High spins transition probability

In Chapter 6 we presented an expression for the transition probability (or intensity, amplitude) of field-swept spectra from randomly oriented simple 5=1/2 systems (Equation 6.4), and we could perhaps tacitly assume (as is generally done in the bioEPR literature) that the expression also holds for effective S = 1/2 systems, such as for the high-spin subspectra defined by the rhombograms discussed in Chapter 5. But what about parallel-mode spectra And how do we compute intensities in complex situations like for systems in the B S B B intermediate-field regime Clearly, we need a more generic approach towards intensity calculations. 

Figure 12.3 outlines the essential features of the PASADENA/PHIP concept for a two-spin system. If the symmetry of the p-H2 protons is broken, the reaction product exhibits a PHIP spectrum (Fig. 12.3, lower). If the reaction is carried out within the high magnetic field of the NMR spectrometer, the PHIP spectrum of the product consists of an alternating sequence of enhanced absorption and emission lines of equal intensity. This is also true for an AB spin system due to a compensating balance between the individual transition probabilities and the population rates of the corresponding energy levels under PHIP conditions. The NMR spectrum after the product has achieved thermal equilibrium exhibits intensities much lower than that of the intermediate PHIP spectrum. 

The visible spectra of oxyHr and metIh N3 are dominated by ligand-to-metal charge transfer bands from the hydroperoxide or azide anions, but otherwise they are similar to those of the synthetic complexes (Rgure 2) (38). The d-d transitions observed at 700 and KXX) nm are more intense than usually observed for high-spin iron(llI) complexes, probably due to the strong antiferromagnetic coupling interaction (38,40). 

Figure A1.6 Nuclear spin states and spectrum for product 3. The upper spin state, a, has abnormally high population. The probability of the downward transition, a — f, is greater than the probability of the upward transition, and emission occurs. The spectrum shows an inverted emission peak, E. From S. H. Pine, J. Chem. Educ.f 49, 664 (1972). Reproduced by permission of the Division of Chemical Education.

---