Nucleus-independent Chemical Shift Indices Research Articles

Novel insight on aromaticity enhancement of pyridone heterocycles using the anomeric effect-Schleyer hyperconjugation aromaticity: A DFT and NBO study

Considering the value of 2-pyridone and 4-pyridone heterocyclic groups in organic chemistry, in this study, the anomeric effect and Schleyer hyperconjugation aromaticity for the first time have been used to increase the aromaticity of these heterocycles. The anomeric effect in tetrahydropyrans containing 2-pyridone or 4-pyridone ring on the anomeric carbon in axial and equatorial conformers has been investigated using the DFT-B3LYP/6–311++G(d,p) calculation method. On the other hand, the aromaticity of 2-pyridone and 4-pyridone in the considered conformers was investigated by nucleus-independent chemical shift indices (NICS) and the harmonic oscillator model of electron delocalization index (HOMED). The results illustrate that placing 2-pyridone and 4-pyridone in the axial position increases their aromaticity through the anomeric effect. Furthermore, connecting the ring from the carbon atom to the anomeric position is better than the nitrogen atom for aromaticity increment. Aromaticity indices show that the anomeric effect-Schleyer hyperconjugation interaction is more robust in the presence of nitrogen atoms than oxygen or sulfur atom.

Efficient synthesis, spectroscopic characterization and DFT based studies of novel 1-amide 4-sulfonamide-1,2,3-triazole derivatives

In the present study, for the first time 1-amide 4-sulfonamide-1,2,3-triazole scaffolds were synthesized by using an azide-alkyne Huisgen cycloaddition reaction. The target products were obtained in moderate to good yields (45–75%) by using catalytic CuI and green system H2O/EtOH. The easy availability of the inexpensive starting materials, avoiding isolation and handling of hazardous organic azides and mild reaction conditions make this method a valuable tool for generating functionalized 1,2,3-triazole derivatives. The unambiguous characterization of synthesized compounds was accomplished by using various spectroscopic techniques such as 1H NMR, 13C NMR, and FT-IR. The information regarding optimized geometry, were obtained by applying DFT/B3LYP-6-31G(d) method. The electrophilicity index, 1H and 13C chemical shift values, lithium and sodium ion affinities of the desired product 3b have been also calculated by the mentioned method. As a whole, the calculated results were found in close agreement to that of experimental data. The studies revealed that the compound 3b possesses good Li+ and Na+ affinity and cation π interaction plays a vital role in the complexation of 3b. For the first time, nucleus–independent chemical shift index was used to confirm the cation π interaction of 3b.

Preparation and molecular structures of N′-(2-heteroarylmethylidene)-3-(3-pyridyl)acrylohydrazides

Abstract The crystal and molecular structures of N′-(2-furylmethylidene)-3-(3-pyridyl)acrylohydrazide and N′-(2-thienylmethylidene)-3-(3-pyridyl)acrylohydrazide are reported, and the influence of the type of the heteroatom on the aromaticity of the aromatic rings is discussed. Both molecules are nearly planar. The geometry of the acrylohydrazide arrangement is comparable to that of homologous compounds. Density functional theory (DFT) calculations were performed in order to analyze the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecule. The most significant changes were observed in the values of the N–N and C–N bond lengths. The harmonic oscillator model of aromaticity index, calculated for the furan and thiophene rings, demonstrated a noticeable increase in aromaticity in comparison to isolated rings and their DFT-calculated structures. By contrast, the nucleus independent chemical shift index indicated a decrease in aromatic character of the rings containing heteroatoms.

Direct C2-arylation of quinoline N-oxides by boronic esters; a molecular approach on the efficient metal-free method in C–C cross-coupling reactions

Kinetic and thermodynamic aspects of the cross-coupling reaction of quinoline N-oxides by various boronic esters in the gas and solvent phases were investigated, theoretically. Two types of boronic esters were considered for the reaction. There are various substitutes in types 1 and 2 which bonded to oxygen and boron atoms, respectively. For investigation of the chemical kinetic aspects of both types, different characters of the rate-determining step (RDS) were considered. Electron density of the involved bonds at the TS, natural charge density of the atoms and donor–accepter orbital interactions are the criteria which were used for the RDS justification of type 1. Nucleus-independent chemical shifts index was applied for description of type 2 behavior in cross-coupling reactions. Theoretical evaluation reveals that a large charge density on the oxygen atom of the O–B bond leads to a higher ability of proton abstraction which reduces ∆G ≠ values in type 1 boronic esters. Furthermore, a large aromaticity character in the quinoline ring at the TS in type 2 boronic esters causes a decrease in the ∆G ≠ value. Finally, based on the obtained results, the ability of proton abstraction of type 1 and the aromaticity character of type 2 boronic esters are the main factors in the energy barrier of the studied reactions.

A Comparative Study of the Two Isomers of Bicalicene

ABSTRACTTwo isomers of bicalicene are possible. The trans-isomer is a stable aromatic compound, known since the 1980s. The cis-isomer has never been experimentally prepared and has so far attracted little attention of theoretical chemists. The properties of cis-bicalicene are now studied theoretically, using the B3LYP/6-311G(d,p) quantum-chemical method. All data obtained (geometric, energetic, as well as those based on harmonic oscillator model of aromaticity and nucleus-independent chemical shift indices of local aromaticity) unequivocally indicate that the “missing” isomer of bicalicene is an unstable antiaromatic species, and that the lack of its existence is no surprise whatsoever.

DFT and experimental studies on structure and spectroscopic parameters of 3,6-diiodo-9-ethyl-9H-carbazole

The first report on crystal and molecular structure of 3,6-diiodo-9-ethyl-9H-carbazole is presented. Experimental room-temperature X-ray and 13C chemical shift studies were supported by advanced theoretical calculations using density functional theory. The 13C nuclear magnetic shieldings were predicted at the non-relativistic and relativistic level of theory using the zeroth-order regular approximation. Theoretical relativistic calculations of chemical shifts of carbons C3 and C6, directly bonded to iodine atoms, produced a reasonable agreement with experiment (initial deviation from experiment of 44.3 dropped to 4.25 ppm). The changes in ring aromatic character were estimated via a simple harmonic oscillator model of aromaticity and nucleus-independent chemical shift index calculations. A good linear correlation between experimental and theoretically predicted structural and NMR parameters was observed.

Theoretical study of the oxidation mechanisms of thiophene initiated by hydroxyl radicals.

The mechanisms for the oxidation of thiophene by OH radicals under inert conditions (Ar) have been studied using density functional theory in conjunction with various exchange-correlation functionals. These results were compared with benchmark CBS-QB3 theoretical results. Kinetic rate constants were estimated by means of variational transition state theory (VTST) and the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Effective rate constants were calculated via a steady-state analysis based upon a two-step model reaction mechanism. In line with experimental results, the computed branching ratios indicate that the most kinetically efficient process involves OH addition to a carbon atom adjacent to the sulfur atom. Due to the presence of negative activation energies, pressures larger than 10(4)bar are required to reach the high-pressure limit. Nucleus-independent chemical shift indices and natural bond orbital analysis show that the computed activation energies are dictated by changes in aromaticity and charge-transfer effects due to the delocalization of lone pairs from sulfur to empty π(*) orbitals. Graphical Abstract CBS-QB3 energy profiles for the reaction pathways 1-3 characterizing the oxidation of thiophene by hydroxyl radicals into the related products.

Theoretical study of the oxidation mechanisms of naphthalene initiated by hydroxyl radicals: the OH-addition pathway.

The oxidation mechanisms of naphthalene by OH radicals under inert (He) conditions have been studied using density functional theory along with various exchange-correlation functionals. Comparison has been made with benchmark CBS-QB3 theoretical results. Kinetic rate constants were correspondingly estimated by means of transition state theory and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Comparison with experiment confirms that, on the OH-addition reaction pathway leading to 1-naphthol, the first bimolecular reaction step has an effective negative activation energy around -1.5 kcal mol(-1), whereas this step is characterized by an activation energy around 1 kcal mol(-1) on the OH-addition reaction pathway leading to 2-naphthol. Effective rate constants have been calculated according to a steady state analysis upon a two-step model reaction mechanism. In line with experiment, the correspondingly obtained branching ratios indicate that, at temperatures lower than 410 K, the most abundant product resulting from the oxidation of naphthalene by OH radicals must be 1-naphthol. The regioselectivity of the OH(•)-addition onto naphthalene decreases with increasing temperatures and decreasing pressures. Because of slightly positive or even negative activation energies, the RRKM calculations demonstrate that the transition state approximation breaks down at ambient pressure (1 bar) for the first bimolecular reaction steps. Overwhelmingly high pressures, larger than 10(5) bar, would be required for restoring to some extent (within ∼5% accuracy) the validity of this approximation for all the reaction channels that are involved in the OH-addition pathway. Analysis of the computed structures, bond orders, and free energy profiles demonstrate that all reaction steps involved in the oxidation of naphthalene by OH radicals satisfy Leffler-Hammond's principle. Nucleus independent chemical shift indices and natural bond orbital analysis also show that the computed activation and reaction energies are largely dictated by alterations of aromaticity, and, to a lesser extent, by anomeric and hyperconjugative effects.

A study on the aromaticity of [n]phenacenes and [n]helicenes (n = 3–9)

We studied the global and local aromaticity of [n]phenacenes and [n]helicenes for n=3–9 using the topological resonance energy and the magnetic resonance energy methods. The local aromaticity of the individual rings was studied using the bond resonance energy, the circuit resonance energy, and the ring current methods. Our results were compared with the results obtained by others who used the para-delocalization index, the harmonic oscillator model of aromaticity, and the nucleus-independent chemical shift indices. We found that sextet rings in the Clar structure are the main source of aromatic stabilization.

Is Al2Cl6 Aromatic? Cautions in Superficial NICS Interpretation

In this article, we employed the induced magnetic field method to show that the Al2X6 (X = F, Cl, Br, I) clusters cannot be classified as aromatic systems. Interestingly, even nucleus independent chemical shift (NICS) reveals the same conclusion when analyzed in greater detail, showing that a superficial analysis of this index can easily lead to incorrect interpretations. In view of the fact that the NICS index is extensively used by computational and theoretically oriented experimental chemists, this is an important warning against superficial analyses, as it can lead to erroneous chemical interpretation.

CLUSTERGEN: a program for molecular cluster generation from crystallographic data

A new program,CLUSTERGEN, for molecular cluster generation is introduced.CLUSTERGENprovides the quantum mechanics/molecular mechanics (QM/MM) input files for program packages such asADF[Baerendset al.(2012). Vrije Universiteit, Amsterdam, The Netherlands] andGAUSSIAN[Frischet al.(2009). Gaussian Inc., Pittsburgh, Pennsylvania, USA]. Additionally, it prints out a standardCRYSTAL[Dovesiet al.(2009). University of Turin, Italy] input and, in general, facilitates file-format manipulation. TheCLUSTERGENprogram is supported by an extensive manual and a user-friendly graphical interface. The code is freely available and carefully commented, which makes it easily modifiable. Exemplary applications ofCLUSTERGENconcerning QM/MM calculations and derivation of nucleus-independent chemical shift indices are demonstrated.

A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C70−2x(BN)x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated 13C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. 13C Chemical shifts (δiso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from −25.29 to −8.80) demonstrate that all the substituted species are aromatic, but less than C70 (−27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral 3He NMR chemical shifts.

Is graphene aromatic?

We analyze the chemical bonding in graphene using a fragmental approach, the adaptive natural density partitioning method, electron sharing indices, and nucleus-independent chemical shift indices. We prove that graphene is aromatic, but its aromaticity is different from the aromaticity in benzene, coronene, or circumcoronene. Aromaticity in graphene is local with two π-electrons delocalized over every hexagon ring. We believe that the chemical bonding picture developed for graphene will be helpful for understanding chemical bonding in defects such as point defects, single-, double-, and multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities, and new materials that are derivatives of graphene.

Probing the aromaticity of the [(HtAc)3(μ2-H)6], [(HtTh)3(μ2-H)6],+, and [(HtPa)3(μ2-H)6] clusters

In this study we report about the aromaticity of the prototypical [(H(t)Ac)(3)(μ(2)-H)(6)], [(H(t)Th)(3)(μ(2)-H)(6)](+), and [(H(t)Pa)(3)(μ(2)-H)(6)] clusters via two magnetic criteria: nucleus-independent chemical shifts (NICS) and the magnetically induced current density. All-electron density functional theory calculations were carried out using the two-component zeroth-order regular approach and the four-component Dirac-Coulomb Hamiltonian, including scalar and spin-orbit relativistic effects. Four-component current density maps and the integration of induced ring-current susceptibilities clearly show that the clusters [(H(t)Ac)(3)(μ(2)-H)(6)] and [(H(t)Th)(3)(μ(2)-H)(6)](+) are non-aromatic whereas [(H(t)Pa)(3)(μ(2)-H)(6)] is anti-aromatic. However, for the thorium cluster we find a discrepancy between the current density plots and the classification through the NICS index. Our results also demonstrate the increasing influence of f orbitals, on bonding and magnetic properties, with increasing atomic number in these clusters. We think that the enhanced electron mobility in [(H(t)Pa)(3)(μ(2)-H)(6)] is due the significant 5f character of its valence shell. Also the participation of f orbitals in bonding is the reason why the protactinium cluster has the shortest bond lengths of the three clusters. This study provides another example showing that the magnetically induced current density approach can give more reliable results than the NICS index.

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.

Aromaticity and Relative Stabilities of Azines

The most refined nucleus-independent chemical shift index (NICS(0)(πzz)) and the extra cyclic resonance energies (ECREs), based on the block localized wave function (BLW) method, show that the aromaticity of all azines is like that of benzene. The same is true for aza-naphthalenes relative to naphthalene. The lower relative energies of isomers with vicinal N's are due to the weakness of NN bonds rather than to reduced aromaticity.

Aromaticity in spin-polarized systems: Can rings be simultaneously alpha aromatic and beta antiaromatic?

The partition of the multicenter electron delocalization indices and the nucleus independent chemical shift indices into alpha and beta contributions in open-shell systems has been performed. In general it is shown that a full understanding of aromaticity in these systems cannot be achieved by restricting the calculations to the global properties but by dissecting these properties into alpha and beta terms. The 4n+2- and 4n-aromaticity rules for singlet and triplet annulenes, respectively, reduce to a general aromaticity rule when the alpha and beta terms are studied separately. This new rule allows us to extend the concept of conflicting aromaticities to radical systems that are simultaneously alpha-aromatic and beta-antiaromatic or vice versa. The existence of such systems is demonstrated here by means of multicenter electron delocalization indices and nucleus independent chemical shifts. Finally, the global aromatic/antiaromatic character of these radical systems is estimated by means of aromatic stabilization energy, which is shown to be either slightly positive or slightly negative, thus reflecting the small aromatic/antiaromatic character of these radicals and reinforcing the conclusions obtained with aromaticity indices.

Multidimensionality of delocalization indices and nucleus independent chemical shifts in polycyclic aromatic hydrocarbons

The aromaticity and local-aromaticity of a large set of polycyclic aromatic hydrocarbons (PAHs) is studied using multicenter delocalization indices from generalized population analysis and the popular nucleus independent chemical shift (NICS) index. A method for the fast computation of the NICS values is introduced, using the so-called pseudo-pi-method. A detailed examination is made of the multidimensional nature of aromaticity. The lack of a good correlation between the NICS and the multicenter delocalization indices is reported and the grounds discussed. It is shown through a thorough statistical analysis that the NICS values arise not only from local aromaticity of the benzenoid rings, but also from other circuits. It is shown that the NICS indices do not reveal the individual aromatic nature of a specific ring, contrary to the delocalization indices.

The Relationship Between Aromaticity and Charge‐Transfer Transitions: A Combined Study of Square‐Planar Metal Complexes Based on DFT, NMR and Nucleus‐Independent Chemical Shifts

AbstractThe aromaticity of square‐planar metal complexes, in this case mixed diiminodithiolate complexes of group VIII metals, has been investigated by 1H and 1H‐15N NMR spectroscopy, both experimentally and theoretically, as well as NICS indices. The electron‐donating/‐attracting ability of the substituent group and the metal ion interact with each other and control the aromaticity of the chelated rings and, as a consequence, the extent of the delocalization in the molecular plane. Evolution of the nucleus‐independent chemical shift (NICS) of the ligands upon complexation is consistent with the change in their electron distribution. At the same time, and in analogy with the EMF in closed circuits, a positive difference of the NICS value (ΔNICS) between two points (where HOMO and LUMO are located) can be envisioned as the driving force for charge‐transfer transitions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)