Oxygen nucleus, spin

In the previous discussion, the electron-nucleus spin system was assumed to be rigidly held within a molecule isotropically rotating in solution. If the molecule cannot be treated as a rigid sphere, its motion is in general anisotropic, and three or five different reorientational correlation times have to be considered 79). Furthermore, it was calculated that free rotation of water protons about the metal ion-oxygen bond decreases the proton relaxation time in aqua ions of about 20% 79). A general treatment for considering the presence of internal motions faster than the reorientational correlation time of the whole molecule is the Lipari Szabo model free treatment 80). Relaxation is calculated as the sum of two terms 8J), of the type... 

Figure 3.8. Molecular orbitals for the oxygen atom, with indication of their quantum numbers (main, orbital angular momentum and projection along axis of quantisation). Shown is the oxygen nucleus and the electron density (where it has fallen to 0.0004 it is identical for each pair of two spin projections), but with two different shades used for positive and negative parts of the wavefunction. The calculation uses density functional theory (B3LYP) and a Gaussian basis of 9 functions formed out of 19 primitive Gaussian functions (see text for further discussion). The first four orbitals (on the left) are filled in the ground state, while the remaining ones are imoccupied.

Proton magnetic resonance studies have also shown the presence of metal-hydrogen species in cyanide solutions of rhodium, platinum, and iridium (Table IX). In particular, the addition of CN- to a boiled aqueous solution of rhodium trichloride, followed by reduction with sodium boro-hydride, yields a solution that contains an Rh—H complex in moderately high concentrations and is stable in the absence of oxygen for several years (108). The observed coupling of the proton with the Rh10 nucleus (spin ) confirms the presence of an Rh—H bond (108). 

Coupling of the spin, which is largely localised on the oxygen nucleus, with the nitrogen nucleus (I = 1) results in the characteristic 1 1 1 triplet. Splitting from the protons in the molecule is not resolvable. 

Carbon-12, like oxygen-16, is not NMR-active. However, only 1.1% of the total carbon in a molecule consists of the spin- 2 carbon-13 isotope, so that the sensitivity of this nucleus is much lower. Thus rather than using only perhaps 8 or 16 pulses, as in many proton experiments, we shall now require hundreds or even thousands of pulses, depending on the solute concentration. 

The series of molecules which has guided us through this book so far was chosen for a good reason it allowed us to discuss in detail the most important nuclei, the proton and carbon-13, while demonstrating the effect of a very important heteronucleus , phosphorus-31, on the spectra of the two key nuclei. In addition, we could discuss the NMR investigation of this heteronucleus, which exists in 100% natural abundance and has a spin of Vi> and in contrast of oxygen-17, a low-abundance nucleus with a spin greater than Vi. 

Experiments whereby reduced enzyme was reoxidized by O2 enriched in (which has a magnetic nucleus) showed that an oxygen species ended up close to the Ni-based unpaired spin in the ready as well as in the unready state (Van der Zwaan et al. 1990). It could only be removed by full reduction and activation of the enzyme. The crystal structure (see Chapter 6) shows an oxygen atom close to nickel and it is... 

NMR observes the chemistry of only the proton nucleus (though it can observe many other nuclei independently). This means that hetero and metallic chemistry cannot be observed directly. Thus, sulfur, nitrogen, oxygen, and metals cannot be directly analyzed by NMR, though secondary correlations can be obtained from the proton chemistry of the sample. In combination with electron spin resonance (ESR) analyzers that can operate in the fringe fields of the NMR magnet the presence of paramagnetic metals and free radicals can be quantified. 

As anticipated in Sections 2.2.2 and 3.1, the unpaired electrons should not be considered as point-dipoles centered on the metal ion. They are at the least delocalized over the atomic orbitals of the metal ion itself. The effect of the deviation from the point-dipole approximation under these conditions is estimated to be negligible for nuclei already 3-4 A away [31]. Electron delocalization onto the ligands, however, may heavily affect the overall relaxation phenomena. In this case the experimental Rm may be higher than expected, and the ratios between the Rim values of different nuclei does not follow the sixth power of the ratios between metal to nucleus distances. In the case of hexaaqua metal complexes the point-dipole approximation provides shorter distances than observed in the solid state (Table 3.2) for both H and 170. This implies spin density delocalization on the oxygen atom. Ab initio calculations of R m have been performed for both H and 170 nuclei in a series of hexaaqua complexes (Table 3.2). The calculated metal nucleus distances in the assumption of a purely metal-centered dipolar relaxation mechanism are sizably smaller than the crystallographic values for 170, and the difference dramatically increases from 3d5 to 3d9 metal ions [32]. The differences for protons are quite smaller [32]. 

---