Harmonic Oscillator Measure Of Aromaticity Research Articles

Influence of external electric field on properties of Cyclotriparaphenyl[6]carbon

Cyclotriparaphenyl[6]carbon is a hypothetical derivative compound of cyclo[12]carbon, in which three CC triple bonds are replaced by three benzene rings. Comprehensive investigations of influence of external electric field (EEF) on cyclotriparaphenyl[6]carbon have been performed from theoretical aspect in this work. The maximum appropriate applied electric field strength is 0.0265 a.u., electron detachment will occur when EEF exceeds this value. When the EEF is in the range of 0–0.0265 a.u., with the increase of EEF, the shape of cyclotriparaphenyl[6]carbon turns from a circle to oval, the HOMO-LUMO gap gradually decreases from 6.046 eV at 0 a.u. to 2.322 eV at 0.0265 a.u., and its absorption band extends to VL (visible light) region and near-infrared region little by little. When the EEF are 0.025 a.u. and 0.0265 a.u., its maximum absorption wavelengths are 1220.6 nm and 1383.5 nm, respectively. HOMA (Harmonic oscillator measure of aromaticity), AV1245 and electron localization function (ELF) calculation results indicate that cyclotriparaphenyl[6]carbon has no aromaticity in the EEF range of 0–0.0265 a.u.. The compound exhibitspromising potential forapplication inthefield of colordevelopingcompound materials.

The relationships between direct substituents, aromaticity and kinetic stability of pentazole ring

The two directly substituted pentazoles, N5NH2 and N5COOH, were designed to theoretically study the relationships between substituents, aromaticity and kinetic stability thereof. Analysis shows their kinetic stability order described by the energy separations between the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUMO), viz. HOMO-LUMO gap, is line with the aromaticity orders obtained from nucleus-independent chemical shift (NICS), harmonic oscillator measure of aromaticity (HOMA), Bird aromaticity index (I) and isochemical shielding surface (ICSS), which is N5NH2 > N5COOH. But this order is contrary to the heat stability order according to Ref [1] The inconsistency between heat stability and kinetic stability reveals the fact that stability of compounds is a complex character and different according to the compound property of the study. The consistency of kinetic stability order with the aromaticity orders described by NICS, NICSSZZ, HOMA, I and ICSS indicates these widely accepted methods for aromaticity estimating are reliable for the two given compounds. As for the effect on aromaticity of pentazole ring from substituent, electron-donating substituent have stronger effect than electron-withdrawing substituent.

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

Analysis of Biphenylene and Benzo{3,4}cyclobuta{1,2-c}thiophene Molecular Orbital Structure using the Huckel Method

The Huckel method is an old fashion method to predict the molecular orbital and energies of  electrons in a conjugated molecule. Although Huckel`s theory`s approximations are relatively crude, its general results are still reasonable compared to the advanced computing method and experimental results for many molecules. This paper describes the Huckel calculation of biphenylene and benzo{3,4}cyclobuta{1,2-c}thiophene using the HuLis software. The benzo{3,4}cyclobuta{1,2-c}thiophene is a derivative of biphenylene, in which case one of the benzene rings is replaced by a thiophene ring. This change produces new electronic properties that are interesting to study. This work focused on calculating those molecules on energy levels diagrams, linear combination coefficient of molecular orbitals, molecular orbital shape, energy gap, resonance energy, bond-order, bond length, and charge distribution (π electron population). Besides, we calculate the harmonic oscillator measure of aromaticity (HOMA) parameter to study the Huckel method`s validity.

Feniltriklorosilanın Delokalize Olmuş pi−pi* Orbital Etkileşimleri ve Stabilizasyon Enerjileri

In this study, the optimized Phenyltrichlorosilane (PTS, C6H5SiCl3) using Density Functional Theory (DFT) with B3LYP/6-311++G(d,p) level was studied using natural bond orbital (NBO) theory with NBO 3.1, as integrated in Gaussian09 program. It was determined that there are only pi-pi* transitions for PTS. Donor-acceptor interactions and stabilization energies for these transitions were calculated. The highest transition energy occurred in the orbital interaction of the pi(C1-C6)→pi*(C2-C3) transition and was calculated ca. 99.32 kJ mol-1, which is in a delocolized state. From the NBO calculation results, it was determined that hybridization occurred by settling in p-orbitals of electrons. Natural charges for PTS were calculated and it was determined that the strongest polarization was between Si and C3 atoms. Harmonic Oscillator Measure of Aromaticity (HOMA) index was calculated for the PTS ring.

Total non-lewis structures: An application to predict the stability and reactivity of linear and angular polyacenes

In the present work, the total non-Lewis structure (TNLS) is introduced for describing the stability and reactivity of linear and angular polyacenes. TNLS of a molecule is derived from the natural bond orbital theory representing the antibonding orbitals and/or electron delocalization of π∗ orbitals. To verify this application, we obtained the TNLS values of thirteen linear and angular polyacenes and evaluated with other quantitative aromaticity probes based on reactivities, energetics, geometrics, and magnetic properties. Our results show that there is a remarkable second-order polynomial correlation between TNLS and seven popular global measures of aromaticity including hardness, resonance energy, aromatic stabilization energy, harmonic oscillator measure of aromaticity, magnetic susceptibility exaltation, global magnetic characteristics, and mean polarizability. By increasing the TNLS values of the systems the reactivity increases and stability decreases. It is worth mentioning that the angular polyacenes produce less TNLS values than the linear systems with the same number of benzene rings. Therefore, it can be stated that TNLS values confirmed with Clar’s rule which in general prove the angular polyacenes are more stable than linear ones.

Interplay of Biradicaloid Character and Singlet/Triplet Energy Splitting for cis-/trans-Diindenoacenes and Related Benzothiophene-Capped Oligomers as Revealed by Extended Multireference Calculations.

The biradicaloid character of different types of polycyclic aromatic hydrocarbons is an important quality that guides the development of new materials with interesting magneto-optical properties. Diindenoacene-based systems represent such a class of promising compounds. In this work, the three types trans-diindenoacenes, cis-diindenoacenes, and trans-dicyclopentaacenobis(benzothiophenes) have been studied by means of advanced ab initio methods. The descriptors singlet/triplet splitting energy (ΔES-T), effective number of unpaired electrons (NU), unpaired electron density, and the harmonic oscillator measure of aromaticity have been used to characterize the biradicaloid properties of these species. For all trans structures, low-spin singlet ground states were found, in agreement with previous investigations. Increasing the length of the inner acene chain decreased the ΔES-T and strongly increased the NU values of the singlet state. The cis-diindenoacenes displayed a greatly increased biradicaloid character, indicating enhanced chemical instability. The thiophene rings in the trans-dicyclopentaacenobis(benzothiophenes) structures were found to simultaneously restrict the unpaired electron density from extending into the terminal six-membered rings and confine the unpaired electron density found in the core benzene rings.

Tuning the UV spectrum of PAHs by means of different N-doping types taking pyrene as paradigmatic example: categorization via valence bond theory and high-level computational approaches.

Tuning of the electronic spectra of carbon dots by means of inserting heteroatoms into the π-conjugated polycyclic aromatic hydrocarbon (PAH) system is a popular tool to achieve a broad range of absorption and emission frequencies. Especially nitrogen atoms have been used successfully for that purpose. Despite the significant progress achieved with these procedures, the prediction of specific shifts in the UV-vis spectra and the understanding of the electronic transitions is still a challenging task. In this work, high-level quantum chemical methods based on multireference (MR) and single-reference (SR) methods have been used to predict the effect of different nitrogen doping patterns inserted into the prototypical PAH pyrene on its absorption spectrum. Furthermore, a simple classification scheme based on valence bond (VB) theory and the Clar sextet rule in combination with the harmonic oscillator measure of aromaticity (HOMA) index was applied to arrange the different doping structures into groups and rationalize their electronic properties. The results show a wide variety of mostly redshifts in the spectra as compared to the pristine pyrene case. The most interesting doping structures with the largest red shifts leading to absorption energies below one eV could be readily explained by the occurrence of diradical VB structures in combination with Clar sextets. Moreover, analysis of the electronic transitions computed with MR methods showed that several of the low-lying excited states possess double-excitation character, which cannot be realized by the popular SR methods and, thus, are simply absent in the calculated spectra.

THEORETICAL AND EXPERIMENTAL INFRARED SPECTRUM ANALYSIS OF 1-ETHYL-3-METHYLIMIDAZOLIUM CATION

In this study, molecular structure of 1-ethyl-3-methylimidazolium cation (EMIM+) was undertaken at the DFT(B3LYP)/6-311++G(2d,2p) level of approximation using the GAUSSIAN 09 suit of programs. EMIM has two conformers (EMIM-1 and EMIM-2) with minimum energies. EMIM-1 and EMIM-2 were found C1 (double degenerated by-symmetry form) symmetry. Normal coordinate analyses were performed with the BALGA program. Experimental infrared spectra of EMIM were obtained by increasing temperature (between room temperature and 150oC) and compared with calculations. Assignment of the spectra and normal coordinate analyses to characterize the vibrations in EMIM conformers were undertaken. p-electron delocalization through the aromaticity index Harmonic Oscillator Measure of Aromaticity (HOMA) of EMIM calculated using Kruszewski and Krygowski definations.

Aromaticity and Electron Density of Hypericin.

The influence of the substituents on the geometry of the central ring system of hypericin has been analyzed. Substitution that causes flattening of the hypericin central rings is connected with introducing the aromatic character of the empty rings. All the hypericin rings have an aromatic character illustrated by the Harmonic Oscillator Measure of Aromaticity (HOMA), Nucleus Independent Chemical Shift (NICS), Fluctuation Index (FLU), and Ellipticity Index (EL) indices. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analyses performed on 7,14-dihydrophenanthro[1,10,9,8-opqra]perylene, its substituted analogues, and hypericin show an influence of this substitution on electron density of the central rings.

Theoretical Study on the Geometry, Aromaticity, and Electronic Properties of Benzo[3,4]cyclobutathiophenes and Their Homologues.

The geometry, aromaticity, and electronic properties of benzo[3,4]cyclobutathiophenes (BCTs) and their homologues have been examined theoretically using density functional theory calculations. The harmonic oscillator measure of aromaticity and nucleus-independent chemical shift analyses revealed the aromaticity characteristics of the two regioisomers benzo[3,4]cyclobuta[1,2-b]thiophene and benzo[3,4]cyclobuta[1,2-c]thiophene. When the aromaticity of one of the six-π-electron rings increases, it concomitantly decreases in the other ring. The anti-aromaticity of the four-membered ring varies depending on the π-electron density of the shared bond with the thiophene ring. This leads to a large difference of the highest occupied molecular orbital-lowest unoccupied molecular orbital gap between the isomers. Linear BCT homologues show medium diradical characters and the smallest EGap values. In the angular and branched homologues, the π-electrons of central benzene rings are localized avoiding the shared bonds, which results in a nonaromatic character. These data were compared to those of the parent hydrocarbons. Because of the diene character of the thiophene ring, the number and position of annulated thiophenocyclobutadieno moieties significantly influence the aromaticity and EGap values of BCT homologues. The present study does not only provide insight into the aromaticity and the properties of organic compounds containing four-membered rings but also affords helpful design guidelines of novel organic semiconductors.

Possible lower energy isomer of carbon clusters Cn (n = 11, 12) via particle swarm optimization algorithm: Ab initio investigation

The low energy structures of neutral carbon clusters Cn (n = 11, 12) are predicted by means of particle swarm optimization and quantum chemistry calculation. The energy and geometrical structure of these isomers are further optimized by DFT, MP2 and CCSD computation methods. The relative Gibbs free energy below 5000 K was calculated, which indicates that C11 clusters are more sensitive to temperature. We also illustrate the computational simulated IR spectra obtained at B3LYP/cc-pVTZ level, which shown the vibrational modes and vibrational regions of different structural isomers. Combining nuclear independent chemical shift (NICS) and harmonic oscillator measure of aromaticity (HOMA), we have analyzed the aromaticity of the three single-ring structures and found that C11 cluster is not aromatic, while the ground stated structure of C12 is anti-aromaticity. Chemical bond analysis of the ground state is performed by Adaptive Natural Density Partition (AdNDP) injunction with electron localization function (ELF), it is found that the 2c-2e bonds of C11 and C12 planar ring structures presented strong and weak alternations, respectively, and their multi-central bonds were strongly dependent on the symmetry of molecular configuration. Finally, topological and natural bond orbital (NBO) analysis indicate that there is no electrostatic interaction in the Cn (n = 11, 12) clusters, and the high angular momentum orbital has a primary contribution to negative natural valance electron configuration. From the electronic configuration, we can see that the natural hybridized orbit of C11 and C12 clusters is mainly composed of sp hybrids.

Avoiding pitfalls of a theoretical approach: the harmonic oscillator measure of aromaticity index from quantum chemistry calculations

The concept of the harmonic oscillator measure of aromaticity (HOMA) is based on comparing the geometrical parameters of a studied molecule with the parameters for an ideal aromatic system derived from bond lengths of the reference molecules. Nowadays, HOMA is routinely computed combining the geometries from quantum chemistry calculations with the experimentally based parameterization. Thus, obtained values of HOMA, however, are bound to suffer from inaccuracies of the theoretical methods and strongly depend on computational details. This could be avoided by obtaining both the input geometries and the parameters with the same theoretical method, but efficiency of the error compensation achieved in this way has not yet been probed. In our work, we have prepared a benchmark set of HOMA values for 25 cyclic hydrocarbons, based on the all core CCSD(T)/cc-pCVQ(T)Z geometries, and used it to investigate the impact of different choices of the exchange–correlation functionals and basis sets on HOMA, calculated against the experimentally based (HOMAEP) or the consistently calculated (HOMACCP) parameters. We show that using HOMAEP leads to large and unsystematic errors, and strong sensitivity to the choice of XC functional, basis set, and the experimental data for the reference geometry. This sensitivity is largely, although not completely attenuated in the consistent approach. We recommend the most suitable functionals for calculating HOMA in both approaches (HOMAEP and HOMACCP), and provide the HOMA parameters for 25 studied exchange–correlation functionals and two popular basis sets.

Patterns of local aromaticity in graphene oxyradicals

Two families of polyaromatic hydrocarbon (PAH) oxyradicals are investigated using density functional theory (DFT) and the semi-empirical PM6 method. These families result from the edge oxidation of substrates that involve only zigzag edges and those that include both zigzag and armchair edges. Oxyradical stability is shown to correlate with local aromatic character of six-atom rings characterized by the harmonic oscillator measure of aromaticity (HOMA) and with the distribution of HOMA values in molecules. It is demonstrated, that oxidation at the edge has a non-local effect on the structure of PAHs and leads to distinguishable types of HOMA patterns that are common for both families of PAHs.

On the factors affecting tautomerism: consequences of N-substituents (Me/NR2) in structures derived from salicylaldimines

This computational organic chemistry study was performed using the B3LYP and the Hartree–Fock methods. Other methods were compared using two model compounds, and these methods are: second-order Møller–Plesset, fourth-order Møller–Plesset, quadratic configuration interaction with single and double excitation and coupled cluster single and double excitations. The study included discussions of the following aspects: the molecular structures, the harmonic oscillator measure of aromaticity, the energy differences between the tautomers, selected infrared stretching frequencies, the atomic charges (Mulliken and natural population analysis), selected NMR chemical shifts, the molecular orbital diagrams and the R groups. The calculations revealed that the electronic structure of salicylaldimines is sensitive to this substitution and may change the physical organic aspects including tautomerism significantly. The results illustrate that the proton–electron delocalisation as a sub-molecular event might be a controllable process when a powerful H-bond exists. This is a new discovery in the area of tautomerism-based functional materials that will help in fine-tuning their properties.

Effects of group 14–16 heteroatoms on the aromaticity of benzene at DFT level

Benzene aromaticity and energy gap (Δ E HOMO–LUMO) are compared and contrasted to those of 11 heterobenzenes containing group 14–16 atoms (Si, Ge, Sn, N, P, As, Sb, O +, S +, Se +, and Te +), at B3LYP/6-311++G ∗∗ level. Four diverse quantitative aromaticity indices are used: the nucleus independent chemical shifts (NICS), the aromatic stabilization energies (ASE), harmonic oscillator measure of aromaticity (HOMA), and Bird aromaticity index. The aromaticity appears with a general decrease in going from the top to the bottom of the periodic table (columns 14–16). The main exception is thiopyranium for showing more aromaticity than pyranium. Some inconsistencies are found among results of various indices employed.

Z/E-Photoisomerizations of Olefins with 4nπ- or (4n + 2)π-Electron Substituents: Zigzag Variations in Olefin Properties along the T1 State Energy Surfaces

[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields.

Investigation on the aromaticity of 1,3,4-thiadiazole-2-thione and its oxygen analogs including their tautomeric forms

The aromaticity of 1,3,4-thiadiazole-2-thione and its oxygen analogs including their tautomeric forms, and 1,3,4-thiadiazole, 1,3,4-oxadiazole, furan, thiophene, cyclopentadiene, the cyclopentadienyl anion and the cyclopentadienyl cation has been examined by harmonic oscillator measure of aromaticity (HOMA), aromatic stabilization energy (ASE), differential heat of hydrogenation (Δ ΔHH), and nucleus independent chemical shifts (NICS (0)), calculated at the HF/6-31G**, B3LYP/6-31G**, B3LYP/6-311G**, and B3LYP/6-311++G** levels. A principal component analysis (PCA) of the data set generated from these aromaticity results has been presented and compared with similar analyses in the recent literature. It has been shown that aromaticity is at least a two-dimensional phenomenon, independent of the level of the computational method employed. We observed that the sulfur-containing heterocyclic compounds are more aromatic than their oxygen analogs according to geometrical aromaticity measurements. In addition, our calculations of the energies of the tautomeric forms showed that the carbonyl and thiocarbonyl tautomers are more stable than the hydroxy and mercapto forms, which is in good agreement with the experimental findings in the literature.