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)-acridinimin e (9-AI), were examined at the ab initio Hartree-Fock (HF) and density functional (DFT) levels of theory with the 6-31G* basis sets. Solven t (hexane, CH3CN, H2O) effects were included in ab initio HF optimizat ions through the self-consistent reaction field (SCRF) technique. Subs equent Hessian calculations followed by the normal-mode analyses revea led all harmonic frequencies to be positive, thus confirming the valid ity of the geometry optimizations. The energies of the molecules at st ationary points corresponding to ab initio HF geometries were suppleme nted with the second-order Moller-Plesset (MP2) electron correlation c orrection. Standard routines utilizing relationships of statistical th ermodynamics enabled determination of enthalpies of formation (supplem ented further with corrections arising from isogyric, hydrogenation, a nd isodesmic processes) and entropies (heat capacities) at selected te mperatures, as well as constants revealing equilibrium between two tau tomeric forms. Other physicochemical characteristics, such as bond ord ers, dipole moments, and energies of the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbitals were also obtained from the oretical calculations. Thermochemical data indicate that 9-AA and 9-AI should coexist at ambient temperature. This is also confirmed by a co mparison of experimental IR and Raman spectra with harmonic frequencie s derived theoretically. H-1 and C-13 chemical shifts obtained at the GIAO level of theory correlate only qualitatively with relevant experi mental NMR data and do not exclude the existence of tautomeric phenome na. The distributions of atomic partial charges and electrostatic pote ntial around the molecules differ noticeably, which implies that 9-AA and 9-AI may behave differently with respect to biomolecules.