AB quartet

Not fully resolved. second order analysis. m- multiplet. 

B. Nicotinamide and Flavin Coenzymes.—High-frequency (220 MHz) H n.m.r. spectroscopy shows that there are differences in conformation between oxidized and reduced pyridine coenzymes. A preliminary report on the P n.m.r. spectra of NAD+ and NADH confirms these observations, as the spectrum of NAD+ consists of an AB quartet while there is only a single resonance discernible in the spectrum of NADH. 

Ethers and sulfides bearing the CF3CHFCF2- group are readily prepared by addition of the alcohol or thiol to hexafluoropropene. Some data from their fluorine spectra are given in Scheme 6.16, along with proton data. With the CHF carbon being chiral, the CF2 group should appear as an AB quartet. 

If the nuclei interact with each other and give an AB quartet signal, then kc and AG are given by the following equations ... 

As an extension of this work, Atkinson and co-workers (123) prepared l-dibenzylamino-l,2-dihydro-2-quinolone (78) and 1 -(/V-benzy l-N-carboxy-methyl)amino-l, 2-dihydro-2-quinolone (79). The benzylic protons of 78 showed an AB quartet that did not coalesce up to 180°C, and 79 was resolved into optical isomers. The E, for racemization was 26.2 0.4 kcal/mol. Various attempts were made to elucidate the possible pathways for isomerization in these quinolone derivatives (123). Radical dissociation, a sigmatropic shift followed by rotation, and restricted rotation about the S—N bond were excluded. The aforementioned authors (123) also excluded the possibility of nitrogen inversion and preferred restricted rotation about the N—N bond as an explanation for the existence of stereoisomers. They supported this explanation by examining the steric effects... 

The barrier to rotation was 24.8 kcal/mol at 48°C, and the population ratio was 2.0 in chloroform-, which is the statistical value. The size of the peri substituent has an important effect on the barrier to rotation about the CH2—C(9) bond, because if a 1,4-dimethoxybenzeno bridge is introduced in place of the 1,4-dimethylbenzeno, the coalescence of the AB quartet due to the benzylic CH2 protons is observed at 167°C corresponding to a free energy of activation of 22 kcal/mol, which is too low for isolation of the atropisomers at room temperature. 

As mentioned in Sect. IV-B, enantiotopic groups even in achiral substances may show nonequivalence. For example, this property sometimes can be used to distinguish meso compounds from their chiral isomers. Thus in the presence of TFPE, the meso isomer of dimethyl 2,3-diaminosuccinate (46) shows two equally intense methoxy singlets and an AB quartet for the now diastereotopic methine hydrogens (88). This coupling clearly shows 46 to be the... 

The IR spectrum of dihydroarcyriarubin B (352) showed the presence of carbonyl bands at v ax 1710 and 1770 cm , indicating the presence of a dihydromaleimide function. The H-NMR spectrum showed the presence of an AB quartet at 5 4.48 and 4.53 assignable to the methylene protons of the dihydromaleimide ring. 

In symmetrical pavines, the C-2 and C-8 methoxyls will appear as a singlet of six hydrogens just as the C-3 and C-9 methoxyls. The former pair, which is proximate to the bridgehead, will appear more downfield by about 0.07-0.1 ppm with respect to the latter (1,25). The methylenedioxy protons center around 8 5.8-5.9 as an AB quartet (J = 1-1.5 Hz). The A-methyl protons appear almost invariably between 8 2.49 and 2.57 in CDCI3 solutions and at 8 2.35 in DMSO solutions. 

The spectral data provide information about the structure of a-acetoxydibenzylnitrosamine (VII). The high frequency (1780 cm ) for the carbonyl group in the infrared spectrum (Fig. 9) is consistent with this structure ( ) and the introduction of the chiral center at the benzylic position causes the methylene of the other benzyl ic substituent to be diastereotopic and appear in the nmr spectrum as an AB quartet. The center of the quartet... 

Reaction of the platinum complex with a molar excess of racemic acrylamide 7 leads to four distinct AB quartets in the 31P-NMR spectrum (with associated platinum satellites) due to the four possible diastereomeric species. These are formed by nonselective binding of the Re-and 57-face of both enantiomers. Such behaviour may be expected when the stereogenic center under examination is remote from the metal center. After reaction of the ethene complex with carvone 8 the metal is bound with complete stereoselectivity to the less hindered Si- 5/-face of the endocyclic, electron-poor double bond. Spectra of optically pure and racemic carvone complexes are shown in Figure 14. 

Complications may arise if the two olefinic protons are coincidentally isochronic so that the coupling constant is difficult to obtain due to the extremely low intensity of the outer AB quartet lines. In such cases higher magnetic fields or addition of a mono- or binuclear LSR may be helpful. Another difficulty may arise from spin-spin coupling of the olefinic protons with other nuclei in the substituents so that each olefinic signal may be a second-order multiplet. In this case decoupling experiments106 are recommended. 

This complex is soluble in dichloromethane, chloroform, benzene, and acetone, and sparingly soluble in hexane and diethyl ether. The H NMR spectra of this complex indicate that the six-membered palladated 2-(2-pyridinylmethylJphenyl-C1, N moiety shows a fluxional motion above — 30 °C. In the low-temperature limiting spectrum measured in chloroform- at — 35 °C, the methylene protons are observed at S 4.45 as an AB quartet [Ad = 0.93, 2J(hh) = 14 Hz], which changes to a sharp singlet at S 4.51 in the high-temperature limiting spectrum at 55 °C. The cyclopalladated structure of the 2-(2-pyridinylmethyl)phenyl-C1, N moiety is confirmed by the fact that the aromatic protons appear as two sets of ABCD patterns at — 35 °C.7... 

Cycl[3,2,2]azines are diatropic. The proton magnetic resonance (PMR) spectrum of the parent compound (la) consists of an A2B multiplet from the protons of the six-membered ring and one AB quartet from the two pairs of protons of the five-membered rings. 

---