The molecular solid [Mo^V₁₂O₃₀(μ₂-OH)₁₀H₂{Ni^II(H₂O)₃}₄]^.14H₂O, abbreviated Ni₄Mo₁₂,  is comprised of magnetic clusters in which four Ni²⁺ centers are antiferromagnetically coupled, making it an excellent model compound with which to study the evolution of a non-magnetic molecular solid through a series of magnetic field-induced level crossings and saturation of the magnetization in the high field ferromagnetic state.  In particular, we sought to test the suggestion that molecular structure (and thus interactions between spins) may be changing with applied magnetic field in Ni₄Mo₁₂. The rational is that an applied magnetic field may change local structure, which modifies orbital overlap, which can change the magnetic exchange parameter J. Our magneto-infrared measurements (Fig. 1) do provide evidence for small field-induced local distortions of the lattice in this system and at least a partial justification for invoking field-dependent magnetic parameters. The field-induced change in the localized H₂O mode on the O attached to the Ni site is particularly evident (Fig. 1(c)). This result is consistent with our previous magneto-optics work indicating a small change in the Ni²⁺ crystal field environment at 30 T.

In response to the development of new theoretical methods to treat magnetic molecules that go beyond the traditional LDA + U and the need to benchmark these DMFT (dynamic mean field theory) based approaches on complex systems, we measured the optical properties of (pyH)₃[Mn₄O₃Cl₇(OAc)₃]^.2MeCn, henceforth Mn₄ (Fig. 2). Based upon these calculations, we assign the 1.8 eV peak to a superposition of Mn³⁺ to Mn⁴⁺ charge transfer excitations of the distorted [Mn₄O₃Cl]⁶⁺ core and d to d on-site excitations of the Mn centers. We find an optical gap of 1.18 ± 0.08 eV, in excellent agreement with DMFT predictions.