Absolute shieldings predicted

Shielding constants reported in experimental studies are usually shifts relative to a standard compound, often tetramethylsilane (TMS). In order to compare predicted values to experimental results, we also need to compute the absolute shielding value for TMS, using exactly the same model chemistry. Here is the relevant output for TMS ... 

The predicted absolute shielding value for the central carbons in butane is 176.3 ppm, which is what we will use as the reference value, subtracting the computed shielding values for the outer carbons in butane and for each type of carbon in the other two compounds from it. 

Predicted 235U Absolute Shieldings. Calculated uranium shieldings for selected molecules are collected in Table V. 

The conformational dependence of the 33S isotropic a in dimethyldisulphide, (CH3)2S2, has been studied by HF calculations using the IGLO method.121 (CH3)2S2 is a good model for the disulphide linkage in polypeptides and proteins. 33S a has been calculated as a function of the dihedral angle C-S-S-C. Predicted chemical shifts vary between —84.4 ppm (< C-S-S-C = 0°) and —42.3 ppm (Calculated values refer to CS2 and cannot be converted to the SO4 scale, since the absolute shielding of the SQj anion has not been reported... 

The performance of DFT for the prediction of absolute shieldings is documented in Table VI.6, where we compare results from uncoupled and coupled DFT calculations with those obtained using the more empirical schemes of Malkin et al. [78] and Wilson et al. [79]. When comparing those results with the best available ab initio data and values derived from... 

Of particular interest in connection with halide ion relaxation rates, is the observation that there are marked upfield shifts of the ions resonances when D2O is substituted for H2O (see Section 6.5). According to the simple method by which Deverell [245 246] accounts for electronic distortion effects, one would then expect to observe larger field gradients with H2O as solvent than with D2O. Since the absolute shielding is not accurately known (see Section 6.1) it is not possible to predict quantitatively the relative effect on the quadrupole relaxation rate. Estimates give a change of the order of 5 %. 

To obtain the predicted shift for the carbon atom in methane, we subtract its absolute value from that of the reference molecule, resulting in a predicted shift of -3.9 ppm, which is in reasonable agreement with the experimental value of -7.0. Note the sign convention for shifts a negative number indicates that there is more shielding in the specified molecule than in the reference molecule, and a positive number indicates that there is less shielding than in the reference molecule. 

The calculated shift of C-3 is 39.1 ppm as compared to an observed value of 39.45 ppm. Prediction of carbon chemical shifts using the Grant-Paul relation (4.1) is a practical aid in assigning the carbon signals of larger alkyl groups, e.g. in cholestane derivatives (Section 5.2.2). - Other increment systems have been proposed [201, 202], as well as an absolute scale for carbon shielding [203],... 

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