Proton absorption

In the 1H NMR spectrum, the acidic -C02H proton normally absorbs as a singlet near 12 5. As with alcohols (Section 17.11), the -C02H proton can be replaced by deuterium when D20 is added to the sample tube, causing the absorption to disappear horn the NMR spectrum. Figure 20.6 shows the H NMR spectrum of phenylacetic acid. Note that the carboxyl proton absorption occurs at 12.0 8. 

In heavy water 3 M with respect to D2SO4, a lower limit of k > 10 l.mole. sec at 26 °C has been proposed by Dietrich and Wahl and Larsen and Wahl as a result of an nmr investigation of the position of the proton absorption bands of mixtures of the iron (III) complexes and iron (II) complexes. This limit applies to the exchange between iron (III) and iron (II) complexes of l.lOphe-... 

Vogtle F, Gestermann S, Kauffmann C et al (1999) Polypropylene amine) dendrimers with peripheral dansyl units protonation, absorption spectra, photophysical properties, intraden-drimer quenching, and sensitization processes. J Am Chem Soc 121 12161-12166... 

In particular, DP9 presented ambiguities associated with the proton assignments from which the 13C assignments were derived. Thus, it was necessary to use the COSY method to assign the proton absorptions first. Homonuclear COSY NMR spectroscopy allowed unambiguous assignment of proton chemical shifts in all cases. 

The H-NMR spectra of pavines can provide appreciable assistance in structural elucidation. The oxygenation pattern of a pavine may be deduced from a careful examination of the methine (H j) and methylene (H. f) proton absorptions (Ic). In the case of 2,3,8,9 substitution, the abc and def protons furnish two superimposable ABX patterns. A doublet integrating for two protons at the lower field end of the system at approximately 8 4.0 represents the bridgehead protons, Ha and Hj. At 60 MHz, it appears as if these protons are coupled to only one of the neighboring protons (J = 6 Hz) (18,20,25). Furthermore, the geminal hydrogens couple to each other with a coupling constant of 17 Hz (18,20). On... 

Hand-shaken dispersions of phospholipids thus may be a more reasonable choice for comparison with membranes, and except in rare cases of highly unsaturated systems the high resolution NMR spectra of such dispersions do not exhibit high resolution proton absorption. Hence, for most membranes, NMR can indicate immobilization of fatty acid chains but not whether chains are immobilized by binding onto proteins or by tight packing into bilayers. Broad line NMR may be useful, especially when a phase transition is present. 

The 100 MHz H-NMR spectrum of polypropylene oxide is too complex to be analyzed accurately even when decoupled from methyl protons. It is reasonable to ascribe the complexity of the spectrum to the spin coupling between hydrogen atoms linked to contiguous carbon atoms in the main chain. If this interpretation is correct, the spectrum should be simplified by substituting the methine proton by a deuterium atom. In fact, the spectrum of polypropylene oxide-a-d was simpler than that of the undeuterated one. Methyl protons lie at higher field and methylene protons at lower field. The stereoregularity was analyzed on the basis of the spectrum of the methylene proton absorption other than that of the methyl proton absorption. 

Fig. 3.61 Proton magnetic resonance spectrum of pyrrole-2-carboxylic acid in DzO/NaOD solution reference DSS, and sweep width 500 Hz. Inset expansion of aromatic proton absorptions, sweep width 50 Hz.

This increase has two-fold implications. First, it implies that the fractured dendrimer is more flexible than the intact dendrimer and second, that the fractured dendrimer may expand due to an increase in positive charge at lower pH, a quantity that was described as essential for PEI mediated transfection. Indeed, when branched PEI was subjected to the same set of experiments, the same three-fold increase in viscosity was observed, lending support to the idea that PAMAM dendrimers act as proton sponges. Evidence of lysosomal buffering capability of PAMAM dendrimers was shown by Kukowska-Latallo et al. (1996) when they observed that the efficiency of G5-EDA dendrimers was enhanced by the addition of chloroquine, while the same molecule could not enhance transfection of G10-EDA dendrimers, which contain 40-fold more surface amine groups for proton absorption. 

The NMR spectrum of cularidine summarized in [73] (57) shows typical 12a-angular proton absorption at <5 4-40 and vicinal coupling... 

In the spectrum of the cancentrine model compound [171] one of the methoxyl group of protons absorbs at low frequency (5 3-38) compared with absorption for the other methoxyl protons at <5 3 85, 3-85, and 3 90. This low frequency signal was assigned to the 7-methoxyl protons. In the hydrogenated derivative the lowest frequency methoxyl proton absorption is at <5 3-65. The low frequency aromatic proton absorption ((56-59) in [171] was assigned to the 8-proton. Both low frequency shifts are attributed to the shielding effects of ring c. (127)... 

The spectrum of the undeuterated compound consists of a triplet for the a protons, a quintet for the (3 protons—assuming equal coupling for all protons—and a triplet for the y protons. For the deuterated compound, the a-proton absorption would be absent, the /3-proton absorption would appear, at modest resolution, as a slightly broadened triplet, and the y-proton absorption would be unaffected. Actually, at very high resolution, each peak of the /3-proton triplet would appear as a very closely spaced quintet (/H C C D 1 Hz) since 2 nl + l = 2X2xl + l = 5, where n is the number of D nuclei coupled to the /3 protons. 

Cycloalkanes and Saturated Hetero-cyclics The chemical shifts of the CH2 groups in monocyclic alkanes are given in Table 4.7. The striking feature here is the strong shift to the right of cyclopropane, analogous to the shift of its proton absorptions. 

Overlap of proton absorptions is common, but absolute coincidence of nonequivalent 13C peaks is quite rare with a high-resolution instrument. Now, select the most likely molecular formula(s) from Appendix A of Chapter 1 for comparison and determine the index of hydrogen deficiency for each. In addition to difficulties caused by unresolved or overlapping peaks, discrepancies may appear between the selected molecular formula(s) and the H and 13C counts because of the presence of elements of symmetry. But this information also contributes to an understanding of the molecular structure. 

The high field position of the aliphatic proton absorption in tabersonine and vindoline may be due to increased shielding by ring currents from the aromatic ring (7, 51b). 

The three alkaloids named in the title (XXXII, XXXIII, and XXXIV) are respectively the A -formyl, -acetyl, and -propionyl derivatives of aspidospermidine (Section II, E). Demethoxypalosine (XXXIV) has been isolated from Aspidosperma limae (40) and A. discolor (40a) and was characterized as an iVa-acyldihydroindole by its UV-spectrum (Table III) and IR-absorption at 5.89 p. A strong band in the IR-spectrum at 13.1 p indicated an unsubstituted benzene ring. The foregoing information was confirmed and the substance was shown to belong to the aspidospermine group by NMR- and mass spectrometry. In the NMR-spectrum (Table IV) the 17-proton absorption is found at 8.13 well downfield from the three-proton multiplet due to the other aromatic protons which is centered at 7.07 8. This shift is due to the proximity of the carbonyl group of the iVa-propionyl group. In the aliphatic part of the spectrum, absorptions which are characteristic of the... 

NMR-absorption at 8.72 8 (Na-H) and 3.78 8 (CO2C//3). NMR-spectro-scopy also showed that the ethylidene group of stemmadenine was still present. The absorption in the 4.0 8 region (which may be assigned to protons allylic to one double bond and next to nitrogen, or to protons allylic to two double bonds) showed two single proton absorptions. Position 3 (partial structure CCXIX) is therefore substituted and can only be linked to C-7, while C-16 may be linked to positions 15 or 21... 

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