Nucleus-independent Chemical Shift Research Articles

On the NICS limitations to predict local and global current pathways in polycyclic systems

Here, we analyze the possibility of predicting local and global current densities in a series of bicyclic hydrocarbons with 4n and 4n + 2 π-electrons from the nucleus-independent chemical shifts (NICS) computations.

Relating nucleus independent chemical shifts with integrated current density strengths.

Indices based on the nucleus independent chemical shift (NICS) are the most frequently used in analysis of magnetic aromaticity. The magnetically induced current density, on the other hand, is a key concept in defining magnetic aromaticity. The integrated current strength (current strength susceptibility) was found to be a very useful tool in aromaticity studies. There is widely accepted notion that the properly chosen NICS-based index can provide information on the current density strength and direction in a molecule of interest. In this work, a detailed numerical testing of the relationship between the integrated bond current strength and the most employed NICS indices was performed for a set of 43 monocyclic aromatic molecules. Based on the statistical data analysis, the relationship between the bond current strength and its π and σ electron components, on one side, and the isotropic NICS (NICSiso and NICSπ,iso) and zz-component of the NICS tensor (NICSzz and NICSπ,zz), on the other side, was examined. It was found that between the NICSπ,zz(1) and π-electron bond current strenghts there is very good linear correlation. Quite surprisingly, it was revealed that the NICSiso(1) and NICSzz(1) are not correlated with the π electron bond current strengths. On the other hand, a reasonably good linear correlation was found between the NICSzz(1) and total bond current strengths.

DFT Based QSAR Studies of Phenyl Triazolinones of Protoporphyrinogen Oxidase Inhibitors

The quantitative structure activity relationships (QSARs) have been investigated on a series of substituted phenyl triazolinones having protoporphyrinogen oxidase (PPO) inhibition activities. The density functional theory (DFT) method is applied to calculate the quantum chemical descriptors. The derived QSAR model is based on three molecular descriptors namely highest occupied molecular orbital (HOMO) energy, electrophilic group frontier electron density (FgE) and nucleus independent chemical shift (NICS). The best QSAR model has a square correlation coefficient r2 =0.886 and cross-validated square correlation coefficient q2 = 0.837.

Novel phenolic compounds by DFT: Electronic effects on antioxidant activity of 4‐vinylphenol derivatives

AbstractConsidering the biological importance of phenolic compounds and their antioxidant activities, we have reached for 10 novel 2,6‐diX‐4‐vinylphenol derivatives (X = NMe2, NH2, OMe, Me, H, Br, Cl, F, CN, and CF3, 1NMe2‐10CF3), at the B3LYP/6‐311++G** level of theory. To evaluate their antioxidant efficiency, the OH bond dissociation energy (BDE) and vertical ionization potential (IPV) are investigated for all structures in gas, water, and benzene phases, using conductor‐like polarized continuum model (CPCM) via B3LYP, LC‐ωPBE, M05‐2X, and M06‐2X functionals. The results indicate that in going from electron‐withdrawing groups (EWGs) to electron‐donating groups (EDGs), the BDE and IPV values decrease which may suggest the increasing efficiency of antioxidants via hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms, respectively. The calculated rate constants (krxn) for reactions between 1NMe2‐10CF3 with ·OOH and·OH radicals indicate that 1NMe2 shows the highest one. The nucleus‐independent chemical shift (NICS) index, energies of highest occupied and lowest unoccupied molecular orbitals (EHOMO and ELUMO, respectively), and natural bond orbital (NBO) analysis provide relevant results to understand the nature of antioxidant activity and stability of their corresponding radicals. The lowest BDE and IPV values are observed in gas and water phases, respectively. Structure 1NMe2 turns out as the most efficient antioxidant for showing the lowest values of BDE and IPV and highest values of NICS, EHOMO, second‐order perturbation energy (E2) and natural charge. Spin densities and electrostatic potential (ESP) maps appear consistent with the obtained results. The overall order of antioxidant efficiency in gas, water, and benzene phases is 1NMe2 > 2NH2 >3OMe > 4Me > 5H > 6Br > 7Cl > 8F > 9CN > 10CF3.

Prediction of azulene-based nanographene-like materials

Azulene-based nanographene-like materials are predicted and studied to reveal the molecular properties of these new materials. C48H18/(G-48), C68H22/(G-68), and C88H26/(G-88) nanographenes and their isomers based on azulene molecule (azulphene) as A-48, A-68, and A-88 were studied to infer general properties for large azulphene systems. Nanotubes from azulphenes are also obtained and studied. The isoelectronic graphene and azulphene materials have significant differences concerning, e.g., electric dipole, aromaticity, bond length, and bond stress. In the present studies, it was used the Density Functional Theory (DFT) and including the version of Grimme's D dispersion for the different spin multiplicities: singlet closed-shell (CS) and open-shell (OS), triplet, and quintet. For example, the ground-state of A-(48-68-88) structures is a singlet. The UV–visible spectra of all compounds exhibit maximum absorption peaks attributed to the electronic transition π→π*. The IR spectrum of G-88 and A-88 shows an intense set of absorption peaks in the range [600:1700 cm−1]. The following IR frequencies could be used to indicate azulphene structures: 1124, 1162, 1436, 1542, and 1597 cm−1. The azulphene sheets have a non-uniform distribution of the electron density, unlike graphene systems, which makes them promising candidates for regioselective chemical modification. The nucleus independent chemical shift calculations show that the five-membered rings are aromatic, and the seven-membered rings are antiaromatic similar to the azulene molecule. Our study also showed smaller diameter tubes based on azulene (C57NTs) might be more stable than their conventional C6NTs isomer.

Investigation the role of single vacancy defected on the performance of born carbide nanotube as an aspirin drug sensor

Density functional theory calculations were employed to investigate the adsorption potential of boron carbide nanotube (BC3NT) to Acetylsalicylic acid (ASA) molecules. Also, the binding energy, electronic structure, charge transfer and nucleus independent chemical shift (NICS) analyses were carried out. It was found that perfect BC3NT could not firmly adsorb the ASA molecules. The introduction of single vacancy (SV) defects in theBC3NT lattice significantly improvedthe interaction between the ASA molecules with Ead of about −16.92 kcal/mol. Furthermore, by the adsorption of ASA molecules onto the SV-BC3NT surface, it was anticipated that the electrical conductivity of SV-BC3NT-ASA complexes was 33.28 times greater than that of perfect BC3NT. The Ead value increases dramatically as the solvent dielectric constant increases and then changes smoothly when the dielectric constant is lower than 30. Eventually, after the adsorption of ASA molecules, the magnetic properties of SV-BC3NT were considerably increased according to NICS calculations.

Dative or coordinative carbon−boron bond in boron trapped N-heterocyclic carbenes (NHCs)? An answer given on the magnetic criterion

Abstract The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of isolated as well as B–C bond length varied model compounds (BR3 trapped NHCs) have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually the anisotropy effects measurable in 1H NMR spectroscopy) are employed to qualify and quantify the present dative vs. coordinative bond character of the boron−carbon bond in the trapped NHCs. Results are confirmed by bond lengths and 11B/13C chemical shift variations in the BR3 trapped NHCs.

On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes

The problem of aromaticity in heterocyclic rings of uracil and its 5-halogenoderivatives (5XU) was analyzed theoretically by calculating modified harmonic oscillator model of aromaticity (HOMA) for Heterocycle Electron Delocalization (HOMHED), nucleus-independent chemical shift parameters (NICS) and the so-called scan experiments, using helium-3 atom as a magnetic probe. The impact of halogen electronegativity on C5 atom’s NBO charges was also investigated. Water, as a polar environment, has a negligible impact on 5XU aromaticity. The most stable diketo tautomer shows a very low aromaticity while the “rare” dihydroxy form (tautomer No 6) is aromatic and resembles benzene. This is in agreement with traditional drawing of chemical formula of uracil’s six-membered ring, directly showing three alternating single and double bonds in its tautomer No 6. No good correlation between magnetic and geometric indexes of aromaticity for the studied 5XU tautomers was found. Linear correlation between the magnitude of NICS minimum, as well as the distance of the minimum above uracil ring plane center from 3He NMR chemical shift scan plot with respect to halogen electronegativity were observed. A strong linear dependence of magnetic index of aromaticity and the electronegativity of 5X substituent was observed.

Multinuclear magnetic resonance studies of five silver(I) trinuclear pyrazolate complexes

Experimental multinuclear determination of the chemical shifts, especially 15N and 109Ag, of five silver(I) trinuclear pyrazolate complexes, (PzAg)3, coupled with ZORA (zeroth-order regular approximation) and GIAO (gauge-independent atomic orbital) calculations, has shed new light on these compounds. An almost complete collection of spin-spin coupling constants has been measured for the first time for several of these compounds. The aromaticity of (PzAg)3 was examined using NICS (nucleus-independent chemical shift) and ACID (anisotropy of the induced current density) approaches reaching the conclusion of its absence.

Estimation of reaction kinetics for aromatic and heterocycles nitration in mixed acids through computational chemistry approach

AbstractNitration of aromatic compounds and heterocycles in mixed acid environment is one of the regularly performed large‐scale reactions in the chemical industry. Although the reaction mechanism of nitration of aromatics in mixed acid is well established, the development of a methodology for the evaluation of kinetics of exothermic aromatic nitrations in a simplified and accurate way is necessary. Here we evaluate the applicability of a kinetic evaluation methodology based on Hammett's acidity function, acidity function, and empirical activity coefficient for the nitration reaction. The contributions from the functional groups on the aromatic ring and quantitative structure‐activity relationships technique are considered for these evaluations. Natural bond orbital and magnetic index nucleus independent chemical shift analyses were carried out to obtain the substitution constants. The rate constant and activation energy values were evaluated at various temperatures and sulfuric acid strengths. The results were validated by comparing with the experimental data from the literature for several molecules. The effect of various functional groups (viz carbonyl, carboxyl, methyl, and amine) substituted on the benzene ring was also evaluated. For a few identified substrates, the data were used for estimation of residence time needed for complete conversion in continuous stirred tank reactor and in a plug flow reactor to quantify the effect of substitution constant and strength of sulfuric acid. The approach will help select a suitable reactor.

Structure, stability and bonding of the leapfrog B24 0 ,±1,±2.

Two new structural motifs of the B24 clusters are constructed by use of the leapfrog transformation. The resulting leapfrog B24 has either a bowl shape with a square vacancy or a quasi-planar 2D close-packed triangular boron sheet. The neutral and ionic forms of the latter are found to be more stable than their homologous leapfrog bowl clusters, with the exception of the dicationic B24 +2 . While the leapfrog isomer is less stable than the tubular double ring in the neutral state, it becomes competitive in some ionic states. The nucleus independent chemical shift, electron localization function, ring current maps and the electronic structure of leapfrog B24 clusters reveal them to behave as aromatics.

Predicted Reversal in N-Methylazepine/N-Methyl-7-azanorcaradiene Equilibrium Upon Formation of Their N-Oxides.

Density functional calculations and up to five different basis sets have been applied to the exploration of the structural, enthalpy and free energy changes upon conversion of the azepine to the corresponding N-oxide. Although it is well known that azepines are typically much more stable than their 7-azanorcaradiene valence isomers, the stabilities are reversed for the corresponding N-oxides. Structural, thermochemical as well as nucleus-independent chemical shift (NICS) criteria are employed to probe the potential aromaticity, antiaromaticity and nonaromaticity of N-methylazepine, its 7-azanorcaradiene valence isomer. For the sake of comparison, analogous studies are performed on N-methylpyrrole and its N-oxide.

A Neutral Three‐Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four‐Membered BSi2N‐Ring

AbstractWe report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three‐membered disilaborirane. The disilaborirane is synthesized by a facile one‐pot reductive dehalogenation of amidinato‐silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three‐membered silicon‐boron‐silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first‐principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si−Si bond of disilaborirane to obtain a four‐membered heterocycle 1‐aza‐2,3‐disila‐4‐boretidine derivative. Both the heterocycles are fully characterized by X‐ray crystallography.

A Neutral Three-Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four-Membered BSi2 N-Ring.

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three‐membered disilaborirane. The disilaborirane is synthesized by a facile one‐pot reductive dehalogenation of amidinato‐silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three‐membered silicon‐boron‐silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first‐principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si−Si bond of disilaborirane to obtain a four‐membered heterocycle 1‐aza‐2,3‐disila‐4‐boretidine derivative. Both the heterocycles are fully characterized by X‐ray crystallography.

Observation of “Outlaw” Dual Aromaticity in Unexpectedly Stable Open-Shell Metal Clusters Caused by Near-Degenerate Molecular Orbital Coupling

The Huckel’s rule, Baird’s rule, and electronic shell closure model are classical and well-established concepts in chemistry, which have long been employed in rationalizing the aromaticity/antiarom...

ANALYSIS OF LOCALIZED ORBITALS IN AZABORA DERIVATIVES OF [8] ANNULENE: IN THE VIEWPOINT OF AROMATICITY AND INDUCED RING CURRENTS

The electron density distributions among a series of [8] annulene (both ions and molecules) and its azabora derivatives, including its ions [BnNnC(8–2n) H8 (n = 1, 2, 3, 4)], are investigated by NBO and NMR analyses. The (4n+2)π and 4nπ systems (Huckel′s Rule) in these compounds are discussed via the localized orbital localization and electron localized function. A diatropic ring current (aromatic) and paratropic current (anti-aromatic) are distinguished. The natural hybrid orbital (NHO) direction and bond bending deviations from the line of nuclear centers are exhibited for understanding the states of π and σ orbitals. For $${{\text{B}}_{2}}{{\text{N}}_{\text{2}}}{{\text{C}}_{4}}\text{H}_{8}^{2-}$$ , $${{\text{B}}_{\text{4}}}{{\text{N}}_{\text{4}}}\text{H}_{8}^{2-}$$ , and $${{\text{B}}_{\text{4}}}{{\text{N}}_{\text{4}}}\text{H}_{8}^{2+}$$ NBO calculations reveal that these structures are the dominant Lewis structures. In this work, for each NAO functions (core, valence, or Rydberg) the orbital occupancy and the orbital energies are discussed. In addition, the nucleus-independent chemical shifts and statistical nucleus independent chemical shifts confirm the amounts of aromaticity and antiaromaticity in these rings.

Heteroatom effects on aromaticity of five-membered rings in acenaphthylene analogs.

The pattern of cyclic conjugation was thoroughly studied in the series of N- and P-acenaphthylene derivatives using several different aromaticity indices: the energy effect (ef), multicenter delocalization index (MCI), harmonic oscillator model of aromaticity (HOMA) index, and nucleus independent chemical shifts (NICS). The Kekulé-structure-based reasoning predicts that there would be no cyclic conjugation in the "empty" five-membered heteroatom-containing rings in the studied molecules. It was found that according to the ef, MCI, and HOMA values, the extent of cyclic conjugation in the pentagonal rings is strongly influenced by the number and mutual arrangement of the hexagonal rings. In addition, it was revealed that in some of the examined molecules, the intensity of cyclic conjugation in the "empty" pentagons is even stronger than that of some hexagonal rings within the same molecule. The obtained results refute what one would expect based on "chemical intuition," which is usually strongly rooted to the Kekulé structures.Graphical abstract.

On the pivotal roles of non-covalent interactions in governing the conformational stability of halo-salicylic acids: an “atoms-in-molecules” perspective

The present contribution elucidates the non-covalent interactions present within the molecular frameworks of a miscellany of mono and dihalo-salicylic acid derivatives from the viewpoint of the “atoms-in-molecule” (AIM) formalism. The pivotal roles of hydrogen bonds (HBs) and other short-contact non-covalent interactions, such as H···H and O···O/O···X (X = F, Cl, Br), toward the conformational preferences of the studied aromatic systems have been depicted in detail. Moreover, the modulations of the π-aromaticity of the studied systems in the presence of the concerned non-covalent interactions have been ascertained employing the nucleus-independent chemical shift (NICS) descriptor.

Substituted ketenes offer exceptional carbon bases in gas phase: Computational study by density functional theory method

AbstractIn the present study, ketene derivatives with cyclopropene and methylenecyclopropene substituents have been investigated by B3LYP/6‐311+G(d,p) level of theory to evaluate their suitability as powerful carbon superbases. The protonation at C (sp) site provides new neutral organic bases with proton affinities (PAs) = 879–1,218 kJ/mol, in which some are more basic than 1,8‐bis(dimethylamino)‐naphthalene, whose gas‐phase PA of 1,028 kJ/mol is considered the threshold of superbasicity. The PAs of designed ketenes were amplified by substitution of electron‐releasing groups such as methyl and dimethylamino on the molecular framework. Two indices of aromaticity, nucleus‐independent chemical shift and harmonic oscillator model of aromaticity, were also used for elucidation of ketene basicity, which reveals that a cyclopropene ring on the proposed ketene derivatives becomes aromatic upon protonation.

Facile Synthesis of Aryl-Substituted Cycloarenes via Bismuth(III) Triflate-Catalyzed Cyclization of Vinyl Ethers

Cycloarenes are an essential class of polycyclic aromatic hydrocarbons with unique electronic structure, but their synthesis is very challenging. Herein, we report a facile synthetic strategy prima...