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

3D global aromaticity in a fully conjugated diradicaloid cage at different oxidation states

Aromaticity is a vital concept that governs the electronic properties of π-conjugated organic molecules and has long been restricted to 2D systems. The aromaticity in 3D π-conjugated molecules has been rarely studied. Here we report a fully conjugated diradicaloid molecular cage and its global aromaticity at different oxidation states. The neutral compound has an open-shell singlet ground state with a dominant 38π monocyclic conjugation pathway and follows the [4n + 2] Hückel aromaticity rule; the dication has a triplet ground state with a dominant 36π monocyclic conjugation pathway and satisfies [4n] Baird aromaticity; the tetracation is an open-shell singlet with 52 π-electrons that are delocalized along the 3D rigid framework, showing 3D global antiaromaticity; and the hexacation possesses D₃ symmetry with 50 globally delocalized π-electrons, showing [6n + 2] 3D global aromaticity. Different types of aromaticity were therefore accessed in one molecular cage platform, depending on the symmetry, number of π-electrons and spin state.

The effect of anion complexation on the aromatic properties of aromatic and antiaromatic porphyrinoids

The effect of anion complexation on magnetically induced current densities and excitation energies of antiaromatic molecular rings has been investigated by calculations on expanded antiaromatic porphyrinoids including orangarin, rosarin, amethyrin and on a theoretically predicted strongly antiaromatic hydrocarbon ring.

Borataalkene or boratabenzene? Understanding the aromaticity of 9-borataphenanthrene anions and its central ring

The recently isolated 1 is isoelectronic to 2 and shows reactivity in central ring that resembles both (cyclo)alkene and aromatic ring. Evaluation of local aromaticity reveal that central ring is best described as a non-aromatic following Clar’s rules.

Singlet and triplet states of the sandwich-type Be2B6 and Be2B7+ clusters. A test for the electron counting rules of aromaticity

The studied complexes exhibit double aromaticity in their triplet states in line with the predictions of Hückel and Baird's rules.

Study of the influence of intermolecular interaction on classical and reverse substituent effects in para-substituted phenylboranes

The substituent effect and the reverse substituent effect in para-substituted phenylboranes and the influence of the intermolecular interaction of H⋯B type with either silane or methylsilane on the latter of these effects are extensively studied.

Induced magnetic field in sp-hybridized carbon rings: analysis of double aromaticity and antiaromaticity in cyclo[2N]carbon allotropes

The induced magnetic field of C2N (N = 3-14) carbon rings was dissected to contributions from out-of-plane and in-plane π orbitals revealing two concurrent long range shielding or deshielding cones as a manifestation of the dual aromatic and antiaromatic character of C4n+2 and of C4n rings respectively. Aromaticity based on the magnetic criterion was evaluated with regard to the bonding pattern and geometrical characteristics that elucidate the influence of bond length and bond angle alteration on out-of-plane and in-plane magnetic responses. Ground state polyynic geometries of C4n+2 rings exhibit comparable shielding cones to annulenes, decreasing the magnetic response with regard to the ring size and similar πout and πin diatropicity. Transition state cumulenic rings display increased aromaticity expressed by a very strong constant magnetic response and augmented πout diatropicity with regard to πin. The variations of the induced magnetic field are explained on the basis of frontier orbital interactions through rotational excitations, which enable further rationalization of the aromatic/antiaromatic behavior.

Three Queries about the HOMA Index

HOMA (Harmonic Oscillator Model of Aromaticity) is a simple, successful, and widely used geometrical aromaticity index. However, HOMA can also be used as a general molecular descriptor appropriate for any type of molecule. It reaches the global maximum for benzene, whereas the potent magnetic aromaticity NICS index has no lower or upper limits. Hence, questions arise and go beyond mere differences between the geometric and magnetic aspects of aromaticity: (1) Does a molecule of aromaticity greater than that of benzene, but undisclosed by the HOMA definition, exist? (2) Can the Kekuléne cyclohexatriene moiety with HOMA = 0 exist as a part of a larger system? (3) Can the geometrical aromaticity index be defined better? Our answer to the first query is "It is not likely enough", to the second, "Why not define HOMA using a less mysterious molecule than cyclohexatriene?", and to the third, "It is possible to construct another fair geometrical index, but is it better for evaluating aromaticity?" To find these answers, we have studied: (1) the HOMA and NICS indices of over 50 hexahomosubstituted benzenes, (2) the HOMA, as well as EN and GEO, indices of over 100 triply fused hexasubstituted benzenes, and (3) the HOMA and new Geometrical Auxiliary Index (GAI) , of different unsaturated and saturated, aromatic and aliphatic hydrocarbons including all alkane constitutional isomers composed of up to nine carbon atoms.

Substituent effects on the stability of the four most stable tautomers of adenine and purine

Substituent effects at the C2-, C8- and N-positions of adenine and purine in their four the most stable tautomers are studied by means of B97D3/aug-cc-pvdz computation applying substituents of varying electronic properties: NO₂, CN, CHO, Cl, F, H, Me, OMe, OH and NH₂. The substituent effect is characterized by the substituent effect stabilization energy (SESE) and substituent Hammett constant σ. For adenine, SESE is obtained with purine as the reference system. Additionally, for both adenine and purine, SESE characteristics are estimated with benzene, imidazole and amino-pyrimidine as reference systems, when possible, taking into account substitution in topologically equivalent positions. The role of a C6-NH₂ group in adenine in modifying the substitution effect is observed and discussed. Additionally, the proximity effect for some asymmetric substituents (e.g. CHO, OMe) is recognized and meticulously analyzed.

Formation of S-alkyl thiophenium ionic liquids: mechanistic rationale and structural relationships

The quaternization of thiophenes through S-alkylation reactions with iodoalkanes in the presence of silver salts opens the door to a new family of room-temperature ionic liquids, yet relatively unexplored in terms of chemical reactivity and applications. This computational study provides a mechanistic rationale that accounts for their formation. The results are consistent with the calculated energy barriers of the corresponding transition structures. Calculations indicate that the geometry at the sulfur atom goes from flat to tetrahedral during salt formation, and the electron delocalization of thiophene is greatly reduced, if not lost, as inferred from aromaticity indexes. Moreover, the rationale explains the influence of polarizable anions on S-alkylation and why alkyl substitution at the α-position of thiophenes gives rise to more stable species than unsubstituted derivatives.

The electronically excited states of cyclooctatetraene-An analysis of the vacuum ultraviolet absorption spectrum by ab initio configuration interaction methods

A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV-5.3 eV. Theoretical vertical excitation energies, determined by configuration interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiconfiguration self-consistent field methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading configurations. The 7a₁6e^* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.

A study on the aromatic conjugation pathways and the ring currents of bridged [18]annulenes

In this article, the topological resonance energy (TRE) method was used to investigate the global aromaticity of a set of [18]annulene-derived compounds which were obtained by replacing either two, four, or all six of the inner hydrogen atoms of [18]annulene with bridges (oxygen, imino-, sulfur, or combinations of the three). Our TRE results show that all the mono-bridged, di-bridged, and tri-bridged [18]annulenes are globally aromatic systems with positive TREs and show relatively larger aromaticity in comparison with the [18]annulene. The aromaticity of each compound was explained using the topological charge stabilization (TCS) rule. The bond resonance energy (BRE) and circuit resonance energy (CRE) methods were used to evaluate local aromaticity. Our BRE and CRE results show that incorporation of five-membered heterocyclic rings changes the main conjugation pathway of the bridged [18]-annulenes. The local aromaticities arising from the five-membered heterocyclic rings (6π) contribute strongly to global aromaticity. However, the ring current (RC), which arises from the 18π annulene-like pathway structures, is the primary determinant of the magnetic properties of the molecule. Our RC results are in good agreement with available ¹H-NMR chemical shift data.

The topological resonance energy method was used to investigate the global aromaticity of a set of [18]annulene-derived compounds obtained by replacing two, four, or all six of the inner hydrogen atoms with bridges (oxygen, imino-, sulfur, or combinations of the three).

An electronic properties investigation to interpret the substituent constants of monosubstituted benzene derivatives

In the present work, the π and π^∗-electronic nature of substitution constants (σ) of the twenty-two monosubstituted benzene derivatives (MSBDs) are estimated in terms of the para-delocalization index (PDI) and total non-Lewis structure (TNLS), respectively. Since these compounds are aromatic, the other descriptors of aromaticity such as nuclear independent chemical shifts and aromatic stabilization energy have been examined. Because of no considerable variation for the π and π^∗-electron delocalization in the ring systems, a very weak correlation has been demonstrated between all aromaticity indices. Also, none of these descriptors has a linear correlation with the values of σ for both electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) in a simultaneous relationship. We now propose the usage of the dipole moments values of molecules, considering their directions, by multiplying the PDI and TNLS values as probes to interpret σ values for benzene and MSBDs since the dipole moment can affect the π and π^∗-electron delocalization. It is interesting to note that there is a remarkable linear correlation between our suggested probes, ±μPDI and ±μTNLS, and σ_p, σ_m, and σ⁺/σ^- for EDGs and EWGs in simultaneous linear relationships. Also, these probes have a relative relationship with Kirkwood and Westheimer equation. Additionally, the regression coefficient between ±μPDI and ±μTNLS is 0.989.

Non-aromatic annulene-based aggregation-induced emission system via aromaticity reversal process

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. Although AIE has attracted increasing attention due to the significant applications in biomedical and optoelectronics, an in-depth understanding of the excited-state intramolecular motion has yet to be fully developed. Here we found the non-aromatic annulene derivative of cyclooctatetrathiophene shows typical AIE phenomenon in spite of its rotor-free structure. The underlying mechanism is investigated through photoluminescence spectra, time-resolved absorption spectra, theoretical calculations, circular dichroism as well as by pressure-dependent fluorescent spectra etc., which indicate that the aromaticity reversal from ground state to the excited state serves as a driving force for inducing the excited-state intramolecular vibration, leading to the AIE phenomenon. Therefore, aromaticity reversal is demonstrated as a reliable strategy to develop vibrational AIE systems. This work also provides a new viewpoint to understand the excited-state intramolecular motion behavior of lumiongens.

Substituent effects on the aromaticity of benzene-An approach based on interaction coordinates

Benzene and 23 monosubstituted and 32 disubstituted derivatives of benzene were optimized for minimum energy structures using the B3LYP/cc-pVTZ method. The force fields of all the compounds were evaluated at their optimized geometries using the same method and basis set. In order to understand the effect of substitution(s) on the aromaticity of benzene, the aromaticity index based on interaction coordinates (AIBIC) values were computed for each and the change from the benzene value was obtained. This difference, the substituent effect based on interaction coordinates (SEBIC), quantifies the effect of the substituent on the aromaticity of benzene ring satisfactorily. It is found that the AIBIC of disubstituted benzenes (XC₆H₄Y) could be predicted well by adding the respective SEBIC(C₆H₅X) and SEBIC(C₆H₅Y) values to the AIBIC of benzene. The projected force fields of the meta and para fragments of the monosubstituted benzenes when chosen properly contain the information about the directing influence of the substituent in terms of the electron density based on interaction coordinates (EDBIC). When the EDBIC(para) > EDBIC(meta) relative to benzene, the substituent is ortho-para directing, while when the reverse is true, it is meta directing. The effect of conformational changes on aromaticity has been studied using aminophenols and dihydroxybenzenes. The additivity rule and the EDBIC concept work adequately well in that the methods can have several useful practical applications that will benefit various areas of science. A good understanding of the substituent effects and the ability to predict them should add a new dimension to the applications of AIBIC.

The ionic states of cyclooctatetraene: Analysis of a new experimental photoelectron spectrum by ab initio and density functional methods

A synchrotron-based study of the photoelectron spectrum (PES) for cyclooctatetraene (COT) is reported, and this has been subjected to theoretical analysis in unprecedented detail. Weak vibrational structure was observed on the lowest ionization energy (IE₁), but the peaks generally show very broad features. Multiconfiguration self-consistent field study confirms that the adiabatic IE (AIE) sequence is 1²A₁ < 1²B₁ < 2²A₁ < 1²A₂ < 2²B₁ < 3²A₁. The Tamm-Dancoff approximation gives an acceptable interpretation of the PES below 20 eV. Vibrational analysis of the PES bands by Franck-Condon methods predicts well-defined vibrational structure for these ionic states. The principal contributors to the PES envelopes are the a₁ modes, and only a few are responsible for the overall shape of most bands. The high density of vibrational states, together with the known D_2d ⇆ D_2d interconversion process, where the C=C and C-C bonds interchange, is attributed to the lack of the observed structure. The transition state (TS) structures for the interconversion above of several ionic states of COT have been elucidated. The intrinsic reaction coordinate procedure gave a planar TS for the X¹A₁ (D_2d) ⇆ ¹A_1g (D_4h) ⇆ X¹A₁ (D_2d) process; this shows alternating C-C (1.4791 Å), C=C (1.3261 Å), and C-H (1.0780 Å) bond lengths. The planar TS is a very shallow maximum, with energy varying with the method used, Hartree-Fock 0.575 eV and second order Møller-Plesset 0.653 eV. A polynomial containing quadratic, quartic, and sextic terms gives an effectively exact fit to the surface. The lowest ionic state of COT (X²A₁) shows a similar TS for the process X²A₁ (D_2d) ⇆ ²B_2u (D_4h). This structure has alternating C-C (1.4366 Å), C=C (1.3572 Å), and C-H (1.0756 Å) bonds.

Bonded to Carbon or Nitrogen? This is a Question on the Regioselectivity in Hyperconjugative Aromaticity

In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. Although it has been extensively investigated in various reactions, the regioselectivity of hyperconjugative aromaticity on either main group systems or transition metal ones remains elusive due to the challenge of synthesizing the target products. Here we report a joint theoretical and experimental study on this issue. Theoretical calculations predicted that electron-withdrawing groups prefer an attachment to the sp³-hybridized carbon atom rather than the nitrogen atom in indoliums. For the electron-donating groups, the two isomers bonded to the sp³-hybridized carbon or nitrogen atom are almost isoenergetic. When both sp²- and sp³-hybridized carbon and nitrogen atoms in the five-membered ring of indoliums are considered, the isomer with the polyaurated substituents bonded to the sp³-hybridized carbon atom is thermodynamically more stable than that with the polyaurated substituents bonded to the sp³-hybridized nitrogen atom. This prediction is reasonably verified by experimental observation. Bond dissociation energy is found to be more important than aromaticity in rationalizing such a preference. Our findings could help experimentalists to design and realize more novel hyperconjugative aromatics.

Electron Delocalization in Planar Metallacycles: Hückel or Möbius Aromatic?

In this work the relationship between the formal number of π-electrons, d-orbital conjugation topology, π-electron delocalization and aromaticity in d-block metallacycles is investigated in the context of recent findings concerning the correlation of π-HOMO topology and the magnetic aromaticity indices in these species. It is demonstrated that for π-electron rich d-metallacycles the direct link between aromaticity, the number of π-electrons and the frontier π-orbital topology does not strictly hold and for such systems it is very difficult to unambiguously associate their aromaticity with the "4n+2" (Hückel) and "4n" (Möbius) rules. It is also shown that the recently proposed electron density of delocalized bonds (EDDB) method can successfully be used not only to quantify and visualize aromaticity in such difficult cases, but also - in contrast to magnetic aromaticity descriptors - to provide a great deal of information on the real role of d-orbitals in metallacycles without the ambiguity of bookkeeping of electrons in the π-subsystem of the molecular ring. Interestingly, some of the metallacycles studied cannot be classified exclusively as Hückel or Möbius because they have a hybrid Hückel-Möbius or even quasi-aromatic nature.

Application of the Extended HOMED (Harmonic Oscillator Model of Aromaticity) Index to Simple and Tautomeric Five-Membered Heteroaromatic Cycles with C, N, O, P, and S Atoms

The geometry-based HOMA (Harmonic Oscillator Model of Aromaticity) descriptor, based on the reference compounds of different delocalizations of n- and π-electrons, can be applied to molecules possessing analogous bonds, e.g., only CC, only CN, only CO, etc. For compounds with different heteroatoms and a different number of CC, CX, XX, and XY bonds, its application leads to some discrepancies. For this reason, the structural descriptor was modified and the HOMED (Harmonic Oscillator Model of Electron Delocalization) index defined. In 2010, the HOMED index was parameterized for compounds with C, N and O atoms. For parametrization, the reference molecules of similar delocalizations of n- and π-electrons were employed. In this paper, the HOMED index was extended to compounds containing the CP, CS, NN, NP, PP, NO, NS, PO, and PS bonds. For geometrical optimization of all reference molecules and of all investigated heterocompounds, the same quantum–chemical method {B3LYP/6-311+G(d,p)} was used to eliminate errors of the HOMED estimation. For some tautomeric systems, the Gn methods were also employed to confirm tautomeric preferences. The extended HOMED index was applied to five-membered heterocycles, simple furan and thiophene, and their N and P derivatives as well as for tautomeric pyrrole and phosphole and their N and P derivatives. The effects of additional heteroatom(s) in the ring on the HOMED values for furan are parallel to those for thiophene. For pyrroles, aromaticity dictates the tautomeric preferences. An additional N atom in the ring only slightly affects the HOMED values for the favored and well delocalized NH tautomers. Significant changes take place for their rare CH forms. When intramolecular proton-transfer is considered for phosphole and its P derivatives, the PH tautomers seem to be favored only for 1,2,3-triphosphole/1,2,5-triphosphole and for 1,2,3,5-tetraphosphole. For other phospholes, the CH forms have smaller Gibbs energies than the PH isomers. For phosphazoles, the labile proton in the favored form is linked to the N atom. The PH forms have smaller HOMED indices than the NH tautomers but higher than the CH ones.

Borepin Rings as "Sigma-Free" Reporters of Aromaticity within Polycyclic Aromatic Scaffolds

The definition and measurement of local and global aromaticity in fused ring polycyclic aromatic compounds is a complex issue. Historically, these types of molecules have been explored in this capacity by way of experimental (NMR, thermochemistry) and computational (NICS, HOMA) analyses. We previously showed how borepin rings with [ b, f] arene fusions can be used as experimental magnetic aromaticity reporters via the remaining protons attached to the borepin rings. In this report, we describe a joint experimental and computational analysis of several borepin-containing polycyclic aromatic molecules in order to draw conclusions about the influence of ring fusion on aromaticity. We find that the borepin ring within these extended structures is a unique motif with limited σ-contribution to aromaticity while still displaying a wide range of structural and magnetic aromatic character.

Classical and Reverse Substituent Effects in Substituted Anthrol Derivatives

The substituent effect in 1-, 2-, and 9-anthrols is studied by means of B3LYP/6-311++G(d,p) computation, taking into account substituents (X): NO2, CN, OH and NH2 located in all positions except the adjacent ones. The substituent effect is characterized by approaches based on quantum chemistry: The charge of the substituent active region (cSAR), substituent effect stabilization energy (SESE) and the charge flow index (CFI) describing flow of the charge from X to the fixed group (or vice versa) as well as substituent constants σ. Changes in properties observed in the fixed group (OH) are described by cSAR(OH). Mutual interdependences are found between these descriptors. The HOMA index is used to describe an effect of a substituent on aromaticity of an anthrol hydrocarbon skeleton and of individual rings. In all cases, the classical (influence of X on the properties of OH) and reverse (influence of OH on the properties of X) substituent effects are studied. The latter is clearly documented by the cSAR approach.

Does aromaticity account for an enhanced thermodynamic stability? The case of monosubstituted azaborines and the stereoelectronic chameleonism of the NH2 group

This work was initiated by the increasing interest in BN/CC isosterism and by the long-lasting interest in the concepts of aromaticity and substituent effects. We have theoretically examined the aromaticity and stability of monosubstituted BN isosters of benzene, the three isomeric azaborines. The results provide insight into the effect of substitution on two basic molecular properties, which are influenced, here, by the substituent effects and by the B/N relationship in the ring. The results, along with other examples in the literature, also warn chemists that the general belief that aromaticity accounts for enhanced thermodynamic stability is not always true. The stability of cyclic, conjugated compounds depends on several effects, and only one of them is aromaticity. In addition, our calculations predict a switching of electronic properties of the NH2 group from the usual p-electron donor to a π-electron acceptor when it is moved from the B/C atoms to the nitrogen atom in all isomers, or C6 in 1,3-azaborine. This is the result of the conformational change that places the NLP in the plane of the ring and the NH bonds in a favourable spatial position to act as acceptors of π-electron density.

Global and local aromaticity of acenes from the information-theoretic approach in density functional reactivity theory

In this work, we report a systematic study on the global and local aromaticity of acenes using a series of model structures from 2-acene to 11-acene. A recently developed ansatz, an information-theoretic approach coached into density functional reactivity theory has been employed, which essentially provides different density functionals characterizing the molecular electron density distribution. Based on the correlation analysis of six conventional aromaticity indices with eight information-theoretic quantities, we examined the aromaticity of acenes from both global and local perspectives. From the global aromaticity viewpoint, our results suggest that different descriptors based on various physicochemical properties are intrinsically dependent. A novel laminated feature ruling local aromaticity of acenes has been unveiled, from which we found that the distance from the terminal rings plays the critical role. Based on the shape of the correlation plots between the conventional aromaticity indices and information-theoretic quantities, the latter could be separated into three subgroups. The seemingly contradictory results from global and local aromaticity perspectives not only present us the uniqueness of the acene systems but all demonstrate the effectiveness of the information-theoretic approach from density functional reactivity theory. Besides strengthening the validity of a series of new aromaticity descriptors, our results should lead to more clear insights into the chemical significance of the information-theoretic quantities.

Impact of heteroatoms (S, Se, and Te) on the aromaticity of heterocirculenes

Computations reveal the structural and energetic aspects of aromaticity in heterocirculenes.

Baird's Rule in Substituted Fulvene Derivatives: An Information-Theoretic Study on Triplet-State Aromaticity and Antiaromaticity

Originated from the cyclic delocalization of electrons resulting in extra stability and instability, aromaticity and antiaromaticity are important chemical concepts whose appreciation and quantification are still much of recent interest in the literature. Employing information-theoretic quantities can provide us with more insights and better understanding about them, as we have previously demonstrated. In this work, we examine the triplet-state aromaticity and antiaromaticity, which are governed by Baird's 4n rule, instead of Hückel's 4n + 2 rule for the singlet state. To this end, we have made use of 4 different aromaticity indexes and 8 information-theoretic quantities, examined a total of 22 substituted fulvene derivatives, and compared the results both in singlet and triplet states. It is found that cross-correlations of these two categories of molecular property descriptors enable us to better understand the nature and propensity of aromaticity and antiaromaticity for the triplet state. Our results have not only demonstrated the existence and validity of Baird's rule but also shown that Hückel's rule and Baird's rule indeed share the same theoretical foundation because with these cross-correlation patterns we are able to distinguish them from each other simultaneously in both singlet and triplet states. Our results should provide new insights into the nature of aromaticity and antiaromaticity in the triplet state and pave the road toward new ways to quantify this pair of important chemical concepts.

An examination of the nature of localized molecular orbitals and their value in understanding various phenomena that occur in organic chemistry

While canonical molecular orbitals have been used in computational chemistry for almost a century, the use of localized molecular orbitals is relatively new, and generating them has been difficult until recently. This has impeded their routine use in modeling chemical systems and reactions so that, even though localized molecular orbitals can now be generated easily, their usefulness in interpreting chemical phenomena has not been properly appreciated. Localized molecular orbitals can provide useful insights into chemical phenomena such as two-electron bonds, π delocalization, and lone pairs. A potentially important application would be interpreting the phenomena that occur in chemical reactions, in particular those reactions which can be described using the Lewis curly-arrow electron pushing convention. This paper considers how canonical and localized molecular orbitals are generated, their usefulness and limitations, and some issues that could be considered controversial regarding their nature, and it presents examples of the usefulness of LMOs in describing six chemical systems and one reaction.

Tetrel Bond between 6-OTX₃-Fulvene and NH₃: Substituents and Aromaticity

Carbon bonding is a weak interaction, particularly when a neutral molecule acts as an electron donor. Thus, there is an interesting question of how to enhance carbon bonding. In this paper, we found that the –OCH₃ group at the exocyclic carbon of fulvene can form a moderate carbon bond with NH₃ with an interaction energy of about −10 kJ/mol. The –OSiH₃ group engages in a stronger tetrel bond than does the –OGeH₃ group, while a reverse result is found for both –OSiF₃ and –OGeF₃ groups. The abnormal order in the former is mainly due to the stronger orbital interaction in the –OSiH₃ complex, which has a larger deformation energy. The cyano groups adjoined to the fulvene ring not only cause a change in the interaction type, from vdW interactions in the unsubstituted system of –OCF₃ to carbon bonding, but also greatly strengthen tetrel bonding. The formation of tetrel bonding has an enhancing effect on the aromaticity of the fulvene ring.

Aromaticity Determines the Relative Stability of Kinked vs. Straight Topologies in Polycyclic Aromatic Hydrocarbons

It is well-known that kinked phenacenes are more stable than their isomeric linear acenes, the archetypal example being phenanthrene that is more stable than anthracene by about 4–8 kcal/mol. In previous studies, the origin of the higher stability of kinked polycyclic aromatic hydrocarbons (PAHs) was found to be better π-bonding interactions, i.e., larger aromaticity, in kinked as compared to linear PAHs. Some years ago, however, Dominikowska and Palusiak (2011) found that dicationic linear anthracene is more stable than the dicationic kinked phenanthrene. Therefore, these authors showed that, in some cases, the linear topology in PAHs can be preferred over the kinked one. Our results using energy decomposition analyses in combination with the turn-upside-down approach show that the origin of the higher stability of dicationic anthracene is the same as in the neutral species, i.e., better π-bonding interactions. A similar result is found for the kinked and straight pyrano-chromenes. We conclude that the aromaticity is the driving force that determines the relative stability of kinked vs. straight topologies in PAHs.

Weak interactions in furan dimers

Dimers of furan, 2,3-dihydrofuran, 2,5-dihydrofuran and tetrahydrofuran were investigated with the use of theoretical methods to determine the interactions that keep the molecules together. The QTAIM and NCI methods confirmed that for furan dimers the C-H⋯O hydrogen bond and stacking interactions can form the dimers with similar energy. For 2,3-dihydrofuran, 2,5-dihydrofuran and tetrahydrofuran, the decisive mechanism of dimer formation is the stacking interaction between the furan rings.

Convergent Covalent Organic Framework Thin Sheets as Flexible Supercapacitor Electrodes

Flexible supercapacitors in modern electronic equipment require light-weight electrodes, which have a high surface area, precisely integrated redox moieties, and mechanically strong flexible free-standing nature. However, the incorporation of the aforementioned properties into a single electrode remains a great task. Herein, we could overcome these challenges by a facile and scalable synthesis of the convergent covalent organic framework (COF) free-standing flexible thin sheets through solid-state molecular baking strategy. Here, redox-active anthraquinone (Dq) and π-electron-rich anthracene (Da) are judiciously selected as two different linkers in a β-ketoenamine-linked two-dimensional (2D) COF. As a result of precisely integrated anthraquinone moieties, COF thin sheet exhibits redox activity. Meanwhile, π-electron-rich anthracene linker assists to improve the mechanical property of the free-standing thin sheet through the enhancement of noncovalent interaction between crystallites. This binder-free strategy offers the togetherness of crystallinity and flexibility in 2D COF thin sheets. Also, the synthesized porous crystalline convergent COF thin sheets are benefited with crack-free uniform surface and light-weight nature. Further, to demonstrate the practical utility of the material as an electrode in energy-storage systems, we fabricated a solid-state symmetrical flexible COF supercapacitor device using a GRAFOIL peeled carbon tape as the current collector.

Bond Length Equalization with molecular aromaticity-A new measurement of aromaticity

A new method to measure the amount of aromaticity is presented through the process of Bond Length Equalization (BLE). Degree of Aromaticity (DOA), a two-dimensional intensive quantity including geometric and energetic factors, as a new measurement of aromaticity is proposed. The unique characteristic of DOA and the formation of DOA will be displayed. The calculation of the geometrical optimization, DOA, Nucleus Independent Chemical Shifts (NICS) and Ring Stretching Vibration Raman Spectroscopy Frequency (RSVRSF) for the aromatic ring molecules - G_nH_n^m(G = C, Si, Ge, n = 3, 5-8, m = +1, -1, 0, +1, +2) were calculated using the method of Density Functional Theory (DFT). The correlation between radius angle and molecular energy is absolute quadratic in the process of BLE. As the increasing of the number of ring atoms, the value of DOA decreasing gradually, the aromaticity decreased gradually, which was a same conclusion as NICS and RSVRSF concluded.

Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

With their versatile molecular topology and aromaticity, porphyrinoid systems combine remarkable chemistry with interesting photophysical properties and nonlinear optical properties. Hence, the field of application of porphyrinoids is very broad ranging from near-infrared dyes to opto-electronic materials. From previous experimental studies, aromaticity emerges as an important concept in determining the photophysical properties and two-photon absorption cross sections of porphyrinoids. Despite a considerable number of studies on porphyrinoids, few investigate the relationship between aromaticity, UV/vis absorption spectra and nonlinear properties. To assess such structure-property relationships, we performed a computational study focusing on a series of Hückel porphyrinoids to: (i) assess their (anti)aromatic character; (ii) determine the fingerprints of aromaticity on the UV/vis spectra; (iii) evaluate the role of aromaticity on the NLO properties. Using an extensive set of aromaticity descriptors based on energetic, magnetic, structural, reactivity and electronic criteria, the aromaticity of [4n+2] π-electron porphyrinoids was evidenced as was the antiaromaticity for [4n] π-electron systems. In agreement with previous studies, the absorption spectra of aromatic systems display more intense B and Q bands in comparison to their antiaromatic homologues. The nature of these absorption bands was analyzed in detail in terms of polarization, intensity, splitting and composition. Finally, quantities such as the average polarizability and its anisotropy were found to be larger in aromatic systems, whereas first and second hyperpolarizability are influenced by the interplay between aromaticity, planarity and molecular symmetry. To conclude, aromaticity dictates the photophysical properties in porphyrinoids, whereas it is not the only factor determining the magnitude of NLO properties.