Angle-dependent Mِssbauer study of the ground and second new metastable state in Na2[Fe(CN)5NO]·2H2O single crystals

We report the results of an angle-dependent Mössbauer experiment with sodium nitroprusside single crystal cuts populated to the maximum possible value with the second new metastable state SII of the nitrosyl [Fe(CN)5NO]2− ion by light irradiation with 1064 nm. All Mössbauer parameters are determined: isomer shift IS=+0.194(3) mm/s, quadrupole splitting ΔEQ=+2.862(3) mm/s, asymmetry parameter of the electric field gradient (EFG) tensor η=0.17(5), orientation angle of the main component of the EFG α=37.3(5)°, orientation angle of the mean square displacement tensor ω=2(7)°, and Lamb–Mössbauer factors for all principal crystallographic directions fLMa=0.70(2), fLMb=0.57(2), and fLMc=0.74(3), respectively. The experimentally measured values are compared with the results of three independent density functional theory calculations. Only the simulated isomer shift and the absolute value of the quadrupole splitting agree quantitatively with the experimentally measured values. The contradiction between some experimentally measured and calculated values is explained by the static side-on conformation used in the calculations for SII. Probably, the ON-group is in reality almost free to rotate around the average symmetry axis and performs bending vibrations between the isonitrosyl structure confirmed for the first new state SI and the static sidewards bonded conformation proposed for SII.