Measurement of 13C Relaxation Times

The stacked series of selectively proton-decoupled 13C NMR spectra with varying decoupling frequency is conveniently recorded fully computer-controlled, as can be seen for nicotine in Fig. 2.25 (b). The series clearly shows that 13C signals, e.g. those of C-2, C-6, C-4, C-5 and C-4, are unequivocally assigned if the protons attached to these carbons do not overlap with others in the H NMR spectrum. For overlapping proton resonances (e.g. the pairs 2 -H, 5 -H and 3 -H, CH3) more than one carbon is affected by decoupling, of course, and other assignment aids such as two-dimensional CH correlation (Section 2.10) have to be taken into account (e.g. for the pairs C-2, C-5 and C-3, CH3 of nicotine in Fig. 2.25 (b)). 

The offsets necessary for selective proton decoupling do not have to be measured, provided the decoupling frequency of TMS protons is known and the proton shifts of the compound are available from the literature. Tn this case, the decoupling frequency offsets are calculated from the proton shifts. This is performed for the protons of 6-methoxy-a-tetralone. The result of decoupling experiments with these values is shown in F ig. 2.25 (c). A complete assignment of all protonated carbons is achieved. 

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A bound divalent metal ion, usually Mn2+, is required in the transcarboxylation step. A possible function is to assist in enolization of the carboxyl acceptor. However, measurement of the effect of the bound Mn2+ on 13C relaxation times in the substrate for pyruvate carboxylase indicated a distance of 0.7 ran between the carbonyl carbon and the Mn2+, too great for direct coordination of the metal to the carbonyl oxygen.68 Another possibility is that the metal binds to the carbonyl of biotin as indicated in Eq. 14-11. Pyruvate carboxylase utilizes two divalent metal ions and at least one monovalent cation.683... 

Fig. 2.29. Measurement of 13C spin-lattice relaxation times by saturation-recovery ...

Using single-frequency and noise-modulated resonance and off-resonance proton decoupling, 7] relaxation time measurements, relaxation reagents like Gd (fod)3 and specifically deuterated compounds, all the carbons in retinal isomers, the model compounds a-and /i-ionone, and vitamin A and its isomers [165, 555-557] were assigned. The olefinic ring carbons (C-5 and C-6) could be identified on the assumption that the 13C relaxation times are dominated by intramolecular dipole-dipole interactions with neighboring protons and that the same rotational correlation time characterizes the interactions for both carbons. Consequently the ratio of T/s for C-5 and C-6 can be estimated from eq. (5.1)... 

Through measurements of 13C spin-lattice relaxation times, a number of noteworthy motional characteristics related to overall molecular tumbling, hydrogen... 

Besides measurements of proton relaxation in crowded 3H NMR spectra indirectly through measurements of the much less crowded 13C or 29Si spectra356, INEPT can be used to speed up the direct measurements of 29Si spin-lattice relaxation357. The pulse sequence used for INEPT-enhanced measurement of spin-lattice relaxation times is just a minor modification of decoupled refocused INEPT (Section V.B) ... 

The surfactant properties of alkali metal sulphonates lead to interesting motional properties in solution that can be studied by H and 13C relaxation times. Franses and Miller67 observed segmental motion and phase transitions by 13C NMR in chloro-form/water solutions when studying the surfactant SHBS 29. For the lamellar liquid-crystalline phase formed by SHBS and water, and linewidth measurements indicated a pronounced motional gradient, resulting from anisotropic motion, as the head group is approached. 

The 50.31 MHz 13C NMR spectra of the chlorinated alkanes were recorded on a Varian XL-200 NMR spectrometer. The temperature for all measurements was 50 ° C. It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the NMR data, carbon-13 Tj data were recorded for all materials using the standard 1800 - r -90 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu)3SnH, (n-Bu)3SnCl, DCP, TCH, pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of DCP and TCH in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4-chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the NMR analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 Tj. The only exception to this is heptane where the shortest T[ is 12.3 s (delay = 2.5 ). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the TCH reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical... 

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