10-Methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulfonates bearing alkyl substituents at the benzene ring were synthesized, purified, and identified. In the reaction with OOH(-) in basic aqueous media, the cations of the compounds investigated were converted to electronically excited 10-methyl-9-acridinone, whose relaxation was accompanied by chemiluminescence (CL). The kinetic constants of CL decay, relative efficiencies of light emission, chemiluminescence quantum yields, and resistance toward alkaline hydrolysis were determined experimentally under various conditions. The mechanism of CL generation is considered on the basis of thermodynamic and kinetic parameters of the reaction steps predicted at the DFT level of theory. The chemiluminescence efficiency is the result of competition of the electrophilic center at C(9) between nucleophilic substitution by OOH(-) or OH(-) and the ability of the intermediates thus formed to decompose to electronically excited 10-methyl-9-acridinone. Identification of stable and intermediate reaction products corroborated the suggested reaction scheme. The results obtained, particularly the dependency of the "usefulness" parameter, which takes into account the CL quantum yield and the susceptibility to hydrolysis, on the cavity volume of the entity removed during oxidation, form a convenient framework within which to rationally design chemiluminescent 10-methyl-9-(phenoxycarbonyl)acridinium cations.

The reaction between 9-carboxy-10-methylacridinium phenyl ester (CMAPE) and hydrogen peroxide leading to the formation of electronically excited 10-methyl-9-acridinone has been examined at the semiempirical PM3 level of theory. The reaction mechanisms proposed in the literature were verified and the details of the rather complicated processes leading to the emission of light (chemiluminescence) explained by these theoretical studies. Particular reaction steps were characterized at the thermodynamic and kinetic levels. The influence of the solvent was studied by means of the PM3-COSMO method (aqueous medium). The bottleneck steps determining the kinetics of the overall process were identified. Structural modifications of CMAPE that could improve its analytical usefulness were indicated on the basis of theoretical considerations.

Properties of multiply-charged hexafluorometallates MF6 3-, M = Sc, Y, La, ZrF6 2-, and TaF6 -, have been examined at the SCF, MP2, and Becke3LYP levels of theory using the SBKJ pseudopotentials and extended one-electron basis sets. For MF6 3-, M = Sc, Y, La, our results suggest a different assignment of vibrational transitions than those quoted by Nakamoto1 and recorded by von Becker et al. For the series of isoelectronic complexes, we document that the discrepancy between the theoretical gas phase vibrational frequencies and those measured in condensed phase experiments increases as the Becke3LYP HOMO binding energies become more positive. The frequencies of stretching modes measured in condensed phase experiments are systematically higher than those calculated for the gas phase anions. We suggest that a partial charge transfer from the multiply-charged anions to countercations is responsible for the stiffening of vibrational modes in condensed phase environments. The vertical electron detachment energies were determined at the outer valence Green's function, MP2, and B3LYP levels. The binding energies of hexafluorometallates with respect to the energies of atomic ions were also calculated and only a small contribution of the electron correlation effects was found.

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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 Møller−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. 1H and 13C 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.

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Diflavonol is a molecule that can exist in neutral or anionic form and in several tautomeric forms in ground and excited states. Absorption and emission spectroscopy combined with theoretical calculations have shown that only one tautomer of neutral diflavonol exists in the ground state, but two exist in the excited state. In the latter case, one is the tautomer originating from the ground state tautomer, which exists in strongly protic solvents, the other is the phototautomer occurring in weakly protic or aprotic solvents as a result of the intramolecular transfer of one proton. The OH groups present in diflavonol and involved in weak intramolecular hydrogen bonds exhibit a proton-donating ability reflected by the experimental values of acidity constants or theoretical enthalpies and free energies of proton detachment. The electronically excited molecule is a relatively strong acid when it loses one proton. With increasing basicity of the medium, monoanionic and dianionic forms occur which exhibit spectral characteristics and an emission ability different from those of neutral diflavonol. These interesting features of diflavonol open up possibilities for the analytical use of the compound and its application as a spectral probe sensitive to the properties of liquid phases.

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