| Abstract
| - Two tautomeric molecules, 9-acridinamine (9-AA) and9(10H)-acridinimine (9-AI), were examined at theabinitio Hartree−Fock (HF) and density functional (DFT) levels oftheory with the 6-31G** basis sets.Solvent(hexane, CH3CN, H2O) effects were includedin ab initio HF optimizations through the self-consistentreactionfield (SCRF) technique. Subsequent Hessian calculations followedby the normal-mode analyses revealedall harmonic frequencies to be positive, thus confirming the validityof the geometry optimizations. Theenergies of the molecules at stationary points corresponding to abinitio HF geometries were supplementedwith the second-order Møller−Plesset (MP2) electron correlationcorrection. Standard routines utilizingrelationships of statistical thermodynamics enabled determination ofenthalpies of formation (supplementedfurther with corrections arising from isogyric, hydrogenation, andisodesmic processes) and entropies (heatcapacities) at selected temperatures, as well as constants revealingequilibrium between two tautomeric forms.Other physicochemical characteristics, such as bond orders, dipolemoments, and energies of the lowestunoccupied (LUMO) and highest occupied (HOMO) molecular orbitals werealso obtained from theoreticalcalculations. Thermochemical data indicate that 9-AA and 9-AIshould coexist at ambient temperature. Thisis also confirmed by a comparison of experimental IR and Raman spectrawith harmonic frequencies derivedtheoretically. 1H and 13C chemical shiftsobtained at the GIAO level of theory correlate onlyqualitativelywith relevant experimental NMR data and do not exclude the existence oftautomeric phenomena. Thedistributions of atomic partial charges and electrostatic potentialaround the molecules differ noticeably, whichimplies that 9-AA and 9-AI may behave differently with respect tobiomolecules.
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