Molecules magnetic properties

Nuclear Magnetic Resonance (NMR) is based on the fact that certain nuclei exhibit a magnetic moment, oriented by a magnetic field, and absorb characteristic frequencies in the radioftequency part of the spectrum. The spectral fines of the nuclei are highly influenced by the chemical environment, i.e. the structure and interaction of the molecules. Magnetic properties of nuclei have been known since 1924 and the first Nuclear Magnetic Resonance experiment was performed in 1945. 

A major weakness of valence bond theory has been its inability to predict the magnetic properties of molecules. We mentioned this problem in Chapter 7 with regard to the 02 molecule, which is paramagnetic, even though it has an even number (12) of valence electrons. The octet rule, or valence bond theory, would predict that all the electrons in 02 should be paired, which would make it diamagnetic. 

The molecular orbitals are arranged in order of increasing energy. The relative energies of these orbitals are ordinarily deduced from experiment Spectra and magnetic properties of molecules are used. 

The use of selective deuteration is a powerful tool in electron spin resonance (ESR) experiments, in order to establish unequivocal assignments of experimental spectra of radicals. The reason for this is, as is well known, the difference in magnetic properties between the deuteron and the proton, which can be exploited to distinguish chemically inequivalent hydrogens in the molecule. 

Electronic and Magnetic Properties of Chiral Molecules and Supramolecular Architectures... 

It should be noted that diffuse functions which are necessary for a good description of the magnetic properties of anions, have been found needless when computing the susceptibilities of the neutral molecules. 

Anton, J., Ericke, B. and Engel, E. (2004) Noncollinear and collinear relativistic density-functional program for electric and magnetic properties of molecules. Physical Review A, 69, 012505-1-012505-10. 

Nuclear magnetic resonance spectroscopy is a technique that, based on the magnetic properties of nuclei, reveals information on the position of specific atoms within molecules. Other spectroscopic methods are based on the detection of fluorescence and phosphorescence (forms of light emission due to the selective excitation of atoms by previously absorbed electromagnetic radiation, rather than to the temperature of the emitter) to unveil information about the nature and the relative amount specific atoms in matter. 

This effect of the position of the lone pair is relevant not only in the case of phthalocyaninato ligands, but also can be a clue to the intriguing behaviour of the [Dy(DOTA)] complex, where the rotation of a water molecule changes the magnetic properties [13]. A PCE or an REC model cannot account for the effect of such a rotation, but an LPEC model would predict a dramatic effect, since the change in the position of the lone pair effectively means a completely different geometry. 

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