Magnetic Circular Dichroism - Example On A and B Terms

Example On A and B Terms

To have an A- and B-term in the MCD spectrum, a molecule must contain degenerate excited states (A-term) and excited states close enough in energy to allow mixing (B-term). One case exemplifying these conditions is a square planar, d8 complex such as ₂Pt(CN)₄. In addition to containing A- and B-terms, this example demonstrates the effects of spin-orbit coupling in metal to ligand charge transfer (MLCT) transitions. As shown in figure 1, the molecular orbital diagram of ₂Pt(CN)₄ reveals MLCT into the antibonding π* orbitals of cyanide. The ground state is diamagnetic (thereby eliminating any C-terms) and the LUMO is the a_2u. The dipole-allowed MLCT transitions are a_1g-a_2u and e_g-a_2u. Another transition, b_2u-a_2u, is a weak (orbitally forbidden singlet) but can still be observed in MCD.

Because A- and B-terms arise from the properties of states, all singlet and triplet excited states are given in figure 2.

Mixing of all these singlet and triplet states will occur and is attributed to the spin orbit coupling of platinum 5d orbitals (ζ ~ 3500 cm−1), as shown in figure 3. The black lines on the figure indicate the mixing of 1A_2u with 3E_u to give two A_2u states. The red lines show the 1E_u, 3E_u, 3A_2u, and 3B_1u states mixing to give four E_u states. The blue lines indicate remnant orbitals after spin-orbit coupling that are not a result of mixing.