Magnetic of nucleus

Fig. 2. Interaction of nucleus (electron) with static magnetic field, Bq, where the bulk magnetization, M, is (a) parallel to Bq and to the -axis, and (b), upon apphcation of a 90° radio frequency pulse along x, M perpendicular to Bq and to the -axis. See text.

Here Iais the magnetic moment of nucleus A and Ra is the position (the nucleus is the natural Gauge origin). Adding this to the external vector potential in eq. (10.62) and expanding as in (10.63) gives... 

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

If two magnetically nonequivalent nuclei I and K are present in the spin system, the transition frequency of nucleus I is shifted by an additional second order term48 55)... 

NMR 13C spin-lattice relaxation times are sensitive to the reorientational dynamics of 13C-1H vectors. The motion of the attached proton(s) causes fluctuations in the magnetic field at the 13C nuclei, which results in decay of their magnetization. Although the time scale for the experimentally measured decay of the magnetization of a 13C nucleus in a polymer melt is typically on the order of seconds, the corresponding decay of the 13C-1H vector autocorrelation function is on the order of nanoseconds, and, hence, is amenable to simulation. 

RULE 2 Interaction of nucleus A with a group of n magnetically equivalent nuclei X (of spin IX), produces a multiplet of (2nx, lx+ 1) peaks,... 

The diamagnetic contribution to the shielding tensors can be defined as the derivative of this Hamiltonian with respect to the magnetic moment Uk=YkIk of nucleus K and the external magnetic field... 

This technique involves transfer of polarization from one NMR active nucleus to another [166-168]. Traditionally cross polarization (CP) was employed to transfer polarization from a more abundant nucleus (1) to a less abundant nucleus (S) for two reasons to enhance the signal intensity and to reduce the time needed to acquire spectrum of the less abundant nuclei [168]. Thus CP relies on the magnetization of I nuclei which is large compared to S nuclei. The short spin-lattice relaxation time of the most abundant nuclei (usually proton) compared to the long spin-lattice relaxation time of the less abundant nuclei, allows faster signal averaging (e.g., Si or C). CP is not quantitative as the intensity of S nuclei closer to 1 nuclei are selectively enhanced. Nowadays CP has been extended to other pairs of... 

Fig. 1. Energy levels of nucleus with 7 = in an external magnetic field. Arrows denote transitions caused by rf field wavy line denotes net transitions due to spin-lattice relaxation.

Heteronuclei such as 13C (this magnetically active nucleus has 1.1% natural abundant) and 15N (0.3% natural abundance) are routinely measured with modem NMR spectrometers. Proton decoupled 13C NMR spectra in natural abundance exhibit singlets for each specific carbon atom, which are easier to count than overlapping multiplet lines in H NMR. ID 13C NMR can be used to investigate whether a peptide exhibits a single set of lines or a double (or more) set, which indicate conformational or configurational isomers (see Section 7.5.3). However, ID 13C NMR is rather insensitive and if there is not enough material or the solubility is low, more sensitive techniques have to be applied. 

The magnetogyric ratio is then the ratio between the magnetic moment and the angular momentum of a rotating particle and has a characteristic value for each type of nucleus ... 

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