Quaternary carbons relaxation

Table 4. Characteristic temperatures and molecular weights evaluated from H and (of quaternary carbons) relaxation data of linear polymers [33, 35, 139, 140]. PE, polyethylene PIP, polyisoprene PIB, polyisobutylene PS, atactic polystyrene PTHF, polytetrahydrofuran PDMS, polydimethylsiloxane. Literature data for critical molecular weights determined with rheology are also listed for comparison and completeness...

Carlomagno, T., Sanchez, V. M., Blommers, M. J. )., Griesinger, C. Derivation of dihedral angles from CH-CH dipolar-dipolar cross-correlated relaxation rates a C-C torsion involving a quaternary carbon atom in epothilone A bound to tubulin. Angew. Chem. Int. Ed. 2003, 42, 2515-2517. 

Obviously we cannot however simply correlate the signal intensities with the presence of attached protons. So relaxation must also play a very important role. Relaxation times T, for carbon atoms also depend on whether these are protonated or not, and while T, for methyl or methylene groups may only be a few seconds, it may be as long as around 2 min for quaternary carbons Now the choice of an ideal relaxation delay becomes impossible, and so we have to make compromizes, which result in the large variations in signal intensity. 

In Problem 49 we recorded the carbon-13 spectrum using a relaxation delay of 25 sec with the shorter delays we tend to use routinely the signals due to the quaternary carbons would have been almost invisible ... 

As mentioned earlier, the irradiation of directly bonded hydrogen atoms increases the intensity of C signals through the NOE, which means that CH, CH2 and CH3 groups usually have a relatively high intensity in a NMR spectrum, while we can often recognize quaternary carbon atoms because of their low intensity. This difference in intensity is due to the different relaxation rates, and is the reason we do not integrate C NMR spectra. 

Sensitivity enhancement is applied to recognise weak signals of slow relaxing, quaternary carbons in noisy C spectra, or to measure the weak H signals of low concentration impurities. 

Quaternary carbon nuclei e.g. CO carbons in metal carbonyls) display particularly large spin-lattice relaxation times, so that relaxation reagents such as chromium acetyl-ace tonate must be added to the sample solution in order to obtain sufficient signal noise within reasonable accumulation times. 

The information obtained from the model studies was then applied to the spectra of sinomenine (38), codeine (39), and thebaine (40), the spectra of which are also recorded in Table VII. Except for the assignment of the quaternary carbon atoms in 39 and 40 the other assignments followed readily from model studies and from application of shift parameters. The quaternary carbon atoms of codeine had already been examined by Wehrli (55) using spin lattice relaxation time (7 ) measurements. In this way an unambiguous assignment of the signals at C-3, C-4, C-ll, and C-12 was achieved. 

To understand how the CP/MAS experiment can be applied to quantitative analysis, the magnetization transfer and relaxation processes, which directly affect the signal intensity, must be considered. Cross polarization is mediated by H-13C dipolar interactions, so the magnetization buildup will occur at different rates for different types of carbons. In general, the cross polarization rate (Tcp1) increases with the degree of protonation.21 22 That is, TcP is generally shorter for methylene and methine carbons than for quaternary carbons, Fig. 12.6. Motional modulation... 

In addition to these solvent peaks, we can count 26 peaks in the spectrum. Because there are 27 carbons in the cholesterol molecule (three are lost in the biosythesis from a triterpene precursor), there must be one peak that accounts for two carbons. The tallest peak (labeled 1, m ) in fact corresponds to two different carbons with nearly identical chemical shifts. The most downfield peaks ( aa and z ) are in the olefin/aromatic region of the 13 C spectrum (120-140 ppm), so they must correspond to C-5 andC-6. Peak aa is less intense ( shorter ) than all of the other peaks because of slow relaxation it must be a quaternary carbon. We will see that the proximity of protons is the primary means of relaxation of 13 C nuclei, so carbons lacking a proton relax much more slowly and give less intense peaks, especially if... 

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