SMMs have received much attention owing to their quantum tunneling and slow relaxation of the magnetization. This has led to an intense activity by synthetic chemists to consistently manufacture molecular systems suitable for detailed study by physicists. A further aim is to produce and characterize new molecules with the goal of identifying features relevant to enhance their properties. Upon ligand substitution, the barriers of a series of SIMs are significantly enhanced compared to that in the parent complex, representing a promising system to probe the relaxation dynamics. Here we propose to use the MERLIN time of flight spectrometer to perform direct measurements of the energy level scheme of these complexes, to find clear cut evidence that the change in auxiliary group leads to large anisotropic barriers, and investigate the correlations between structure and magnetic properties.