Justyna Dominikowska

Aromaticity from the viewpoint of molecular geometry: application to planar systems.

to Planar Systems Tadeusz M. Krygowski,*,† Halina Szatylowicz,*,‡ Olga A. Stasyuk,‡ Justyna Dominikowska, and Marcin Palusiak †Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland ‡Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Department of Theoretical and Structural Chemistry, Faculty of Chemistry, University of Łod́z,́ Pomorska 163/165, 90-236 Łod́z,́ Poland

EL: the new aromaticity measure based on one-electron density function

Ellipticity of the bond, being the quantity which numerically reflects how far the given chemical bond has elliptic cross-section, may be used to estimate π-electron contribution in bonding. We make use of that fact and develop a new measure of aromaticity—EL index. Since ellipticity is available from calculations on one-electron density function, EL can be used for both theoretical and experimental data. The EL measure is normalized to make interpretation of this parameter as easy and comfortable as possible. We compare EL values with the values of other commonly used aromaticity measures, such as HOMA, PDI, FLU, and NICS. It appears that the indications of EL are in agreement with indications of other indices and general expectations.

Cyclooctatetraene in metal complexes—planar does not mean aromatic

The cyclooctatetraene (COT) ring may act as an effective π-type ligand in metal complexes and very often in such complexes it adopts the planar structure, which in fact is not the most favourable conformation when compared with the free COT molecule. Such planarization is usually considered as an effect of COT ring aromatization due to charge transfer from the metal centre into the COT ligand and reorganization of the π-electron structure from a 4n to 4n+2 Huckel system. In this paper it has been shown that the conformational changes of the COT ring, leading to its planarization and partial bond equalization, in the first order are connected with the complexation efficiency and not with the aromatization of COT itself. Such a conclusion was drawn on the basis of statistical analysis of X-ray data collected in the Crystal Structure Database (47 COT rings were taken into consideration, all found in the selected highest quality X-ray measurements) and advanced quantum-chemical calculations, including analysis of the electron density distribution made in the framework of Atoms-in-Molecules Quantum Theory.

The role of the long-range exchange corrections in the description of electron delocalization in aromatic species

In this article, we address the role of the long‐range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange‐correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long‐standing debate in the literature. The presented results indicate that the noncorrected exchange‐correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteria.

Cyclooctatetraene dianion—an artifact?

Cyclooctatetraene in its dianionic form (COT2−) is considered to be partially or fully aromatic due to the fact that, unlike its neutral counterpart, it adopts planar structure with CC bonds equalized. However, some authors report that this dianion is neither planar nor aromatic. Thus, we performed a detailed analysis of the COT2− case. The influence of several technical parameters on the result of calculations on COT2− was investigated. It appears from our analysis that the use of some specific level of approximation may lead to very misleading results in which the COT ring occurs in its neutral structure, in fact being neither planar nor aromatic. Additionally, our results may suggest that COT2− dianion is rather an artificial structure (being the result of specific basis set description) and should not occur in experimental conditions.

Source of Cooperativity in Halogen-Bonded Haloamine Tetramers.

Inspired by the isostructural motif in α-bromoacetophenone oxime crystals, we investigated halogen-halogen bonding in haloamine quartets. Our Kohn-Sham molecular orbital and energy decomposition analysis reveal a synergy that can be traced to a charge-transfer interaction in the halogen-bonded tetramers. The halogen lone-pair orbital on one monomer donates electrons into the unoccupied σ*N-X orbital on the perpendicular N-X bond of the neighboring monomer. This interaction has local σ symmetry. Interestingly, we discovered a second, somewhat weaker donor-acceptor interaction of local π symmetry, which partially counteracts the aforementioned regular σ-symmetric halogen-bonding orbital interaction. The halogen-halogen interaction in haloamines is the first known example of a halogen bond in which back donation takes place. We also find that this cooperativity in halogen bonds results from the reduction of the donor-acceptor orbital-energy gap that occurs every time a monomer is added to the aggregate.

The effect of benzoannulation on the transition state and the proton transfer equilibrium in di(2-pyridyl)methane derivatives

The tautomeric properties of di(2-pyridyl)methane and its benzoannulated derivatives were studied using a computational approach (M05/6-31G(2d,p)). Our analysis showed that the degree of cyclic π-electron delocalization in benzene and pyridine rings is directly connected to the effect of resonance present in quasi-rings formed by intramolecular hydrogen bonds of the N–H⋯N type. This direct relation can be explained using two concepts, namely, the concept of Clars aromatic sextet and the Leffler–Hammond concept originally developed for the explanation of the energy relation between ground-state and transition-state structures corresponding to proton transfer in H-bridges. Application of these two different concepts allows us to explain in detail the role of intramolecular hydrogen bonding in polycyclic aromatic hydrocarbons containing N heteroatoms.

Why 1,2-quinone derivatives are more stable than their 2,3-analogues?

Abstract In this work, we have studied the relative stability of 1,2- and 2,3-quinones. While 1,2-quinones have a closed-shell singlet ground state, the ground state for the studied 2,3-isomers is open-shell singlet, except for 2,3-naphthaquinone that has a closed-shell singlet ground state. In all cases, 1,2-quinones are more stable than their 2,3-counterparts. We analyzed the reasons for the higher stability of the 1,2-isomers through energy decomposition analysis in the framework of Kohn–Sham molecular orbital theory. The results showed that we have to trace the origin of 1,2-quinones’ enhanced stability to the more efficient bonding in the π-electron system due to more favorable overlap between the SOMOπ of the ·C4n−2H2n–CH·· and ··CH–CO–CO· fragments in the 1,2-arrangement. Furthermore, whereas 1,2-quinones present a constant trend with their elongation for all analyzed properties (geometric, energetic, and electronic), 2,3-quinone derivatives present a substantial breaking in monotonicity.

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.

Aromaticity Induced by Electric Field: The Case of Polycalicenes.

Local and global π-electron delocalization occurring in planar poly-1,7-[N]calicenes is investigated with use of 10 aromaticity measures based on different physical properties. Systematic change of aromatic character is observed along chains of connected calicene units. Multidimensionality of the aromaticity phenomenon is studied with use of principal component analysis (PCA). The structural characteristics are compared with the properties of the isolated calicene molecule exposed to external electric fields of various intensities. Interrelations between the value of electric field applied and physical properties of the calicene molecule are discussed in the context of calicene unit affected by its surroundings in polycalicene chains. The patterns of global π-electron delocalization are described in graph theory terminology, and interconnections between local and global aromaticity in these systems are established.