Magnetic multiple resonance theory

For quantum chemistry, first-row transition metal complexes are perhaps the most difficult systems to treat. First, complex open-shell states and spin couplings are much more difficult to deal with than closed-shell main group compounds. Second, the Hartree—Fock method, which underlies all accurate treatments in wavefunction-based theories, is a very poor starting point and is plagued by multiple instabilities that all represent different chemical resonance structures. On the other hand, density functional theory (DFT) often provides reasonably good structures and energies at an affordable computational cost. Properties, in particular magnetic properties, derived from DFT are often of somewhat more limited accuracy but are still useful for the interpretation of experimental data. 

The effective Hamiltonian used by Saykally, Evenson, Comben and Brown [62] contains terms which we have already met in this chapter, and which we will therefore deal with fairly briefly, with appropriate references to the details given elsewhere, particularly in this section. The theory has been developed in a number of papers, particularly by Brown, Kopp, Malmberg and Rydh [63], Brown and Merer [64], who dealt with n states of triplet and higher spin multiplicity, and Steimle and Brown [65] who specifically addressed the theory of the A-doubling of CO in the 3 n state. The theory of the Zeeman interactions follows closely that developed to analyse the magnetic resonance spectra of OH by Brown, Raise, Kerr and Milton [66]. All of these... 

Principles of the technique. In section 7.4 we discussed in detail the spontaneous decay of excited atoms by the simultaneous emission of two electric dipole photons. Consideration of arguments similar to those used in Chapter 9 in the derivation of the Einstein relations show that, corresponding to this spontaneous decay process, there must also exist stimulated transitions involving the simultaneous emission or absorption of two photons. Such multiple-quantum transitions were observed experimentally in radiofrequency resonance experiments by Brossel et al. (1954) and Kusch (1954), and a theory of this effect for magnetic dipole... 

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