Accurate experimental characterization of the labile N-Cl bond in N-chloro-N'-(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 hrs 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