A Theoretical Study of Structural, Electronic, and Vibration Properties of Acetylcholine Chloride and Bromide

The ever-growing capabilities of computing systems and algorithms make the first-principle methods extremely important in such research fields as biologically active compounds and complex biological systems. They also facilitate the target search and prediction of new forms. The purpose of the study was to define the structural parameters and basic electronic and vibration characteristics of crystal acetylcholine halides (ACh-Hal). 
The study featured acetylcholine chloride and bromide (ACh-Hal). The theoretical analysis was based on the density functional theory (DFT) with gradient Perdew-Burke-Ernzerhof exchange-correlation functional (PBE) and a semi-empirical scheme for dispersion-accounting density functional (DFT+D3). 
The calculations made it possible to define the optimized parameters of the crystal lattice: a = 9.765 Å, b = 15.217 Å, c = 6.274 Å for orthorhombic acetylcholine chloride (ACh-Cl); a = 10.883 Å, b = 13.304 Å, c = 7.077 Å, 109.21° for monoclinic bromide (ACh-Br). The calculations also revealed atomic coordinates, bond lengths, valence, and torsion angles. As for the electronic structure, ACh-Hal proved to be dielectrics with a band gap of 4.734 and 4.405 eV. The effective charges of carbon atoms were highly anisotropic. The calculated vibration spectra confirmed the experimental data in the entire interval, while the dependence on the mass of the anion was especially prominent in the region of lattice vibrations. 
The first-principle DFT+D3 scheme made it possible to obtain reliable data on the basic properties of complex organic and biologically active systems.