Proton spin-lattice relaxation time direction

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. 

On-line SFE-NMR coupling was also reported [151,152], SFE provides some degree of separation by means of solubility and affinity to the matrix. This offers the possibility of transferring analytes directly from the extraction into the NMR probe. Drawbacks in the acquisition of SFE-NMR and SFC-NMR spectra are the elongated spin-lattice relaxation times 7) of protons and the pressure dependence of H NMR chemical shifts [153]. 

For carbons possessing a directly bonded proton, the carbon-13 spin lattice relaxation time,, spin-spin relaxation time,... 

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) ... 

Selected spin-lattice relaxation times are shown in Fig. 1. Carbon nuclei bonded to protons in typical complexes of the type reviewed here frequently have short spin-lattice relaxation times (7 ) and hence relatively large tilt angles can be used in routine FT measurements. Tx is considerably longer for carbon nuclei not directly bonded to protons, e.g., a value of 56 sec has been measured for the meso-allyl carbon atom in 6, which accounts, in part, for the low intensity of the signals of such nuclei under standard measurement conditions. 

For the partially deuterated benzoic acid (C6D5COOH), the solid state H NMR spectrum is dominated by the intra-dimer H- H dipole-dipole interaction. In a single crystal, both tautomers A and B are characterised by a well-defined interproton vector with respect to the direction of the magnetic field (Fig. 1). Proton motion modulates the H- H dipole-dipole interactions, which in turn affects the H NMR lineshape and the spin-lattice relaxation time. It has been shown that spin-lattice relaxation times are sensitive to the proton dynamics over the temperature range from 10 K to 300 K, and at low temperatures incoherent quantum tunnelling characterises the proton dynamics. A dipolar splitting of about 16 kHz is observed at 20 K. From the orientation dependence of the dipolar splitting, the... 

Si spin-lattice relaxation times for organosilicon compounds are generally greater than 20 s. Even in cases where there are directly attached protons which contribute to a very efficient DD mechanism in... 

In addition, the formal unit itself offers a new opportunity for monitoring chain motion relative to the polycarbonates since the carbonate unit contains no protons. The spin-lattice relaxation times, T s of all protons and all carbons with directly bonded protons are reported for the polyformal. Also the carbon and proton Tj s are measured at two different Larmor frequencies to expand the frequency range covered by the study. 

Relaxation parameters of interest for the study of polymers include 1) 13C and H spin-lattice relaxation times (T1C and T1H), 2) the spin-spin relaxation time T2, 3) the nuclear Overhauser enhancement (NOE), 4) the proton and carbon rotating-frame relaxation times (T p and T p), 5) the C-H cross-relaxation time TCH, and 6) the proton relaxation time in the dipolar state, T1D (2). Not all of these parameters provide information in a direct manner nonetheless, the inferred information is important in characterizing motional frequencies and amplitudes in solids. The measurement of data over a range of temperatures is fundamental to this characterization. 

The sensitivity of the - C NMR experiment is limited by the often very long spin-lattice relaxation times of the - C nuclei. Values of T of over 1000 s have been measured in solid poly(ethylene). " It is likely that - C nuclei involved in rigid cross-link structures also possess very long C T relaxation times. As a result pulse repetition times, and hence total scan times are often prohibitively long, especially when the aim is to observe peaks due to low concentrations of products of irradiation. The technique of cross-polarization reduces this problem since the spin temperature of the protons and not the - C nuclei has to reequilibrate before pulse repetition. The H Ti in proton-rich condensed systems is usually much shorter than T. In addition the signal-to-noise in the CPMAS spectrum is increased by a factor of up to four compared with spectra obtained by direct excitation of the spins, due to the larger Boltzmann population of the proton nuclei. 

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). 

The direct NMR method for determining translational difiFusion constants in liquid crystals was described previously. The indirect NMR methods involve measurements of spin-lattice relaxation times (Ti,Ti ),Tip) [7.45]. Prom their temperature and frequency dependences, it is hoped to gain information on the self-diflPusion. In favorable cases, where detailed theories of spin relaxation exist, difiFusion constants may be calculated. Such theories, in principle, can be developed [7.16] for translational difiFusion. However, until recently, only a relaxation theory of translational difiFusion in isotropic hquids or cubic solids was available [7.66-7.68]. This has been used to obtain the difiFusion correlation times in nematic and smectic phases [7.69-7.71]. Further, an average translational difiFusion constant may be estimated if the mean square displacement is known. However, accurate determination of the difiFusion correlation times is possible in liquid crystals provided that a proper theory of translational difiFusion is available for liquid crystals, and the contribution of this difiFusion to the overall relaxation rate is known. In practice, all of the other relaxation mechanisms must first be identified and their contributions subtracted from the observed spin relaxation rate so as to isolate the contribution from translational difiFusion. This often requires careful measurements of proton Ti over a very wide frequency range [7.72]. For spin - nuclei, dipolar interactions may be modulated by intramolecular (e.g., collective motion, reorientation) and/or intermolecular (e.g., self-diffusion) processes. Because the intramolecular (Ti ) and intermolecular... 

The spin-lattice relaxation times are listed as the relaxation time multiplied by the number of directly bonded protons. 

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