NMR Study of Solvation Effect on the Geometry of Proton-Bound Homodimers of Increasing Size.

Hydrogen bond geometries in the proton-bound homodimers of quinoline and acridine derivatives in an aprotic polar solution have been experimentally studied using 1H NMR at 120 K. The reported results show that an increase of the dielectric permittivity of the medium results in contraction of the N···N distance. The degree of contraction depends on the homodimer's size and its substituent-specific solvation features. Neither of these effects can be reproduced using conventional implicit solvent models employed in computational studies. In general, the N···N distance in the homodimers of pyridine, quinoline, and acridine derivatives decreases in the sequence gas phase > solid state > polar solvent.

Ligand Design for Site-Selective Metal Coordination: Synthesis of Transition-Metal Complexes with η6 -Coordination of the Central Ring of Anthracene.

A polycyclic aromatic ligand for site-selective metal coordination was designed by using DFT calculations. The computational prediction was confirmed by experiments: 2,3,6,7-tetramethoxy-9,10-dimethylanthracene initially reacts with [(C5 H5 )Ru(MeCN)3 ]BF4 to give the kinetic product with a [(C5 H5 )Ru]+ fragment coordinated at the terminal ring, which is then transformed into the thermodynamic product with coordination through the central ring. These isomeric complexes have markedly different UV/Vis spectra, which was explained by analysis of the frontier orbitals. At the same time, the calculations suggest that electrostatic interactions are mainly responsible for the site selectivity of the coordination.

Conformationally locked chromophores as models of excited-state proton transfer in fluorescent proteins.

Members of the green fluorescent protein (GFP) family form chromophores by modifications of three internal amino acid residues. Previously, many key characteristics of chromophores were studied using model compounds. However, no studies of intermolecular excited-state proton transfer (ESPT) with GFP-like synthetic chromophores have been performed because they either are nonfluorescent or lack an ionizable OH group. In this paper we report the synthesis and photochemical study of two highly fluorescent GFP chromophore analogues: p-HOBDI-BF2 and p-HOPyDI:Zn. Among known fluorescent compounds, p-HOBDI-BF(2) is the closest analogue of the native GFP chromophore. These irrreversibly (p-HOBDI-BF(2)) and reversibly (p-HOPyDI:Zn) locked compounds are the first examples of fully planar GFP chromophores, in which photoisomerization-induced deactivation is suppressed and protolytic photodissociation is observed. The photophysical behavior of p-HOBDI-BF2 and p-HOPyDI:Zn (excited state pK(a)'s, solvatochromism, kinetics, and thermodynamics of proton transfer) reveals their high photoacidity, which makes them good models of intermolecular ESPT in fluorescent proteins. Moreover, p-HOPyDI:Zn is a first example of "super" photoacidity in metal-organic complexes.

Mutual influence of cyclopentadienyl and carbonyl ligands in cymantrene: QTAIM study.

The phenomenon of bond length alternation in the cyclopentadienyl (Cp) ligand of cymantrene (eta(5)-C(5)H(5))Mn(CO)(3) was investigated using high-resolution X-ray diffraction analysis (XRD) and quantum chemical calculations. It was shown that the trans-effect of strong field CO ligands causes the redistribution of electron density in the Cp ring, and it manifests in atomic charges and energy of its carbon atoms. The angle C(Cp)-Mn-CO is characteristic for this feature and can be used for the prediction of bond lengths and the charge distribution in the aromatic fragment in various piano-stool-type complexes.

Estimation of dissociation energy in donor-acceptor complex AuCl x PPh3 via topological analysis of the experimental electron density distribution function.

The high-resolution X-ray diffraction analysis and plane-wave density functional theory were applied to the investigation of charge density distribution in the donor-acceptor complex of AuCl with PPh3. The approach allows us to estimate the atomic charges, the charge transfer, the energy of weak interactions (Au...H, Au...C, H...Cl, etc.), and Au-P bond energy directly from the experimental data.