Nucleus-independent Chemical Shift Values Research Articles

An Investigation of Electronic Structure and Aromaticity in Medium‐Sized Nanoclusters of Gold‐Doped Germanium

The electronic property and aromaticity of endohedrally doped and clusters are investigated using the density‐functional theory (DFT) within the hybrid B3LYP method. The calculated results reveal that the two clusters have high thermodynamic stability reflected by reaction energy. At the same time, it could be hoped that their high stability may arise from the closed‐shell spherical aromaticity with eight π‐electrons satisfying the counting rule with Nπ = 1. A popular nucleus‐independent chemical shifts (NICSs) calculation on basis of magnetic shieldings is also performed to confirm the aromaticity of the three‐dimensional nanoclusters with largely negative NICS values. In addition, the electronic features and chemical bonding of the two clusters are analyzed with the help of the density of states (DOS) and electron localization function (ELF), and the majority of Ge–Ge bonds on the cage show more covalent characters.

Theoretical Analysis of [5.7]nCyclacenes: Closed-Shell Cyclacene Isomers

The [5.7](n)cyclacenes represent a novel class of all sp(2)-hybridized carbon structures. In contrast to the isomeric [n]cyclacenes, [5.7](n)cyclacenes are predicted at the B3LYP/6-31G* level of theory to have stable, closed-shell singlet ground state configurations. Predicted geometries, electronic structures, band gaps, nucleus-independent chemical shift (NICS) values, and strain energies for this new family of cyclic conjugated molecules are presented.

A computational NICS and 13C NMR characterization of BN-substituted 60C fullerenes

DFT calculations are applied to devise NMR parameters of heterofullerenes C 60−2 x (BN) x ( x=1, 2, 3, 6, 9, 12, 15, 18, 21, and 24). The calculated chemical shielding tensors at the sites of 13C nuclei in the first and second neighbor of the doped atoms are perturbed and other distant atoms are not influenced significantly. Chemical shielding of the second neighbor of nitrogen and boron nuclei is significantly shifted to upfield and downfield (the second neighboring effect), respectively. Nucleus independent chemical shifts (NICS) at the cage center of BN-substituted fullerenes demystifies that substituted species are more aromatic than 60C. The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral 3He NMR chemical shifts.

Probing the structural and electronic properties of lanthanide-metal-doped silicon clusters: M@Si 6 (M = Pr, Gd, Ho)

Density functional calculations are performed to investigate the electronic and structural properties of M@Si 6 (M = Pr, Gd, Ho). Generalized Koopmansʼ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). Subsequently, a detailed comparison between previously published experimental PES spectra and the present theoretical simulations helps to identify the ground state structures. The most stable M @ Si 6 − (M = Pr, Gd, Ho) is a three-dimensional structure with the lanthanide-metal atom sitting on top (or bottom) of the regular pentagonal bipyramid. Instead, the neutral species show the impurity as a four-coordinate atom in the equatorial plane of pentagonal bipyramid. Moreover, the nucleus independent chemical shift (NICS) is discussed at various points at and over or under the center of the Si 5 ring plane, and the negative NICS values support the aromatic character of the M @ Si 6 − (M = Pr, Gd, Ho) clusters.

Theoretical investigation on geometries and aromaticity of heterocyclic platinabenzenes

The electronic structure and properties of the heterocyclic platinabenzenes isomers have been investigated using the hybrid density functional B3LYP theory. Basic measures of aromatic character were derived from the structure and nucleus-independent chemical shift (NICS). The energetic criterion suggests that the ortho-isomer enjoy conspicuous stabilization where heteroatom is P or As. But the meta-isomer is most stable isomer, where heteroatom is N. The NICS values calculated at several points above the ring center to gave the result consistent with that cased on the relative energy. The atoms in molecules analysis indicates a correlation between NICS(1.0) and the electron density of ring critical point in all species. The natural bond orbital analysis has been used to study the bond characterizations in all species.

Structure and bonding of three-coordinate N-heterocyclic carbene nickel nitrosyl complexes: Theoretical study

The structure and bonding of the for C3N3H2X2Ni(Cp)NO (X = H, F, Cl, Br) and their linkage isomers C3N3H2X2Ni(Cp)ON has been studied by carrying out density functional theory. The bonding nature of NiC bonds has been further explored by means of AIM method and natural bond orbital (NBO) analysis. Nucleus-independent chemical shift (NICS) values calculated at several points above ring center indicate aromaticity of heterocyclic cycle. Also, the effect of substitution (X = F, Cl, Br, CN) in N-heterocyclic carbene on the properties of complex has been shown.

Molecular interactions in methylimidazolium tetrafluoroborate ionic liquid ([Mim +][BF 4−]): Structures, binding energies, topological properties and NMR one- and two bonds spin–spin coupling constants

Molecular interactions in methylimidazolium tetrafluoroborate ionic liquid ([Mim +][BF 4 −]) have been investigated using B3LYP, B3PW91 and MP2 methods with a wide range of basis sets. Binding energy, topological properties of electron density, charge transfer, nucleus-independent chemical shift (NICS) and NMR one- and two bonds spin–spin coupling constants were calculated. With five preferential binding sites in the vicinity of the Mim + ring, five ion pairs ( A–E) with three intermolecular hydrogen bonds were found on the potential energy surface. The most stable ion pairs are formed via N–H and C–H bonds of Mim + and B–F bonds of BF 4 −. Ion pairs have electronic binding energies (BEs) in the range of − 335.6 to − 402.9 kJ/mol at MP2/6-311++G(2d,2p) level and − 328.1 to − 383.6 kJ/mol at B3LYP/AUG-cc-pVTZ level. NBO analysis confirms that the charge transfer takes place from BF 4 − to Mim +. The NICS values reveal the aromaticity of imidazole ring. The results show a correlation between absolute value of 1h J(H⋯F) and electron density at H⋯F bond critical point.

Ni(C6H4-nFn)(CO)2(C6H4=benzyne, n=1-4) 착물의 구조 및 화학결합

Ni<TEX>$(C_6H_{4-n}F_n)(CO)_2$</TEX> (<TEX>$C_6H_4$</TEX>=benzyne, n=1-4) 착물의 전자구조 및 성질을 혼성 밀도 함수 B3LYP 이론을 이용하여 조사하였다. Benzyne 고리에 대한 방향족 성질과 벤젠핵과 무관한 화학적이동(NICS)을 분석하였다. 일-, 이- 및 삼-플루오르화 착물 중에서 3-F, 3, 6-F, 및 4-H는 각기 가장 안정한 이성질체였다. 고리 중심 상단의 여러 점에서 계산한 NICS 수치는 이들 착물의 상대 에너지에 의해 구한 값들과 일치하였다. 분자-내-원자(AIM) 분석에서 얻어진 Ni-C 결합거리는 모든 화학종에 대한 고리 임계점의 전자밀도(<TEX>${\rho}_{rcp}$</TEX>)와 좋은 상관성을 보였다. The electronic structure and properties of Ni<TEX>$(C_6H_{4-n}F_n)(CO)_2$</TEX> (<TEX>$C_6H_4$</TEX>=benzyne, n=1-4) complexes have been investigated using hybrid density functional B3LYP theory. Both aromatic natures and nucleus independent chemical shift (NICS) of the benzyne rings have been analyzed. Among mono-, di-, and tri-fluorinated complexes, 3-F, 3,6-F, and 4-H are the most stable isomers, respectively. NICS values calculated at the several points above the ring centers are consistent with those based on the relative energies of the complexes. The atoms in molecules (AIM) analysis indicates that Ni-C bond distance is well correlated with the electron density of a ring critical point (<TEX>${\rho}_{rcp}$</TEX>) in all species.

Aromaticity and hydrogen storage capability of planar [formula omitted] and [formula omitted] rings

Theoretical investigation on planar N 6 4 - and N 4 2 - rings has been performed. The aromaticity of the rings has been analyzed through their nucleus independent chemical shift (NICS) values as well as the σ- and π-contribution of the Z-component of the induced magnetic field. Counter to a previously reported result [Chem. Phys. Lett. 432 (2006) 331], the N 6 4 - ring is found to be a local minimum and aromatic with planar D 6h symmetry. The N 4 2 - ring exhibits conflicting aromaticity like Al 4 4 - . Both the N 6 4 - and N 4 2 - rings can be stabilized in presence of suitable counter cations and systems like N 6Ca 2 and N 4Li 2 can trap molecular hydrogen.

DFT Study on Electronic Structures and Spectroscopic Properties of Oligo(silanylenediethynylanthracene)

Poly(silanylenediethynylanthracene) (PSDEA) exhibits a hole-transporting ability experimentally. In order to simulate the property of PSDEA, a series of silanylenediethynylanthracene oligomers were designed. The structures of these oligomers were optimized by using density function theory at B3LYP/6-31G(d) level. The energy gaps of the oligomers decrease with the increase in the chain length. The energy gaps of the oligomers also decrease in the presence of the electron-withdrawing group on the anthracene ring. The 13C chemical shifts and nucleus independent chemical shifts (NICS) at the anthracene ring center in the oligomers were calculated at B3LYP/6–31G level. The chemical shifts of the carbon atoms connected with the nitryl group changed upfield, compared with those of the carbon atoms without the nitryl group. The aromaticity at the anthracene ring center decreases in the presence of the electron-withdrawing group, whereas increases with the increase in the number of the silanylene units. The most sensitive location for calculating the NICS values is 0.1 nm above the anthracene plane.

Stability and aromaticity of nH(2)@B(12)N(12) (n=1-12) clusters.

Standard ab initio and density functional calculations are carried out to determine the structure, stability, and reactivity of B12N12 clusters with hydrogen doping. To lend additional support, conceptual DFT-based reactivity descriptors and the associated electronic structure principles are also used. Related cage aromaticity of this B12N12 and nH2@B12N12 are analyzed through the nucleus independent chemical shift values.

The first nonmetal-centered binuclear sandwich-like complexes based on the tetraatomic species E2−4 (E = N, P, As, Sb, Bi) and boron atoms

The first nonmetal-centered binuclear sandwich-like complexes, B2(η4-E4)2 (E = CMe, N, P, As, Sb, Bi; Me = CH3), have been investigated by density functional theory (DFT) to explore whether the nonmetal element can be at the center to form stable sandwich-like complexes. The stable conformer for each species is the D4d staggered one, in which there exists strong interaction between the two boron atoms. Natural bonding orbital (NBO) analysis indicates that the boron–boron bonds are all σ single bonds, which are predicted to be derived mostly from the s and pz orbitals of the boron atoms. The boron–boron bond dissociation energies are higher than that of the Zn–Zn bond in the synthesized complex Cp*ZnZnCp* (Cp* = C5Me5; Me = CH3) suggesting that these nonmetal-centered sandwich-like complexes may be synthesized in future experiments. According to the energy decomposition analysis (EDA), the boron–boron bond is much weaker than the boron–ligand bond and the ability of the E2−4 ligands to stabilize the boron–boron bonds is in the order of N2−4 > P2−4 > As2−4 > Sb2−4 > Bi2−4. Nucleus-independent chemical shift (NICS) values reveal that E2−4 (E = CMe, N, P, As, Sb) rings of B2(η4-E4)2 exhibit characteristics of π aromaticity at 1.0 A above the ring center, whereas Bi2−4 rings of B2(η4-Bi4)2 possess antiaromaticity. In addition, the differences in the bonding nature between the nonmetal-centered and metal-centered binuclear sandwich-like complexes were also investigated. In these nonmetal-centered complexes B2(η4-E4)2, the interactions between the boron atoms and the E2−4 ligands are more than half covalent, while in the metal-centered binuclear sandwich-like complexes, the metal–ligand interactions are mainly ionic.

Relativistic scalar and spin–orbit density functional calculations of the electronic structure, NICS index and ELF function of the [Re2(CO)8(μ-BiPh)2] and [Re2(CO)8(μ-BiPh2)2] clusters

Abstract Relativistic scalar and spin–orbit density functional calculations of the electronic structure, Nucleus-Independent Chemical Shift (NICS) index and ELF function of the [Re 2 (CO) 8 (μ-BiPh) 2 ] and [Re 2 (CO) 8 (μ-BiPh 2 ) 2 ] clusters are reported. We show here that the [Re 2 (CO) 8 (μ-BiPh) 2 ] cluster has large negative NICS values in the region defined by the Re–Bi–Re–Bi four-membered ring and the ELF function shows significant electron delocalization density in the center of the metallic ring, thus indicating an aromatic cluster. In contrast the Re–Bi–Re–Bi four-membered ring in the [Re 2 (CO) 8 (μ-BiPh 2 ) 2 ] cluster has negligible paratropic ring currents and the ELF function shows a low-density region within the metallic ring indicating that aromaticity is switched off. However, the phenyl ligands in both clusters show the expected aromatic character.

Low-valent alkaline-earth metals in all-metal dinuclear metallocenes M 2(η 5-E 5) 2 (M = Be, Mg, Ca; E = Sb, Bi)

A series of all-metal dinuclear alkaline-earth metallocenes, M 2(η 5-E 5) 2 (M = Be, Mg, Ca; E = Sb, Bi), have been calculated by the density functional theory (DFT). Natural Bonding Orbital (NBO) analysis indicates that the metal–metal bonds of the title compounds are all single bonds with each metal in its +1 oxidation state, and the bonding between the metal and the all-metal ligand is mainly ionic. For both M 2(η 5-Sb 5) 2 and M 2(η 5-Bi 5) 2, the metal–ligand bonds are very strong and the lighter alkaline-earth metal has stronger metal–ligand bonding. By comparing the dissociation energies of metal–metal bond and metal–ligand bond for each M 2(η 5-E 5) 2, the single metal–metal bond is much weaker than the metal–ligand bond with the same metals. The Mg–Mg bond is the strongest in the M 2(η 5-Sb 5) 2 and M 2(η 5-Bi 5) 2 species. Nucleus-independent chemical shifts (NICS) values suggest the planar E 5 - exhibits characteristics of aromaticity in these M 2(η 5-E 5) 2 species. All the NICS values decrease in the order of Be 2(η 5-E 5) 2 > Mg 2(η 5-E 5) 2 > Ca 2(η 5-E 5) 2 except for the NICS (1.0) values of the M 2(η 5-Bi 5) 2 (M = Be, Mg, Ca) species. The absolute values of NICS (0.0), NICS (0.5) and NICS (1.0) for M 2(η 5-Sb 5) 2 are larger than those of the corresponding M 2(η 5-Bi 5) 2.

Dications of Benzylidenefluorene and Diphenylmethylidene Fluorene: The Relationship between Magnetic and Energetic Measures of Antiaromaticity

Oxidation of m- and p-substituted benzylidene fluorenes to antiaromatic dications was attempted by electrochemical and chemical means. Electrochemical oxidation to dications was successful for benzylidene fluorenes with p-methoxy, p-methyl, p-fluoro, and unsubstituted phenyl rings in the 3-position; attempts to oxidize the m-substituted derivatives via electrochemistry were unsuccessful. Chemical oxidation with SbF(5)/SO(2)ClF gave the dication of 9-[(4-methoxyphenyl)methylene]-9H-fluorene cleanly; oxidation of all other substituted benzylidene fluorenes resulted in mixtures of products. The excellent linear relationship between the chemical shifts calculated by the GIAO method and the experimental shifts for the p-methoxy-substituted benzylidene fluorene dication suggests that the calculations satisfactorily reflect the magnetic properties of this dication and potentially those of the other dications studied. The redox potentials from electrochemical oxidation, a measure of the stability of the dications, showed a good linear relationship with another measure of stability, the calculated difference in energy between each dication and its neutral precursor. The dications of benzylidene fluorenes were less stable than the dications of diphenylmethylidene fluorenes; within each type of compound, dications with p-substituted phenyl rings were more stable than dications with m-substituted phenyl rings and dications with phenyl rings substituted with electron-donating groups were more stable than dications with phenyl rings substituted with electron-withdrawing groups. The antiaromaticity of the fluorenyl system was assessed through the nucleus-independent chemical shift (NICS) that was also calculated by the GIAO method. The plot of the NICS values per square area versus the calculated energy difference for the dications showed a moderate degree of linearity; the plot of NICS values per square area versus the oxidation potentials was less linear. Thus, a suggestive, but not conclusive, relationship between magnetic and energetic measures of antiaromaticity was observed.

Control aromaticity in the thermal decomposition of 2,5-dihydrofuran, 2,5-dihydrothiophene and 3-pyrroline: a kinetic and thermodynamic study via DFT

A theoretical study of the thermal decomposition kinetics of 2,5-dihydrofuran (1), 2,5-dihydrothiophene (2), and 3-pyrroline (3) has been carried out at the B3LYP/6-31++G**, B3PW91/6-31++G** and MPW1PW91/6-31++G** levels of theory. Our results show that the MPW1PW91/6-31++G** method is in good agreement with the available experimental values. The nucleus independent chemical shift (NICS) values of all reactants, TSs and products indicate that all studied structures are aromatic and the studied reactions are controlled kinetically and thermodynamically by the change of the aromaticity. Based on the optimized ground state geometries using the MPW1PW91/6-31++G** method, the natural bond orbital analysis (NBO) of donor–acceptor (bonding-antibonding) interactions revealed that by the increase of electronegativity of atom \( {\text{X }}\left( {{\text{O}},{\text{ S}},{\text{ NH}}} \right),{\text{ LP}}\left( {\text{e}} \right)_{\text{X1}} \to \sigma_{{{\text{C}}_{ 2} - {\text{H}}_{ 6} }}^{*} \) resonance energies and also, the HOMO-LUMO energy-gaps in the ground state structures increase. The results also suggest that in compounds 1–3, the hydrogen elimination are controlled by LP(e) → σ* resonance energies.

Boron fullerenes with 32-56 atoms: irregular cage configurations and electronic properties

Unbiased search by first-principles simulated annealing revealed irregular cage configurations for medium-sized B n clusters (namely, boron fullerene) with n=32-56, which are more stable than the previously proposed symmetric cages. The stability of these irregular cages can be understood by the three-center bonds as well as the polygonal holes on the cage surface. The delocalized distribution of molecular orbitals as well as the negative nucleus-independent chemical shifts (NICS) values for each boron cage indicates strong aromaticity.

A DFT Study on the Magnetostructural Property of Ferromagnetic Heteroverdazyl Diradicals with Phenylene Coupler

We have theoretically designed five different m-phenylene coupled high-spin bis-heteroverdazyl diradicals and their analogous p-phenylene coupled low-spin positional isomers. The geometry-based aromaticity index, harmonic oscillator model of aromaticity (HOMA) values for both the couplers (local HOMA), and the whole diradicals (global HOMA) have been calculated for all the diradicals. We also qualitatively relate these HOMA values with the intramolecular magnetic exchange coupling constants (J), calculated using a broken symmetry approach within unrestricted density functional theory framework. Structural aromaticity index HOMA of linkage specific benzene rings in our designed diradical systems shows that the aromatic character depends on the planarity of the molecule and it controls the sign and magnitude of J. The predicted J values are explained on the basis of spin polarization maps, average dihedral angles, and magnetic orbitals. The effect of the spin leakage phenomenon on magnetic exchange coupling constant and that on HOMA values of certain phosphaverdazyl systems has been explicitly discussed. In addition, a similar comparison is made between the calculated exchange coupling constants and corresponding HOMA values. The main novelty of this work stands on the consideration of the aromatic behavior by means of the geometrical index HOMA. We also estimate another aromaticity index, nucleus independent chemical shift (NICS) values for the phenylene coupler in each diradical to measure aromaticity and compare its value with that of HOMA. The ground state stabilities of these diradicals have also been compared.

Theoretical Study of the Interactions Between Borthiin and Fluorinated Borthiins with Difluorine

The nature of borthiin and fluorinated borthiin interactions with difluorine was studied with ab initio calculations. The interaction energies were calculated at the B3LYP/6-311+G(d,p) level. Vibrational and AIM analyses of the complexes indicate that the variation of densities correlates well with the obtained interaction energies. Also, the nucleus independent chemical shift (NICS) values have been used for studying aromaticity in species. These values suggest that aromaticity increases in complexes.

A quantum chemical investigation on methylated pyridines

The structural features of the methylated pyridine series have been studied by quantum chemical methods. The aromatic character of these molecules is derived from the frontier molecular orbital energies, harmonic oscillator model of aromaticity (HOMA), and nucleus independent chemical shift (NICS) values. Hybrid density functional theory with 6-311++G(d,p) basis set was employed for all calculations.