Quantum Chemical Calculations of Monomer–Dimer Equilibria of Aromatic <i>C</i>‑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 <i>C</i>-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 <i>E</i>- and <i>Z</i>-dimers have several different conformers, which all have different solvation energies. While in the gas phase the nitrosobenzene <i>E</i>-dimer is more stable that its <i>Z</i>-form, in chloroform, the <i>Z</i>-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