Proton chemical shifts additivity rules

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

Again, collections of H shifts exist which can be searched manually e.g., methine and methylenes. More usefully, proton chemical shifts have also been the subject of SCS or additivity rules. Prestsch and co-workers devised... 

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. 

Pyrazine and its derivatives have been extensively studied by proton and NMR spectroscopy and conflicting reports on the reliability of additivity rules and/or correlation of chemical shifts with calculated electron densities have appeared. 

From the 13C chemical shift data collected in Table 4.1, Grant and Paul [85] deduced their additivity rule for the 13C chemical shifts of alkanes. The signal assignments for the alkanes, also given in Table 4.1, are based on signal intensities and proton decoupling experiments. 

To assign H and C spectra of model compound and Pi s, tentative chemic shifts were determined using substituent additivity rules. The APT spectrum allows to determine the luimber of protons attached to each carbon. 

Chanical-Shift Ranges of Some Nuclei Reference Standards for Selected Nuclei H and C Chemical Shifts of Useful NMR Solvents Proton NMR Absorptions of Major Functional Groups Some Useful H Coupling Constants Additivity Rules in C NMR Correlation Tables Alkanes Alkenes Alkynes... 

The deshielding of protons and carbon by substituents has given rise to attempts to predict the chemical shift by application of empirical additivity rules. While these can be useful, they are, at best, only approximate. Thus, as shown in Table 5.3 for the chlorinated derivatives of methane, there is not a simple incremental value to... 

More recently, the method has been applied to the prediction of H NMR chemical shift. The program automatically searches for an appropriate substructure for each proton in the compound from a dataset of 728 substructures. A base value is assigned and using the additivity rules and 2000 substituent increments, a chemical shift is predicted. Results from a test set of 200 spectra gave rise to chemical shift estimates for over 90% of the protons bonded to carbon with a mean deviation of —0.06 ppm and a standard deviation of 0,18 ppm. 

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