Structural features of tricarbonyl(N-methyl-2-pyridinecarboxyamide)chloro-rhenium(I)-potential precursor of radiopharmaceuticals

Structural characteristics of the novel [fac-Re(CO)₃L]Cl complex, where L denotes the N-methyl-2-pyridinecarboxyamide, are presented. Molecular structure of the complex has been established by means of X-ray single crystal diffraction and compared with quantum mechanical calculations. It has been shown that pyridinethioamide is softer base than pyridineamide. As a result of different Re^I–S and Re^I–O bond covalent shortenings (reflecting their diverse ionic/covalent character ratio) difference of the bond lengths decreases from a theoretical value of 0.45 Å to the value of 0.28 Å.     

Molecular similarity based on atomic electrostatic potential

We propose a new similarity measure operating in the space spanned by the potential values, evaluated at atoms constituting the benzene ring and the COOH group in para-substituted benzoic acids and at benzene ring atoms in monosubstituted benzenes. The similarity measures are equivalent to the Euclidean distance between points in that space. Only the distances between the potentials at corresponding atoms in different molecules are included. The distances for benzene rings were very similar, regardless of whether they were calculated in para-substituted acids or in monosubstituted benzenes. As reference reactions, dissociation of benzoic acids and nitration of monosubstituted benzenes have been used. The effects of reduction of dimensionality of the potential space on the comparison of similarity measures with the free energies of the reference reactions have been investigated. It became obvious that the potentials at individual atoms in molecules of the acids and monosubstituted benzenes are mutually correlated to a high degree.     

Complex formation in the indolo[2,3-b]quinolines--methylene blue systems in aqueous solutions

Changes in the absorption spectra of methylene blue (MB) aqueous solutions containing an indoloquinoline can be described by assuming complex formation between MB and the indolinoquinoline. At the low concentration range of the two reactants a simple 1:1 complex stoichiometry was postulated on the basis of the Benesi-Hildebrand, Scott and Scatchard linearization of spectroscopic data and augmented by the formation of an isosbestic point. The estimated association constants were correlated with the ab initio calculated electronic properties of indolo[2,3-b]quinolines.     

Application of natural bond orbital analysis to delocalization and aromaticity in C-substituted tetrazoles.

Energies of two tautomeric forms of 10 tetrazole derivatives substituted at C5 were established by DFT/B3LYP calculations carried out at the 6-311++G level. In each case the calculated energy of the 2H-tautomer was lower than that of the 1H. Furthermore, three geometric aromaticity indices of both forms were calculated, as were the values of nuclear independent nuclear shift and aromatic stabilization energy. The electronic properties were evaluated with the help of the natural bonding orbital theory. Following this a new pi-delocalization parameter, the root-mean square of pi-electron density localized on the atoms of the five-membered tetrazole ring, SDn, was introduced. It was concluded that the electronic delocalization can be described equally well by three different parameters: SDn, the extent of the transfer of electron density from the p(z) orbital of one nitrogen to the rest of the pi electron system, and population of two antibonding pi orbitals. Arguably, the information provided by the electronic parameters is similar to that contained in the geometric (structural) aromaticity indices except for tetrazole substituted by -BH(2).     

Calculation of acidic dissociation constants in water: solvation free energy terms. Their accuracy and impact

Three polarizable continuum models, DPCM, CPCM, and IEFPCM, have been applied to calculate free energy differences for nine neutral compounds and their anions. On the basis of solvation free energies, the pK_a values were obtained for the compounds in question by using three thermodynamic cycles: one, involving the combined experimental and calculated data, as well as two other cycles solely with calculated data. This paper deals with the influence of factors such as the SCRF model applied, choice of a particular thermodynamic cycle, atomic radii used to build a cavity in the solvent (water), optimization of geometry in water, inclusion of electron correlation, and the dimension of the basis set on the solvation free energies and on the calculated pK_a values. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Geometrical,electrical, and energetic parameters of hetero-disubstituted cumulenes and polyynes in the presence and absence of the external electric field

Structural Chemistry - Cumulenes and polyynes have the potential to be applied as linear, sp-hybridized, one-dimensional all-carbon nanowires in molecular electronics and optoelectronics. The...     

Platinum(II) complexes with thiourea derivatives containing oxygen,sulfur or selenium in a heterocyclic ring: computational studies and cytotoxic properties

Platinum(II) complexes with 1-(2-oxolanylmethyl)-2-thiourea, 1-(2-thiolanylmethyl)-2-thiourea and 1-(2-selenolanylmethyl)-2-thiourea were synthesized in order to compare their cytotoxic activities with those of the free thioureas. Their equilibrium geometries and bonding energies in the gas phase and in solution were calculated using density functional theory at the MPW1PW/LanL2DZ level. The IR spectra of the complexes and their free ligands were compared. Stability of the complexes in 0.9% saline aqueous solution was tested by means of HPLC. The cytotoxic activities of the species against five human cell lines were evaluated, as well as the antimicrobial activities against twelve bacterial strains.     

Correlations of NBO energies of individual hydrogen bonds in nucleic acid base pairs with some QTAIM parameters

Structural Chemistry - DNA and RNA base-pairs are the most important systems containing multiple hydrogen bonds. Characterizing the energy of individual intermolecular interactions in such systems...     

The response of electronic and energetic properties of conjugated vs aromatic molecules to an external uniform electric field

The susceptibility of electric and energetic properties of two sets of molecules to perturbation in a uniform electric field was investigated. The molecules of one set were deca-1,3,5,7-pentaene terminated with two functional groups, R₁ and R₂; those of the second set were 4-R₁-4′-R₂-p-diphenylbenzenes, with R₁ and R₂ being the same as in the polyene set. The polyene and aromatic molecules had similar lengths of the system of conjugated bonds between the two most extreme carbon atoms. Dipole moments were used as determinants of the overall charge transfer within the molecules. The electric field was directed along the main (longest) axes of the molecules from the negative pole to the positive. Comparison of the effect of the charge relocation in both sets of conjugated molecules revealed that the charge transfer imposed by the electric field was more efficient in the polyenes than in the aromatic compounds; however, for the molecule pairs with the same R₁ and R₂, reversal of the dipole moment direction falls at the same field strength. Parabolic dependence of the molecules’ energy as a function of field strength can also be interpreted in terms of the response of electron density to an electric field.

     

Electronic properties and aromaticity of substituted diphenylfulvenes in the ground (S<Subscript>0</Subscript>) and excited (T<Subscript>1</Subscript>) states

In the present account, we investigate electronic properties of diphenylfulvene and its derivatives substituted in phenyl rings. The results were compared with the analogous properties of fulvene and its derivatives with the same substituents at the exocyclic carbon atom. All properties were evaluated and compared in the ground electronic S₀ state and in the first excited T₁ triplet state. These properties are dipole moments, charges, number of π electrons, and aromaticity of the fulvenic, five-membered ring in the two sets of compounds. The latter property was estimated by the harmonic oscillator model for aromaticity (HOMA) index and, for the fulvenes group, by the calculation of aromatic stabilization energy in both electronic states. It was also investigated whether Baird’s rule alone can account for the aromaticity differences in the two electronic states.