(Almost) Perfect heteronuclear system-NMR relaxation theory vs experiment

1H and 19F spin-lattice relaxation studies were performed for partially deuterated 3-fluoroaniline-2,4,6-d3,ND2 (FC6HD3ND2) in the frequency range from 10 kHz to 20 MHz (referring to 1H resonance frequency) at 208, 218, 228, and 238 K. The molecules contain single 1H and 19F nuclei in geometrically equivalent positions, and therefore, this compound was selected as an example of a heteronuclear (1H, 19F) spin system. The well-known spin relaxation theory developed for a model system including two spins (that can be exemplified by a single molecule of 3-fluoroaniline-2,4,6-d3,ND2) has been adopted (extended) to "real" systems (including many molecules) by taking into account relaxation pathways associated with the intermolecular 1H-1H, 19F-19F, and 1H-19F magnetic dipole-dipole interactions. The theoretical framework was applied to interpret the 1H and 19F spin-lattice relaxation data aiming to assess how accurately the model reproduces the experimental results. The relaxation theory predicts the bi-exponential relaxation processes for heteronuclear spin systems; the bi-exponentiality is, however, rarely observed experimentally. The reason for this effect was discussed with 3-fluoroaniline-2,4,6-d3,ND2 as an example.