Relaxation time, spin-lattice transverse

After B, is turned off, nuclei can change their nuclear spin orientations through two types of relaxation processes. Spin-lattice (longitudinal) relaxation (governed by relaxation time 7, ) involves the return of the nuclei to a Boltzmann distribution. Spin-spin (transverse) relaxation (governed by relaxation time 72 or 7 ) involves the dephasing of the bundled nuclear spins. Normally 7 < T2 < 7,. 

Ti = spin-lattice relaxation times T2 = spin-spin relaxation time = quadrupolar relaxation time 7"sc transverse scalar relaxation time d = chemical shift y = gyromagnetic ratio a = shielding constant... 

The term relaxometry is normally used in context with techniques for the measurement of spin-lattice relaxation times in general. Transverse relaxation and effects due to residual dipolar couplings will be considered in the next section. 

The relaxation rates of the individual nuclei can be either measured or estimated by comparison with other related molecules. If a molecule has a very slow-relaxing proton, then it may be convenient not to adjust the delay time with reference to that proton and to tolerate the resulting inaccuracy in its intensity but adjust it according to the average relaxation rates of the other protons. In 2D spectra, where 90 pulses are often used, the delay between pulses is typically adjusted to 3T] or 4Ti (where T] is the spin-lattice relaxation time) to ensure no residual transverse magnetization from the previous pulse that could yield artifact signals. In ID proton NMR spectra, on the other hand, the tip angle 0 is usually kept at 30°-40°. 

Longitudinal and transverse relaxations have been assumed by Bloch et al. [6] to be first-order rate processes. Following this assumption, the increase of Mz to M0 and the decay of Mx and My to zero may be expressed in terms of spin-lattice and spin-spin relaxation times, T, and T2 ... 

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