Nuclear magnetic resonance spectroscopy precession

Fig. 1. Nuclear magnetic resonance spectroscopy. Precession of the axis of the magnetic moment of a nucleus aligned with (a) and against (b) the magnetic field of an external magnet whose poles are indicated by N and S.

Nuclear magnetic resonance spectroscopy first aroused the chemist s Interest when the discovery was made that the exact nuclear precession frequency is dependent upon the chemical environment of the nucleus. The displacement of the resonance frequency relative to an arbitrary standard is commonly referred to as chemical shift. Without this property, NMR would be without practical utility to the chemist as an analytical tool and it would probably long be extinct. 

In c.w.-n.m.r. spectroscopy, a relatively weak, but rapidly oscillating, magnetic field is produced on the x axis by the application of a continuous, low-powered radiofrequency (r.f.) to the transmitter coil(s). As this radiofrequency approaches the resonance frequency, the magnetization vector is very slightly tipped out of the z axis, and precesses about this axis. When this frequency of precession is matched by the r.f. applied (the resonance condition), some of the individual, nuclear moments undergo transitions to the less-stable energy-level represented by precession about the — z direction, accompanied by absorption of energy from the transmitter coil. 

Electron spin echo spectroscopy (ESE) monitors the spontaneous generation of microwave energy as a function of the timing of a specific excitation scheme, i.e. two or more short resonant microwave pulses. This is illustrated in Fig. 7. In a typical two-pulse excitation, the initial n/2 pulse places the spin system in a coherent state. Subsequently, the spin packets, each characterized by their own Larmor precession frequency m, start to dephase. A second rx-pulse at time r effectively reverses the time evolution of the spin packet magnetizations, i.e. the spin packets start to rephase, and an emission of microwave energy (the primary echo) occurs at time 2r. The echo ampHtude, as a fvmction of r, constitutes the ESE spectrum and relaxation processes lead to an irreversible loss of phase correlation. The characteristic time for the ampHtude decay is called the phase memory time T. This decay is often accompanied by a modulation of the echo amplitude, which is due to weak electron-nuclear hyperfine interactions. The analysis of the modulation frequencies and ampHtudes forms the basis of the electron spin echo envelope modulation spectroscopy (ESEEM). 

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