Proton Spin-Lattice Relaxation Experiments

These protons then show same relaxation rates, providing the same value however, protons that are in different environments or which are far apart relax independently of one another [13]. Consequently, a partially miscible or immiscible polymer blend would demonstrate, respectively, either a partial averaging of the relaxation rates or no averaging at all. 

The extent of mixing of polymer blends on a nanometer scale can be determined by examining the spin-lattice relaxation performance of the H nuclei. This has been achieved previously in blends of cellulose/poly(vinyl alcohol) (PVOH) [14], wheat proteins/PVOH [15], starch/poly(caprolactone) (PCL) [16] and starch/crosslinked poly(acrylic acid) [17]. 

Recently, Yoshitake et al. [18] reported the miscibility of cellulose acetate (CA) with poly(acryloyl morpholine) (PACMO), in which CA materials with an acetyl degree of substitution (DS) of 1.80-2.95 were used. In these studies, Yoshitake and coworkers used solid-state C CP/MAS NMR measurements to estimate the scale of homogeneity for PACMO/CA miscible blends. In the following, the acetyl DS adopted for the CA component is 2.18 throughout, unless otherwise specified. 

A quantitative analysis of the mixing scale in the PACMO/CA (DS = 2.18) blends was carried out through measurements of proton spin-lattice relaxation times in the rotating frame (T ). values can be obtained by practically fitting the decaying carbon resonance intensity to the following single-exponential function  

An effective path length I of the spin diffusion in a time (T ) is given by the following equation [21]  

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Moistening the sample assists proton spin-lattice relaxation. Experiments in the authors laboratory gave a value of Ti(H) = 0.05 s for a sample of Pinus radiata wood containing 45% moisture this rose to 0.75 s when the sample was... 

It appears that purification of commercially available solvents is sometimes required for the complete elimination of impurity resonances. Occasionally, these impurities may be turned into advantage, as in the case of C2D2CI4 where the (known) C2DHCI4 content may be used as an internal standard for quantitation. Thus, removal of every impurity peak is not always essential for identification and quantitative analysis of stabilisers in PE. Determination of the concentration of additives in a polymer sample can also be accomplished by incorporation of an internal NMR standard to the dissolution prepared for analysis. The internal standard (preferably aromatic) should be stable at the temperature of the NMR experiment, and could be any high-boiling compound which does not generate conflicting NMR resonances, and for which the proton spin-lattice relaxation times are known. 1,3,5-Trichlorobenzene meets the requirements for an internal NMR standard [48]. The concentration should be comparable to that of the analytes to be determined. 

Regarding quantitation in the CP/MAS experiment, for peak areas to accurately represent the number of nuclei resonating, one of the conditions that must be met is that the time constant for cross polarization must be significantly less than the time constant for proton spin lattice relaxation in the rotating fi ame, Tch or Tnh TipH. Other factors affecting quantitation in CP/MAS have been discussed in several reviews (28-33). Since no analyses of the spin dynamics were performed in this study, the solid state spectra presented in this manuscript will be interpreted only semiquantitatively. 

The barrier to rotation of CH3 groups, V 5.0 kJ/mol, was first estimated from a correlation between barrier contributions of other groups and the minimum internuclear distance for various organic compounds (using d(Ge-C) = 1.98 A) [3]. Very low values based on H NMR relaxation measurements (2.7 kJ/mol) [34] and experimental and calculated entropies (3.1 kJ/ mol) [57] cannot be correct see also [90, p. 19]. Consistent values for solid Ge(CH3)4 have been obtained from far-infrared studies [52], neutron scattering [80,90,91], and proton Zeeman spin-lattice relaxation experiments [94] (barrier height V and torsional ground level Eq in... 

Hartmann and Hahn [39] showed that CP can be achieved when two rf fields Bin and B c = 4Bih are simultaneously applied. Jn/jc = 4, so, when B c = 4Bm energy is transferred between them, or they are cross-polarized, because Tc ic = Th ih (which is called the Hartmann-Hahn match of heteronuclear rotating-frame frequencies). In the CP experiment C nuclei obtain their spin polarization from H nuclei, so, not only do the shorter proton spin-lattice relaxation times determine the repetition rate of... 

Since the nuclei obtain their polarization from the spins, it is the proton spin-lattice relaxation time (T ) which determines the repetition rate of the CP experiment. This circumvents the problem of the long C values normally found in solids. In addition, the C signal shows an enhancement in its intensity, which can be as large as yn/yc = 4. The CP experiment results in both a time-saving and an improvement in the signal-to-noise ratio in the C NMR spectrscopy of solid samples. 

Early measurements of the proton spin-lattice relaxation revealed almost free rotation of the methyl groups [83]. However, the tunnelling bands observed in the 300 xeV range (see Figure 8.21) are quite below the frequency anticipated for almost free rotation ( 675 xeV). Moreover, since all methyl groups in the crystal experience the same effective potential the rather complex spectrum must be interpreted in terms of dynamical correlation between indistinguishable quantum rotors. 

Spin-lattice relaxation and transient nuclear Overhauser enhancement (tnOe) experiments with C- and H-labelled substrates have been used for the conformational analysis of several mono- and di-saccharides. An investigation of the molecular dynamics of polyciystalline mono- and di-saccharides involved measurements of the temperature dependencies of proton spin-lattice relaxation times and second moments of n.m.r. lines. ... 

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). 

The second separation method involves n.O.e. experiments in combination with non-selective relaxation-rate measurements. One example concerns the orientation of the anomeric hydroxyl group of molecule 2 in Me2SO solution. By measuring nonselective spin-lattice relaxation-rat s and n.0.e. values for OH-1, H-1, H-2, H-3, and H-4, and solving the system of Eq. 13, the various py values were calculated. Using these and the correlation time, t, obtained by C relaxation measurements, the various interproton distances were calculated. The distances between the ring protons of 2, as well as the computer-simulated values for the H-l,OH and H-2,OH distances was commensurate with a dihedral angle of 60 30° for the H-l-C-l-OH array, as had also been deduced by the deuterium-substitution method mentioned earlier. 

The "decrease of the spin temperature means an increase of population difference between the upper and lower energy spin states and consequently an increased sensitivity of the NMR experiment. From Equation (25), the temperature of dilute spins has been lowered by a factor 7x/y1 h, that is, V4 when X = 13C. This means an increased sensitivity of the FID resonance experiment equal to about 4 for the 13C nuclei. Because the X signal is created from the magnetization of dilute nuclei, the repetition time of NMR experiment depends on the spin-lattice relaxation time of the abundant spin species, protons, which is usually much shorter than the spin-lattice relaxation times of the dilute nuclei. This, a further advantage of cross polarization, delay between two scans can be very short, even in the order of few tens of milliseconds. 

The essence of the ASAP method is based on the different spin-lattice relaxation behaviour of these two types of protons. In the case of a HMBC experiment, the acceptor protons are those directly bound to 13C with large spin-spin couplings they are the spins that give rise to the final spectrum. By contrast, the donor protons have negligible couplings to 13C and are therefore essentially unaffected by this polarization transfer sequence, which simply returns them to the z axis. In their method, Kupce and Freeman have proposed to replace the usual relaxation delay with a short cross-polarization (HOHAHA) interval. This offers a... 

Fig. 1. Pulse sequences for determining spin-lattice relaxation time constants. Thin bars represent tt/2 pulses and thick bars represent tt pulses, (a) The inversion-recovery sequence, (b) the INEPT-enhanced inversion recovery, (c) a two-dimensional proton-detected INEPT-enhanced sequence and (d) the CREPE sequence. T is the waiting period between individual scans. In (b) and (c), A is set to (1 /4) Jm and A is set to (1 /4) Jm to maximize the intensity of IH heteronuclei and to (1/8) Jm to maximize the intensity of IH2 spins. The phase cycling in (c) is as follows 4>i = 8(j/),8(-j/) 3 = -y,y A = 2(x),2(-x) Acq = X, 2 —x), X, —X, 2(x), —x, —x, 2(x), —x, x, 2 —x),x. The one-dimensional version of the proton-detected experiment can be obtained by omitting the f delay. In sequence (d), the phase 4> is chosen as increments of 27r/16 in a series of 16 experiments.

C spin-lattice relaxation times of individual nuclei can also be measured by PFT 13C H experiments using a 90°, r, 90", r,... pulse train and noise modulation of the proton decoupling frequency. This method is known as progressive saturation [43] and is based on the following concept. 

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