Hartree-Fock and density functional methods and IR and NMR spectroscopies in the examination of tautomerism and features of neutral 9-acridinamine in gaseous and condensed media

Two tautomeric molecules, 9-acridinamine (9-AA) and 9(10H)-acridinimine (9-AI), were examined at the ab initio Hartree-Fock (HF) and density functional (DFT) levels of theory with the 6-31G** basis sets. Solvent (hexane, CH3CN, H2O) effects were included in ab initio HF optimizations through the self-consistent reaction field (SCRF) technique. Subsequent Hessian calculations followed by the normal-mode analyses revealed all harmonic frequencies to be positive, thus confirming the validity of the geometry optimizations. The energies of the molecules at stationary points corresponding to ab initio HF geometries were supplemented with the second-order Moller-Plesset (MP2) electron correlation correction. Standard routines utilizing relationships of statistical thermodynamics enabled determination of enthalpies of formation (supplemented further with corrections arising from isogyric, hydrogenation, and isodesmic processes) and entropies (heat capacities) at selected temperatures, as well as constants revealing equilibrium between two tautomeric forms. Other physicochemical characteristics, such as bond orders, dipole moments, and energies of the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbitals were also obtained from theoretical calculations. Thermochemical data indicate that 9-AA and 9-AI should coexist at ambient temperature. This is also confirmed by a comparison of experimental IR and Raman spectra with harmonic frequencies derived theoretically. H-1 and C-13 chemical shifts obtained at the GIAO level of theory correlate only qualitatively with relevant experimental NMR data and do not exclude the existence of tautomeric phenomena. The distributions of atomic partial charges and electrostatic potential around the molecules differ noticeably, which implies that 9-AA and 9-AI may behave differently with respect to biomolecules.