NICS Value Research Articles

Highly Robust and Antiaromatic Rhenium(I) Rosarin.

1ReH•Cl, a highly robust and antiaromatic rhenium(I) complex of triarylrosarin, is synthesized. The 1H NMR spectrum of 1ReH•Cl shows upfield-shifted pyrrole protons and highly downfield-shifted inner protons that confirm its antiaromatic nature, with density functional theory calculations strongly supporting this interpretation. Antiaromatic 1ReH•Cl absorbs from the UV to near-IR region of the optical spectrum; cyclic voltammetry, thin-layer UV-vis spectroelectrochemistry, and spin-density distributions clearly reveal that the rosarin backbone of 1ReH•Cl undergoes redox chemistry. The X-ray structure of 1ReH•Cl shows a fully coordinated and protonated inner cavity that effectively prevents proton-coupled electron transfer when treated with an acid. A remarkably negative NICS(0) value, clockwise anisotropy of the induced current density ring current, and the aromatic shielded inner cavity in the 2D ICSS(0) map reveal that the T1 state of 1ReH•Cl is aromatic based on Baird's rule.

Synthesis and Studies of Fused Benzo-Benzisapphyrins.

A series of rare examples of fused benzo-benzisapphyrins were synthesized readily by (3 + 2) condensation of benzodipyrrole-derived diol and para-benzitripyrrane in the presence of 0.5 equiv of TFA in CH2Cl2 under inert atmosphere conditions accompanied by DDQ oxidation in open air. The crude compounds were separated by basic alumina column chromatography and afforded pure fused benzo-benzisapphyrins in 20-22% yields. The fused sapphyrins were characterized in detail by high-resolution mass spectrometry (HRMS) and one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy. The 1H NMR spectra recorded at both 298 and at 233 K clearly exhibited the presence of a strong diatropic ring current in benzo-benzisapphyrins, and the macrocycles are of aromatic nature. The DFT-optimized structure of benzo-benzisapphyrin revealed that the macrocycle was planar to a great extent due to the rigid structure of the dibenzopyrrole moiety, and the NICS(0) value of -11.2 ppm supports the aromatic nature of macrocycles. The absorption spectra of benzo-benzisapphyrins showed three weak Q bands approximately in the region of 650-900 nm and a strong Soret band at 480 nm, along with a shoulder band at ∼510 nm. The diprotonated derivative generated by the addition of excess TFA to the benzo-benzisapphyrin macrocycle exhibited bathochromically shifted absorption bands compared to the free base macrocycle.

Synthesis and Structural Properties of NIR‐Absorbing Pyridine‐Containing Heptaphyrins

Four examples of stable nonaromatic pyridine containing heteroheptaphyrins (pyrithiaheptaphyrins) 2-5 were synthesized in 8-13% yields by [5+2] condensation of newly synthesized pyridine-based pentapyrrane 8 and bithiophene diol 9 a-d. The X-ray crystallographic analysis of macrocycle 2 proved that the macrocycle assumes a highly planar structure with two inverted thiophene rings. The heteroheptaphyrins 2-5 are asymmetric and showed a greater number of resonances in 1 H NMR spectra compared to our previously reported symmetric heterohexaphyrin (pyrithiahexaphyrin) 1 c. Most of the macrocyclic core protons in pyrithiahepaphyrins 2-5 experienced upfield/downfield shifts compared to pyrithiahexaphyrin 1 c indicating the alteration of π-conjugation in the macrocycles. The absorption bands were significantly red-shifted and located in the NIR region in macrocycles 2-5 compared to 1 c supporting the increase of π-delocalization. The theoretical studies support the experimental findings and NICS(0) value supports the non-aromaticity of the macrocycles.

Lewis acid-catalyzed formation of triply S-confused hexaphyrin (1.0.1.0.1.0) and its structure-property correlations

A simple rigid precursor termed as a “diheterole” bearing thiophene linked to [Formula: see text]-pyrrole at 4-position was prepared using a three-step synthetic strategy. This functionalized diheterole was allowed to undergo acid-catalyzed condensation in the presence of Lewis acid (BF3 [Formula: see text] Et2O) to produce the [2+2+2] cyclotrimer 1 as a major product with trace formation of 2and higher homologues.Various 1D and 2D NMR analyses along with theoretical investigations indicate that the molecule 1 exhibits nonaromaticity due to its non-conjugated annulenic structure. The optical properties and electronic structures were analyzed using UV-vis absorption spectroscopy and time-dependent density functional theory calculations. Detailed structural analyses revealed that the conformationally rigid triply S-confused hexaphyrin 1 adopts nearly planar geometry with three thiophene rings inverted. Upon protonation, the individual rings are tilted largely due to the steric congestion by both [Formula: see text]-CH’s and pyrrolic NH’s inside the core. The calculated NICS(0) values of +1.73 ppm for the protonated species, 1 [Formula: see text] 3H[Formula: see text] indicates the distinct non-aromatic feature as observed in 1 (NICS value of +2.34 ppm).

An Anomalous Antiaromaticity That Arises from the Cycloheptatrienyl Anion Equivalent

A fused azulene dimer with helicene‐like structure prepared by simple Suzuki–Miyaura coupling between naphthalene and azulene derivatives shows absorption in the near‐IR region up to 900 nm owing to the effective π‐expansion. The newly formed seven‐membered ring exhibits a positive NICS(1) value that arises from the antiaromatic cycloheptatrienyl anionic structure.

δ-Acidity of benzene in [(benzene)RuII(N-N)Cl]+. Crystal structures, nuclear magnetic resonance spectra and nucleus independent chemical shifts

The ligands L1 and L2 are 1:1 condensates of acetylacetone and 2-hydrazinopyridine and 2-hydrazinobenzothiazole respectively. Reactions of these ligands with [RuCl2(C6H6)]2 in 2:1 M proportions in the presence of excess NH4PF6 yield two complexes − dark red [(C6H6)Ru(L1)Cl]PF6 (1) and yellow [(C6H6)Ru(L2)Cl]PF6 (2). Their X-ray crystal structures have been determined. They are 1:1 electrolytes in dimethylsulfoxide (dmso). The bonding in the complexes involves significant back donation from the filled dxy orbital of the metal to the empty π∗ orbital of coordinated benzene, as confirmed by the visualization of the HOMOs in 1 and 2 obtained from the density functional theory (DFT) calculations. As a consequence, in the 1H NMR spectra in dmso-d6 the benzene protons are shifted to lower values of 6.23 (in 1) and 6.37 (in 2) from 7.26 ppm of the free benzene. Thus the protons of the ligated benzene become less deshielded compared to free benzene. Further a concomitant lengthening of the CC bonds in the coordinated benzene by ∼0.04 Å is observed. A pseudo-octahedral geometry of Ru(II) in 1 and 2 is stipulated from a d-d transition which gives the 10 Dq value of 24,500 in 1 and 24,000 cm−1 in 2. The NICS(0) value, an index of aromaticity, calculated at the center of the benzene ring by GIAO method at BP86/LanL2DZ level, of −6.53 ppm in free benzene becomes more negative in 1 (−20.36 ppm) and 2 (−21.45 ppm). Thus coordinated benzene becomes more aromatic from the point of view of NICS. In addition when NICS is varied as a function of the distance (up to 0.8 Å) above the center of the benzene ring, it increases in complexes 1 and 2 whereas in unbound benzene it decreases.

Twisted Polycyclic Aromatic Hydrocarbon with a Cyclooctatetraene Core via Formal [4+4] Dimerization of Indenofluorene

A polycyclic aromatic hydrocarbon having a twisted cyclooctatetraene (COT) core was prepared by a formal [4+4] dimerization of an in situ generated indeno[2,1-<i>a</i>]fluorene derivative. The compound consists of two indenofluorene frameworks connected with two double bonds and adopts a twisted geometry to avoid intramolecular steric repulsion. It exhibits a long-wavelength absorption band with a maximum at 634 nm extending to about 850 nm, which is consistent with the moderate antiaromaticity of the twisted COT ring as indicated by the NICS value (+11.9). A mechanism for cycloaddition reaction is proposed in terms of stepwise bond formations due to the diradical character of the indenofluorene framework, on the basis of the completely different reaction product obtained from the extended benzo analogue.

Nucleus-independent chemical shift analysis of the electronic states of the (CO)4, (CS)4, and (CSe)4 molecules.

NICS(1) calculations have been performed on the 8π and 10π singlet states and the 9π triplet state of (CO)4, (CS)4, and (CSe)4. The results show that transfer of electrons from the b2g σ MO into the a2u π MO decreases the NICS(1) value, indicating an increase in the diamagnetic ring current. The decreases in the calculated NICS(1) values are substantially larger in (CO)4 than in (CS)4 or (CSe)4. This finding is rationalized by the larger coefficients on the carbons in the a2u MO of (CO)4 than in the a2u MOs of (CS)4 and (CSe)4.

Synthesis and antiaromatic properties of highly planar dithiaamethyrin.

2,5-Bis(4-propyl-2-pyrrolyl)thiophene was reacted with aryl aldehydes to afford large porphyrinoids; [24]dithiaamethyrin(1.0.0.1.0.0) and [36]trithianonaphyrin(1.0.0.1.0.0.1.0.0). X-ray crystallography of the dithiaamethyrin revealed a highly planar ring structure with mean plane deviation (MPD) value of 0.053 Å. A large positive NICS(0) value (+13.9 ppm) calculated for this planar 24π system unambiguously indicates an antiaromatic character that is consistent with a remarkably low field (1) H chemical shift of the inner NH proton at 24.0 ppm. The magnitude of the paratropic ring current effect in the 24π amethyrin framework in the free base form and the diprotonated form was discussed on the basis of the MPD value and the NICS value. The present dithiaamethyrin is much more strongly antiaromatic than the dodecaalkylamethyrin in their free base forms, but the order inversed in their diprotonated forms.

A Computational Investigation of the Electronic Properties of Partially Hydrogenated Fullerenes C60Hn (n = 18, 20, 24, 36 and 48)

A DFT study is performed to investigate the electronic and magnetic properties of partially hydrogenated fullerenes C60Hn (n = 18, 20, 24, 36, and 48) based on NMR parameters, NICS, and NPA analysis. A few peaks appear in the 13C NMR pattern of each C60Hn, showing the electrostatic environments of C nuclei are divided into a few layers that are related to three local structures around C atoms. Since, hexagons and pentagons in the zone of hydrogenated carbons are puckered, the range of isotropic chemical shifts (δiso) for each group of carbons are caused by different curvatures at the corresponding sites. Partially hydrogenation of fullerenes results in isolated cyclohexatriene fragments with almost planarity surface, in some cases, which yields 13C δiso of 130.7–132.4 ppm. Moreover, cyclohexatriene pole ring with the calculated NICS value of −8.9 ppm at its center, similar to that of benzene (−8.0), can be considered as benzenoid fragment. Magnetic properties are also examined inside the compounds, indicating that NICS values are strongly negative in the middle of all the cages, except for C60H24 and C60H48. The predicted NICS values may be also useful for identification of the hydrogenated fullerenes through their endohedral 3He NMR chemical shifts.

Constrained Clar Formulas of Coronoid Hydrocarbons

Aromatic character of coronoid hydrocarbons is greatly influenced by the shapes of outer and inner peripheries. The most aromatic rings in coronoids are jutting benzene rings on the armchair edges, if any. Clar formulas of many coronoids conform to the aromaticity patterns. However, placement of all aromatic sextets on highly aromatic rings is sometimes forbidden by the presence of the central cavity. The magnitude of aromatic stabilization energy due to macrocyclic conjugation [SSE(mc)] and the NICS(1) value at the center of the cavity strongly depend on the structure of the superposed Clar formula. Localization of π-electrons in fixed aromatic sextets effectively suppresses macrocyclic conjugation. The sign of SSE(mc) is determined by the number of carbon atoms that form the hub cycle.

COMPUTATIONAL NICS AND 13C NMR ChARACTERIZATION OF SUBSTITUTION PATTERNS OF C60-nNn FULLERENES (n = 1–12)

DFT calculations are applied to evaluate the effects of atomic arrangements of dopant atoms on electronic features of the most stable structures of C 60−n N n(n = 1–12) fullerenes. Our study reveals that 13 C isotropic chemical shifts (δiso) of the nuclei at C–N pentagon–hexagon (ph) junctions appear at downfield values while there are no tangible values of δiso for the nuclei at C–N hexagon–hexagon (hh) junctions; the carbon sites attached to the first neighbors of nitrogen at hh junctions (the second neighboring effect) yield upfield values of δiso. Moreover, compensation between diatropic and paratropic ring currents leads to slightly less negative NICS value in C59N heterofullerene (-3.6) compared to the parent fullerene C60 (-4.25). However, with incorporating more nitrogen atoms into the cage the aromaticity first increases, up to the 66π-system C54N6 and C53N7 , yielding the most negative NICS values of -26.3 and -26.8, respectively, and then decreases so that NICS finally reaches the value of -6.6 ppm for C48N12 heterofullerene. On the basis of distinct value predicted for each heterofullerene, one expects that NICS values may also be useful for identification of the molecules through their endohedral 3 He NMR chemical shifts.

Exploring magnetic properties and Curved π-Conjugation of BxNyCz nanotubes using density functional theory

The aromaticity and magnetic properties of armchair (6,6) and zigzag (10,0) BxNyCz nanotubes (BNNTs) have been investigated by computing NICS values based on density functional theory. In addition, curved π-conjugation of selected NTs has been analyzed with π-orbital axis-vector (POAV) method. To evaluate magnetic property along the NT Z-axis, NICS values computed at several distance from NT center with a step size of 1Å. In order to examine the effect of NT geometry as well as presence of Boron–Nitrogen atoms on magnetic properties, BxNyCz nanotubes with two different geometries including zigzag (10,0) and armchair (6,6) NT have been selected. Generally, the most magnetic property exhibited at the center of NTs and it tends to be reduced with increasing distance from center of NTs. Among of NTs investigated here, the zigzag CNT (10,0) exhibited the most central negative NICS value and therefore the most magnetic property. Instead BNNTs are the least deshielded ones. The calculated magnetic value presented very sensitive result to NT geometry. We attempted to explain overall resultant ring current – aromaticity – of NTs using strain energy of constituent hexagonals experienced by curvature of π-orbitals together with geometry of NT.

Density functional theory study on the structural, reactivity, and electronic properties of all mono-, di-, tri-, tetra-, and penta-fluoroanilines as monomers for conducting polymers

Electrical and structural properties of mono-, di-, tri-, tetra-, and penta-fluoroanilines as candidate monomers for new conducting polymers have been investigated using hybrid density functional theory (B3LYP/6–311+G**) based methods. The effects of the number and position of the fluorine atoms on the electrical and structural properties of fluoroanilines and their radical cations have also been investigated. The values of the vibrational frequencies, charge and spin-density distributions, ionization potentials, dipole moments, electric polarizabilities, HOMO-LUMO gaps, and the NICS values of these compounds have been calculated and analyzed as well. The results showed that the double bonds in 2-fluoroaniline and 2,5-difluoroaniline are more delocalized compared with other fluoroanilines; therefore, these molecules have the most aptitude for the electropolymerization reactions. The frequency analysis showed that the electrochemical stability of 2-fluoroaniline is greater than the other fluoroanilines. Also, this molecule possesses the largest NICS value compared to the other fluoroanilines. Consequently, 2-fluoroaniline has the largest ring current and the highest conductivity among all other monomers. Based on the results obtained, 2-fluoroaniline and 2,5-difluoroaniline are the best candidate monomers among all fluoroanilines for the synthesis of corresponding conducting polymers.

Borazine-Embedded Coronene—A DFT Study

Two isomeric coronene derivatives having embedded borazine-like moiety are considered for DFT treatment at various levels. The stabilities and some quantum chemical properties of them are discussed. Also, NICS(0) value of each ring of these structures has been obtained. When the borazine moiety occupies a peripheral location, the core ring is the most aromatic one which is followed by the borazine-like ring. However, when the borazine moiety located at the core position, its aromaticity seems to be much less than the aromaticity of any peripheral hexagonal rings.

Electronic structure, bonding, and properties of Sn mGe n ( m + n ⩽ 5) clusters: A DFT study

Electronic structures of tin–germanium binary clusters up to five atoms obtained by searching global optimization of the energy surface using Metropolis Monte Carlo (MMC) and genetic algorithm (GA) followed by bond strength propensity model (BSPM) and by local optimization using density functional theory (DFT) are reported. Geometries and properties of the most stable cation, neutral, and anion clusters are calculated at the B3LYP/aug-cc-pVTZ-pp level of theory. The average binding energy per atom is calculated to study its dependence on the size of the cluster. The HOMO–LUMO energy gap, ionization potential (IP), electron affinity (EA), and vertical detachment energy (VDE) are also computed for the most stable isomer of each cluster. The reactivity descriptors such as chemical potential, chemical hardness, and electrophilicity index of the lowest-energy isomers of these clusters are estimated to study their relative stabilities. The one-step fragmentation patterns of the clusters are studied to determine the relative strength of the Ge–Ge, Sn–Ge, and Sn–Sn interactions by computing the fragmentation energies. A four membered ring formed by the Sn 2Ge 2-b cluster shows π-aromaticity with a NICS(1) value of −13.2 ppm.

Synthesis of a Novel Lithocene that has Aromatic-Like Nature with Nonaromatic Rings

Recrystallization of 1,2-dilithiobistannole 1 with [12]-crown-4 afforded novel lithocene 2, which is the first metallocene in which the two cyclopentadienyl (Cp)-like ligands are directly bonded without any bridging group. The high-field 7Li resonance and negative NICS(1) value suggest considerable aromatic character of 2, while the stannole rings are nonaromatic with CC bond alternation (see scheme).

Dehydrotropylium-Co2(CO)6 Ion: Generation, Reactivity and Evaluation of Cation Stability

The dehydrotropylium-Co(2)(CO)(6) ion was generated by the action of HBF(4) or BF(3)⋅OEt(2) on the corresponding cycloheptadienynol complex, which in turn has been prepared in four steps from a known diacetoxycycloheptenyne complex. The reaction of the cycloheptadienynol complex via the dehydrotropylium-Co(2)(CO)(6) ion with several nucleophiles results in substitution reactions with reactive nucleophiles (N>1) under normal conditions, and a radical dimerisation reaction in the presence of less reactive nucleophiles. Competitive reactions of the cycloheptadienynol complex with an acyclic trienynol complex show no preference for generation of the dehydrotropylium-Co(2)(CO)(6) ion over an acyclic cation. DFT studies on the dehydrotropylium-Co(2)(CO)(6) ion, specifically evaluation of its harmonic oscillator model of aromaticity (HOMA) value (+0.95), its homodesmotic-reaction-based stabilisation energy (≈2.8 kcal mol(-1)) and its NICS(1) value (-2.9), taken together with the experimental studies suggest that the dehydrotropylium-Co(2)(CO)(6) ion is weakly aromatic.

Ring currents in boron and carbon buckyballs, B80 and C60

The ipsocentric method at the coupled Hartree-Fock level is used for the calculation of magnetically induced ring currents in the boron buckyball B(80), for both I(h) and distorted T(h) geometries. A close similarity between the current patterns in boron and carbon buckyballs is noted, but with a higher current density in B(80). Paratropic currents on the pentagons are predominant in the boron buckyball, and the central NICS value is positive. These observations support the conclusion that B(80) should be considered (weakly) anti-aromatic. The largest orbital contributions to the ring currents in both molecules are identified and related to specific excitations in the frontier orbital region.

Theoretical study on structures and properties of lantern-like compounds derived from ammonia borane

The structures, stability, energy gaps, aromaticities, electronic spectra and enthalpies of formation of nineteen lantern-like compounds derived from ammonia borane have been investigated theoretically. The viable structures of the 19 lantern-like compounds are presented. The energy gap of H 3 NB ( B ) 5 BNH 3 - is 1.53 eV, which is the smallest, and the energy gaps of other compounds are between 2.81 and 5.88 eV. The centers of the cages of these compounds have strong aromaticity except for H 3NB(C 2H 2) 3BNH 3, H 3NB(C 2H 4) 3BNH 3, H 3NB(C 6H 6)BNH 3 and H 3NB(B 3N 3H 6)NBH 3. Specially, the NICS(0) value of H 3NB(B) 4BNH 3 is −78.62, showing the center of the cage has very strong aromaticity. The wavelengths of the origin band of the electronic transition of these complexes H 3NB(B) 4BNH 3 and H 3 NB ( B ) 5 BNH 3 - are greater than 700.00 nm, while the wavelengths of the other complexes are between 220.00 and 422.00 nm. The enthalpies of formation of the 19 compounds at 0 and 298 K are given, which the enthalpies of formation of H 3NB(B 3N 3H 6)NBH 3, H 3NB(BH) 10BNH 3 and H 3NB(X) 3BNH 3 (X = O, S) are negative, while those of other compounds are positive.