Microwave frequency effect studies on EPR line widths and critical testing of theoretical simulations are reported in this paper for a model system in solution. The aim was to determine the effective spin Hamiltonian parameters for copper complexes with nitrogen ligands under a broad range of motion conditions. Direct inclusion of superhyperfine interaction terms in algorithms used in the computer program for slow motion is not simple because it greatly increases matrix dimensions, affecting numerical stability and computational time. The solution proposed here consists of the use of the convolution theorem under the reasonable assumption of purely isotropic hyperfine interaction for nitrogens. Nonsecular terms used in the spin Hamiltonian to describe the line shape of the "bare" electron-metal nucleus system were explicitly included since they are not negligible for transition metal ions at low frequency. Spin-rotational contribution was also taken into account. The model used in this paper was a monomeric copper complex of glycyl-L-histidine previously characterized in solution. A simple polynomial fitting procedure was used to remove baseline drift.