Application of the quasi-spectral density function of (15)N nuclei to the selection of a motional model for model-free analysis.

Parameters used in model-free analysis were related to simulated spectral density functions in a frequency region experimentally obtained by quasi-spectral density function analysis of (15)N nuclei. Five kinds of motional models used in recent model-free analyses were characterized by a simple classification of the experimental spectral density function. We demonstrate advantages and limitations of each of the motional models. To verify the character of the models, model selection using experimental spectral density functions was examined.