T2 time

The next step after apodization of the t time-domain data is Fourier transformation and phase correction. As a result of the Fourier transformations of the t2 time domain, a number of different spectra are generated. Each spectrum corresponds to the behavior of the nuclear spins during the corresponding evolution period, with one spectrum resulting from each t value. A set of spectra is thus obtained, with the rows of the matrix now containing Areal and A imaginary data points. These real and imagi-... 

MHz relative to the protons in Mc4Si at exactly 100 MHz. The effect of quadrupole line broadening is attenuated to some extent by the magnitude of / (/= 9/2), and the line widths are among the narrowest of all quadrupolar nuclei studied to date. One would expect efficient relaxation via the quadrupole moment, giving very short T2 times and consequently broad NMR signals. Also if the electronic distribution around the nucleus is symmetrical, sharp resonances can be obtained. 

Figure 22 Effect of emulsifiers (E) and hydrocolloids (H) on properties of bound water in ice cream mix (T2 time and percentage of hydrogen atoms with low T2).

Rearranging Equation 7-131 and integrating from tx to t2 time passes from 0 to 0 gives ... 

The term 2D NMR, which stands for two-dimensional NMR, is something of a misnomer. All the NMR spectra we have discussed so far in this book are two dimensional in the sense that they are plots of signal intensity versus frequency (or its Fourier equivalent, signal intensity versus time). By contrast, 2D NMR refers to spectroscopic data that are collected as a function of two time scales, tx (evolution and mixing) and t2 (detection). The resulting data set is then subjected to separate Fourier transformations of each time domain to give a frequency-domain 2D NMR spectrum of signal intensity versus two frequencies, Fx (the Fourier transform of the t time domain) and F2 (the Fourier transform of the t2 time domain). Thus, a 2D NMR spectrum is actually a three-dimensional data set ... 

As with Ti relaxation, T2 relaxation has a strong dependence upon the molecular correlation time. Unlike U relaxation, T2 relaxation does not reach a minimum and then increase, but continues to decrease, as shown in Fig. 3. Therefore large, slowly tumbling molecules have very short T2 times. This poses a great challenge in the study of large molecules or molecules in the solid state since the lifetime of the signal is very short and the linewidths are very broad. 

Above we quantify the observed decoherence with the / -parameter. Here we comment on the physical grounds for the decoherence. A well known parameter for describing the decay of the transverse spin components Jy and Jz is the T2-time defined by... 

A typical study is one on butadiene rubber, treated with varying amplitudes of ultrasound.51 Here lH and 13C T2 measurements were made, the questions asked being, what happens to the structure of the material at a molecular level as a result of treatment with ultrasound. lH T2 times are known to be dependent on interchain dynamics and, in particular, are... 

Fig. 42. The form of the DQ-DQ correlation experiment.68 DQ coherence is excited and reconverted using a /-encoding pulse sequence such as BABA. In the experiment, conducted under rapid MAS, t is set equal to t2 and the reorientation of a homonuclear dipolar tensor is monitored through the DQ rotor-encoded spinning sidebands that emerge from Fourier transforming in the t — t — t2 time domain.

Fig. 43. Calculated sideband patterns (corresponding to the Fourier transform of the t — ti — t2 time domain data from the pulse scheme in Fig. 42) from a DQ-DQ exchange experiment, where the sideband pattern has been divided by a similar pattern obtained from the same experiment with a very short mixing time.68 This ensures that the sidebands purely reflect the mobile sites in the sample. The patterns are calculated assuming a two-site exchange through angle Ady. (a) Z)yTex = 1. (b) DijT ex— 1.5, where rex is the DQ excitation time and Dy is the dipole coupling constant between spins i and j.

Figure 23 The relaxation process associated with the T2 time constant. The external magnetic field Bo, which is not exphcitly shown, has the same direction as the z axis, a) an r.f. pulse flips the magnetization vector M by an angle a away from the equilibrium position b) once the pulse turned off the magnetization freely precess around the z axis, c) the same situation as in the previous figure but viewed as a projection in the x-y plane d) not...

The same is true if we consider the time required for the FID signal to vanish (Figure 7d,g) when we would again obtain the T2 rather than the T2 time constant. 

Scorch time (t2) time for two units rise above Li (minutes) Characterises processing safety... 

In conclusion, in this section we presented the formal expressions for the absorption lineshape [Eq. (70)] and for spontaneous Raman and fluorescence spectroscopy. For the latter, we derived Liouville space expressions in the time and the frequency domain [Eqs. (74) and (75)], an ordinary correlation function expression [Eq. (76)], and, finally, the factorization approximation resulted in Eqs. (77) and (78). The factorization approximation is expected to hold in many cases for steady-state experiments and for time-resolved experiments with low temporal resolution. It is possible to observe a time-dependent shift of spontaneous emission lineshapes using picosecond excitation and detection [66-68]. This shift arises from the reorganization process of the solvent and also from vibrational relaxation that occurs during the t2 time interval. A proper treatment of these effects requires going beyond the... 

Both the spin-lattice relaxation time Tj and the spin-spin relaxation time T2 vary among different types of tissue, and Ti is always larger than T2. In addition, the Ti relaxation time depends on the field strength Bo, whereas the T2 time is independent. The signal amplitude depends on the timing of the experiment and the relaxation times Ti and T2( ). It can also be influenced by endogenous contrast mechanisms such as diffusion or blood-oxygenation. 

Figure 11 Intensity ratio for two resonances with different T2 times (72 and 72°) with respect to the tps/T2 and tps/72° ratios (cf. Equation (15)). The delay tps is the time in the pulse sequence where the T2 relaxation takes place in a 2D pulse experiment.

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