Nuclear Independent Chemical Shift Research Articles

Insights on the aromaticity of imidazolylidene carbenes by means of DFT calculations

The effect of sixteen substituents on the structure of imidazolylidene carbenes was studied using five aromaticity criteria: Singlet-Triplet energy gap (STG), a modified version of the Harmonic Oscillator Model of Aromaticity (HOMA) geometric descriptor, the Aromatic Stabilization Energy (ASE) based on the energy of hydrogenation of the carbenes, the Nuclear Independent Chemical Shift (NICS) and the Aromatic Fluctuation Index (FLU). These calculations were conducted at the B2PLYP/aug-cc-pVDZ//B3LYP/6-311+G∗ level of theory. Even if these five descriptors are intended to quantify the “aromaticity” of the substituted carbenes, the correlation between them is far from being statistically significant. After excluding some “outliers”, a detailed analysis reveals three important correlations: (i) STG and ASE, (ii) HOMA and ASE and (iii) NICS(1)zz and FLU. The first one supports the well-known connection between the STG and the energy of hydrogenation of carbenes. The second is strongly influenced by the interplay between energetic and “steric” factors associated with each substituent group. The last one is clearly due to the extent of the electronic delocalization in these heterocyclic compounds.

Synthesis, Optical Properties, and Electronic Structures of Tetrakis(pentafluorophenyl)tetrathiaisophlorin Dioxide.

The synthesis, structure, optical and redox properties, and electronic structure of tetrakis(pentafluorophenyl)tetrathiaisophlorin dioxide (12) are reported. Oxidation of tetrakis(pentafluorophenyl)tetrathiaisophlorin (11) with dimethyldioxirane afforded the oxidized product, which was the tetrathiaisophlorin with two thiophene 1-oxide moieties (12). More significant nonplanarity and greater bond length alternation in 12 than those of 11 were observed by X-ray structural analysis. The absorption spectrum of 12 contains two bands at λ=348 and 276 nm, with a weak tail that extends to λ≈650 nm. Analysis of the magnetic circular dichroism spectrum of 12, based on Michl's 4N-perimeter model and molecular orbital calculations, indicate that the broad band at λ=348 nm appears to contain N2 and P2 bands, and 12 is classified as a 4nπ system, similar to 11. The nuclear-independent chemical shift values and (1) H NMR spectroscopy data indicate that 12 has more antiaromatic character than 11.

Aromaticity of the doubly charged [8]circulenes.

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes. The gauge including magnetically induced current (GIMIC) method has been employed for calculating the current density susceptibilities. The aromatic character and current pathways are deduced from the calculated current density susceptibilities showing that the neutral [8]circulenes have two concentric pathways with aromatic and antiaromatic character, respectively. The inner octatetraene core (the hub) is found to sustain a paratropic (antiaromatic) ring current, whereas the ring current along the outer part of the macrocycle (the rim) is diatropic (aromatic). The neutral [8]circulenes can be considered nonaromatic, because the sum of the ring-current strengths of the hub and the rim almost vanishes. The aromatic character of the doubly charged [8]circulenes is completely different: the dianionic [8]circulenes and the OC-, CH-, CH2-, SiH-, GeH-, SiH2-, and GeH2-containing dicationic species sustain net diatropic ring currents i.e., they are aromatic, whereas the O-, S-, Se-, NH-, PH- and AsH-containing dicationic [8]circulenes are strongly antiaromatic. The present study also shows that GIMIC calculations on the [8]circulenes provide more accurate information about the aromatic character than that obtained using local indices such as nuclear-independent chemical shifts (NICSs) and (1)H NMR chemical shifts.

1,2‐Phosphaborines: Hybrid Inorganic/Organic P–B Analogues of Benzene

AbstractPhotolysis of the cyclic phosphine oligomer [PPh]5 in the presence of pentaarylboroles leads to the formation of 1,2‐phosphaborines by the formal insertion of a phenylphosphinidene fragment into the endocyclic CB bond. The solid‐state structure features a virtually planar central ring with bond lengths indicating significant delocalization. Appreciable ring current in the 1,2‐phosphaborine core, detected in nuclear independent chemical shift (NICS) calculations, are consistent with aromatic character. These products are the first reported 1,2‐BPC4 conjugated heterocycles and open a new avenue for BP as a valence isoelectronic substitute for CC in arene systems.

1,2-Phosphaborines: hybrid inorganic/organic P-B analogues of benzene.

Photolysis of the cyclic phosphine oligomer [PPh]5 in the presence of pentaarylboroles leads to the formation of 1,2-phosphaborines by the formal insertion of a phenylphosphinidene fragment into the endocyclic CB bond. The solid-state structure features a virtually planar central ring with bond lengths indicating significant delocalization. Appreciable ring current in the 1,2-phosphaborine core, detected in nuclear independent chemical shift (NICS) calculations, are consistent with aromatic character. These products are the first reported 1,2-BPC4 conjugated heterocycles and open a new avenue for BP as a valence isoelectronic substitute for CC in arene systems.

Photomagnetic and nonlinear optical properties in cis‐trans green fluoroprotein chromophore coupled Bis‐imino nitroxide diradicals

The current study extends an earlier investigation (Bhattacharya, et al., Phys. Chem. Chem. Phys. 2012, 14, 6905) to further explore various photomagnetic and optical properties of bis‐imino nitroxide, that is, (IN)2‐based green fluorescent protein (GFP) chromophore coupled diradicals revealing new significant features. The conversion mechanisms of selected trans‐isomers into their corresponding cis‐conformers are discussed in detailed using a number of recently‐developed density functional theory (DFT) functionals based on the Minnesota suite of DFT‐models as well as using some other DFT functionals developed earlier. To provide a more in‐depth analysis of variations in magnetic properties as trans‐conformers (singlet ground‐state) convert into their cis‐analogues (triplet ground‐state), the changes in exchange magnetic coupling constants J are compared with the variation of the selected aromaticity indices. The aromaticity indices include the nuclear independent chemical shift [NICS(0)] values calculated at the center of ring structures and the harmonic oscillator model of aromaticity. Furthermore, the investigation of static nonlinear optical response properties in the (IN)2‐based GFP chromophore coupled diradicals reveal unusually large static first hyperpolarizabilities for these systems which is highly significant for practical applications in optics and optoelectronics. © 2015 Wiley Periodicals, Inc.

Silicon doping on nanotubular fullerene D5h–C90 from first principles

This work presented theoretical studies of nanotubular fullerene C90 upon the silicon substitutional doping by means of density functional theory combined with self-consistent field molecular orbital method. The doping effect on geometrical structure, relative stability, defect formation energy, electronic property, nonlinear optical property and aromaticity of the tube-shaped C90 fullerene has been addressed systemically and in details with B3LYP method. It is observed that the silicon atoms exhibit extroversion trend in the doped structure. The obtained defect formation energy is in the range of 4.861–5.231eV for the doped C90 studied, and the substitutionally doping the C90 by Si atom is more favorable to occur near the cap region according to the calculated defect formation energies. The distributions for the highest occupied molecular orbital of the doped C90 are approach to that of the pure C90 fullerene. However, the lowest unoccupied orbital of most doped C90 exhibit obvious localization at the silicon site. To study the nonlinear optical property of the doped structures, the third-order nonlinear optical constant, γ is computed with finite-field method. The enhanced values of nonlinear optical constant of the fullerenes upon silicon doping are observed and understand by γ density analysis. Moreover, the doping effect on the aromaticity of the nanotubular is also studied based on the probe of nuclear independent chemical shift.

The discussion of descriptors for the QSAR model and molecular dynamics simulation of benzimidazole derivatives as corrosion inhibitors

The corrosion inhibition performances of 20 protonated benzimidazole derivatives were studied using theoretical methods. Nuclear Independent Chemical Shift (NICS), the measurement of aromaticity, demonstrated good correlation with inhibition efficiencies and was used as a descriptor. Binding energies were calculated on the basis of molecular dynamics simulations using a realistic corrosive environment. Some improved descriptors correlate well with experimental inhibition efficiencies. A reliable nonlinear quantitative structure–activity relationship model was constructed by a support vector machine approach. The correlation coefficient and root-mean-square error were 0.96 and 6.79%, respectively. Additionally, six new benzimidazole molecules were designed, and their inhibition efficiencies were predicted.

Nucleus-independent chemical shift profiles along the intrinsic distortion path for Jahn-Teller active molecules. Study on cyclopentadienyl radical and cobaltocene

The aromatic/antiaromatic behavior of the cyclopentadienyl anion (Cp-), bis(?5-cyclopentadienyl)iron(II) (Fe(Cp)2), as well as of the Jahn-Teller (JT) active cyclopentadienyl radical (Cp?) and bis(?5-cyclopentadienyl)cobalt(II) (Co(Cp)2) has been investigated using Density Functional Theory (DFT) calculations of the Nuclear Independent Chemical Shifts (NICS). According to the NICS values, pentagon ring in Fe(Cp)2 is more aromatic than isolated Cp-. The NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for Cp? and Co(Cp)2 showing antiaromaticity, which decreases with increasing deviation from high symmetry D5h to low symmetry (LS) C2v. Changes in the NICS values along the IDP revealed that Co(Cp)2 in the LS nuclear arrangement has aromatic character, in contrast to the case of Cp?

Theoretical study of relationships between structural, optical, energetic, and magnetic properties and reactivity parameters of benzidine and its oxidized forms.

Structural, topological, optical, energetic, and magnetic properties and reactivity parameters of benzidine, its radical cation, and its dication as well as molecular complexes of the benzidine dication with the F(-), Cl(-), Br(-), I(-), NO3(-), HSO4(-), and H2PO4(-) anions were calculated at the B3LYP/6-311++G(2d,2p) level of theory in the CH2Cl2 medium. The CAM-B3LYP functional (as the most reliable one) and the 6-311++G(3df,3pd) basis set were used for the UV-vis absorption spectra prediction. The obtained spectral results are in a good agreement with available experimental data. A number of the calculated global and local molecular properties, including several recently developed ones, (in general, more than 20 parameters), namely, λmax, the bond lengths and orders (l and LA,B), adiabatic ionization energy (IEad), global electrophilicity index (ω), condensed electrophilic Fukui functions (f(+)) and dual descriptor (ΔfA), van der Waals molecular volume, nuclear independent chemical shifts (NICS) and QTAIM topological parameters were estimated in the critical points of the C(1)-C(1'), C(2)-C(3), and C(4)-N bonds as well as at the ring critical point. These quantities were found to be in a strong linear dependence (R(2) > 0.99 in most cases) with the number of detached electrons (Nel) from the benzidine molecule up to formation of the dication (Nel = 2). On one hand, a position of the long-wave absorption band (λCT) corresponding to the anion-to-cation charge transfer in the neutral complexes of the benzidine dication with anions, correlates with the Mulliken electronegativity of the anion (R(2) = 0.8646) and its adiabatic ionization energy (R(2) = 0.8054). On the other hand, the correlations with the anion charge in the complexes and the anion isotropic polarizability are rather poor (R(2) = 0.6392 and 0.3470, respectively). On the ground of the obtained strong relationships, one may recommend the calculated molecular properties as potentially preferable descriptors for the benzidine-based compounds in terms of the QSAR methodology.

First‐principle simulations on silicon-doped armchair single‐walled carbon nanotubes of various diameters

This work presented theoretical studies of structures, energies and properties of armchair carbon nanotubes (CNTs) upon the silicon substitutional doping by using first-principle calculations. The doping effects on quantitative description from viewpoint of the pi-orbital axis vector theory, relative stability, defect formation energy, electronic structure, nonlinear optical property and aromaticity of the tubes has been addressed systemically and in details. The obtained pyramidalization angles of the silicon are much larger than those of carbon according to the pi-orbital axis vector analysis. The results of defect formation energy suggest that the Si-doping would be contained easier in small CNTs. A quantitative relation about the curvature-dependent defect formation energy is obtained. The silicon atoms exhibit large distributions for the frontier molecular orbital of doped tubes. As for the nonlinear optical property, the hyperpolarizability of the tubes is dramatically enhanced upon silicon defect. The doping effect on the aromaticity is also studied. It is found that the aromaticity/anti-aromaticity transition is even occurred at the selected positions based on the probe of nuclear independent chemical shift.

Ab initio study of dehalohydrogenation reaction of 2-halo-2,3-dihydrophosphinine

Decomposition of 2-fluoro-2,3-dihydrophosphinine (1), 2-chloro-2,3-dihydrophosphinine (3), 2-bromo-2,3-dihydrophosphinine (5) to phosphinine was investigated using Molecular orbital and density functional theory. Study on the B3LYP/6-311+G** level of theory revealed that the required energy for the decomposition of compounds 1, 3, and 5 to phosphinine is 30.56 kcal·mol−1, 28.23 kcal·mol−1, and 24.03 kcal·mol−1, respectively. HF/6-311+G**//B3LYP/6-311+G** calculated barrier height for the decomposition of compound 1, 3, and 5 to phosphinine is 57.56 kcal·mol−1, 37.26 kcal·mol−1, and 30.77 kcal·mol−1, respectively. Also, MP2/6-311+G**//B3LYP/6-311+G** results indicated that the barrier height for the decomposition of compound 1, 3, and 5 to phosphinine is 46.59 kcal·mol−1, 47.28 kcal·mol−1, and 42.57 kcal·mol−1, respectively. Natural bond orbital (NBO) population analysis and nuclear independent chemical shift (NICS) results showed that, reactants are non-aromatic but products of elimination reaction are aromatic, C-H and C-X bonds are broken and H-X bond is appear.

DFT investigation of the influence of Jahn–Teller distortion on the aromaticity in square-planar arsenic and antimony clusters

Density functional theory (DFT) calculations were performed to investigate aromaticity of tetra atomic metalloid clusters, As42− and Sb42−. The careful analysis of nuclear independent chemical shifts (NICS) revealed strong σ antiaromatic and week π aromatic character of investigated species. This unexpected behavior is explained through the analysis of antagonistic paratropic and diatropic contributions, and with detailed adaptive natural density partitioning (AdNDP) analysis. Furthermore, we investigated aromatic/antiaromatic behavior of Jahn–Teller (JT) active species As4− and Sb4−. NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) showing strong antiaromaticity which decreases with increasing deviation from D4h to D2h symmetry.

From building blocks of proteins to drugs: a quantum chemical study on structure–property relationships of phenylalanine, tyrosine and dopa

Density functional theory and ab initio methods have been employed to address the impacts of hydroxyl (OH) group substitutions on the physico-chemical properties of levodopa (or L-dopa) against the natural amino acids, phenylalanine and tyrosine. L-dopa, which is an important therapeutic drug for Parkinson's disease, shares structural homology with the amino acids, whose structures differ only by OH substitutions in their phenyl side chains. It is revealed that the backbone geometries of the aromatic molecules do not show apparent OH-dependent differences; however, their other molecular-level properties, such as molecular dipole moment, electronic properties and aromaticity, change significantly. The core binding energy spectra indicate that the atom sites that undergo modifications exhibit large energy shifts, so as to accommodate the changes in the intra-molecular chemical environment of the molecules. The binding energies of the modified C 1s sites in the molecules shift as much as 1.8 eV, whereas the electronic changes in their O 1s spectra happen in the higher energy region (ca. 536 eV). The valence spectra provide enhanced insights into the reactivity and chemical properties of the aromatic molecules. The impacts of OH moieties on the valence spectra are predominantly focussed in the energy band <16 eV, where the frontier molecular orbitals display much reorganization and energy shifts from the amino acids to L-dopa. Of the three molecules, L-dopa also has the least HOMO–LUMO energy gap, which can readily explain its proactivity as a drug compound. Furthermore, the nuclear independent chemical shift calculations suggest that L-dopa also has more aromaticity features than those of the amino acids. The OH groups, therefore, play a more prominent role in shaping the physico-chemical properties of L-dopa, which significantly improve its drug potency.

Theoretical study of neutral bismuth-copper, bismuth-silver and bismuth-gold clusters

The study of alloy clusters is particularly interesting since the importance of the electronic and geometrical contributions to the stability can be investigated by examining the influences of substituting different atoms in a cluster. In this work we study the equilibrium geometries, relative stability, and electronic properties including the ionization energy (IE), electron affinity (EA), reactivity, aromaticity, and gap of small Bi m Cu n (bismuth/copper) clusters and their cations with m + n \hbox{$\leqslant$} ⩽ 5. The work is complemented with the study of small neutral bismuth/silver (Bi m Ag n ) and bismuth/gold (Bi m Au n ) clusters. The study was done using density functional theory with the M06l functional. Comparison of neutral Bi m Cu n clusters with the cationic Bi m Cu n + and neutral Bi m Ag n and Bi m Au n clusters show only small distortions of the geometries, except for Bi2Cu2 +, Bi1Cu4 +, Bi2Cu3 +, Bi3Cu2 +, Bi4Cu1 +, Bi2Ag2, Bi3Ag, Bi3Ag2, Bi2Ag3, Bi2Au2, Bi3Au and Bi2Au3. Our results on stability for cationic Bi m Cu n + clusters are consistent with experimental report. The electronic affinity for odd number of total atoms of Bi m Cu n , Bi m Ag n and Bi m Au n increases with increasing number of copper, silver or gold atoms. Moreover, it is found that the Bi m Au n are more stable than Bi m Cu n and Bi m Ag n structures. Nuclear independent chemical shift analysis confirms the presence of sigma and pi aromatic/antiaromatic character.

A bond, ring and cage resolved Poincaré–Hopf relationship for isomerisation reaction pathways

We introduce a new tool for reaction pathway analysis based on a chemical bonding density descriptor obtained from the higher resolution Poincaré–Hopf relation. To remove the degeneracy of the Poincaré–Hopf relationship for critical points of the electron density, we introduce the concepts of bonding cardinality and characterise the topology of rings and cages. This allows us to relate the changes in the relative energetic stability along an isomerisation reaction pathway with changes in the topology of the electron density. To demonstrate the utility of this approach, we consider the complete circuit of the reaction pathway connecting the various Möbius and Hückel structures in Au and/or Pd bis-metallated octaphyrins. Where the topological feature ‘cages’ exist in adjacent topologies, the degeneracy in the Poincaré–Hopf relationship is completely removed even for differences in relative energies of only 0.1 kcal/mol. Metallation of the Möbius and Hückel structures was found to increase the topological complexity and bonding density. Combining Pd and Au nuclei in the octaphyrin complexes reduced the smoothness of the potential energy surface. The presence of long, weak C–H contacts correlates in all cases with the lowest energy enantiomers and the most negative nuclear-independent chemical shift values of the reaction pathways in Au and/or Pd bis-metallated octaphyrins.

Density functional investigation of X@BnHn (X = B+, Be; n = 5–8)

The stability of X@BnHn (X=B+, Be; n=5–8) is investigated by density functional theory. Geometry optimization shows that both the radius of the center atom and the ring size make contribution to the stability of X@BnHn structure. All the optimized structures are thermodynamics stable according to the large HOMO–LUMO gap, bonding energy, vertical ionization potential and vertical electron affinity. Nuclear independent chemical shift reveals that the (quasi-)planar structures are π aromatic, which is confirmed by chemical bonding analysis.

Rattling Motion of Alkali Metal Ions through the Cavities of Model Compounds of Graphyne and Graphdiyne

We study the passage of the alkali metal ions (Li(+), Na(+), and K(+)) through some conjugated carbon-based ring systems, starting from C12H6 and C24H12 (tribenzocyclyne; TBC), which serve as model compounds for graphyne to some of the higher analogues, C26H12, C28H12, and C30H12, the model systems for graphdiyne. The motion of the ions through cyclic carbon clusters, C12 and C14, is also investigated. The potential for the motion of the ions from one side of the ring to the other through the cavities of the molecules is a symmetric double well in most cases, while it is a rather flat potential in others, arising due to the free motion of the ions through the cavities. Electrostatic potential (ESP) analyses reveal that the ions bind to the ring systems at the most negative regions of ESP. The estimated energy barriers for the motion of Li(+) through C12H6 and C24H12 are 4.7 and 4.3 kcal mol(-1), respectively, and are comparable to the barrier for the classic case of umbrella-like inversion in ammonia. Transmission of Li(+) through C26H12, C28H12, C30H12, C12, and C14 rings is barrierless. We predict that the rattling motion of Li(+) through the model compounds of graphyne and graphdiyne should be experimentally observable. We also model the effectively one-dimensional motion of the ions through the rings using discrete variable representation (DVR) and calculate the energy levels of the complexes in the symmetric double well potentials. The molecular orbital analyses and the nuclear independent chemical shift (NICS) values for the rings suggest distinct trends based on the (4n + 2/4n) π electron count, leading us to propose two neutral complexes, (C12H6)Li2 and (C24H12)Li2, that are highly stable with binding energies of 400 and 356 kcal mol(-1), respectively.

Nucleus-Independent Chemical Shifts and Aromaticity in Hexanuclear Cluster Complexes [Mo6X8]n-(X = S, Se, and Te;n= 0 and 4) of Chevrel Phases

Molecular diamagnetism and multicentered X-Mo-Mo and Mo-Mo-Mo interactions in hexanuclear cluster complexes [Mo(6)X(8)](n-) (X = S, Se, and Te; n = 0 and 4) are studied using calculations of nuclear independent chemical shifts (NICS) and the methods "atoms in molecules" and electron localization function. It is shown that [Mo(6)X(8)](4-) complexes are characterized by large negative NICSs related to the aromaticity of Mo-Mo bondings and to diatropic currents in Mo(6) metallic core of the cuboctahedral clusters. The calculated currents can participate in the mechanism of nucleation of the vortex lattice of macroscopic Abrikosov's electronic whirls responsible for the magnetic high-field stability of the superconducting currents in type II superconductors.

A theoretical study of the parent NH-benzazoles (benzimidazoles, indazoles and benzotriazoles): geometries, energies, acidity and basicity, NMR properties and molecular electrostatic potentials

The three parent NH-benzazoles, benzimidazole, indazole and benzotriazole, have been studied theoretically at the B3LYP/6-311++G(d,p) level. Optimized geometries have been compared with those obtained by X-ray crystallography, energies with studies about tautomerism, acidbase properties with pKas, and chemical shifts with those reported in the literature. As aromaticity probe, Schleyer's Nuclear Independent Chemical Shifts (NICS) were used and represented in two 3D isosurfaces of the electron density.