Extended study of crystal structures, optical properties and vibrational spectra of polar 2-aminopyrimidinium hydrogen phosphite and three centrosymmetric salts - bis(2-aminopyrimidinium) sulfate monohydrate and two 2-aminopyrimidinium hydrogen sulfate polymorphs

This study aimed primarily at completing and extending the characterization of the crystallographic, spectroscopic and optical properties of polar, biaxial, optically negative 2-aminopyrimidinium(1+) hydrogen phosphite. Besides the redetermination of the low-temperature crystal structure (space group P2₁), high-quality single crystals of this salt were grown from an aqueous solution, and their optical properties were studied. The determination of the refractive indices in the wavelength range of 435–1083 nm showed anomalous dispersion of the refractive indices, resulting in a point of uniaxiality. The crystal allows phase matching for collinear second harmonic generation (SHG) processes of both type I and type II in a broad wavelength range. SHG properties were studied for powdered size-fractioned samples and oriented single-crystal cuts. The optical damage threshold experiments confirmed excellent optical resistance - at least 220 TWm⁻² and 70 TWm⁻² for 800 and 1000 nm irradiation, respectively. The low-temperature crystallographic study was also extended for three monoclinic salts of 2-aminopyrimidine and sulfuric acid - i.e. bis(2-aminopyrimidinium(1+) sulfate monohydrate (space group P2₁/n) and two polymorphs of 2-aminopyrimidinium(1+) hydrogen sulfate (both with space group P2₁/c). The vibrational spectra of all title compounds were assigned using single-molecule quantum chemical computations (including Potential Energy Distribution analysis) in combination with the nuclear site group analysis. Spectroscopic results concerning sulfates of 2-aminopyrimidine provided valuable “reference” materials for the vibrational spectroscopic study and also addressed the question of their polymorphism. An optimal computational approach employing solid-state DFT calculations has also been sought to model the vibrational spectra of 2-aminopyrimidinium (1+) hydrogen phosphite crystals.