Dipole collectivity nuclei

A ID nuclear overhauser experiment (NOE) (Gaggelli and Valensin, 1993) is an important and valuable variation on the simple ID NMR experiment that provides spatial relationship between each nucleus in the structure. Coupling constants observed in a ID H NMR spectra provide connectivity for directly bonded nuclei whereas, a ID NOE experiment identifies nuclei that are close in space (<6A). Briefly, a ID NOE experiment requires the addition of a second lowpowered rf pulse that selectively saturates a specific peak in the NMR spectrum. The saturated peak becomes a null in the spectrum and any other nuclei that are coupled through space via a dipole-dipole interaction to the saturated peak will experience a change in peak intensity. A ID NOE experiment requires collecting two NMR spectra, with and without saturation, to monitor changes in peak intensity. A summary of common ID NMR experiments and their applications are listed in Table 12.6. 

In a collection of helium atoms, for example, the average distribution of the electrons about each nucleus is spherically symmetrical as shown in A Figure 11.4(a). The atoms are nonpolar and so possess no permanent dipole moment. The instantaneous distribution of the electrons, however, can be different from the average distribution. If we could freeze the motion of the electrons at any given instant, both electrons could be on one side of the nucleus. At just that instant, the atom has an instantaneous dipole... 

The existance of crystalline forms of noble gases, such as argon, proves that attractive forces must exist between non-polar molecules. In 1930 F. London showed that a collection of nonpolar molecules only lack polarity when viewed over a period of time. An instantaneous photograph of the molecules would show a distortion of the electrons relative to the nucleus sufficient to cause a temporary dipole moment. That instantaneous dipole moment causes induced dipoles in surrounding molecules which results in an attractive force between the non-polar molecules. The potential energy of the interaction was shown by London to be ... 

Weak, secondary forces, resulting from molecular dipoles, also act between materials. They are often classified according to the nature of the interacting dipoles. Keesom orientation forces act between permanent dipoles, London dispersion forces between transient dipoles, and Debye induction forces between a permanent and an induced dipole, see O Tables 2.1 and O 2.2. These are collectively known as van der Waals forces (but note alternative usage of this term, O Table 2.2), and occur widely between materials. They are much less dependent upon specific chemical structure than primary bonds. Indeed, dispersion forces are universal. They only require the presence of a nucleus and of extranuclear electrons, so they act between all atomic and molecular species. 

---