Nucleus-independent Chemical Shift Values Research Articles

Theoretical study of the solvent and substitution effects on the structure and properties of iridatropylium cations: [C7H6Ir(PX3)3]+; X = H, Me, F

The structures and properties of a iridatropylium cations [C7H6Ir(PX3)3]+; X = H, Me, and F have been rJT″ explored using theoretical methods. The influence of solvent on the structural parameters, dipole moments, and frontier orbital energies was studied. These calculations were performed for different solvents, i.e., cyclohexane, dichloromethane, tetrahydrofuran, chlorobenzene, and chloroform on the basis of self-consistent reaction field (SCRF) theory. Also, substituent effect of phosphine ligands on the structure and properties was investigated. Aromaticity of these seven-membered rings was characterized by nucleus independent chemical shift (NICS) values.

Effect of benzo-annelation on local aromaticity in heterocyclic conjugated compounds.

The effect of benzo-annelation on the local aromaticity of the central ring of acridine (1), 9H-carbazole (2), dibenzofuran (3), and dibenzothiophene (4) was analyzed by means of the energy effects (ef), pairwise energy effects (pef), multicenter delocalization index (MCI), electron density at ring critical points (ρ(r(C))), harmonic oscillator model of aromaticity (HOMA), and nucleus independent chemical shifts (NICS). According to energetic, electron delocalization, and geometrical indices, angular benzo-annelation increases, whereas linear benzo-annelation decreases, the extent of the local aromaticity of the central ring containing heteroatoms. The local aromaticity of the central heterocyclic ring in the examined molecules can significantly vary by applying different modes of benzo-annelation. The NICS values do not always support the results obtained by the other aromaticity indices and, in some cases, lead to completely opposite conclusions.

Structure and bonding of novel paddle-wheel diiridium polynitrogen compounds: A stronger iridium–iridium bonding by density functional theory

The equilibrium geometries, thermochemistry, and bonding of diiridium polynitrogen compounds Ir2(N5)4 were predicted using density functional theory. For Ir2(N5)4 the preferred structure was a novel paddle wheel structure. Natural bonding orbital (NBO) analysis indicated that the bonding between the metal atom and the five-membered ring was predominantly ionic for Ir2(N5)4 species. In addition, Nucleus independent chemical shift (NICS) values confirmed that the planar N5− exhibited aromaticity in Ir2(N5)4. The dissociation energies into mononuclear fragments for Ir2(N5)4 were predicted to be 155.1 (149.7) kcal/mol, but Ir2(N5)4 is thermodynamically unstable with respect to dissociation into Ir2(N5)3 + N5. Our most remarkable structural finding is the extremely short Iridium–Iridium distance (2.394 Å, MPW1PW91) predicted for the paddle wheel structure of Ir2(N5)4. The new class of potential compounds was predicted stable enough for practical applications as high-energy density materials (HEDMs).

Characterizations of Novel Binuclear Transition Metal Polynitrogen Compounds: M2(N5)4 (M=Co, Rh and Ir)

Theoretical studies on a series of binuclear transition metal pentazolides M2(N5)4(M=Co, Rh and Ir) predict Paddlewheel-type structures with very short metal-metal distances suggesting high-order metal-metal multiple bonds. Natural Bonding Orbital (NBO) analysis have indicated that the bonding between the metal atom and the five-membered ring is predominantly ionic for each M2(N5)4species, and a high-order metal-metal multiple bonding exists between the two metal atoms, in addition, the presence of the delocalized π orbital plays an important role in the stabilization of this metal-polynitrogen species. Nucleus independent chemical shift (NICS) values confirm that the planar N5exhibits aromaticity in these M2(N5)4species. The values of NICS(0.0), NICS(0.5) and NICS(1.0) for Co2(N5)4are larger than those of the other two M2(N5)4species (M=Rh and Ir), with the order of Co2(N5)4>Rh2(N5)4>Ir2(N5)4. The dissociation energies into Mononuclear Fragments for M2(N5)4(M=Co, Rh and Ir) are predicted to be 82.9 (85.7), 139.9 (113.2), and 155.1 (149.7) kcal/mol, respectively. However, the dissociation energies for the loss of one pentazolato group from the M2(N5)4analysis have indicated that the Co2(N5)4is relatively higher at ~40 kcal/mol. Thermochemistry suggests Co2(N5)4to be a viable species.

Theoretical studies on benzo[1,2,4]triazine-based high-energy materials

Density functional theory calculations of 13 aminonitro compounds based on the benzo[1,2,4]triazine fused-ring system were performed. The geometries of all 13 species were optimized at the B3LYP/6-31G(d) level of theory. In order to refine the energy values, single-point energy calculations of the species were made at the B3LYP/6-311++G(2df,2p) level. The gas-phase heats of formation of the species considered were calculated using the atom equivalent method. Condensed-phase heats of formation were calculated utilizing the heats of sublimation of the designed molecules, as evaluated during the present study. With the help of the WFA program, crystal densities of the designed compounds were predicted using the geometry of the molecule optimized at the B3PW91/6-31G(d,p) level. The stabilities and impact sensitivities of all of the compounds are discussed in the present paper in terms of the bond dissociation energy (BDE) of the trigger linkage (the longest C-NO₂ bond) and the available free space per molecule (∆V) in the unit cell of each compound. A nucleus-independent chemical shift (NICS) study was performed to assess the aromaticities of the designed molecules, and the NICS(1) values determined 1 Å above and below the plane of the ring were found to be -7.9 to -10.5, respectively, for the benzene ring and -10.7 to -11.4, respectively, for the triazine ring in the designed fused-ring molecules, showing that both rings retain their aromaticities when undergoing substitution by nitro groups. Detonation parameters of the species were calculated, and the results suggest that the designed compounds possess comparable values to those of the commercial explosives TNT and RDX. Furthermore, results suggest that the designed compounds may be less sensitive than many nitroaromatic and nitramine explosives. Thus, the results obtained during the present study imply that the designed compounds may be used as safe explosive materials, and could be potential alternatives to TNT and RDX.

Evaluation of the Aromaticity of a Non-Planar Carbon Nano-Structure by Nucleus-Independent Chemical Shift Criterion: Aromaticity of the Nitrogen- Doped Corannulene

Substitution of two or four carbon atoms by nitrogen in the corannulene molecule as a carbon nanostructure was done and the obtained structures were optimized at MP2/6-31G(d) level of theory. Calculations of the nucleus-independent chemical shift (NICS) were performed to analyze the aromaticity of the corannulene rings and its derivatives upon doping with N at B3LYP/6-31G(d) level of theory. Results showed NICS values in six-membered and five-membered rings of two and four N atoms doped corannulene are different and very dependent to number and position of the N atoms. The values of the mean NICS of all N-doped structures are more positive than intact corannulene that show insertion of N atom to the structures causes to decreasing aromaticity of them.

The NICS‐XY‐Scan: Identification of Local and Global Ring Currents in Multi‐Ring Systems

Nucleus-independent chemical shift (NICS)-based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi-ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS-based method for the determination of local and global ring currents in conjugated multi-ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICSπZZ , but it is shown that at a height of 1.7 Å above the molecular plane, NICSZZ provides the same qualitative picture. This method, namely the NICS-XY-scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.

Theoretical and computational investigation of meta-phenylene as ferromagnetic coupler in nitronyl nitroxide diradicals

We predict the magnetic exchange coupling constant (J) for 27 m-phenylene-based nitronyl nitroxide (NN) diradicals with nine different substituents in three unique (common ortho, ortho–meta and common meta) positions on the coupler unit by using the broken-symmetry density functional methodology. For all investigated diradicals, J values are computed using B3LYP, B3LYP-D3 and M06-2X functionals with 6–311+G(d,p) basis set. The JM06-2X value is larger than JB3LYP and closer to the observed value for the unsubstituted species. Substitutions at common ortho position always produce a greater angle of twist between the spin source and the coupler units. When the twist angle is very large, the nature of intramolecular magnetic interaction changes from ferromagnetic to antiferromagnetic. In these cases, the coupler–NN bond order becomes small. Substitution at the common meta position of m-phenylene in the diradical has little steric and hydrogen-bonding effects. Electron-withdrawing groups reveal a specific trend for single-atom substitution. An ortho substitution generally decreases J and a meta substitution always increases J with a decreasing −I effect. Variation of J with planarity as well as Hammett constant is investigated. The nucleus-independent chemical shift value is found to decrease from the corresponding mono-substituted phenyl derivatives. The dependence of J on these factors is explored.

σ-Aromaticity in Hexa-Group 16 Atom-Substituted Benzene Dications: A Theoretical Study

C6I6(2+) has been reported to have a σ-aromatic character since removal of two σ anti-bonding electrons localized on iodines results in fulfilling Hückel (4n+2) rules for I6(2+) as well as C6 parts. To search for molecules possessing similar character, hexa-group 16 atom-substituted benzene dications C6(ChH)6(2+) (Ch = S, Se, Te) and their derivatives are examined for aromatic character by using nucleus-independent chemical shift (NICS). For these dications, in which iodines in C6I6(2+) are replaced by group 16 atoms, negative NICS values larger in magnitude than for benzene are found when a σ anti-bonding orbital localized on group 16 atoms is unoccupied. To clarify the origin of large negative NICS values, they are decomposed into individual molecular orbitals. It has been shown that both π bonding orbitals on C6 and σ bonding orbitals on Se6 or Te6 contribute to the negative NICS values, indicating that the aromaticity of these dications have a substantial σ character as well as π characters. Aromaticity of group 14 and 15 atom-substituted benzene dications is also discussed.

Quantum‐chemical analyses of aromaticity, UV spectra, and NMR chemical shifts in plumbacyclopentadienylidenes stabilized by Lewis bases

We carried out a series of zeroth-order regular approximation (ZORA)-density functional theory (DFT) and ZORA-time-dependent (TD)-DFT calculations for molecular geometries, NMR chemical shifts, nucleus-independent chemical shifts (NICS), and electronic transition energies of plumbacyclopentadienylidenes stabilized by several Lewis bases, (Ph)2 ((t) BuMe2 Si)2 C4 PbL1 L2 (L1, L2 = tetrahydrofuran, Pyridine, N-heterocyclic carbene), and their model molecules. We mainly discussed the Lewis-base effect on the aromaticity of these complexes. The NICS was used to examine the aromaticity. The NICS values showed that the aromaticity of these complexes increases when the donation from the Lewis bases to Pb becomes large. This trend seems to be reasonable when the 4n-Huckel rule is applied to the fractional π-electron number. The calculated (13)C- and (207)Pb-NMR chemical shifts and the calculated UV transition energies reasonably reproduced the experimental trends. We found a specific relationship between the (13)C-NMR chemical shifts and the transition energies. As we expected, the relativistic effect was essential to reproduce a trend not only in the (207)Pb-NMR chemical shifts and J[Pb-C] but also in the (13)C-NMR chemical shifts of carbons adjacent to the lead atom.

ZnBi4]3– Pentagon in K6ZnBi5: Aromatic All-Metal Heterocycle

The first aromatic all-metal heterocycle, [ZnBi4](3-), found in the metallic salt, K6ZnBi5, has been synthesized and structurally characterized. The exactly planar [ZnBi4](3-) pentagon with six π electrons coupled with multiply bonded Zn-Bi and Bi-Bi bonds, multicentered π-conjugated bonding, and negative nucleus-independent chemical shift values reveals its aromatic character. The metallic nature of K6ZnBi5 has been established by Pauli-type temperature-independent paramagnetism and theoretical analysis of the band structure and total/partial density of states.

Efficient Synthesis of Novel Pyranoquinoline Derivatives from Simple Acetanilide Derivatives: Experimental and Theoretical Study of their Physicochemical Properties using DFT Calculations

A convenient reaction of 2-chloroquinoline-3-carbaldehyde derivatives and dimedone in the presence of KF-Al2O3 for the synthesis of useful pyranoquinolines is described. Reasonable yields (41-50%), easily available starting materials and less expensive efficient catalyst are the key features of the present method. A mechanism was proposed for the reaction course. Attribution of the chemical shifts was made with the help of the density functional theory (DFT) calculations. The computed nuclear magnetic resonance (NMR) chemical shifts are in good agreement with available experimental data. The nucleus-independent chemical shift (NICS) values were used as quantitative measures for the relative aromatic character in pyranoquinolines. The calculated NICS values obtained for the phenyl group of pyranoquinoline compounds are smaller than that of benzene.

Aromatic Stabilization Energy and Magnetic Properties in Fulvalenes: Is There a Connection Between These Two Aromaticity Indices?

The nucleus-independent chemical shift (NICS) and Coulombic energy of 15 j,k-fulvalenes (j, k = 3, 5, 7, 9, 11) were investigated. It was found that in all of the hetero derivatives (j ≠ k) charge transfer between the rings occurs. It occurs when the one ring contains 4n + 1 and the other 4n + 3 π electrons, forming two rings containing numbers of π electrons closer to 4n + 2, and also when both rings contain either 4n + 3 or 4n + 1 π electrons, forming a partially aromatized ring and a partially antiaromatized ring. Both types of charge transfer are associated with aromatic stabilization energy. The NICS values are consistent with the above-described partial aromatization and antiaromatization. A semiquantitative relationship between the aromatic stabilization energy and NICS is given.

A study of the aromaticity of the heterofullerene C30X6 and C24X12 (X = B, N) analogs

The aromaticity of the heterofullerene isomers C30X6 and C24X12 (X = B, N), formed from the initial C36 fullerene with D6h symmetry, has been investigated using both the topological resonance energy (TRE) and the percentage topological resonance energy (%TRE) methods. Our results obtained by the TRE and %TRE methods were compared with the nucleus-independent chemical shift (NICS) values at the cage center. The local aromaticity was studied using the bond resonance energy (BRE) method and compared with the NICS values of the individual rings. The local aromaticity orderings of the different hexagons were obtained using the BRE method. According to the BRE results, we can predict that the C–C bonds shared by two pentagons are stabilized by the introduction of nitrogen atoms, whereas the C–C bonds shared by two hexagons are destabilized by the introduction of boron atoms. The relative orders of the global and local aromaticity are highly dependent upon the types and numbers of the heteroatoms in the structure.

Conformational Change from a Twisted Figure‐Eight to an Open‐Extended Structure in Doubly Fused 36π Core‐Modified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity

Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and -NH protons in the (1)H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by N-H···O, N-H···F, and C-H···F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state.

Theoretical Study on the Electronic Structure and Third-Order Nonlinear Optical Properties of Open-Shell Quinoidal Oligothiophenes

The chain-length dependences of geometric and electronic structures and third-order nonlinear optical (NLO) properties of open-shell quinoidal oligothiophenes (QTs) are investigated from the viewpoint of diradical character (y) using the broken-symmetry density functional theory method. The results are compared with those of closed-shell oligothiophenes. The bond-length alternation and the nucleus-independent chemical shift values are evaluated to clarify the chain-length dependence of aromaticities of the terminal and middle thiophene rings in the oligomer chain, which is closely related to the variation in the diradical character through the quinoidal/aromatic resonance structure. The y values of these QTs ranging from monomer to hexamer are found to have a wide variety of y values [0 (closed-shell state) ≤ y ≤ 1 (pure diradical state)]. The γ values per unit of the open-shell QTs having intermediate y are found to be significantly enhanced as compared to those of the closed-shell counterparts. The feature of y dependence of γ in open-shell QTs is found to be in good agreement with so-called “y–γ correlation”, which has been proposed on the basis of the simple two-site diradical model using the valence configuration interaction method. These results reveal the relationships among the terminal substituent group, molecular structure and size, and diradical character, which contribute to building design guidelines for highly efficient third-order NLO materials based on the open-shell one-dimensional oligomers.

Particle on a Boron Disk: Ring Currents and Disk Aromaticity in B202–

The B20(2-) cluster is predicted to exhibit a planar sheet-like structure with a circular circumference. Orbital plots and energy correlations demonstrate the close correspondence between the electronic structure of B20(2-) and the Bessel functions describing the waves of a quantum mechanical particle confined to a disk. The π-band of B20(2-), and its B19(-) congener, contains 12 π-electrons, forming a (1σ)(2)(1π)(4)(1δ)(4)(2σ)(2) configuration, which corresponds to a "disk aromaticity" electron count. The analogy not only applies to the π-band, but also extends to the 50 valence σ-electrons. The occupied σ-orbitals are assigned on the basis of radial and angular nodes of the scalar disk waves. The magnetic response of the cluster was examined by Nucleus Independent Chemical Shift (NICS) values and current density calculations based on the ipsocentric model. B20(2-) is found to exhibit a remarkable inner paratropic current in the σ-channel and an outer diatropic current in the π-channel. The orbital excitations responsible for the antiaromaticity in σ and the disk-aromaticity in π are identified.

Theoretical identification of the lowest energy structure of C70−n Si n , n = 1, 2, 6, 10, and 20 heterofullerenes

We have applied DFT calculations to devise stable atomic arrangements of C70−n Si n heterofullerenes with n = 1, 2, 6, 10, and 20. In general, it seems that among the considered arrangements, C atoms that are located at the polar caps of C70 prefer to be substituted by Si atoms. In the case of C68Si2, an extensive isomer search shows that the most stable configuration is that in which the Si atoms are located at the para positions of a hexagon in a polar cap. In the highly Si-doped C70 fullerenes, C and Si atoms form separate subnetworks. Doping ten Si atoms in each cap of C70, following the most stable structure of C60Si10, leads to three distinct subnetworks, a tubular carbon belt C50 surrounded by two Si10 caps. C atoms in the tubular belt preserve the conjugated pattern of distances typical of carbon nanotubes in a regular arrangement. Nucleus-independent chemical shifts (NICS) calculated at the cage centers of heterofullerenes (−21.8 to −11.0) demonstrate that all the substituted species are aromatic, but less than C70 (−27.3). The calculated NICS values may be useful for the characterization of these heterofullerenes through their endohedral 3He chemical shifts. .

Exploration on the structure, stability, and isomerization of planar C n B5 (n = 1−7) clusters

The structures, stabilities, nature of bonding, and potential energy surfaces of low-energy isomers of planar CnB5 (n = 1−7) have been systematically explored at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) level. Incremental binding energy (IBE) and second order energy difference (Δ2E) analyses demonstrate that CnB5 clusters with even n have relatively higher stability. The nature of bonding in these clusters is discussed based on valence molecular orbital (VMO), and Mayer bond order (MBO). Hückel (4n + 2) rule and nucleus-independent chemical shift (NICS) values suggest that the ground states of C3B5, C4B5, and C7B5 have π aromaticity. VMO, electron localization function (ELF), adaptive natural density partitioning (AdNDP), and NICS analyses reveal the double aromaticity of C3B5 cation. CB5 and C3B5 are stable both thermodynamically and kinetically based on isomerization analysis. In addition, the simulated IR spectra are expected to be helpful for future experimental studies of these clusters.

Solvent effects on guanidinium-anion interactions and the problem of guanidinium Y-aromaticity

We have calculated the complexes formed by guanidine/guanidinium and HCl/Cl−, HNO3/NO3− and H2SO4/HSO4− both in the gas and aqueous Polarizable Continuum Model (PCM) phase to understand the effect that solvation has on their interaction energies. In the gas phase, the cation–anion complexes are much more stable than the rest; however, when PCM-water is considered, this energetic difference is not as large due to the extra stabilization that the ions suffer when in aqueous solution. All the complexes were analyzed in terms of their AIM and NBO properties. In all cases, water solvation seems to “dampen” those properties observed in the gas phase. The values of Nucleus Independent Chemical Shift (NICS)(1) and NICS(2) indicate a huge influence of the proximity of the carbon atom for short distances; thus, the 3D NICS values on the van der Waal isosurfaces have been used to evaluate the possible Y-aromaticity of the guanidinium system. The isosurface in this system is more similar to cyclohexane than to benzene as indication of poor aromaticity. Copyright © 2013 John Wiley & Sons, Ltd.