Slavko Radenković

We investigated halogenated cycloalkanes in which halogen substituents are attached to each carbon atom, forming a cyclic framework. The dicationic forms of these molecules were found to exhibit σ-electron delocalization along the halogen-atom ring.

The photochemistry of heteroaromatic compounds depends on the character of their lowest electronically excited states, which are of either n,π* or π,π* type. For species with 4n + 2 π-electrons, the latter type of states can be antiaromatic to various extents according to Baird's rule and, thus, highly reactive. The n,π* type of states, on the other hand, will have an odd number of π-electrons leading to an unclear character, spanning from aromatic to antiaromatic. Six-membered ring (6-MR) heteroaromatics with in-plane lone-pair orbitals (n_σ, herein n) have either n,π* or π,π* states as their lowest vertically excited states, but regular five-membered ring (5-MR) heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as these states. However, this is different for cyclic π-conjugated (potentially aromatic) 5-MR carbenes that have the n orbitals at the divalent C atom. Also, 3-MR species have n,π* states as their lowest vertically excited states. Herein, we reveal which factors determine which type of vertical excited state is the lowest in energy for various heteroaromatics. The important factors are (i) the electronegativity of the heteroatom(s), (ii) the valence angle at the heteroatom impacting the lone-pair orbital energy, (iii) the number of π-orbitals and π-electrons, (iv) the degree of (anti)aromatic character of the n,π* state, (v) the electronegativity of atoms adjacent to the heteroatom, and (vi) the spatial extent of the n orbital affecting the intraorbital electron repulsion. Our findings point toward the development of a rational systematics for prediction of which heteroaromatics have n,π* as the first vertical excited states and which ones have π,π* states as these.

We proposed a modified NDDO-based approach for calculating magnetically induced current densities (MICDs), which significantly reduces computational time without compromising accuracy.

The development of more modern and effective strategies for teaching chemical bonding theory is a key objective within the chemistry education community. A solid understanding of this concept is essential for comprehending a range of chemistry topics, including the physical and chemical properties of substances, chemical thermodynamics, and polymers – all of which play a vital role in real-world applications. Integrating valence bond (VB) theory, with its strong interpretative capabilities, into the teaching of chemical bonding through information and communication technologies (ICT) can help simplify complex ideas and enhance students' comprehension. This paper presents a structured approach to explaining charge-shift bonding (CSB) within the VB theory framework by calculating the contributions of covalent and ionic VB structures, bond dissociation energy, and resonance energy. Additionally, it introduces unique teaching materials for university teachers and students, combining contemporary teaching tools with classical VB theory. The study involved 47 chemistry students, and the effectiveness of the practical training was assessed using three questionnaires, two knowledge tests (pre-test and post-test), and one survey. Results showed that MSc and PhD students were able to successfully grasp the fundamental concepts of CSB using the proposed approach, while undergraduate students encountered more difficulty. The practical class can serve as a valuable supplement to traditional upper-level instruction on chemical bonding, helping educators and students stay informed about more accurate and modern explanations of bonding in certain molecules.

The doubly oxidized hexaiodobenzene [C₆I₆]²⁺ is a well-known example of a double aromatic molecule, exhibiting both π- and σ-aromaticity. In this study, a series of periodo-monocyclic molecules and their doubly oxidized forms were systematically investigated to explore the origin of their double aromaticity. These molecules were employed to provide insights into how the size and aromaticity of the central carbon atom ring influence the aromaticity of the resulting doubly oxidized structures. The knowledge gained in this study was subsequently applied to model periodo-derivatives of polycyclic (anti)aromatic hydrocarbons in which oxidation can induce additional σ-electron cyclic delocalization along the macrocyclic iodine ring, thus also achieving their double aromaticity.

In this study, the bonding and electronic structure of the HNBeCO molecule were investigated using high level classical Valence Bond (VB) theory. Starting from a full-structure calculation involving 175 VB structures, we identified and selected only seven dominant VB structures to describe the bonding between nitrogen and beryllium in HNBeCO. Our results showed that the employed set of VB structures accurately predicts the bond dissociation energy, closely matching the reference values obtained from CCSD(T) calculations. This study highlights the importance of both static and dynamic correlation in describing multiple bonding in beryllium-containing compounds. Notably, we confirmed that the N-Be bond in HNBeCO exhibits a triple bond character.

This study investigates π-delocalization, π-bonding situations, and aromaticity of the pentazolate anion ([cyclo-N₅^-], (a)), which was detected by Christe et al. in 2002. To gain a broader perspective, we investigated the iso-π-electronic species [cyclo-P₅^-] (b) and [cyclo-(CH)₅^-] (c). VB analyses reveal that the three studied molecules display significant resonance stabilization, as indicated by their high resonance energy values. A comprehensive analysis of aromaticity was conducted using electronic and magnetic aromaticity indices, revealing that all three anions exhibit strong π-aromaticity and relatively weak σ-aromaticity.

Some years ago, Jishan Wu reported the synthesis of 8MC and 10MC, two homologues of the cyclopenta‐ring‐fused oligo(m‐phenylene) macrocycles mMC, each behaving as an annulene‐within‐an‐annulene (AWA). This was a surprising result as the AWA behavior is rare. Both molecules have a partial polyradical character, enforced by the quest for restoring some aromatic character of benzene rings. However, that restoration brings back some coupling between the two annulenes. Indeed, we found that the geometry and the magnetically induced currents indicate that, while 8MC does have an AWA character, this is not the case of the larger 10MC. Limitations of the design strategy of AWA molecules should be taken into account in future attempts to prepare novel large coronenes.

Heteroaromatic molecules are found in areas ranging from biochemistry to photovoltaics. We analyze the n,π* excited states of 6π-electron heteroaromatics with in-plane lone pairs (n_σ, herein n) and use qualitative theory and quantum chemical computations, starting at Mandado's 2n + 1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n + 2)π-electron species has 2n + 2 π_α-electrons and 2n + 1 π_β-electrons (or vice versa) and becomes π_α-antiaromatic and π_β-aromatic. Yet, the antiaromatic π_α- and aromatic π_β-components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n,π* states (³n,π*), which are most readily analyzed, but also singlet n,π* states (¹n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the π_β-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti)aromatic residual. Among isomers, the one that has the most aromatic residual in ³n,π* is often of the lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as their first excited states (T₁ and S₁), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D_2h) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n,π* state and the energy gap between the lowest n,π* and π,π* states (R² = 0.42), while it is stronger for monosubstituted pyrazines (R² = 0.84).