Resul Sevinçek

Aromaticity balance, π-electron cooperativity and H-bonding properties in tautomerism of salicylideneaniline: the quantum theory of atoms in molecules (QTAIM) approach.

Topological analysis based on DFT calculations regarding proton transfer reaction in salicylideneaniline (SA) was performed to scrutinize possible changes in the intramolecular H-bond, π-electron delocalization and aromaticity levels of certain fragments. Quantum chemical calculations and natural bond orbital (NBO) analyses were carried out over a tautomeric ensemble whose members correspond to the molecules at different stages in tautomeric interconversion of SA. The elaboration of intramolecular hydrogen bonding in terms of the relevant topological parameters and the interpretation of certain dependencies regarding its strength were examined. The results show that delocalization index (DI) between donor and acceptor atom δ(O,N) is a useful topological parameter for describing H-bond strength, which is influenced by π-delocalization level within quasiaromatic chelate ring, indicating its resonance-assisted character. NBO analyses reveal that lone-pair (LP) population on N center also affects the strength of intramolecular H-bond in SA. Furthermore, π-electron transfer accompanying intramolecular proton migration in SA is brought into being through formally vacant non-Lewis type LP* orbital on the tautomeric proton. As a result of this, tautomeric protons in molecular entities near TS have hypovalent character due to the lack of electron population in the bonding orbital relative to that in LP* orbital. While H-bonds in the tautomeric ensemble of SA are predominantly partial covalent, molecular entities close to transition state have the strongest covalent H-bonds. The most important result is also that there are linear correlations between the orders of bonds (hydroxyl and amine) involving intramolecular H-bond and electron density values at the relevant BCPs due to partially covalent character of these bonds, contrary to exponential behavior as for purely covalent bonds. Quasiaromatic chelate ring formation is established not only to compel a reduced aromaticity of salicylidene ring but also to decrease in LP-population on N.

Crystal Structure and Conformational Analysis of 4-[(p-N,N-Dimethylamino) benzylidene]-2-(3,5-dinitrophenyl)oxazole-5-one

ABSTRACT An azlactone derivative, 4-[(p-N,N-dimethylamino)benzylidene]-2-(3,5-dinitrophenyl)oxazol-5-one (DNPO), C18H14N4O6, has been synthesized, and its crystal structure has been investigated by single crystal X-ray analysis and ab initio method. DNPO is monoclinic, with a = 9.3628(4) Å, b = 13.5148(9) Å, c = 13.7701(6) Å, β = 92.921(4)°, Z = 4, Dx = 1.46 g/cm3, μ(MoKα) = 0.112 mm−1, and space group P 121/c1. The whole molecule is almost planar. The crystal structure is stabilized by C–H · · · N type intramolecular, C–H · · · O type intermolecular interactions. To determine the flexibility of DNPO, the selected torsion angle is varied from − 180° to 180° in steps of 10°, and the molecular energy profile is calculated and analyzed.

Changes in ligating abilities of the singlet and triplet states of normal, abnormal and remote N-heterocyclic carbenes depending on their aromaticities

AbstractQuantum chemical calculations at B3LYP/aug-cc-pVTZ level about singlet N-heterocyclic carbene (NHC) ligands, imidazol-2-ylidene, imidazol-4-ylidene, pyrazol-3-ylidene and pyrazol-4-ylidene, and their protonated analogues show that they are considerably aromatic except for pyrazol-3-ylidene. This result is experimentally verified by approximately five thousand NHC transition metal complexes retrieved from the Cambridge Structural Database (CSD). CSD search discloses that NHCs can participate in π-stacking interactions, albeit scarce. Geometry-based HOMA and electronic aromaticity index FLU rather than NICS provide a satisfactory description of the bonding situations in NHC ligands. Singlet state of the normal NHC has electron-deficient aromaticity as compared to those of the abnormal and remote NHCs. Depending on the transition from the singlet to triplet state, NHCs become electron-deficient ligands except for remote NHC. Computational studies regarding electronic nature of free NHC ligands show that the π-electronic population of the formally vacant pπ orbital on the carbene atoms in abnormal and remote NHC is occurred as a result of the aromaticity of NHCs, not as a result of the direct electron donation from LP-orbitals of N atoms to carbene atom according to putative push-pull effect used in understanding the electronic stabilization of normal NHC. Increase in the aromaticity raises σ-donating ability of both imidazol- and pyrazol-based NHC ligands. Free abnormal and remote NHC ligands have higher σ-donation ability than normal NHC ligands. The lack of σ-donating ability of normal NHC is compensated by its relatively high π-accepting ability, whereas π-back donation abilities of abnormal and remote NHCs are prohibited by their almost fully occupied π-orbitals. Aromaticities of the triplet NHC ligands are higher than that of the lowest-lying triplet state of benzene. Increase in the aromaticity of NHC ligands decreases van der Waals shortening in TM-NHC bonds mainly due to diminishing dative character of these bonds. FigureSinglet and triplet states of Arduengo type (normal) NHC showing their electron deficient aromatic characters

2,5-Dihexyl-3,6-diphenyl-pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione.

The asymmetric unit of the title compound, C30H36N2O2, contains one half-molecule, the other half being generated by a crystallographic inversion centre. The crystal structure is devoid of any classical hydrogen bonds however, non-classical C—H⋯O interactions link the molecules into chains propagating in [001] and a C—H⋯π interaction leads to the formation of a two-dimensional network in (011).

Positional isomeric effect on the crystallization of chlorine-substituted N-phenyl-2-phthalimidoethanesulfonamide derivatives.

The ortho-, para- and meta-chloro-substituted N-chlorophenyl-2-phthalimidoethanesulfonamide derivatives, C16H13ClN2O4S, have been structurally characterized by single-crystal X-ray crystallography. N-(2-Chlorophenyl)-2-phthalimidoethanesulfonamide, (I), has orthorhombic (P2(1)2(1)2(1)) symmetry, N-(4-chlorophenyl)-2-phthalimidoethanesulfonamide, (II), has triclinic (P1¯) symmetry and N-(3-chlorophenyl)-2-phthalimidoethanesulfonamide, (III), has monoclinic (P2(1)/c) symmetry. The molecules of (I)-(III) are regioisomers which have crystallized in different space groups as a result of the differing intra- and intermolecular hydrogen-bond interactions which are present in each structure. Compounds (I) and (II) are stabilized by N-H···O and C-H···O hydrogen bonds, while (III) is stabilized by N-H···O, C-H···O and C-H···Cl hydrogen-bond interactions. The structure of (II) also displays π-π stacking interactions between the isoindole and benzene rings. All three structures are of interest with respect to their biological activities and have been studied as part of a programme to develop anticonvulsant drugs for the treatment of epilepsy.

Structural and photophysical characterization, topological and conformational analysis of 2-o-tolyl-4-(3-N,N-dimethylaminophenyl-methylene)-oxazol-5-one

A novel oxazole-5-one derivative 2-o-tolyl-4-(3-N,N-dimethylaminophenylmethylene)-oxazol-5-one (TDPO) C19H18N2O2 is synthesized and characterized and the crystal structure is determined by X-ray crystallography. TDPO is monoclinic in the P21/c space group. The molecule adopts the Z configuration. To enlighten the flexibility of TDPO, the selected torsion angle is varied from −180° to 180° in each 10° separately, and the molecular energy profile is calculated and analyzed by density functional calculations. In addition, Bader’s QTAIM analysis is performed to investigate the intramolecular weak interactions.

Substituent position effect on the crystal structures of N-phenyl-2-phthalimidoethanesulfonamide derivatives

In order to determine the impact of different substituents and their positions on intermolecular interactions and ultimately on the crystal packing, unsubstituted N-phenyl-2-phthalimidoethanesulfonamide, C16H14N2O4S, (I), and the N-(4-nitrophenyl)-, C16H13N3O6S, (II), N-(4-methoxyphenyl)-, C16H16N3O6S, (III), and N-(2-ethylphenyl)-, as the monohydrate, C18H18N2O4S·H2O, (IV), derivatives have been characterized by single-crystal X-ray crystallography. Sulfonamides (I) and (II) have triclinic crystal systems, while (III) and (IV) are monoclinic. Although the molecules differ from each other only with respect to small substituents and their positions, they crystallized in different space groups as a result of differing intra- and intermolecular hydrogen-bond interactions. The structures of (I), (II) and (III) are stabilized by intermolecular N-H...O and C-H...O hydrogen bonds, while that of (IV) is stabilized by intermolecular O-H...O and C-H...O hydrogen bonds. All four structures are of interest with respect to their biological activities and have been studied as part of a program to develop anticonvulsant drugs for the treatment of epilepsy.