Accurate experimental characterization of the labile N{\textendash}Cl bond in N-chloro-N$\prime$-(p-fluorophenyl)-benzamidine crystal at 17.5 K

N-Chloro-N '-(p-fluorophenyl)-benzamidine (NCLBA) is a N-halamine derivative which can easily release chlorine, for example when stimulated by high-energy photons. Despite the rich chemistry performed by N-halamines, the chemical properties of the N-Cl bond are poorly investigated. In this work, we determine the accurate charge density distribution of NCLBA by single crystal X-ray diffraction. A very low temperature (T = 17.5 K), coupled with the low X-ray flux of a fine-focus conventional source, allowed the specimen to survive for longer than 750 h of data acquisition without appreciable diffraction deterioration. Electronic and electrostatic properties of NCLBA are not significantly affected by the crystal field, enabling the derivation of molecular properties from the X-ray experimental data. The N-Cl bond in NCLBA is one of the longest reported to date in available structural studies. In general, the longer the N-halogen bond, the lower the amount of electron sharing in the internuclear region, with the bond approaching its homolytic dissociation limit. The synergy between accurately measured high-order data and low temperature enabled modelling of the residual thermal motion anharmonicity of the molecule's halogen atoms with refinement of Gram-Charlier thermal cumulants at the expense of large parameter correlations, as the data extension is rather short of the Kuhs empirical rule.