allow any conclusions about the general applicability of the Hammett equation, using the same a- and p-values as for benzenes, to be drawn with certainty. The present author has pointed out that large deviations should be expected with strong - -M-substituents, as is also indicated from the rates of alkaline hydrolysis of methyl 5-amino- and 5-acylamino-2-thenoates. From the chemical shifts in the NMR spectra of thiophenes and benzenes it appears that another set of cr-values should be used in the thiophenes series which seems plausible since the transmission of the sub-... 

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