Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 2A / Section A / 16 / c / Solution

The descending line is ${}^3{T}_{1g}(F)$; the low rising line is ${}^3{T}_{2g}$; the steep line is ${}^3{A}_{2g}$; and the branch starting at $15B$ is ${}^3{T}_{1g}(P)$. Their high-field slopes, in the same order, areThese are slopes of the supplied absolute term energies relative to their chosen reference, not slopes of transition energies measured upward from the ground state. The two $T_{1g}$ states share a symmetry and mix, whereas the crossing of $A_{2g}$ and the upper $T_{1g}$ branch is allowed because they have different irreducible representations.