Proton chemical shifts of reference compounds

Table 7.47 Proton Chemical Shifts of Reference Compounds 7.98...

The general absorption pattern of quaternary pavines strongly resembles that of the tertiary analogs with the exception of the expected downfield shifts for each of the protons. In particular, the bridgehead protons will move downfield by about 1-1.5 ppm (32,35). N,N-Dimethyl protons will be observed as a singlet between 8 3.3 and 3.7 (29,32,35). The set of empirical rules deduced for aromatic proton chemical shifts in a tertiary system has been shown to apply also to the quaternary system (29). A listing of aromatic proton chemical shifts of some quaternary pavine bases has been presented as a reference for future studies on similar compounds (29). 

Aromatic character in isoxazoles has been studied from a number of viewpoints, and these studies indicate that although isoxazole may be formally considered an aromatic system, the disposition of the ring heteroatoms modifies this character to an appreciable extent. From a qualitative viewpoint, thermal stability and electrophilic attack at the 4-position may be considered consistent with an aromatic character. Furthermore, NMR chemical shifts of the ring protons are consistent with those of an aromatic compound. References related to these studies may be found in Section 4.16.2.3.4. 

TABLE 10. NMR chemical shifts of methyl protons of various organosulfur compounds (after References 43 and 44)... 

We neatly side-step both of these problems by quoting the chemical shift relative to an agreed reference compound. For example, in the case of proton NMR the reference compound is TMS. If the frequency of the line we are interested in is v (in Hz) and the frequency of the line from TMS is vtms (also in Hz), the chemical shift of the line is computed as ... 

---