Chemical shift empirical calculations

A useful empirical method for the prediction of chemical shifts and coupling constants relies on the information contained in databases of structures with the corresponding NMR data. Large databases with hundred-thousands of chemical shifts are commercially available and are linked to predictive systems, which basically rely on database searching [35], Protons are internally represented by their structural environments, usually their HOSE codes [9]. When a query structure is submitted, a search is performed to find the protons belonging to similar (overlapping) substructures. These are the protons with the same HOSE codes as the protons in the query molecule. The prediction of the chemical shift is calculated as the average chemical shift of the retrieved protons. 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

D chemical structure drawing on Macintosh. ChemWindow for 2D chemical structure drawing. C-13 NMR module for predicting chemical shifts. MS Calculator for obtaining empirical formulas from masses. SciWords for spell checking. Art of Science clip art. Entropy for chemical database management. PCs (DOS and Windows) and Macintosh. 

Inc, [34], is an example of a software package that can calculate 3D geometries, chemical shifts, and coupling constants using semi-empirical approaches (Figure 10.2-2). 

A proton can be (numerically) represented by a series of topological and physicochemical descriptors, which account for the influence of the neighborhood on its chemical shift. Fast empirical procedures for the calculation of physicochemical descriptors are now easily accessible [45. Geometric descriptors were added in the case of some rigid substructures, as well as for rr-systems, to account for stereochemistry and 3D effects. 

The first step for any structure elucidation is the assignment of the frequencies (chemical shifts) of the protons and other NMR-active nuclei ( C, N). Although the frequencies of the nuclei in the magnetic field depend on the local electronic environment produced by the three-dimensional structure, a direct correlation to structure is very complicated. The application of chemical shift in structure calculation has been limited to final structure refinements, using empirical relations [14,15] for proton and chemical shifts and ab initio calculation for chemical shifts of certain residues [16]. 

Calculations have been done at the STO-3G and 4-3IG levels, and the resulting substituent constants correlate well with empirical values derived from ground-state structural parameters, such as C-NMR chemical shifts and IR absorption frequencies. 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

Grant and Paul Chemical Shifts. The technique of obtaining branch content information from NMR for polymers utilizes an empirical relationship given by Grant and Paul [29,79,80]. The Grant and Paul empirical relationship [29,79,80] can be used to calculate the values of the chemical shifts for carbon atoms in the vicinity of a branch point in a hydrocarbon polymer. The empirical relationship was obtained from NMR studies on alkanes. The chemical shift of any carbon atom in a 13C-NMR can be decomposed as a sum of contributions from its nearest five neighboring carbon atoms. The value of the chemical shift for any carbon atom C, is given as,... 

This empirical relationship cannot be used without accounting for some correction terms, which take into account the molecular geometry of the bonded neighbors. This is especially essential when calculating the chemical shift of a branch point carbon atom. These correction terms were given by Grant and Paul to be as follows [29,80]. 

The ambiguity of the interpretation of the pnmr spectrum motivated Ray et al. (1971 and eaurlier papers) to determine and analyse the cnmr spectrum of the triphenylcarbonium ion (as well as some of its derivatives). The chemical shifts they obtained are given in Table 4, and a comparison of the empirical charge densities with those calculated by the HMO and CNDO techniques... 

The high-resolution C spectrum has also been measured, and the following values obtained chemical shift (in acetone) 62.4 ppm upheld from CSg (130.9 ppm downfield from tetramethylsilane) J (geminal) CH, 205 Hz J (vicinal) CH, 13.4 Hz. The chemical shift value is in accord with an empirical equation based on the number and position of the nitrogen atoms in several five-membered heterocyclics, and also reflects the 7T-electron density of the system as calculated by the extended Hiickel method - and by the simple MO method. spectra of v-triazole and of its 1- and 2-methyl derivative have also been obtained. ... 

The chemical shifts of the four methylene carbons depicted in Figure 8 have been calculated using the empirical shift parameters derived from the preceding study on hydrochlorinated... 

A carbon-13 NMR study of four sydnones (1) has been reported. In 4-unsubstituted sydnones (1, R = Me, Ph or PhCHj, R = H), the chemical shift of C-4 is substantially higher than has been measured for carbon in other unsubstituted heterocycles. This shielding of C-4 in the sydnone molecule is consistent with the results of ab initio and semi-empirical calculations which predict a high electron density at C-4. 

In an attempt to relate calculated results to experimental findings for monomeric, lignin model compounds, preliminary work has compared theoretically determined electron densities and chemical shifts reported from carbon-13 nuclear magnetic resonance spectroscopy (62). Although chemical shifts are a function of numerous factors, of which electron density is only one, both theoretical and empirical relationships of this nature have been explored for a variety of compound classes, and are reviewed by Ebra-heem and Webb (63), Martin et al. (64), Nelson and Williams (65), and Farnum (66). 

Empirically determined chemical shift additivity parameters have been determined- for diene-type polymers. The shift contribution of a quaternary carbon which is fllto the carbon in question was not determined by those authors. However, using their additivity parameters and the shift positions of the ( carbons in Figure 8, a value of +15.4 ppm can be estimated for the contribution of a neighboring (0() quaternary carbon. Using this value, the shift positions of the carbons in structures VII and VIII are calculated as shown. If the first 1,2 unit were on the chain in a 2,1 manner, the methylene carbon resonance would be at a considerably higher field, but it would be difficult to estimate its position with any certainty because the quaternary effect... 

---