Currently MRI exams use contrast agents to enhance contrast imaging and so diagnostics. A contrast agent is most of the time a gadolinium complex in which a chelate ligand is used to avoid Gd release, thus preventing toxic ion to spread in patient's body. In this thesis we studied magnetic properties of gadolinium complexes that affect the relaxation of a water molecule coordinated to Gd, which is the source of contrast imaging. We focused on the commercial contrast agent ProHance, and some derivatives. The first step was to compute ab initio molecular dynamics of the different systems and analyse them. Then we calculated using quantum chemistry hyperfine interaction and Zero Field Splitting. The analysis of hyperfine tensors was achieved for each system, and average terms were estimated. The origin of tensors' fast fluctuations in ProHance system was identified thanks to geometrical collectives variables decomposition. ZFS was decomposed into two contributions: static and transient. We can now set up from ab initio all the necessary ingredients for modeling the electronic relaxation time and then the water coordinated molecule proton relaxation for each of the studied systems.
