Uncovering kinetics of nitro-nitrito photoisomerization in [Co(NH3)5NO2]BrNO3 via time-resolved single-crystal X-ray diffraction

Studying dynamic processes in solid state is one of the hot topics in contemporary crystallography. For the laboratory X-ray diffractometers crystal structure determination in most cases possible for the sample in thermodynamic equilibrium only. This limitation arises since diffraction data collection usually takes up to several hours. Most chemical reactions and phase transitions occur much faster and kinetic studies by diffraction methods are impossible in this case. Studying dynamic processes and reaction kinetics in laboratory conditions is possible if special techniques are applied (e.g. pump-probe) or process rate is much slower than data collection time. Therefore, kinetic studies of most processes are only possible with synchrotron radiation. At the same time, modern area X-ray detectors in combination with microfocus X-ray sources allow data collection within minutes. That enables direct measurements of product and reagent concentrations making diffraction kinetic studies possible even in laboratory conditions. In this paper we present a single-crystal X-ray diffraction study of photochemical reaction kinetics for linkage isomerisation [Co(NH3)5NO2]BrNO3 ⇄ [Co(NH3)5ONO]BrNO3. Direct measurement of concentration of NO2/ONO isomers with time resolution ~10 min on continuous sample irradiation has been performed. This study allowed us to determine the relations between unit cell parameters and crystal composition, transformation degrees for different irradiation time, rate constants for direct and reverse processes. The problems encountered and possible improvements of the laboratory diffraction kinetic experiment are also discussed.