Hyperfine coupling of the cyanide ions and crystal water molecules of three dimensional magnetic polycyanides as studied by solid-state 13C- and 2H NMR

Abstract

The hyperfine coupling constant (HFCC) of a CN ligand must be sensitive to its coordination bond and local magnetic structures in three dimensional magnetic polycyanide such as Prussian-blue analogs. The local magnetic structure of Rb0.90Mn1.05[Fe(13CN)6]·3H2O, which exhibits a thermally induced spin phase transition near room temperature with a wide hysteresis, was investigated in the high and low spin phase by solid-state 13C NMR spectrum of the 13CN ligand. Major and minor peaks were observed in the high spin phase FeIII(S=1/2)13CNMnII(S=5/2), while the major and two other peaks were observed in the low spin phase. These results indicate a slightly non-uniform structure of Rb0.90Mn1.05[Fe(13CN)6]·3H2O. The HFCC of a carbon atom was estimated from the slope of the 13C NMR shift as a function of inverse temperature. The HFCC of the Fe13CNMn of Rb0.90Mn1.05[Fe(13CN)6]·3H2O is positive in both spin phases, while that of the FeIII(S=1/2)13CN of (Na0.4K0.6)3[FeIII(S=1/2)(13CN)6] is negative. This result indicates that the effect of the spin of Mn ion exceeds the negative contribution of Fe ion in the FeCNMn system. On the other hand, the HFCC is negative for Na0.5Co1.26[Fe(13CN)6]·nH2O in the high spin phase, where the spin of FeIII(S=1/2) ion dominates the HFCC in the FeIII(S=1/2)CNCoII(S=3/2) system. The positive and negative contributions from MnII(S=5/2) and CrIII(S=3/2), respectively, dominate the 13C NMR shift in different temperature regions in the CrIII(S=3/2)13CNMnII(S=5/2) system of MnII1.5[CrIII(13CN)6]·nH2O.