Vibrational spectra, spin equilibrium

The far-infrared spectrum of Fe(EtPhDtc)3 as a function of pressure shows that the intensity of the band assigned to the 2 V2 state increases relative to that assigned to the 6A t state upon increasing pressure (93b) (38). For the Fe(n-Pr2 -Dtc)3 complex, the Fe—S vibration at 367 cm"1 was assigned to the low-spin-state (6i4j) isomer. On the basis of these results, the spin-state equilibrium was adopted as the true model with a spin-state interconversion rate lower than the vibrational time scale ( 10" 3 sec). 

That is, transitions are normally between states of the same spin. Other selection rules may relate to the geometrical symmetry of the molecule. The molecular transitions seen in the visible and ultraviolet regions of the spectrum must be transitions from one rotational-vibrational-electronic state to another. We shall consider this in detail for a hypothetical diatomic molecule for which the ground state and excited state potential curves are those shown in Figure 10.8. For both electronic state potential energy curves there are sets of vibrational states and rotational sublevels. Notice that the equilibrium distance is not the same for both curves and that the curvature (i.e., the force constant) is not the same either. Thus, there are a different vibrational frequency and a different rotational constant for each electronic state. This has to be taken into account in working out the transition frequencies. 

---