Chemical shifts for

NMR spectra are basically characterized by the chemical shift and coupling constants of signals. The chemical shift for a particular atom is influenced by the 3D arrangement and bond types of the chemical environment of the atom and by its hybridization. The multiplicity of a signal depends on the coupling partners and on the bond types between atom and couphng partner. 

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... 

In this second empirical approach, which has also been used for C NMR spectra, predictions are based on tabulated chemical shifts for classes of structures, and corrected with additive contributions from neighboring functional groups or substructures. Several tables have been compiled for different types of protons. Increment rules can be found in nearly any textbook on NMR spectroscopy. 

A relatively small training set of 744 NMR chemical shifts for protons from 1 20 molecular structures was collected from the literature. This set was designed to cover as many situations of protons in organic structures as possible. Only data from spectra obtained in CDCl, were considered. The collection was restricted to CH protons and to compounds containing the elements C, H, N, 0, S, F, Cl, Br. or I. 

A combination of physicochemical, topological, and geometric information is used to encode the environment of a proton, The geometric information is based on (local) proton radial distribution function (RDF) descriptors and characterizes the 3D environment of the proton. Counterpropagation neural networks established the relationship between protons and their h NMR chemical shifts (for details of neural networks, see Section 9,5). Four different types of protons were... 

An example of the neural network prediction of NMR chemical shifts for a natural product is illustrated in Figure 10.2-7 together with the calculations from other methods. This molecule was chosen as it had been discovered [47]... 

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

Recently. Fourier transform technique allowed the determination in natural abundance of C chemical shifts for some 4-thiazoline-2-thiones. Substituent chemical shifts for methyl and phenyl groups have been collected and discussed (874). For the overcrowded polyalkyl-A-4-thiazoline-2-thiones. the evolution of these chemical shifts furnishes... 

A second 2D NMR method called HETCOR (heteronuclear chemical shift correlation) is a type of COSY in which the two frequency axes are the chemical shifts for different nuclei usually H and With HETCOR it is possible to relate a peak m a C spectrum to the H signal of the protons attached to that carbon As we did with COSY we 11 use 2 hexanone to illustrate the technique... 

Section 13 5 Protons m different environments within a molecule have different chem real shifts, that is they experience different degrees of shielding Chem ical shifts (8) are reported m parts per million (ppm) from tetramethylsi lane (TMS) Table 13 1 lists characteristic chemical shifts for various types of protons... 

NMR The electronegative oxygen of an alcohol decreases the shielding of the car bon to which it is attached The chemical shift for the carbon of the C—OH is 60-75 ppm for most alcohols Carbon of a C—S group is more shielded than carbon of C—O... 

Table 7.43 Estimation of Chemical Shift for Protons of —CHj— and Methine...

TABLE 7.44 Estimation of Chemical Shift for Proton Attached to a Double Bond... 

Substituents on both sides of the double bond are considered separately. Additional vinyl carbons are treated as if they were alkyl carbons. The method is applicable to alicyclic alkenes in small rings carbons are counted twice, i.e., from both sides of the double bond where applicable. The constant in the equation is the chemical shift for ethylene. The effect of other substituent groups is tabulated below. 

The XPS chemical shift, for an atomic core orbital with principal and orbital... 

Table 1. H-nmr Chemical Shifts for Indole and C-nmr Chemical Shifts for Indole...

Analytical and Test Methods. Most of the analytical and test methods described for THF and PTHF are appHcable to OX and POX with only minor modifications (346). Infrared and nmr are useful aids in the characterization of oxetanes and their polymers. The oxetane ring shows absorption between 960 and 980 cm , regardless of substituents on the ring (282). Dinitro oxetane (DNOX) has its absorption at 1000 cm . In addition, H-nmr chemical shifts for CH2 groups in OX and POX are typically at 4.0—4.8 5 and 3.5—4.7 5, respectively (6,347,348) C-nmr is especially useful for characterizing the microstmcture of polyoxetanes. 

Table 2. H-Nmr Chemical Shifts for Selected Pyrazolones and Pyrazolines ...

J3 4 = 3.45-4.35 J2-4 = 1.25-1.7 and J2-5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. C chemical shifts for thiophene, from tetramethylsilane (TMS), are 127.6, C 125.9, C 125.9, and C 127.6 ppm. 

Tables 11 and 12 give some available chemical shifts for azolines and azolidines, respectively. Unfortunately data for many of the parent compounds are lacking, often because the compounds themselves are unknown.

Chemical shifts for aromatic azoles are recorded in Tables 14-17. As for the proton spectra, fast tautomerism renders two of the chemical shifts equivalent for the NH derivatives (Table 14). However, data for the AT-methyl derivatives (Table 15) clearly indicate that the... 

Table 14 C NMR Chemical Shifts for Nitrogenous Azoles (a) NH Derivatives...

Table 16 NMR Chemical Shifts for Azoles Containing Oxygen ...

Table 19 C NMR Chemical Shifts for Non-aromatic Azoles with Two Ring Double Bonds...

Table 20 C NMR Chemical Shifts for Azolines (Non-aromatic Azoles with One Ring Double Bond)...

Chemical shifts for some representatives of the less common class of A -pyrazolines are shown in Table 13. These cyclic enehydrazines are also probably puckered (69) however, no X-ray determination nor conformational study has been carried out to solve this problem. Compound (70) is representative of the rare N(l)-unsubstituted A -pyrazolines prepared by Burger et al. (79T389). 

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