Carbon relaxation time

Numbers are methyl carbon relaxation times in seconds. 

FIG. 2. Carbon relaxation times (s) for 1 m cholesteryl chloride in carbon tetrachloride. (31)... 

When a molecule is rigid and rotates equally well in any direction (isotropically), all the carbon relaxation times (after correction for the number of attached protons) should be nearly the same. The nonspherical shape of a molecule, however, frequently leads to preferential rotation in solution around one or more axes (anisotropic rotation). For example, toluene prefers to rotate around the long axis that includes the methyl, ipso, and para carbons, so that less mass is in motion. As a result, on average, these carbons (and their attached protons) move less in solution than do the ortho and meta carbons, because atoms on the axis of rotation remain stationary during rotation. The more rapidly moving ortho and meta carbons... 

Solid state NMR was used to recognize guest-free TNP matrix and ICs with guests, e.g. benzene, tetrahydrofuran and p-xylene. Carbon relaxation times were used to study the motion mechanism of the guests. P CP/MAS spectra were particularly informative about the symmetry of the phosphazene ring. Figure 20 presents C MAS spectra of TNP ICs with benzene and THF guests in the matrix. 

Although much work remains to be done to fully assess the value of the various carbon relaxation times in understanding the mechanical properties of polymers, the initial results appear promising even if ambiguous in some instances. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

The spin-lattice relaxation time, T/, is the time constant for spin-lattice relaxation which is specific for every nuclear spin. In FT NMR spectroscopy the spin-lattice relaxation must keep pace with the exciting pulses. If the sequence of pulses is too rapid, e.g. faster than BT/max of the slowest C atom of a moleeule in carbon-13 resonance, a decrease in signal intensity is observed for the slow C atom due to the spin-lattice relaxation getting out of step. For this reason, quaternary C atoms can be recognised in carbon-13 NMR spectra by their weak signals. 

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Turning from chemical exchange to nuclear relaxation time measurements, the field of NMR offers many good examples of chemical information from T, measurements. Recall from Fig. 4-7 that Ti is reciprocally related to Tc, the correlation time, for high-frequency relaxation modes. For small- to medium-size molecules in the liquid phase, T, lies to the left side of the minimum in Fig. 4-7. A larger value of T, is, therefore, associated with a smaller Tc, hence, with a more rapid rate of molecular motion. It is possible to measure Ti for individual carbon atoms in a molecule, and such results provide detailed information on the local motion of atoms or groups of atoms. Levy and Nelson " have reviewed these observations. A few examples are shown here. T, values (in seconds) are noted for individual carbon atoms. 

It is evident from Table 2 that the chemical shift data are very similar in both states of aggregation. Only the carbonyl carbon show a small but definite shifts, 2 ppm. In the solution state, in acetone -d6 solution the relaxation times T1 of the pyranose carbon atoms are very similar and only slightly smaller than those of the carbon atom of the methyl group in the acetyl substituent, while the T1-value of the carbon atom of the carbonyl group is considerably higher. 

This conclusion is supported by the experimental result " given by the pulsed-NMR measurement that the spin-spin relaxation time T2 is considerably shorter for the gel than that for the matrix mbber vulcanizate, which of course, indicates that the modulus is considerably higher for the gel than for the matrix mbber. More quantitatively, Maebayashi et al. measured the acoustic velocity of carbon gel by acoustic analysis and concluded that the compression modulus of the gel is about twice that of matrix mbber. Thus, at present, we can conclude that the SH layer, of course without cross-linking, is about two times harder than matrix cross-linked mbber in the filled system. 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

Given the specific, internuclear dipole-dipole contribution terms, p,y, or the cross-relaxation terms, determined by the methods just described, internuclear distances, r , can be calculated according to Eq. 30, assuming isotropic motion in the extreme narrowing region. The values for T<.(y) can be readily estimated from carbon-13 or deuterium spin-lattice relaxation-times. For most organic molecules in solution, carbon-13 / , values conveniently provide the motional information necessary, and, hence, the type of relaxation model to be used, for a pertinent description of molecular reorientations. A prerequisite to this treatment is the assumption that interproton vectors and C- H vectors are characterized by the same rotational correlation-time. For rotational isotropic motion, internuclear distances can be compared according to... 

In the spectrum of fully reductively [ C] methylated glycophorin A, the resonance at 42.8 p.p.m. must correspond to the N, N -di[ C]methylated, N-terminal amino acid residue. The ratio of the integrated intensities of the N, N -di[ C]methylLeu resonance to the N, N -di[ C]methyllysine resonances is 5 1, as expected. The integration values determined were valid, because the recycle times of spectra in Figs. 3B, 3C, and 3D were twice the spin-lattice relaxation-times (Tj values) of those of the di[ C]methyl carbon atoms, and also because the n.O.e. values of the N, N -di[ C]methyl and N, N -di[ C]methyl carbon atoms were equivalent. ... 

The errors in the APT spectra may arise due to (a) variation in the relaxation times of different carbons, (b) wide variation in / h values, and (c) modulations caused by long range C- H couplings. A procedure known as ESCORT (Error Self Compensation Research by Tao Scrambling) has been developed to yield cleaner subspectra with reduced / cross-talk (Madsen et ai, 1986). This involves replacing the normal APT spectral... 

The variation in the Tj relaxation times of the individual carbons can be minimized by keeping the time between excitation and detection constant (Radeglia and Porzel, 1984). 

The low intensities of nonprotonated carbons is usually due to their long relaxation times. The addition of a paramagnetic substance such as... 

Direct H—C dipole-dipole coupling dominating 13C relaxation (for protonated carbons) information about molecular shapes and motion are contained in the experimental spin-lattice relaxation times. 

KINETIC RESULTS FOR DCP AND TCH. The portion of the 50.13 MHz 13C NMR spectra containing the methylene and methine carbon resonances of DCP and the resultant products of its (n-Bu)3SnH reduction are presented in Figure 2 at several degrees of reduction. Comparison of the intensities of resonances possessing similar T, relaxation times (see above) permits a quantitative accounting of the amounts of each species (D,M,P) present at any degree of reduction. 

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