Energetic aspects of cyclic pi-electron delocalization: evaluation of the methods of estimating aromatic stabilization energies

Diels-Alder Reactivities of Benzene, Pyridine, and Di-, Tri-, and Tetrazines: The Roles of Geometrical Distortions and Orbital Interactions

The cycloadditions of benzene and ten different azabenzenes (pyridine, three diazines, three triazines, and three tetrazines) with the ethylene dienophile have been explored with density functional theory (M06-2X) and analyzed with the distortion/interaction model. Activation barriers correlate closely with both distortion energies and interaction energies over an activation energy range of 45 kcal/mol. The replacement of CH with N increases Diels-Alder reactivity due not only to the more favorable orbital interaction, but also to a decrease in distortion energy. The rates of reactions are greatly influenced by the nature of the bonds being formed: two C-C bonds > one C-C bond, and one C-N bond > two C-N bonds. The activation energy of Diels-Alder reactions correlates very well with reaction energies and with the NICS(0) values of the aromatic dienes. The distortion energy of the Diels-Alder reaction transition states mostly arises from the diene out-of-plane distortion energy.

Octahedral aromaticity in 2S+1A1g X6q clusters (X = Li–C and Be–Si, S = 0–3, and q = −2 to +4)

Octahedral aromaticity was found in most clusters of formula X₆^q (X = Li–C and Be–Si) with q = −2 to +4 and spin states ranging from the singlet to the septet that have electronic configurations of closed-shells or open shells half-filled with the same spin electrons.

Aromaticity of the doubly charged [8]circulenes

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes.

Double head-to-tail direct arylation as a viable strategy towards the synthesis of the aza-analog of dihydrocyclopenta[hi]aceanthrylene – an intriguing antiaromatic heterocycle

Straightforwardly prepared head-to-tail bis-imidazo[1,2-a]pyridine displays antiaromaticity and no fluorescence.

Cyclic π-electron delocalization in non-planar linear acenes

Based on the experimental data of newly determined structures ofper-substituted naphthalenes by halogen atoms (F, Cl and Br) and perchloroanthracene, as well as on the molecular modeling we have shown that deviation from planarity leads to relatively small changes in the cyclic π-electron delocalization of acenes.

Computation of Electron Delocalization for Extended Cyclic Conjugated Molecules

Cyclic conjugated molecules have relatively planar conformations due to overlap of adjacent π-orbitals of delocalized electrons and which is strongly correlated with the degree of electron delocalization. We first demonstrate the quantitative relationship between structural heterogeneity and two structural parameters: out-of-plane distances of atoms and torsional angles between neighbouring aromatic moieties. The molecular characteristic-dependent trend of planarity is presented in terms of these two parameters for the number of unit moieties, type and distribution of linkers, and substituting alkyl groups. The method presented may provide a simple yet systematic guide for determining the degree of delocalization of cyclic conjugated molecules.

Unification of ground-state aromaticity criteria – structure, electron delocalization, and energy – in light of the quantum chemical topology

Energy, bond length and electron delocalization are connected within the context of quantum chemical topology theories.

Catalytic Reduction of CO2 by Renewable Organohydrides

Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H(+)/1e(-)/1H(+)/1e(-) reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP(+)/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e(-) reduction. A HT-PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H(+) such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.

Aromaticity of Diazaborines and Their Protonated Forms

Substitution of a CH group in benzene with nitrogen has a little effect on its aromaticity (Wang et al., Org. Lett. 2010, 12, 4824). How does the same type of substitution affect aromatic character of the three isomeric azaborines? Does further protonation change aromaticity of diazaborines? This work is aimed at answering these questions. Such a knowledge should be of interest for further exploration and application of BN/CC isosterism. Aromaticity of diazaborines and their protonated forms is studied with the aid of four aromaticity indices, HOMA, NICS(0)πzz, PDI and ECRE. Generally, NICS(0)πzz and PDI point to similar aromaticity of diazaborines and their parent azaborines, while HOMA and ECRE indicate some changes. Thus, aromaticity of 1,2-azaborine slightly decreases/increases when CH meta/ortho,para to B is substituted with nitrogen. Aromaticity of the most aromatic 1,3-azaborine remains almost unchanged when CH meta to B and N is replaced with nitrogen, and becomes slightly weaker when any other CH group is substituted with nitrogen. Replacement of the CH ortho to N in 1,4-azaborine does not change much its cyclic delocalization, while replacement of the CH ortho to B leads to smaller cyclic delocalization. Protonated forms are either of similar or decreased aromaticity compared with neutral molecules.

First-Principles Prediction of Enthalpies of Formation for Polycyclic Aromatic Hydrocarbons and Derivatives

In this article, the first-principles prediction of enthalpies of formation is demonstrated for 669 polycyclic aromatic hydrocarbon (PAH) compounds and a number of related functionalized molecules. It is shown that by extrapolating density functional theory calculations to a large basis set limit and then applying a group based correction scheme that good results may be obtained. Specifically, a mean unsigned deviation and root mean squared deviation from the experimental enthalpies of formation data of 5.0 and 6.4 kJ/mol, respectively, are obtained using this scheme. This computational scheme is economical to compute and straightforward to apply, while yielding results of reasonable reliability. The results are also compared for a smaller set of molecules to the predictions given by the G3B3 and G3MP2B3 variants of the Gaussian-3 model chemistry with a mean unsigned deviation and root mean squared deviation from the experimental enthalpies of formation of 4.5 and 4.8 kJ/mol, respectively.

Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects

Aromaticity, one of the most important concepts in organic chemistry, has attracted considerable interest from both experimentalists and theoreticians. It remains unclear which NICS index is best to evaluate the triplet-state aromaticity. Here, we carry out thorough density functional theory (DFT) calculations to examine this issue. Our results indicate that among the various computationally available NICS indices, NICS(1)zz is the best for the triplet state. The correlations can be improved from 0.840 to 0.938 when only neutral species are considered, demonstrating the significant effect of the charge on the triplet-state aromaticity. In addition, calculations suggest that five-membered cyclic species with "hyperconjugative" aromaticity (and antiaromaticity) in the S0 state will become antiaromatic (and aromatic) in the T1 state, indicating an important role of hyperconjugation. Finally, a moderate correlation (r(2) =0.708) is identified between the NICS(1)zz values and spin distributions.

Unexpected higher stabilisation of two classical antiaromatic frameworks with a ruthenium fragment compared to the osmium counterpart: origin probed by DFT calculations

Density functional theory (DFT) calculations were carried out to investigate the stability and aromaticity of metallapentalocyclobutadienes. The results reveal unexpected higher stabilisation achieved with a 4d ruthenium fragment compared to the 5d [corrected] osmium counterpart. Moreover, direct 1-3 metal-carbon bonding in the metallabutadiene unit of these two complexes is negligible.

Switching between Aromatic and Antiaromatic 1,3-Phenylene-Strapped [26]- and [28]Hexaphyrins upon Passage to the Singlet Excited State

We have demonstrated aromaticity reversal in the singlet excited states of internally 1,3-phenylene-strapped [26]- and [28]hexaphyrins (P26H and P28H). P26H displays a broad and reduced singlet-excited-state absorption spectrum, whereas P28H exhibits a sharp and intense singlet-excited-state absorption spectrum; both are in contrast to the ground-state absorption spectra, strongly indicating aromaticity reversal in the singlet excited state. Furthermore, magnetic and topological indices of aromaticity such as nucleus-independent chemical shift and harmonic oscillator model of aromaticity values for P26H and P28H also suggest that their singlet excited states become antiaromatic and aromatic, respectively.

Prediction of ROA and ECD Related to Conformational Changes of Astaxanthin Enantiomers

ECD, ROA, and VCD were used to characterize astaxanthin conformers that differ in their arrangements of the β-ionone ring in respect to the chain. We obtained ECD spectra experimentally, and the ECD, ROA, and VCD spectra of both individual conformers and conformation-averaged mixtures were predicted using quantum-chemical calculations at the CAM-B3LYP level of theory using the PCM solvation model. The chiroptical methods employed (particularly ECD and ROA) were considerably more sensitive to conformational changes of astaxanthin compared to "mono-signed" conventional Raman spectroscopy. Strikingly, conformers that are the same optical isomers (e.g., of 3S,3'S-astxanthin), while geometrically nearly mirror images, exhibited sign-inversed ECD and ROA spectra. The conformational sensitivity of these chiroptical methods makes them a promising tool in the study of carotenoids in the natural environment (for instance, in de novo algal or yeast astaxanthin sources).

Isomeric signatures in the fragmentation of pyridazine and pyrimidine induced by fast ion impact

We present fast proton impact induced fragmentations of pyrimidine and pyridazine as an experimental resource to investigate isomeric signatures. Major isomeric imprints are identified for few fragment ions and differences of more than an order of magnitude for the cross sections of fragments of the same mass were measured. The observation of the molecular structure of these isomers gives no apparent indication for the reasons for such substantial differences. It is verified that the simple displacement of the position of one nitrogen atom strongly inhibits or favors the production of some ionic fragment species. The dependency of the fragmentation cross sections on the proton impact energy, investigated by means of time of flight mass spectroscopy and of a model calculation based in first order perturbation theory, allows us to disentangle the complex collision dynamics of the ionic fragments. The proton-induced fragmentation discriminates rather directly the association between a molecular orbital ionization and the fragment-ions creation and abundance, as well as how the redistribution of the energy imparted to the molecules takes place, triggering not only single but also double vacancy and leads to specific fragmentation pathways.

Aromaticity: what does it mean?

Aromaticity/aromatic belongs to one of the most useful and popular terms in organic chemistry and related fields. However, aromaticity is not an unambiguous term; therefore, its definition is enumerative. The criteria are based on energy (increased stability), molecular geometry (very low bond lengths alternation), magnetism (induction of the diatropic ring current by external magnetic field) and reactivity (tendency to maintain π-electron structure in chemical reactions). The energetic criterion is based on resonance energy and aromatic stabilization energy, whereas harmonic oscillator model of aromaticity-on molecular geometry. Magnetism-based criteria are illustrated by local indicators (for individual rings): nucleus independent chemical shifts and proton nuclear magnetic resonance chemical shifts as well as the global aromaticity index-exaltation of the magnetic susceptibility. For selected homo- and hetero-cyclic compounds, illustrative data are presented in tables, which allow the comparison of the above-mentioned indices. Finally, examples of agreements or disagreements between these various aromaticity indices are presented for few representative cases.

Quantifying aromaticity with electron delocalisation measures

Aromaticity cannot be measured directly by any physical or chemical experiment because it is not a well-defined magnitude. Its quantification is done indirectly from the measure of different properties that are usually found in aromatic compounds such as bond length equalisation, energetic stabilisation, and particular magnetic behaviour associated with induced ring currents. These properties have been used to set up the myriad of structural-, energetic-, and magnetic-based indices of aromaticity known to date. The cyclic delocalisation of mobile electrons in two or three dimensions is probably one of the key aspects that characterise aromatic compounds. However, it has not been until the last decade that electron delocalisation measures have been widely employed to quantify aromaticity. Some of these new indicators of aromaticity such as the PDI, FLU, ING, and INB were defined in our group. In this paper, we review the different existing descriptors of aromaticity that are based on electron delocalisation properties, we compare their performance with indices based on other properties, and we summarise a number of applications of electronic-based indices for the analysis of aromaticity in interesting chemical problems.

“Carbo-aromaticity” and novel carbo-aromatic compounds

Recent advances in experimental and theoretical studies ofcarbo-benzene derivatives, along with the proposition of a generalization of the definition of aromaticity to the two-membered π-rings of triple bonds, suggest relevance for the notion of “carbo-aromaticity”.

Geometric and energetic consequences of prototropy for adenine and its structural models – a review

Prototropy for adenine and its convenient models causes parallel changes of geometric (HOMED) and energetic (ΔE) parameters for neutral tautomers.

On the HOMA index of some acyclic and conducting systems

The HOMA index can be useful for acyclic linear systems and design of new conducting polymers.

Aromaticity, closed-shell effects, and metallization of hydrogen

CONSPECTUS: Recent theoretical and experimental studies reveal that compressed molecular hydrogen at 200-350 GPa transforms to layered structures consisting of distorted graphene sheets. The discovery of chemical bonding motifs in these phases that are far from close-packed contrasts with the long-held view that hydrogen should form simple, symmetric, ambient alkali-metal-like structures at these pressures. Chemical bonding considerations indicate that the realization of such unexpected structures can be explained by consideration of simple low-dimensional model systems based on H6 rings and graphene-like monolayers. Both molecular quantum chemistry and solid-state physics approaches show that these model systems exhibit a special stability, associated with the completely filled set of bonding orbitals or valence bands. This closed-shell effect persists in the experimentally observed layered structures where it prevents the energy gap from closing, thus delaying the pressure-induced metallization. Metallization occurs upon further compression by destroying the closed shell electronic structure, which is mainly determined by the 1s electrons via lowering of the bonding bands stemming from the unoccupied atomic 2s and 2p orbitals. Because enhanced diamagnetic susceptibility is a fingerprint of aromaticity, magnetic measurements provide a potentially important tool for further characterization of compressed hydrogen. The results indicate that the properties of dense hydrogen are controlled by chemical bonding forces over a much broader range of conditions than previously considered.

Total synthesis of viridicatumtoxin B and analogues thereof: strategy evolution, structural revision, and biological evaluation

The details of the total synthesis of viridicatumtoxin B (1) are described. Initial synthetic strategies toward this intriguing tetracycline antibiotic resulted in the development of key alkylation and Lewis acid-mediated spirocyclization reactions to form the hindered EF spirojunction, as well as Michael-Dieckmann reactions to set the A and C rings. The use of an aromatic A-ring substrate, however, was found to be unsuitable for the introduction of the requisite hydroxyl groups at carbons 4a and 12a. Applying these previous tactics, we developed stepwise approaches to oxidize carbons 12a and 4a based on enol- and enolate-based oxidations, respectively, the latter of which was accomplished after systematic investigations that revealed critical reactivity patterns. The herein described synthetic strategy resulted in the total synthesis of viridicatumtoxin B (1), which, in turn, formed the basis for the revision of its originally assigned structure. The developed chemistry facilitated the synthesis of a series of viridicatumtoxin analogues, which were evaluated against Gram-positive and Gram-negative bacterial strains, including drug-resistant pathogens, revealing the first structure-activity relationships within this structural type.