The angular distribution of the C–H⋯O interactions of aromatic C–H donors was studied by analyzing data in the Cambridge Structural Database (CSD) and by ab initio calculations. The analysis of the C–H⋯O interactions in the crystal structures from the CSD indicate that aromatic C–H donors do not show strong preference for linear contacts and that the preference depends on the type of the atom or group in the o-position to the interacting C–H group. Namely, the acceptor oxygen atom has possibility for simultaneous C–H⋯O interactions with the hydrogen atom in the o-position to the interacting C–H group. The C–H⋯O interactions of aromatic molecules with two hydrogen atoms in the o-positions do not show preference for linear contacts. Bifurcated interactions are observed in a substantial number of structures. Moreover, in the structures with a substituent in the o-position there is possibility for simultaneous interactions, depending on the nature of the substituent. The results of the ab initio calculations are in accord with the CSD data and show that the stabilization energy is larger for bifurcated than for linear interactions. The calculated energies at the MP2/cc-pVTZ level for linear C–H⋯O interactions of benzene with water, methanol, and acetone are 1.28, 1.47, 1.45 kcal mol⁻¹; while for bifurcated interactions are 1.38, 1.63, and 1.70 kcal mol⁻¹, respectively. Analysis of the data in the CSD and the ab initio calculations indicate that the vicinity of the other possible hydrogen donors in the aromatic molecules causes a small tendency for linear contact in the C–H⋯O interactions. The result that nonlinear interactions are not energetically disfavoured, because of the possibility for simultaneous interactions, can be very important for recognizing C–H⋯O interactions in biomolecules containing aromatic groups, like proteins.

The extension of the aromatic system in the ligand structure (enlarging of the π-electron framework) results in a decrease of the effective band gap energy of surface modified TiO ₂ nanoparticles.

The surface modification of nanocrystalline TiO₂ particles (45 Å) with catecholate-type ligands having different electron donating/electron withdrawing substituent groups, specifically 3-methylcatechol, 4-methylcatechol, 3-methoxycatechol, 3,4-dihydroxybenzaldehyde and 4-nitrocatechol, was found to alter the optical properties of nanoparticles in a similar way to catechol. The formation of the inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and a reduction of the effective band gap, being slightly less pronounced in the case of electron withdrawing substituents. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of the catecholate type (binuclear bidentate binding–bridging) thus restoring six-coordinated octahedral geometry of surface Ti atoms. From the absorption measurements (Benesi–Hildebrand plot), the stability constants in methanol/water = 90/10 solutions at pH 2 in the order of 10³ M⁻¹ have been determined. The binding structures were investigated by using FTIR spectroscopy. Thermal stability of CT-complexes was investigated by using TG/DSC/MS analysis. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain the vibrational frequencies of charge transfer complexes, and the calculated values were compared with the experimental data.

The surface modification of nanocrystalline TiO₂ particles (45 Å) with salicylate-type ligands consisting of an extended aromatic ring system, specifically 3-hydroxy-2-naphthoic acid, 3,5-dihydroxy-2-naphthoic acid and 3,7-dihydroxy-2-naphthoic acid, was found to alter the optical properties of nanoparticles in a similar way to salicylic acid. The formation of the inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and a reduction in the band gap upon the increase in the electron delocalization when including an additional ring. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of a salicylate-type (binuclear bidentate binding-bridging) thus restoring the six-coordinated octahedral geometry of surface Ti atoms. From both absorption measurements in methanol/water = 90/10 solutions and steady-state quenching measurements of modifier fluorescence upon binding to TiO₂ in aqueous solutions, stability constants in the order of 10³ M⁻¹ have been determined at pH 2 and pH 3. Fluorescence lifetime measurements, in the presence and absence of colloidal TiO₂ nanoparticles, indicated that the fluorescence quenching process is primarily static quenching, thus proving the formation of a nonfluorescent CT complex. The binding structures were investigated by using FTIR spectroscopy. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain the vibrational frequencies of charge transfer complexes, and the calculated values were then compared with the experimental data.

The influence of hydrogen bonding on the electrostatic potentials and sensitivity of selected energetic molecules was studied by quantum chemical calculations.