Magnetic resonance multiple quantum transitions

We should, finally, mention also the possibility to Indirectly detect resonance l2X. 2H experiments. A 2D experiment invariably involves two variable time periods (i)(see Fig. 7). During the initial "evolution period" the w.j-information is acquired by the nuclear spins and "stored" in the form of phase or amplitude information which is read out and recorded during the detection period together with the (02-lnformatlon. The indirect detection of the U3. -information permits the study of otherwise inaccessible properties. This feature can be utilized to indirectly observe resonance of low sensitivity nuclei (2JJ and of forbidden transitions (2 3S 35j, i. . multiple quantum transitions and combination lines. In this respect 2D spectroscopy really provides novel information. In particular zero quantum transitions, l.e. transition between energy levels of equal magnetic quantum number had never been observed before (35J(see Fig. 8). 

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... 

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. 

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