Precession nuclear, Larmor

Figure 1.1. Nuclear precession nuclear charge and nuclear spin give rise to a magnetic moment of nuclei such as protons and carbon-13. The vector n of the magnetic moment precesses in a static magnetic field with the Larmor frequency vo about the direction of the magnetic flux density vector Bo...

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

As shown in Fig. 2.1 (b), the nuclear moments still precess with Larmor frequency v0 about the z axis in the xy plane, as does the resultant transverse magnetization (Figs. 2.1(b) and 2.2(b)). In the rotating frame (Section 1.7.3), the transverse magnetization with reference frequency v0 stands while faster or slower components with v( > v0 or v, < v0 will rotate clockwise or counterclockwise, respectively, as shown in Fig. 2.3. 

Plots of J o>) versus oj for different values of Tc are shown in Fig. 8.1. The values of rc should be compared with the reciprocal of the nuclear Larmor frequency w0. For either a very short or very long rc the value of J 0 is relatively small. J(to) reaches its maximum when rc = l/o)0, that is, when the average molecular tumbling frequency is equal to the nuclear precession... 

Whether such interactions occur or not depends on the relative magnitude of two characteristic lifetimes. These are the relaxation time of the electronic spins (tr) and the period of the nuclear Larmor precession (tl). tr may be regarded as the mean time between successive changes in electronic spin orientation. It is generally composed of both spin-lattice and spin-spin contributions of which the former are temperature-dependent and the latter are not. tl is related to the internal magnetic field by... 

Angular frequency, nuclear Larmor precession frequency (in radian s )... 

For F2 = COL, i-C". on resonance, the field Bj (r) rotates around the z-axis coherently with the nuclear Larmor precession described by Equation (2.2.1), whereas B+ (r) rotates in the opposite sense. In a coordinate frame rotating around the z-axis with frequency SI = -f2k (named rotating frame), the field Bj"(f) is stationary, as well as the nuclear spins, while B+ (t) rotates with twice the Larmor frequency. Therefore, only B]" (t) will have an effective influence on the nuclear spins, provided that both fields have magnitude much smaller than that of the static field Bo... 

Larmor frequency The nuclear precession frequency about the direction of Bo. Its magnitude is given by yBo/27T. 

Precession A characteristic rotation of the nuclear magnetic moments about the axis of the applied magnetic field Bo at the Larmor frequencies. Preparation period The first segment of the pulse sequence, consisting of an equilibration delay. It is followed by one or more pulses applied at the beginning of the subsequent evolution period. 

Larmor frequency The exact frequency at which nuclear magnetic resonance occurs. At this frequency, the exciting frequency matches that of the precession of the axis of the spin of the nucleus about the applied magnetic field. 

NUCLEAR MAGNETIC RESONANCE LARMOR PRECESSION LIGAND BINDING ANALYSIS LINE-SHAPE ANALYSIS LOW-BARRIER HYDROGEN BONDS ROLE IN CATALYSIS MAGNESIUM ION (INTRACELLULAR) MAGNETIZATION TRANSFER Nuclear pores,... 

---