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The crystal and molecular structure of pyrrole-2-carboxamide (PyCa) determined by single crystal X-ray diffraction is presented. The trends are in line with the experimental results presented since in crystals there are centrosymmetric dimers of PyCa.

The molecular and crystal structure of pyrrole-carboxylic acid (PCA) that is determined by single-crystal X-ray diffraction is discussed. The Bader theory is applied to confirm the influence of pyrrole moiety on the strength of H-bond interaction.

Low temperature x ray diffraction measurements to validate the existence of H...H intermolecular contact in the crystal and molecular structures of [4-(E)-but-1-enyl)-2,6-dimethoxyphenyl]pyridine-3-carboxylate (BDMP) and [4-(E)-pent-1-enyl)-2,6-dimethoxyphenyl]pyridine-3-carboxylate (PDMP) is discussed. The probability of intramolecular dihydrogen bonding is reported to be supported by the application of the Bader theory.

A theoretical study of singlet state cations of polycyclic aromatic hydrocarbons is performed. Appropriate symmetry suitable for further calculations is chosen for each of the systems studied. The excitation states of such species are obtained by the time dependent density functional theory (TD-DFT) method. The computations are performed using both Pople and electronic response properties basis sets. The results obtained with the use of different basis sets are compared. The electronic transitions are described and the relationships for the lowest-lying transitions states of different species are found. The properties of in-plane and out-of-plane transitions are also delineated. The TD-DFT results are compared with the experimental data available. [ABSTRACT FROM AUTHOR]

The impact of intramolecular noncovalent interaction, H-bonding and Li-bonding, on the properties of substituents communicating through the resonance effect in such molecular systems as salicylaldehyde, o-hydroxy Schiff base, o-nitrosophenol and their lithium analogues is examined. The studies have shown that the relation between intramolecular noncovalent interactions and the [pi]-electron delocalization in the sequence of [pi]-conjugated covalent bonds linking A and B is considered in terms of the Hammett-like substituent effect in which electron-donating and electron-withdrawing properties of substituents are affected by the noncovalent interaction.

The interplay between aromaticity and hydrogen bonding in 1,3-dihydroxyaryl-2-aldehydes is examined by means of quantum-chemical calculations. The studies have shown that the quasi-ring has partially adopted the role of a typical aromatic ring, the position of which has a meaningful influence on the aromaticity of the rest of the rings.

A detailed study about the electronic effects taking place within the molecular system of [sigma]-hydroxy Schiff bases and the relation between localization/delocalization of [pi]-electrons within the whole system is described. The results concluded that [pi]-electron effects play a crucial role in the stabilization of the hydrogen bridge within [sigma]-hydroxy Schiff bases.

A series of o-hydroxyaryl aldehydes are analyzed to discuss the interplay between the resonance-assisted hydrogen bond (RAHB) formation and the aromaticity of the adjacent aromatic rings. Results reveal that both the substituent effect and the effect of resonance assistance of the hydrogen bond are mutually cooperating, and that it is possible to state that both these effects could be treated as a component of the same phenomenon.

Cocrystals of thiourea with 4‐nitropyridine N‐oxide, C5H4N2O3·2CH4N2S, (I), and 3‐bromopyridine N‐oxide, C5H4BrNO·CH4N2S, (II), crystallize in the monoclinic space group P21/c. In the crystals, molecules of both components are linked by N—H...O hydrogen bonds, creating R21(6) synthons. The bromine substituent of the N‐oxide component in (II) is a centre for C—Br...S halogen bonding to the thiourea molecule. Computations based on quantum chemistry methods (quantum theory of atoms in molecules, QTAIM) and atoms in molecules (AIM) theory were performed for a more detailed description of the observed type of halogen‐bonding interaction. [ABSTRACT FROM AUTHOR]