Theoretical investigation of cyromazine tautomerism using density functional theory and MOller-Plesset perturbation theory methods

A computational chemistry analysis of six unique tautomers of cyromazine, a pesticide used for fly control, was performed with density functional theory (DFT) and canonical second-order MOller-Plesset perturbation theory (MP2) methods to gain insight into the contributions of molecular structure to detection properties. Full geometry optimisation using the 6-311++G** basis set provided energetic properties, natural charges, frontier orbitals and vibrational modes. Excitation energies were obtained using time-dependent DFT. Hydrogen location and bond order contribute significantly to the electronic properties. The common cyromazine tautomer possesses the lowest energy, highest band gap energy and highest excitation energy. B3LYP/6-31G** dynamics simulations indicate each tautomer possesses a stable structure with limited rotation about the single bonds. Tautomerisation involving intramolecular hydrogen transfer influences the natural charges of neighbouring atoms and the frontier orbital properties. The excitation energies are highly correlated with band gap energies of the frontier orbitals. The calculated infrared and Raman spectra are suitable for vibrational assignments associated with the chemical structure. The tautomeric forms of cyromazine possess similar spatial properties and significant variation in electronic properties.