Katarina Varga

Crystal disordering and organic solid-state reactions

This work is the extension of our preliminary communication about the relationship between the change in the solid-state structure and the chemical reaction in a crystal. The investigation is a case study about the nature of the adiabatic organic solid-state reactions by kinetic measurements of the processes that occur during the dimerization of aromatic nitroso compounds (p-bromo- and p-iodonitrosobenzenes, respectively) in crystals. From the reaction rates at different temperatures we have calculated the activation energy. Since dimerization is induced by the appearance of crystal deformations on the surface caused by sublimation, we study here the relationship between the rate of sublimation and the rate of dimerization. There are indications that the sublimation of molecules from the surface serves only for the activation of chemical solid-state reaction. Dimerization is also studied under three different topochemical environments: strong topochemistry in the cryogenic conditions where the starting monomers were obtained by photolysis of crystals of the corresponding dimer at 14 K, looser topochemistry in the freshly sublimed crystals of monomers, and randomly distributed monomer molecules in thin layers prepared by cryogenic deposition. It is demonstrated that the change in topochemistry drastically modifies the reaction rate.

Quantum Chemical Calculations of Monomer‒Dimer Equilibria of Aromatic C-Nitroso Compounds

Monomer-dimer equilibria of nitrosobenzene and 2-nitrosopyridine in gas phase and solution were studied by range of quantum chemical methods in an attempt to find the level of theory suitable for modeling dimerization reactions of aromatic C-nitroso compounds in general. The best agreement with the experimental standard reaction Gibbs energies was obtained with a combination of double-hybrid density functionals B2PLYP-D3, PBE0DH, and DSD-PBEB86, and basis sets of triple-ζ quality. Of all other tested functionals, global hybrid PBE0 behaved equally well, and proved to be more than adequate for at least preliminary work. Other tested methods either produced inferior results (MP2, MP4(SDQ), CCSD, G4(MP2), CBS-QBS, CBS-APNO), or were too demanding for practical use (CCSD(T)). Analysis of computationally obtained thermodynamic data reveal intricate details of these reactions. Both E- and Z-dimers have several different conformers, which all have different solvation energies. While in the gas phase the nitrosobenzene E-dimer is more stable that its Z-form, in chloroform, the Z-form is more stable. Gas-phase dimerization entropies are large and negative, so these reactions are strongly temperature dependent. In some cases, like with 2-nitrosopyridines, entropy and enthalpy terms essentially cancel each other out, allowing structural and media effects to significantly influence dimerization equilibria.