C Spin-lattice relaxation times

The technique for measurement which is most easily interpreted is the inversion-recovery method, in which the distribution of the nuclear spins among the energy levels is inverted by means of a suitable 180° radiofrequency pulse A negative signal is observed at first, which becomes increasingly positive with time (and hence also with increasing spin-lattice relaxation) and which 

in the series of Ti measurements of 2-octanol (42, Fig. 2.27) for the methyl group at the hydrophobic end of the molecule, the signal intensity passes through zero at Tq = 3.8 s. From this, using equation 10, a spin-lattice relaxation time of Ti = 5.5 s can be calculated. A complete relaxation of this methyl C atom requires about five times longer (more than 30 s) than is shown in the last experiment of the series (Fig. 2.27) Tj itself is the time constant for an exponential increase, in other words, after T/ the difference between the observed signal intensity and its final value is still 1/e of the final amplitude. 

The main contribution to the spin-lattice relaxation of C nuclei which are connected to hydrogen is provided by the dipole-dipole interaction (DD mechanism, dipolar relaxation). For such C nuclei a nuclear Overhauser enhancement of almost 2 will be observed during H broadband decoupling according to  

Accordingly, the relaxation time of a C atom will increase the fewer hydrogen atoms it bonds to and the faster the motion of the molecule or molecular fragment in which it is located. From this, it can be deduced that the spin-lattice relaxation time of C nuclei provides information concerning four molecular characteristics  

From Cg/ZgNO (problem 4), for example, the empirical formula C Hjo is derived and compared with the alkane formula Cc H2o, a hydrogen deficit of ten and thus of five double-bond equivalents is deduced. If the NMR spectra have too few signals in the shift range appropriate for multiple bonds, then the double-bond equivalents indicate rings (see, for example, a-pinene. Fig. 2.4). 

If smaller NOE enhancements are recorded for certain C nuclei, then other mechanisms (e.g. spin-rotation) contribute to their spin-lattice relaxation 

---

Figure 2.27. Sequence of measurements to determine the C spin-lattice relaxation times of 2-octanol (42) [(CD3)2C0, 75% v/v, 25 °C, 20 MHz, inversion-recovery sequence, stacked plot]. The times at which the signals pass through zero, xo, have been used to calculate, by equation 10, the T values shown above for the nuclei of 2-octanol...

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). 

Table IX. C Spin Lattice Relaxation Times for Neat 1,2,3-Decanetrioia.

The most mobile groups are those with nuclei having (i) the sharpest H resonances (smallest linewidths), or (ii) the longest ( H-decoupled) C spin-lattice relaxation times. 

We have also measured C spin-lattice relaxation times (T ) in the mixed powder in room-temperature MAS experiments, using the saturation-recovery technique. T[ for C ) is 28 5 s, while... 

Fig. 4. Temperature dependence rfthe C spin-lattice relaxation time Tj in KjCao, plotted as T]T versus T. Error bars are standard deviations estimated from five Tj measurements at 298 K.

Fig. 9.17. Plots of C spin-lattice relaxation times Ti against inverse absolute-temperature for C-labeled polyethylene adsorbed on the surface of silica gel [23].

Table 17.6. Temperature dependence of and C spin-lattice relaxation times (Ti/s) of...

Table 24.1. C spin-lattice relaxation times of starch gel (33%) by DD- and CP-MAS NMR method (s, from Ref. [19])...

C Spin lattice relaxation times, Tj, spin-spin relaxation times, T2, and nuclear Overhauser enhancements, NOE, for the a-carbons of PBLG of various molecular weights have been used to study transitions from rigid to flexible forms of this polymer (Allerhand and Oldfield, 1973). Effective rotational correlation times, reff, calculated from 7 - and NOE-values, for the a-carbons were 24-32 nanoseconds for the helical form and approximately 0-8 nanoseconds for the random coil (Allerhand and Oldfield, 1973). The transition from the a-helix to the random-coil of PLM causes the resonances of the a- and carbonyl carbons to move upfield 2-3 and 3-4 ppm respectively (Tadokoro et al., 1973), which is consistent with results obtained for PBLG and PCBO. Further work is required before the reasons for the chemical shift differences between the corresponding carbons in the helical and random-coil forms in deuterochloroform-TFA systems can be elucidated. Plots of chemical shifts and relaxation times vs. pH have been used to study the helix-coil transition of poly-L-lysine hydrochloride in aqueous solution (Saito and Smith,... 

Komoroski, R. A., Peat, I. R., and Levy, G. C. (1975). Biochem. Biophys. Res. Comm. 65, 272. High Field Carbon-13 nmr Spectroscopy. Conformational Mobility in Gramicidin S and Frequency Dependence of 1 3 C Spin-Lattice Relaxation Times. 

Papers on the structure of bleomycin and its zinc complex, the C spin-lattice relaxation times for bleomycin-A2 at 67.89 and the Fe -... 

All NMR experiments were performed on a Varian XL-200 spectrometer at 50.31 MHZ. Relevant instrument settings include 90 degree pulse angle, 1.0 second acquisition time, 0.5 second pulse delay, 238.5 ppm spectral width, and broad band proton decoupling. About 40,000 transients were collected for each spectrum. Temperature was maintained at 40 C. Spin-lattice relaxation time (Tl) and Nuclear Overhauser Enhancement (NOE) values for all C-13 NMR resonances were carefully measured to determine the optimum NMR experimental conditions. The spectral intensity data thus obtained were assured of having quantitative validity. 

Figure 2 shows the C spin-lattice relaxation times (Tx) of Individual proton bearing carbons In methionine-enkephalin at a concentration of 100 mg/ml In D2O. The Ti values multiplied by N, the number of protons directly bonded to the carbon under stud are In good agreement with those reported by Combrisson et al. (1976). 

Table 5. C spin-lattice relaxation times, (s), and spin-spin relaxation times under the proton decoupling and magic angle spinning, T-f (ms), of [2- C] Ala-labeled bacteriorhodopsin ... 

---