Małgorzata Domagała

Conformers of diheteroaryl ketones and thioketones: a quantum chemical study of their properties and fundamental intramolecular energetic effects

The conformational behavior of ten diheteroaryl ketones and thioketones is investigated using various quantum chemical methods. These ketones and thioketones are formed by the disubstitution of formaldehyde and thioformaldehyde with such a heteroaryl group as 2-furanyl, 2-thiophenyl, 2-selenophenyl, 2-pyrrolyl or 1-methyl-2-pyrrolyl. For these compounds, their conformational preference and the energetic ordering of their conformers are determined at the MP2 and B3LYP levels of theory. Energetic barriers resulting from the interconversion between conformations are also estimated. The natural bond orbital (NBO) and interacting quantum atoms (IQA) methods are used to study fundamental intramolecular energetic effects influencing the stability of individual conformers. The results of the two methods indicate the great significance of the effect associated with electron delocalization (in the form of either NBO donor–acceptor interactions or the IQA interatomic exchange–correlation interaction energy) in governing the conformational behavior of the investigated diheteroaryl ketones and thioketones.

Spectroscopic and theoretical studies on some new pyrrol-2-yl-chloromethyl ketones

A novel series of pyrrole-2-yl chloromethyl ketones were synthesized and studied by FT-IR, 1H, 13C NMR spectroscopy and DFT calculations at the B3LYP/6-311++G(d,p) level of approximation. Two stable conformations were detected in solution: s-cis and s-trans forms where the CO group is located on the same side or the opposite side of N–H group, respectively. The conformational stability of these molecules is governed mainly by intermolecular hydrogen bonding interactions. The strength of hydrogen bonds was evaluated on the basis of 1H chemical shift and infrared red shift ΔνN–H of the stretching vibration of N–H proton donating bonds. The quantum theory of ‘atoms in molecules’ as well as the natural bond orbital method were applied to characterize hydrogen bonding interactions.

Substituent Effect in Benzene Dication

It was recently postulated that the benzene ring and its 4n + 2 π-electron analogues are resistant to the substituent effect due to the fact that such systems tend to retain their delocalized character. Therefore, the 4n π-electron dicationic form of benzene should appear to be less resistant to the substituent effect, as compared with its parent neutral molecule. For this reason the effect of substitution on the dicationic form of benzene was thoroughly investigated and the consequences of single and double substitution (of para- and meta-type) were assessed by means of several parameters, including various aromaticity indices and the Substituent Effect Stabilization Energy (SESE) parameter. It is shown that, distinct from neutral benzene, its dicationic form is much more sensitive to the substitution. However, the dicationic benzene itself, as a moiety with a significant deficit of electrons, will be considered as a strongly electron-withdrawing centre, thus interacting in a cooperative way with electron-donating substituents and in an anticooperative way with electron-withdrawing substituents. Clear differences between singlet- and triplet-state dicationic forms of benzene were also found. Triplet state structures seem to be significantly more delocalized, and as a consequence less sensitive to the substituent effect than the singlet state structures. Finally, the para- and meta-type substitution was investigated and it was found that the disubstituted dicationic benzene exhibits significantly different behaviour from that of neutral benzene. Although the difference between para- and meta-substitution can be found for dicationic benzene, the mechanism responsible for such an observation is different from that present in neutral benzene. Finally, it is shown how and why double ionization of benzene reduces its aromatic character in the singlet dication whereas aromaticity is essentially conserved in the triplet dication. The above findings highlight that in the case of charged analogues of benzene the aromaticity indices can be misleading and are to be used with great precaution.

Extremely Strong Halogen Bond. The Case of a Double-Charge-Assisted Halogen Bridge

The stable model of a double (±)charge-assisted halogen bridge has been built on the basis of searches of the Crystal Structure Database. The model, investigated by DFT theory, consists of quinuclidine-like cation derivatives and a set of simple anions. These charged fragments form halogen-bonded complexes of which the energy of complexation in some cases reaches 100 kcal/mol. Even for such strong interactions, the QTAIM characteristics are similar to those of the more classic, relatively weak halogen bonds. An important effect of complexation is the charge transfer measured by means of QTAIM and NBO. It can also be supposed, on the basis of detailed structural and QTAIM analysis, that the delocalization of the charge in a quinuclidine moiety occurs through space and not necessarily along formal bonds. The analysis of only partially charged and fully neutral counterparts of a double (±)charge-assisted halogen bridge shows significantly weaker bonding, being less than 10 kcal/mol.

UV-vis spectra of singlet state cationic polycyclic aromatic hydrocarbons: time-dependent density functional theory study.

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.

Heteroatom and solvent effects on molecular properties of formaldehyde and thioformaldehyde symmetrically disubstituted with heterocyclic groups C4H3Y (where Y = O–Po)

AbstractIn this work several molecular properties of symmetrically disubstituted formaldehyde and thioformaldehyde have been studied using a quantum chemistry approach based on density functional theory. Five-membered heteroaromatic rings containing a single group 16 heteroatom were taken into account as the substituents (i.e., furan-2-yl, thiophen-2-yl, selenophen-2-yl, tellurophen-2-yl, and the experimentally as yet unknown polonophen-2-yl). For the resulting ten formaldehyde and thioformaldehyde derivatives, the geometry, energetics, frontier molecular orbitals, dipole moment and polarizability of their molecules were examined in order to establish the effect of ring heteroatom on these properties. Furthermore, these properties were also determined for the molecules in three solvents of low polarity (benzene, chloroform, and dichloromethane) in order to expand our study to include solvent effects. The dipole moment and polarizability of the investigated molecules show regular variations when the ring heteroatom descends through group 16 and the solvent polarity grows. The heteroatom and/or solvent effects on the part of the studied properties are, however, more complex. An attempt is made to rationalize the observed variations in the molecular properties. The conformational behavior of the investigated molecules was also explored and the conformationally weighted values of dipole moment and polarizability are presented. Graphical abstractSome molecular properties of symmetrically disubstituted formaldehyde and thioformaldehyde