Dipolar nucleus

Spherical, it disturbs a probing electromagnetic field independent of direction. The result is a strong, sharp NMR signal. is a dipolar nucleus. 

It is fortunate that P is a dipolar nucleus. However, it is frequently bonded to quadrupolar nuclei, such as N, Co, Cu, Cu, Pd, Ir, and " Au. The magnitude of complications depends on factors such as natural abundance of the isotope, quadruple moment, and the relative receptivity. 

The standard NMR experiment for a dipolar nucleus like P involves subjecting the probe to an external magnetic field with constant field strength and variable radiofrequency v. In such a field, the phosphorus nuclei can either align themselves parallel or antiparallel to the external field. This can be represented as N or N (see Fig. 1.1). The nuclear magnetic moment // is directly proportional to the spin I of the nucleus. 

When a dipolar nucleus ( P in the present case) is bonded directly to a quadmpolar nucleus ( 0 in the present case), the P nucleus will also be relaxed by virtue of its spin - spin coupling with O. This was termed scalar relaxation of the second kind by Abragam (1961). Such a scalar relaxation... 

When the product r /is sufficiently small, the scalar relaxation dominates the relaxation of P and results in the collapse of the multiplet. Suzuki and Kubo (1964) have calculated the line shape of a dipolar nucleus coupled to a quadrupolar nucleus with / = f, with different magnitudes of T J. Figure 2 shows the O- and 3ip(no)-NMR spectra of P OClj (A), (CHaOsP O (B), (Ph0)3P 0 (C), and PhjP O (D). It can be seen in Fig. 2 that as the O-NMR coupling pattern collapses (decreasing J and/or increasing... 

In equation (bl. 15.24), r is the vector coimecting the electron spin with the nuclear spin, r is the length of this vector and g and are the g-factor and the Boln- magneton of the nucleus, respectively. The dipolar coupling is purely anisotropic, arising from the spin density of the impaired electron in an orbital of non-... 

Usually, nuclear relaxation data for the study of reorientational motions of molecules and molecular segments are obtained for non-viscous liquids in the extreme narrowing region where the product of the resonance frequency and the reorientational correlation time is much less than unity [1, 3, 5]. The dipolar spin-lattice relaxation rate of nucleus i is then directly proportional to the reorientational correlation time p... 

Although the exact mechanism of the Tschitschibabin cyclisation has not been elucidated, it is reasonable, as shown in Scheme 4, to assume a series of reversible steps from the vinylogous ylide (or methylide) to a methine and an enol-betaine intermediate and then finally an irreversible dehydration to the indolizine nucleus. The reaction might be related to the modern electrocyclic 1,5 dipolar cyclization. 

Eq. 16 is an extremely useful criterion for examining the extent of dipolar interaction in a multispin system, and gives the relaxation method a major advantage over the n.O.e. method. The equivalent quantitative test for the n.O.e. experiment requires all but the receptor nucleus to be saturated and this is not readily performed in practice. 

Nuclear Overhauser effect (nOe) The change in intensity in the signal of one nucleus when another nucleus lying spatially close to it is irradiated, with the two nuclei relaxing each other via the dipolar mechanism. Nuclear Overhauser effect correlation spectroscopy (NOESY) A 2D... 

It is well-known that the hyperfine interaction for a given nucleus A consists of three contributions (a) the isotropic Fermi contact term, (b) the spin-dipolar interaction, and (c) the spin-orbit correction. One finds for the three parts of the magnetic hyperfine coupling (HFC), the following expressions [3, 9] ... 

Since the core polarization will give a negative spin density at the iron nucleus, it is expected that in the state the isotropic and dipolar contributions partially cancel each other along the Fe-O bond direction while they will reinforce each other along this direction in the state. 

It is important to understand that the atomic charges refer to atoms that are not spherical. Consequently the centroid of electronic charge of an atom does not in general coincide with the nucleus, and each atom therefore has an electric dipole moment—or, more generally, an electric dipolar polarization (since only the dipole moment of electrically neutral atoms is origin independent). 

This is the beauty of this quantity which provides specifically a direct geometrical information (1 /r% ) provided that the dynamical part of Equation (16) can be inferred from appropriate experimental determinations. This cross-relaxation rate, first discovered by Overhau-ser in 1953 about proton-electron dipolar interactions,8 led to the so-called NOE in the case of nucleus-nucleus dipolar interactions, and has found tremendous applications in NMR.2 As a matter of fact, this review is purposely limited to the determination of proton-carbon-13 cross-relaxation rates in small or medium-size molecules and to their interpretation. 

Aniosotropic hyperfine coupling results primarily from dipolar interactions between a magnetic nucleus and an unpaired electron in a p, d, or f orbital. Such interactions give rise to a Hamiltonian... 

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