Proton methine

Section 13 9 The methyl protons of an isopropyl group appear as a doublet and the methine proton as a septet in compounds of the type (CH3)2CHX... 

Substituent group Methyl protons Methylene protons Methine proton... 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

These three protons split the signal for the methine proton into a quartet. 

FIGURE 13.13 The magnetic moments (blue arrows) of the two possible spin states of the methine proton affect the chemical shift of the methyl protons in 1,1-dichloroethane. When the magnetic moment is parallel to the external field if.o (green arrow), it adds to the external field and a smaller 3 0 is needed for resonance. When it is antiparallel to the external field, it subtracts from it and shields the methyl protons. 

The physical basis for peak splitting in 1,1-dichloroethane can be explained with the aid of Figure 13.13, which exanines how the chemical shift of the methyl protons is affected by the spin of the methine proton. There aie two magnetic environments for... 

Spin of methine proton reinforces a weaker Mq is needed for resonance. Methyl signal appears at lower field. 

In recent years, trimethylsilyl protection has often been used for the methine proton of the acetylenic group because of the mild reaction conditions for desilylation. As a rule, the starting pyrazole trimethylsilyl derivative is mixed up, at room temperature, with a 2 A aqueous solution of NaOH, potash, or methanol solution in ammonia. 

Although [34]octaphyrin 80 fulfills Hiickel s rule, the II NMR spectrum indicates by the high-field shift of the methine protons that the system is nonaromatic. The X-ray structure analysis demonstrates clearly the reason for the lack of aromatic stabilization, namely the nonplanar loop conformation in which the whole macrocycle is twisted similarly to the [32]octaphyrin structure and which is also found for [36]octaphyrin and [40]decaphyrin structures (vide infra). 

The diastereomeric ratio was determined by the NMR integral ratio of the methine protons adjacent to the hydroxyl group (3.33 ppm for the (//-isomer 3.44 ppm for the waw-isomer). 

A = peak area of the methin proton signal of alkenesulfonic acid... 

For the t-butyl radical is positive and necessarily Ag — 0. Since in isobutane Jy is positive and the methine proton came originally from the other radical (i.e. uy is negative),... 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Signals falling in the second spin system represent aromatic methine protons. Starting from the most downfield signal, at 8 8.25, representing the C-8 methine proton (which appears downfield due to the... 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

---