Chemical shift predicting values

They are applicable to compounds in the common NMR solvents - but not in D6-benzene (or D5-pyridine). The substituent effects are additive, but don t place too much reliance on chemical shifts predicted using the table, in compounds where more than two groups are substituted next to each other, as steric interactions between them can cause large deviations from expected values. Note that Table 5.4, like all others, does not cater for solvent shifts, etc ... 

One point worth noting is that the theoretical calculation of chemical shifts predicts absolute chemical shifts, whereas experimental values are reported with respect to some convenient but otherwise arbitrary reference. Although theoretical chemical shift trends in a series of compounds may be compared with experimental trends, it can be useful to also calculate the chemical shift of a solid reference compound for comparison, while realizing that such a calculation will also be subject to some degree of inaccuracy. 

More recently, software has been developed that predicts H, C, and N chemical shift values of proteins from either 3D stmcture files, for example, SHIFTS (6), SHIFTX (7), and SPARTA (8), or from the mere amino acid sequence using SHIFTY (9). First results have been reported on the de novo stmcture determination of proteins using fragment-based chemical shift predictions and molecular modeling (10, 11). 

Papers containing Se NMR data of metal complexes appear in the literature in increasing numbers. Their Se chemical shifts cover the whole range of known values, from ca. —1000 to -1-1500 and the same holds for pertinent coupling constants. Although some trends can be deduced in certain compound families, a general view which would go beyond earlier summaries or allow reliable Se chemical shift prediction is not yet available and may be... 

However, one of the most successfiil approaches to systematically encoding substructures for NMR spectrum prediction was introduced quite some time ago by Bremser [9]. He used the so-called HOSE (Hierarchical Organization of Spherical Environments) code to describe structures. As mentioned above, the chemical shift value of a carbon atom is basically influenced by the chemical environment of the atom. The HOSE code describes the environment of an atom in several virtual spheres - see Figure 10.2-1. It uses spherical layers (or levels) around the atom to define the chemical environment. The first layer is defined by all the atoms that are one bond away from the central atom, the second layer includes the atoms within the two-bond distance, and so on. This idea can be described as an atom center fragment (ACF) concept, which has been addressed by several other authors in different approaches [19-21]. 

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... 

Another important component of Table 18 is substituent chemical shift (SCS) datadenvedforeachoftheSlentnes The SCS is simply thedifferencein F-NMR chemical shifts of the substituted compounds and that of unsubstituted fluoroben zene (-113 5 ppm in CDCI3) These values numerically represent the mfluence a substituent has on the shieldmg or deshieldmg of the fluorine nucleus and depend upon substituent position o, m, orp) Fluonne chemical shifts can be predicted for polysubstituted fluorobenzene systems simply by addmg the SCS value of each substituent to a base value of -113 5 ppm... 

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