Criteria Of Aromaticity Research Articles

Magnetic criteria of aromaticity in a benzene cation and anion: how does the Jahn–Teller effect influence the aromaticity?

The aromatic/antiaromatic behavior of the Jahn–Teller (JT) active benzene cation and anion has been investigated using Density Functional Theory (DFT) calculations of Nuclear Independent Chemical Shifts (NICS) and magnetic susceptibility. NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for the benzene cation showing antiaromaticity which decreases with increasing deviation from D6h to D2h symmetry. Changes in NICS values along the IDP from D6h to C2v in the benzene anion revealed non-aromatic character.

Correlated Electron Dynamics: How Aromaticity Can Be Controlled

In this Article, we show that the aromaticity of a molecule can be turned off by controlling the electron dynamics. We present a controlled switching from the aromatic ground state of benzene to two different nonaromatic states, using a laser pulse. The propagation of the molecular wave function is carried out with the time-dependent configuration interaction method. The laser pulse for switching between the ground and excited states is optimized using optimal control theory. Bond orders and Mulliken charges serve as an aromaticity criterion. The nonaromatic target states exhibit localized bonds and partial charges on the carbon atoms; these localized electrons circulate on an attosecond time scale in the ring system.

Structures, Energetics, and Aromaticities of the Tetrasilacyclobutadiene Dianion and Related Compounds: (Si4H4)2–, (Si4H4)2–·2Li+, [Si4(SiH3)4]2–·2Li+, [Si4(SiH3)4]2–·2Na+, and [Si4(SiH3)4]2–·2K+

In light of the important recent synthesis of a stable tetrasilacyclobutadiene dianion compound by Sekiguchi and co-workers and the absence of theoretical studies, ab initio methods have been used to investigate this dianion and a number of related species. These theoretical methods predict multiple minima for each compound, and most minima contain folded and bicyclic silicon rings. For (Si(4)H(4))(2-), (Si(4)H(4))(2-)·2Li(+), [Si(4)(SiH(3))(4)](2-)·2Li(+), [Si(4)(SiH(3))(4)](2-)·2Na(+), and [Si(4)(SiH(3))(4)](2-)·2K(+), respectively, the energetically lowest-lying structures are designated A-3 (C(2v) symmetry), B-8 (C(1) symmetry), C-1 (C(2) symmetry), D-1 (C(2) symmetry), and E-1 (C(2h) symmetry). None of these structures satisfies both the ring planarity and the cyclic bond equalization criteria of aromaticity. However, all of the representative NICS values of these lowest-lying structures are negative, indicating some aromatic character. Especially, structures C-1 and D-1 of C(2) symmetry effectively satisfy the criteria of aromaticity due to the slightly trapezoidal silicon rings, which are nearly planar with nearly equal bond lengths. SiH(3) substitution for hydrogen in (Si(4)H(4))(2-)·2Li(+) significantly reduces the degree of aromaticity, as reflected in the substantially smaller NICS absolute values for [Si(4)(SiH(3))(4)](2-)·2Li(+) than those of (Si(4)H(4))(2-) and (Si(4)H(4))(2-)·2Li(+). The aromaticity is further weakened in [Si(4)(SiH(3))(4)](2-)·2Na(+) and [Si(4)(SiH(3))(4)](2-)·2K(+) by replacing lithium with the sodium and potassium cations.

Influence of thermal vibrations on aromaticity of benzene

The values of twenty-eight aromaticity indices were estimated for distorted benzene geometries coming from normal modes of thermal vibrations at room temperature. The applied Principal Component Analysis (PCA) revealed that four orthogonal principal components are sufficient for the description of 80% of the population variance. The importance of different aromaticity criteria was confirmed by values of variable loadings. The first principal component has a quite complex composition. Although the highest contribution to PC1 comes from ATI the roles of magnetic susceptibility, REC, EN and A(j) are also non-negligible. The second orthogonal set of variables constituting PC2 comprises PDI and to a lesser extent REC. The importance of θ and EN is emphasized by contributions to PC3. The fourth PC is defined mostly by ΔE. This suggests that geometry alterations encountered during thermal vibrations are associated with changes in electron delocalization and chemical shift. Additionally, electron density changes along normal vibration modes affect also the resonance energy of the ring. Although thermal vibrations of benzene at room temperature do not exceed ±25% and most of analyzed parameters fluctuate within ±10%, the very different molecular properties must be taken into account for proper description of consequences of molecular vibrations on π-electron delocalization in a benzene ring.

Aromaticity of topological isomers of cyclazines and their [14] to [18] annulenes

The structures and magnetic properties of the parent cyclazines [2.2.2], [3.2.2], [3.3.2], [4.2.2], [3.3.3], [4.3.2], their topological isomers and the doubly bridged [14] to [18] annulenes were studied computationally at the B3LYP/6-311+G ** level of density functional theory (DFT). The magnetic criteria Λ (magnetic susceptibility exaltations, MSE), ∑NICS (the summation of nucleus-independent chemical shifts), and 1H NMR chemical shifts, geometry-based HOMA (harmonic oscillator model of aromaticity) index and HOMO–LUMO gaps were used to elucidate their aromatic and anti-aromatic properties. The excellent correlation between Λ and the summation of NICS(0) and NICS(1) demonstrates that these two originally proposed indices are still good indicators of the whole aromaticity of these polycyclic ring systems. The computed NMR chemical shifts are also in good agreement with available experimental data. In general, there is a good relationship between different criteria of aromaticity.

A new approach for aromaticity criterion based on electrostatic field gradient

In this research, electric field gradient (EFG), which is the first derivative of electric field, is applied for evaluation of aromaticity of 89 cyclic organic compounds. In our calculations, DFT procedure (b3lyp) with basis set 6-311++G has been employed, and the obtained electronic structures and frequency test has been done for optimized geometries. The aromaticity evaluated for these compounds by EFG procedure is successfully compared with other well-known indices in literature, especially with nuclear independent chemical shift (NICS). These comparisons show that EFG method of assessment of aromaticity can be used as a rather valid procedure for this purpose. Flexibility and simplicity of EFG make this method a rather easy procedure for assessment of aromaticity. Since EFG method of aromaticity evaluation does not need specific programming and it can be done by known software such as Gaussian, therefore, the availability for everyone to calculate desired aromaticity by this method is one of attractive feature of it similar to NICS.

The applicability of three-dimensional aromaticity in BiSnn− Zintl analogues

Three-dimensional aromaticity is shown to play a role in the stability of deltahedral Zintl clusters and here we examine the connection between aromaticity and stability. In order to gain further insight, we have studied Zintl analogs comprised of bismuth doped tin clusters with photoelectron spectroscopy and theoretical methods. To assign aromaticity, we examine the ring currents induced around the cage by using the nucleus independent chemical shift. In the current study, BiSn(4)(-) is a stable cluster and fits aromatic criteria, while BiSn(5)(-) is found to fit antiaromatic criteria and has reduced stability. The more stable clusters exhibit an aromatic character which originates from weakly interacting s-states and bonding orbitals parallel to the surface of the cluster, while nonbonding lone pairs perpendicular to the surface of the cluster account for antiaromaticity and reduced stability. The effect of three-dimensional aromaticity on the electronic structure does not result in degeneracies, so the resulting variations in stability are smaller than those seen in conventional aromaticity.

Demonstration of “Möbius” Aromaticity in Planar Metallacycles

Möbius aromaticity, predicted by Edgar Heilbronner in 1964, is a stabilizing effect exhibited by 4n electron fully conjugated cyclic molecules (or transition states) with an odd number of orbital phase inversions. Although it has previously been suggested that this effect might also apply to planar metallacycles in which a transition metal employs a d orbital in delta-type binding mode, only very few examples of stable twisted molecules composed of main group elements are known. We report herein, the first computationally confirmed 4npi aromatic planar metallacyclic examples and their building principles. Aromatic stabilization energy (ASE) of a 8pi metalla-cycloheptatriene [Fe(CH)(6)H(2)], with four doubly occupied pi orbitals and a HOMA value of +0.80 (cf. benzene=+1.0), an NICS(0) value of -8.5 (benzene=-9.8, NICS=nucleus independent chemical shift), and with one phase inversion, is +27.5 kcal mol(-1) (about two-thirds of the value for benzene). In contrast, an unknown non-Möbius 1,4-dimetallabenzene [Fe(2)(CH)(4)H(4)], also with 8pi electrons, and without phase inversions, has an ASE of -4.1 kcal mol(-1) and a NICS(0)=+15.6, indicative of antiaromaticity. Aromaticity of the proposed Möbius aromatic metallacycles is confirmed by using magnetic (NICS(0), NICS(1)(zz), delta(1)H) and geometric (HOMA) aromaticity criteria, planarity, and near equalized C-C bond lengths, bonding analysis (Wiberg bond indices, NBO, and NLMO analysis). The role of wave function boundary conditions (periodic vs. antiperiodic) in chemistry is further stressed, being equivalent to Zimmerman's concept of nodal parity for Möbius/Hückel systems.

ChemInform Abstract: The Large closo-Borane Dianions, BnHn2- (n = 13-17) Are Aromatic, Why Are They Unknown?

The relative stabilities of the unknown larger closo-borane dianions B(n)()H(n)()(2)(-) (n = 13-17), were evaluated at the B3LYP/6-31G level of density functional theory by comparing the average energies, E/n, and also by the energies using the model equation: B(n)()(-)(1)H(n)()(-)(1)(2)(-) + B(6)H(10) --> B(n)()H(n)()(2)(-) + B(5)H(9) (n = 6-17). Starting with the small closo-borane, B(5)H(5)(2)(-), the sequential addition of BH groups is represented by formal transfer from B(6)H(10) to build up larger and larger clusters. Most of the energies for these sequential steps are exothermic, but not for the B(12)H(12)(2)(-) to B(13)H(13)(2)(-) and the B(14)H(14)(2)(-) to B(15)H(15)(2)(-) stages. The cumulative total energies (DeltaH(add)) of these BH group additions, based on B(5)H(5)(2)(-) as the reference zero, tend to increase with increasing cluster size. DeltaH(add) indicates that the larger unknown closo-boranes B(13)H(13)(2)(-) to B(17)H(17)(2)(-) are more stable than B(9)H(9)(2)(-), B(10)H(10)(2)(-), and B(11)H(11)(2)(-); this agrees with E/n and with Lipscomb's earlier conclusion based on the PRDDO average energies. B(13)H(13)(2)(-), B(14)H(14)(2)(-), and B(15)H(15)(2)(-) are less stable than B(12)H(12)(2)(-), which has the lowest average energy on a per vertex basis among the closo-borane dianions. However, the total DeltaH(add) treatment indicates the larger B(16)H(16)(2)(-) and B(17)H(17)(2)(-) to be favorable relative to B(12)H(12)(2)(-), because of the larger number of vertexes. The formation of B(13)H(13)(2)(-) from B(12)H(12)(2)(-) is especially unfavorable. The further formation of B(14)H(14)(2)(-) and B(15)H(15)(2)(-) via BH transfer also is endothermic. These are not the only thermodynamic difficulties in building up large closo-borane dianions beyond B(12)H(12)(2)(-). The highly exothermic disproportionation of larger and smaller closo-borane dianions, e.g., B(12+)(n)()H(12+)(n)()(2)(-) + B(12)(-)(n)()H(12)(-)(n)()(2)(-) --> 2B(12)H(12)(2)(-) (n = 1-5), also indicate possible synthetic problems in preparing larger closo-boranes with more than 12 vertexes under condition where smaller boranes are present. All the larger closo-B(n)()H(n)()(2)(-) (n = 13-17) cluster exhibit three-dimensional aromaticity, judging from the computed Nucleus Independent Chemical Shifts (NICS), which range from -30.9 to -36.5 ppm. The trends in NICS values are similar to the variations in the bond length alternations, Deltar. Thus, the qualitative relationships between geometric and magnetic criteria of aromaticity found earlier for the smaller clusters extends to the larger closo-borane dianions, B(n)()H(n)()(2)(-) (n = 13-17).

Theoretical basis for the correlation between energetic, magnetic, and electron density criteria of aromaticity: definition of molecular circuit electric conductance

Using a semiclassical model, a theoretical basis for the correlation between energetic, magnetic, and electron density criteria of aromaticity has been established. The key quantity that links ring current intensity, topological resonance energy, and multicenter electron delocalization is the molecular circuit electric conductance, which is defined as the inverse of the resistance of the molecular circuit to the electron flow when an external electromotive force is exerted on it. The particular case of an external magnetic field applied perpendicularly to the molecular plane is employed here as source of the electromotive force. In addition, it is shown how aromatic stabilization can be estimated quantitatively by means of multicenter electron delocalization in PAHs and nitrogenated polyheterocyclic compounds.

Topological characteristics of the Ring Critical Points and the aromaticity of groups IIIA to VIA hetero-benzenes

The relative aromaticity of the hetero-atom containing benzenes (groups IIIA to VIA) was investigated using the magnetic and energetic criteria of aromaticity. Topological properties of electron density at the Ring Critical Points (RCP) were also studied and the advantages of using these points for evaluation of the NICS index of aromaticity were shown in comparison with the classical criteria. Although, in each group of Periodic Table RCP characteristics are almost in accordance with classical aromaticity criteria, i.e. magnetic and energetic indices of the aromaticity, in general, the relationship between the topological properties of the RCP and the magnetic and energetic indices fails.

Are Thermodynamic and Kinetic Stabilities Correlated? A Topological Index of Reactivity toward Electrophiles Used as a Criterion of Aromaticity of Polycyclic Benzenoid Hydrocarbons

A topological index of reactivity (TIR) of benzenoid hydrocarbons is defined basing on an approximate value of the bicentric localization energies. TIR values correlate with all known (24) Hammett-Streitwieser position constants, based on kinetic data for electrophilic substitution in benzenoid hydrocarbons. The maximum value of the index, denoted by TIR(max), defines the stability of a molecule toward electrophiles. For all 35 nonisoarithmic molecules of benzenoid hydrocarbons for which Hess and Schaad data are known, TIR(max) values correlate with classical numerical characteristics of aromaticity: resonance energy per pi-electron (REPE), HOMO-LUMO gap, and geometry based aromaticity index HOMA. Correlation between TIR(max) and exaltation of magnetic susceptibility is also found for cata-condensed benzenoid hydrocarbons, whereas if the peri-condensed ones are included, no correlation is observed. This can be ascribed to the presence of both paratropic and diatropic rings in perifusenes.

Macrocyclic aromaticity in Hückel and Möbius conformers of porphyrinoids

Macrocyclic aromaticity is one of the key concepts in porphyrin chemistry. The degree of macrocyclic aromaticity and the associated main macrocyclic conjugation pathway in Hückel- and Möbius-type porphyrinoids were determined using our recently proposed procedure based on bond resonance energy (BRE). All porphyrinoids with diatropic and paratropic macrocycles were found to have positive and negative superaromatic stabilization energies (SSEs), respectively. Main macrocyclic conjugation pathways predicted for various porphyrinoids were exactly the same as those predicted from the annulene model for porphyrinoids. Thus, macrocyclic aromaticity of Hückel and Möbius porphyrinoids has been rationalized successfully using an energetic criterion of aromaticity.

Multiphoton Infrared Initiated Thermal Reactions of Esters: Pseudopericyclic Eight-Centeredcis-Elimination

Multiphoton infrared absorption from a focused, pulsed CO(2) laser was used to initiate gas-phase thermal reactions of cis- and trans-3-penten-2-yl acetate. By varying the helium buffer gas pressure, it was possible to deduce the product distribution from the initial unimolecular reactions, separate from secondary reactions in a thermal cascade. Thus, trans-3-penten-2-yl acetate gives 54 +/- 5% of beta-elimination to give trans-1,3-pentadiene, 40 +/- 3% of [3,3]-sigmatropic rearrangement to give cis-3-penten-2-yl acetate and 6 +/- 4% of cis-1,3-pentadiene. Similar irradiation of cis-3-penten-2-yl acetate gives 45 +/- 1% of beta-elimination to give cis-1,3-pentadiene, 32 +/- 2% of [3,3]-sigmatropic rearrangement to give trans-3-penten-2-yl acetate and 23 +/- 2% of trans-1,3-pentadiene. The latter process is an eight-centered delta-elimination, which is argued to be a pseudopericyclic reaction. Although beta-eliminations have been suggested to be pericyclic, B3LYP/6-31G(d,p), MP2 and MP4 calculations suggest that both beta- and delta-eliminations, as well as [3,3]-sigmatropic rearrangements of esters are primarily pseudopericyclic in character, as judged by both geometrical, energetic and transition state aromaticity (NICS) criteria. Small distortions from the ideal pseudopericyclic geometries are argued to reflect small pericyclic contributions. It is further argued that when both pericyclic and pseudopericyclic orbital topologies are allowed and geometrically feasible, the calculated transition state may be the result of proportional mixing of the two states; this offers an explanation of the range of pseudopericyclic and pericyclic characters found in related reactions.

PYRAMIDALITY AND AROMATICITY IN POLYPHOSPHAPHOSPHOLES

Ab initio calculations show that, for the P n( CH )4-n PH with n = 0 - 4, the fully optimized structures have decreasing pyramidality at the tri-coordinated phosphorus atom with n. The interaction between the tri-coordinated phosphorus [Formula: see text] and the carbons–phosphorus π-system [Formula: see text] with different n, the differences of the structure, electron properties and energy among the phospholes indicate that the ring strain and the delocalization of the lone pair electron of tri-coordinated phosphorus should be responsible for this phenomena. Various aromatic criteria such as geometry, magnetism, and energy show that these molecules are aromatic, and their aromaticities increase with the number of phosphorus atoms in the nonplanar pentagons. The electron circulation in the aromatic molecules can promote the delocalization of the lone pair. In contrast to the B3LYP prediction, we draw some similar conclusions, but the planarity and aromaticity of molecules increase largely at the MP2 lever due to the consideration of electron correlation.

Exploring the Stability of Golden Fullerenes

The golden fullerenes Au32, Au50, and Au72 are reinvestigated, using high-level relativistic density functional theory (DFT). Their stability is found to be highly dependent on the electron count fulfilling the criterion of spherical aromaticity, which in turn is sensitive to relativity. Only neutral and cationic Au32 are shown to be stable with respect to alternative, space-filling isomers. Endohedrally doping Au32 with an alkali metal cation significantly increases its thermal stability, facilitating experimental preparation of the hollow cluster.

How the Aromatic 4-Membered Hydrido-Bridged Copper Rings Respond to Successive Nucleophilic Attack?

Electronic structure calculations (B3LYP/6-311+G**) predict that nucleophilic attack of the aromatic cyclo- Cu4(μ-�)4 ring yields ligand-stabilized tetranuclear Cu4 clusters formulated as cyclo-Cu4(μ-�)4Nucn (n = 1-4; Nuc = N2, CO, H2O, NH3 and PH3). Depending on the number of added nucleophiles, the tetranuclear Cu4 clusters adopt planar, bent or 3D tetrahedral geometries. These molecules exhibit aromatic character, which is primarily due to 4s and 3d cyclic elec- tron delocalization over the Cu4 framework (s and d-orbital aromaticity). The aromaticity of the novel ligand stabilized tetranuclear Cu4 clusters was verified by a number of established criteria of aromaticity. In particular, the nucleus- independent chemical shift, NICS(0) and NICS(1) and their out-of-plane components NICSzz(0), NICSzz(1) and the NICS scan pictures are indicative for the aromaticity of the planar, bent and tetrahedral Cu4 clusters. The effect of the substitu- ents on the aromatic character of the Cu4 clusters is also a main concern of the present work. It was found that increasing the number of the attacking nucleophiles increases the diatropic response of the aromatic tetranuclear Cu4 clusters. Moreover, the aromaticity of the 3D structures (spherical aromaticity) is higher than the aromaticity of the planar and bent structures. The effect of the coordinated nucleophiles, Nuc, on the stability, geometry, electronic structure and bonding mode of the cyclo-Cu4(μ-�)4Nucn molecules is also thoroughly discussed.

Triplet‐State Aromaticity of 4nπ‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function

The pi contribution to the electron localization function (ELF) is used to compare 4npi- and (4n+2)pi-electron annulenes, with particular focus on the aromaticity of 4npi-electron annulenes in their lowest triplet state. The analysis is performed on the electron density obtained at the level of OLYP density functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all-carbon annulenes are set up: the span in the bifurcation values DeltaBV(ELF(pi)) should be small, ideally zero, and the bifurcation value for ring closure of the pi basin RCBV(ELF(pi)) should be high (> or = 0.7). On the basis of these criteria, nearly all 4npi-electron annulenes are aromatic in their lowest triplet states, similar to (4n+2)pi-electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D4h and D8h symmetry, respectively, the RCBV(ELF(pi)) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of alpha- and beta-electrons.

Structure and aromaticity of [formula omitted], Bi 5M (M = Li, Na, K) and Bi 5M + (M = Be, Mg, Ca) clusters

The equilibrium geometries, electronic structures, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICS) of Bi 5 - , Bi 5M (M = Li, Na, K), and Bi 5M + (M = Be, Mg, Ca) clusters are calculated by the density functional theory (DFT) method. Our calculation show that the ground states of Bi 5 - , Bi 5M, and Bi 5M + species are predicted to be the D 5 h , C 5 v , and C 5v structures, respectively. In addition, the Bi 5 - unit preserves its structural and electronic integrity in forming the Bi 5M and Bi 5M + complexes. Molecular orbital analysis and NICS show that the planar Bi 5 - ( D 5 h ) anion satisfies the Hükcel rule of 4 n + 2 π electrons and magnetic criteria for aromaticity with six delocalized π electrons and negative NICS values. The dissected NICS suggests that the aromaticity of Bi 5 - ( D 5 h ) arise primarily from the contributions of Bi–Bi σ bonds and Bi–Bi π bonds, while the aromaticity of Bi 5M and Bi 5M + mainly owe to the Bi–Bi σ bonds.

Comparison of molecular structure of alkali metal o-, m- and p-nitrobenzoates

The influence of nitro-substituent in ortho, meta and para positions as well as lithium, sodium, potassium, rubidium and cesium on the electronic system of aromatic ring and the distribution of electronic charge in carboxylic group of the nitrobenzoates were estimated. Optimized geometrical structures were calculated (B3LYP/6-311++G ∗∗). To make quantitative evaluation of aromaticity of studied molecules the geometric (A J, BAC, I 6 and HOMA) as well as magnetic (NICS) aromaticity indices were calculated. Electronic charge distribution was also examined by molecular spectroscopic study, which may be the source of quality criterion for aromaticity. Experimental and theoretical FT-IR, FT-Raman and NMR ( 1H and 13C) spectra of the title compounds were analyzed. The calculated parameters were compared to experimental characteristics of these molecules.