Styrene oxide chemical shift

Figure 3 Selected aggregation states (20A and 20B) of lithiated styrene oxide chemical shifts (ppm) of the carbenoid atom determined in THF at 173 K (relative deshielding with respect to the non-lithiated precursor).

In the oxides of styrene, stilbene, and stilbazole, the chemical shifts of the cis protons of the oxirane ring are larger than those for the trans isomers. This can be explained by the polarization effect of the electric dipole moment of one of the CH protons on the other CH. The anisotropc shielding of the ring in the case of oxiranes substituted with an aromatic group can be utilized well in the determination of the configuration. ... 

When the protons attached to a single carbon are chemically equivalent (have the same chemical shift), the n -I-1 Rule successfully predicts the splitting patterns. In contrast, when the protons attached to a single carbon are chemically nonequivalent (have different chemical shifts), the n -t-1 Rule no longer applies. We shall examine two cases, one in which the n + 1 Rule applies (1,1,2-trichloroethane) and one in which it fails (styrene oxide). 

In two earlier seetions, we introduced the speetra of some other commonly encountered types of systems. The NMR speetra of both styrene oxide (Fig. 5.10) and vinyl acetate (Fig. 5.17) contain examples of AMX systems, where M, being in the middle of the alphabet, indicates a chemical shift intermediate between A and X. That is, aU three ehemical shifts (A, M, and X) are separated widely. trans-Cmnarmc acid (Fig. 5.16) is an example of an AX system. 

Vinyl acetate gives an NMR spectrum typical of a compound with a terminal alkene. Each alkenyl proton has a chemical shift and a coupling constant different from those of each of the other protons. This spectrum, shown in Figure 7.50 is not unlike that of styrene oxide (Fig. 7.31). Each hydrogen is split into a doublet of doublets (four peaks). Figure 7.51 is a graphical analysis of the vinyl portion. Notice that Vbc (trans, 14 Hz) is larger than Vac (cis, 6.3 Hz), and that Vab (gem-inal, 1.5 Hz) is very small—the usual situation for vinyl compounds. 

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