Spin-crossover, definition

Spin-crossover, definition, 32 2 Spin delocalization, polynuclear molecules, 43 182-183... 

Both spin-crossover transitions (HS < LS, FO LS) are first order accompanied by definite jumps of populations, while the cooperative Jahn-Teller transition (HS FO) is weak first-order (very close to a second-order transition). It suggests a possibility of observation of hidden cooperative Jahn-Teller transition (the broken line in Fig. 7) between the metastable HS and FO phases, if the HS phase could be supercooled enough below the spin-crossover transition temperature Tc by a rapid cooling. 

Significant complementary data on the metal sites in proteins have also been obtained using the technique of nuclear inelastic scattering spectroscopy (NIES), also known as nuclear resonance vibrational spectroscopy. NIES probes the vibrational modes which directly involve motion of Mossbauer-active nuclei such as Fe. Thus the spectra are dominated by Fe-ligand modes associated with actual displacement of the Fe atom, such as the low frequency doming modes of heme.The NIES technique has also had a definitive role in the study of Fe(II) mono- and polymeric spin-crossover species and that work has been well reviewed recently. Spin-crossover complexes are the topic of separate discussion in Section 4 below. 

---