Free induction decay precession

Figure 4-9. (Ai Precessing moment vectors in field tfo creating steady-state magnetization vector Afo. with//i = 0. (B) Immediately following application of a 90° pulse along the x axis in the rotating frame. (C) Free induction decay of the induced magnetization showing relaxation back to the configuration in A.

Free induction decay A decay time-domain beat pattern obtained when the nuclear spin system is subjected to a radiofrequency pulse and then allowed to precess in the absence of Rf fields. 

In pulsed NMR, the magnetic field is turned on for the time necessary to rotate the magnetization vector into a plane called the 90° rotation or 90° pulse. The field is turned off and the magnetization vector rotates at a nuclear precession frequency relative to the coil. This induces an NMR signal that decays with time as the system returns to equilibrium. This signal is called the free induction decay (FID). 

Obviously, it is difficult to find a schematic representation for a compound absorbing 10 different frequencies. In such a case, M0 can be dissociated into many vectors, each of which precesses around the field with its own frequency (Fig. 9.7 shows a simplified situation). As the system returns to equilibrium, which can take several seconds, the instrument records a complex signal due to the combination of the different frequencies present, and the intensity of the signal decays exponentially with time (Fig. 9.9). This damped interferogram, called free induction decay (FID), contains at each instant information on the frequencies of the nuclei that have attained resonance. Using Fourier transform, this signal can be transformed from the time domain into the frequency domain to give the classical spectrum. 

Free induction decay (FID) An oscillating voltage recorded as the magnetization vector precesses (rotates) in the laboratory frame. 

The ESE intensity does not depend on t if the precession frequency of each electron spin does not fluctuate during the time interval 2t. However it does more or less fluctuate under the influence of surrounding spins etc., the precession of the spins looses coherence. The refocussing becomes more and more imperfect and the ESE intensity becomes weaker and weaker as the time elapses after the 90° pulse. This behavior of the spin magnetization is called phase relaxation. Both the free induction decay and the phase relaxation are called transverse relaxation. 

Free Induction Decay Static distribution of precession frequency No... 

The transverse relaxation in solids is generally much faster than the longitudinal relaxation, because the transverse relaxation is very sensitive to the fluctuation of the precession frequency. The phase relaxation is often used synonymously for the transverse relaxation which, strictly speaking, includes both the free induction decay and the transverse relaxation of one spin packet. We shall use the phase relaxation exclusively to express the transverse relaxation of one spin packet. 

Fourier transform NMR spectroscopy. The FT NMR spectrometer delivers a radio-frequency pulse close to the resonance frequency of the nuclei. Each nucleus precesses at its own resonance frequency, generating a free induction decay (FID). Many of these transient FIDs are accumulated and averaged in a short period of time. A computer does a Fourier transform (FT) on the averaged FID, producing the spectrum recorded on the printer. 

Modern Fourier transform (FT) NMR spectroscopy excites all the nuclei of interest at once with a radiofrequency (rf) pulse and detects the alternating current produced by the precessing spins in a coil surrounding the sample. This signal, or free induction decay (FID), is collected in the time domain and then processed to the frequency domain to generate the NMR spectrum. 

The relaxation forces present in the system reduce the resultant moment in the x y plane as nuclei precess faster or slower than the mean Larmor frequency. This gives rise to the free induction decay. The process of dephasing of nuclear spin moments in the x y plane can be... 

---