Probing Magnetic Excitations in Co^II Single‐Molecule Magnets by Inelastic Neutron Scattering

Co(acac) 2 (H 2 O) 2 ( 1 , acac = acetylacetonate), a transition metal complex ( S = 3/2), displays field‐induced slow magnetic relaxation as a single‐molecule magnet. For 1 and its isotopologues Co(acac) 2 (D 2 O) 2 ( 1‐ d 4 ) and Co(acac‐ d 7 ) 2 (D 2 O) 2 ( 1‐ d 18 ) in approximately D 4 h symmetry, zero‐field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy M S = ±1/2 ( ϕ 1,2 ) and higher energy M S = ±3/2 ( ϕ 3,4 ) states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1‐ d 4 and 1‐ d 18 . Direct‐geometry, time‐of‐flight Disk‐Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra‐KD transition as a result of Zeeman splitting, M S = –1/2 ( ϕ 1 ) → M S = +1/2 ( ϕ 2 ), in 1‐ d 18 . This is a rare study of the M S = –1/2 → M S = +1/2 excitation in transition metal complexes by INS. Indirect‐geometry INS spectrometer VISION has been used to probe the inter‐KD, ZFS transition, M S = ±1/2 ( ϕ 1,2 ) → M S = ±3/2 ( ϕ 3,4 ) in both 1‐ d 4 and 1‐ d 18 , by variable‐temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations.