Transient versus Static Electron Spin Relaxation in Mn2+ Complexes Relevant as MRI Contrast Agents

The zero,-field splitting (ZFS) parameters of the Mn(EDTA)(H2O).2H(2)O. and [Mn(MeNO(2)A)(H2O)].2H(2)O systems were estimated by using DFT and ab initio CASSCF/NEVPT2 calculations (EDTA = (ethane-1,2-41bis(azanetriy1))tetraatetate; MeNO2A = 2,2'-(7-methyl-1,4,7triazonane-1,4-diyOdiacetate). Subsequent molecular dynamics calculations performed within the atom-centered, density matrix propagation (ADMP) approach provided access to the transient and static ZFS parameters, as well as to the correlation time of the transient ZFS. The calculated ZFS parameters present a reasonable agreement with the experimental values obtained from, the analysis of H-1 relaxation data. The correlation times calculated for the two systems investigated turned out to be very short (tau(c) similar to 0.02-0.05 ps), which shows that the transient ZFS is modulated' by molecular vibrations. On the contrary; the static, ZFS is modulated by the rotation of the complexes in solution, which for the small Complexes investigated here is characterized by rotational correlation times of tau(R) similar to 35-60 ps. As a result, electron spin relaxation in small Mn2+ complexes is dominated by the static ZFS.