NICS Value Research Articles

Aromatization of Fulvene by Complexation with Lithium

At the B3LYP/6-311++G(d,p) level, approach of a lithium atom to a face of the fulvene molecule leads to formation of a complex with binding energy 41 kcal/mol and significant ion-pair character. The fulvene moiety gains a delocalized aromatic cyclic π system, documented by the geometry-based aromaticity index HOMA, and a strong diatropic ring current, visualized by ipsocentric calculation of the π current-density, which leads to an "aromatic" NICS value of -11 ppm.

Scrutinizing the Chemical Nature and Photophysics of an Expanded Hemiporphyrazine: The Special Case of [30]Trithia-2,3,5,10,12,13,15,20,22,23,25,30-dodecaazahexaphyrin

Thirty π-electron-expanded hemiporphyrazines 1a-c have been prepared by crossover condensation reaction of 2,5-diamino-1,3,4-thiadiazole and the corresponding phthalonitrile (3) or diiminoisoindoline (4) derivatives. The expanded azaporphyrin hexamers have been unequivocally characterized by means of spectroscopic, crystallographic, and electrochemical techniques. Weak intramolecular hydrogen bonding imposes a planar conformation to macrocycles. However, the overall electronic delocalization is low, and the nature of the resulting [30]heteroannulene is nonaromatic, as confirmed by NMR studies, XR diffraction analysis, and calculation of the NICS(0) value. Studies on a wide range of physicochemical features including ground, excited, reduced, and oxidized states provide evidence for the wide applicability of these 30 π-electron-expanded hemiporphyrazines in processes involving electron transfer. A key asset of our work is the systematic development of spectroscopic and kinetic markers for the formation and decay of all of the aforementioned species. Thirty π-electron-expanded hemiporphyrazines evolve as broadly absorbing light harvesters with excited state energies of around 2.3 eV that are susceptible to facile one-electron reduction and one-electron oxidation reactions.

Demonstration of “Möbius” Aromaticity in Planar Metallacycles

Möbius aromaticity, predicted by Edgar Heilbronner in 1964, is a stabilizing effect exhibited by 4n electron fully conjugated cyclic molecules (or transition states) with an odd number of orbital phase inversions. Although it has previously been suggested that this effect might also apply to planar metallacycles in which a transition metal employs a d orbital in delta-type binding mode, only very few examples of stable twisted molecules composed of main group elements are known. We report herein, the first computationally confirmed 4npi aromatic planar metallacyclic examples and their building principles. Aromatic stabilization energy (ASE) of a 8pi metalla-cycloheptatriene [Fe(CH)(6)H(2)], with four doubly occupied pi orbitals and a HOMA value of +0.80 (cf. benzene=+1.0), an NICS(0) value of -8.5 (benzene=-9.8, NICS=nucleus independent chemical shift), and with one phase inversion, is +27.5 kcal mol(-1) (about two-thirds of the value for benzene). In contrast, an unknown non-Möbius 1,4-dimetallabenzene [Fe(2)(CH)(4)H(4)], also with 8pi electrons, and without phase inversions, has an ASE of -4.1 kcal mol(-1) and a NICS(0)=+15.6, indicative of antiaromaticity. Aromaticity of the proposed Möbius aromatic metallacycles is confirmed by using magnetic (NICS(0), NICS(1)(zz), delta(1)H) and geometric (HOMA) aromaticity criteria, planarity, and near equalized C-C bond lengths, bonding analysis (Wiberg bond indices, NBO, and NLMO analysis). The role of wave function boundary conditions (periodic vs. antiperiodic) in chemistry is further stressed, being equivalent to Zimmerman's concept of nodal parity for Möbius/Hückel systems.

Multidimensionality of delocalization indices and nucleus‐independent chemical shifts in polycyclic aromatic hydrocarbons II: Proof of further nonlocality

In a recent contribution, we examined the effect of 10- and 14-center circuits on the nucleus-independent chemical shifts NICSs using multicenter bond indices (MCBIs) (Fias et al., J Comput Chem 2008, 29, 358). In this study, the nonlocal contributions to the NICS are further investigated for a larger set of polycyclic aromatic hydrocarbons (PAHs). To achieve this, the NICSs are predicted using the MCBI and compared with ab initio results. The NICSs of the central ring of perylene- and benzo-[ghi]perylene-like fragments and of coronene appear to have other nonlocal contributions than the ones previously studied. It is shown that a model based on the MCBI-ring current maps and the inclusion of new circuits proves the existence and shows the nature of these new nonlocal effects on the NICS. This new model leads to a better understanding of the differences between the NICSs and delocalization indices. The results show that the NICS value is not only significantly influenced by the higher order circuits encircling the ring at which it is evaluated but also by the local aromaticity of the surrounding rings, and occasionally, like in the case of coronene, the NICSs are even influenced by currents farther away in the molecule.

Aromaticity of ionic structures: Investigation and application of NICS value and 4n + 2 rule

At DFT/B3LYP/6-31G** theoretical level, C6H and C (n = 0, −2, and +2), C6H and C (n = 0, ±2, ±4, and ±6), C6H (n = 0–6), as well as C6H6-A and C6-A (A = Be, B, N, O, Mg, Al, Si, S, and Fe) structures were investigated. Comparing NICS values of C6H and C (n = 0, −2, and +2), we discovered that C6H, C6H were antiaromatic, and C6H6, C6, C, C had aromaticity with negative NICS values. According to research of C6H and C (n = 0, ±2, ±4, ±6), C6H (n = 0–6), we sustained that their σ and π orbit were different and the locations of electrons were difficult to confirm in ionic structures. Thus, neither 4n + 2 rule nor NICS values can precisely estimate the aromaticity of ionic structures. Besides, through WBI (NBO) research of C6H6-A and C6-A (A = Be, B, N, O, Mg, Al, Si, S, and Fe) structures, we found that C6H6 was easy to accept electrons, contrarily, C6 was prone to bestowing electrons. Moreover, C6H6 took the symmetrical carbon atoms form feeble interaction or bond, and C6 used all carbon atoms to impact with other atom. C6H6 generated two contrapuntal single bonds with oxygen, sulfur, and nitrogen atoms, whereas C6 molecule formed double bond with oxygen and nitrogen atoms, two conjoint single bonds with sulfur atom. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Facile Formation of a Benzopyrane-Fused [28]Hexaphyrin That Exhibits Distinct Möbius Aromaticity

A benzopyrane-fused [28]hexaphyrin, 2, was prepared by simple heating of [26]hexaphyrin 1 in acetic acid. Fused [28]hexaphyrin 2 features a molecular twist, a distinct diatropic ring current, a large HOMA value, a large negative NICS value, and a large two-photon absorption (TPA) cross section even at room temperature, all of which support the Möbius aromaticity of 2. To the best of our knowledge, 2 is the first macrocycle that acquires distinct Möbius aromaticity without any assistance from metal coordination, temperature control, or protonation.

Energetic, geometric and magnetic analysis of all- syn, trans cyclic cyclopropanes

Structural, energetic and magnetic properties of a series of cyclic cyclopropanes have been investigated on the DFT (B3LYP/6-31G(d)) level. The cyclization of these oligomeric all- syn, trans cyclopropanes, designated [ N]rondelanes, should be possible on energetic grounds, since no substantial destabilization energies were calculated for the larger homologues. However, the high strain for the smaller rondelanes might prevent their successful synthesis. Thermochemical reactions, structural data, and NICS values were investigated in order to test for homoaromatic stabilization. The careful choice of a homoisodesmic reaction revealed weak aromatic stabilization energies for [3] and [7]rondelane, 2.3 kcal/mol and 2.2 kcal/mol, respectively. However, we could not observe a pattern according to the Hückel rule and the stabilization energies are small compared to the stabilization of cyclopropane due to σ-aromaticity. Calculated structural data indicate homoconjugation for the smaller homologues. [3]Rondelane showed a NICS value of −2.54 indicating a weak neutral homoaromaticity.

Configuration Change in [14]Annulene Requires Möbius Antiaromatic Bond Shifting

For both [4n] and [4n + 2]annulenes, cases of configuration change in which the number of trans CC bonds changes by 1 must involve a Mobius π-bond shifting step. The known configuration change in [14]annulene (1, CTCTCTT → 2, CCTCTCT), for which Δtrans = 1, is thus predicted to involve a Mobius antiaromatic transition state for bond shifting. Using unrestricted density-functional theory (DFT), we have located the key transition state for this process, as well as other stationary points involved in the mechanism. CASPT2(14,14)/cc-pVDZ//(U)BH&HLYP/6-311+G** results provide a barrier of 19.3 kcal/mol for the overall process 1 → 2, in good agreement with the known experimental barrier (21.3 kcal/mol). The bond-shifting transition state has substantial singlet diradical character and is strongly antiaromatic on the basis of its large positive NICS value. [14]Annulene thus represents the first example of a concerted reaction for which a Mobius antiaromatic transition state provides the only possible pathway.

Total carbo-Mer of Benzene, Its carbo-Trannulene Form, and the Zigzag Nanotube Thereof

The total carbo-mer of benzene, hexaethynyl carbo-benzene C30H6, has been calculated at the B3PW91/6-31G** level. Its geometrical and magnetic characteristics are compared with those of the C18H6 partial carbo-mers, unsubstituted carbo-benzene, and hexaethynylbenzene. The carbo-[6]trannulene isomer is found to exist as a minimum on the singlet spin state potential energy surface (PES) and is 65.6 kcal.mol(-1) higher in energy than hexaethynyl-carbo-benzene. In the former, a strong cyclic electron delocalization is evidenced from the root-mean-square deviation (rms) of the ring bond lengths and the NICS value computed at the centroid of the trannulene ring. As an alternative to the graphene sheet wrapping process traditionally used to illustrate the construction of carbon nanotubes, a dehydrocoupling-stacking process is invoked for the construction of zigzag nanotubes from trannulene bricks. The process is applied to the carbo-[6]trannulene brick to generate a novel type of acetylene-expanded carbon nanotube, which is a polymer of primitive C60 segments. A C60H6 carbo-meric equivalent of a cyclacene belt is first considered. Two such segments are then formally dehydrocoupled to generate a cylindrical (C60)2H6 molecule, the central part of which is assumed to be a relevant model for the infinite nanotube. Axial and sectional electron delocalization inside the tube models is discussed on the basis of bond length analysis, NICS values, pi MO analysis, and singlet-triplet state energy gap. The capping of the C120 cyclinder is finally addressed by use of carbo-[3]radialenic units.

Royal crown‐shaped electride Li3‐N3‐Be containing two superatoms: New knowledge on aromaticity

The structure and aromaticity of a royal crown-shaped molecule Li(3)-N(3)-Be are studied at the CCSD(T)/aug-cc-pVDZ level. This molecule is a charge-separated system and can be denoted as Li(3) (2+)N(3) (3-)Be(+). It is found that the Li(3) (2+) ring exhibits aromaticity mainly because the Li(3) (2+) ring can share the pi-electron with the N(3) (-3) ring. The 4n+2 electron counter rule can be satisfied for the Li(3) (2+) subunit if the shared pi valence electron of N(3) (3-) subunit is also taken into account. This new knowledge on aromaticity of a ring from the interactions between subunits is revealed first time in this paper. Li(3)-N(3)-Be can be also regarded as a molecule containing two superatoms (Li(3) and N(3)), which may be named as a "superomolecule." Li(3)-N(3)-Be is a new metal-nonmetal-metal type sandwich complex. The N(3) (3-) trianion in the middle repulses the electron clouds of the two metal subunits (mainly to the Li(3) superatom) to generate an excess electron, and thus Li(3)-N(3)-Be is also an electride. This phenomenon of the repulsion results in: (a) the HOMO energy level increased, (b) the electron cloud in HOMO distended, (c) the area of the negative NICS value extended, and (d) the VIE value lowered. So the superomolecule Li(3)-N(3)-Be is not only a new metal-nonmetal-metal type sandwich complex but also a new type electride, which comes from the interaction between the alkali superatom (Li(3)) and the nonmetal superatom (N(3)).

Super-valence phenomenon of carbon atoms in C 20 molecule

Fourteen structures of C 20 are studied at DFT/B3LYP/6-31G* theoretical level. Except ring, bowl, cage and isomer 1 which have been studied before, other isomers have not been reported so far. Calculated results show that the ring has the lowest energy at this level and isomers 1, 2, 3 and 4 have lower energies than that of cage. Analyses of optimized bond lengths, electronic structure indicate that some carbon atoms express super-valence property. In addition, NICS value is consistent with molecular orbital character in denoting aromaticity of C 20 molecule. Delocalization character averts influence of curvature strain, which can well explain the stability of the cage.

Au42: An Alternative Icosahedral Golden Fullerene Cage

AbstractFor Abstract see ChemInform Abstract in Full Text.

σ-Aromaticity in [formula omitted]: Insights from ring-current maps

Putative σ-aromaticity of the clusters H 3 + and Li 3 + is investigated by computation of ring-current maps in the ab initio ipsocentric approach. Although H 3 + shows a marked diatropic ring-current and can be considered σ aromatic on the magnetic criterion, Li 3 + shows no global current and is non-aromatic on this criterion, in spite of its electron count and negative NICS value. The difference in magnetic response is interpreted in terms of orbital energies using the ipsocentric model: in Li 3 + the diatropic HOMO–LUMO excitation is opposed by paratropic excitations to higher unoccupied orbitals; in H 3 + this cancellation does not occur.

Au42: An Alternative Icosahedral Golden Fullerene Cage

We present a new icosahedral gold fullerene, Au42, based on density functional theory calculations. The Au42 fullerene has a nanoscale hollow space that can hold up to 13 Au atoms. The Au42 fullerene also has a larger HOMO-LUMO gap compared to the compact-filling geometries. However, unlike the known gold fullerene Au32, the Au42 fullerene does not satisfy the 2(N + 1)2 aromatic rule and has a positive NICS value at the center of the cage. These two nanometer-sized gold fullerenes can be used as golden cages to accommodate other atoms or molecules for the purpose of studying fundamental chemistry because of their apparently dissimilar chemical characteristics, or they can be used as structural motifs to build highly stable core-shell nanoclusters or novel cluster-assembled materials.

Relationship between electrophilicity index, Hammett constant and nucleus-independent chemical shift

Inter-relationships between the electrophilicity index (Ω), Hammett constant (óp @#@) and nucleusindependent chemical shift (NICS (1) — NICS value one ångstrom above the ring centre) have been investigated for a series of meta- and para-substituted benzoic acids. Good linear relationships between Hammett constant vs electrophilicity and Hammett constant vs NICS (1) values have been observed. However, the variation of NICS (1) against CO shows only a low correlation coefficient.

Cage-delocalised syn-periplanar bishydrazine radical cations and dications—en route to σ-bishomoaromatic 4N 2+ systems?

One- and two-electron oxidation of a caged, proximate, syn-periplanar bishydrazine leads to highly persistent, 4N/(7)6e (radical) cations—with dominant electron delocalisation along the cage bonds (CV, EPR, B3LYP). A NICS value of −20.6 is evidence for strong electron delocalisation in the caged dication.

A valence bond study of the Bergman cyclization: geometric features, resonance energy, and nucleus-independent chemical shift (NICS) values

The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1, enediynes), which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT, respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space [CASSCF(6 x 6)] computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6 x 6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non-synchronous; the TS is geometrically approximately 80% product-like and electronically approximately 70% reactant-like. While the pi resonance in the TS is very small, it is large (64.4 kcal mol(-1)) for 2 (cf. benzene=61.5 kcal mol(-1)). As a consequence, substituents operating on the sigma electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional sigma resonance in 2 results in unusually high values for the nucleus-independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to sigma resonance to which the NICS procedure is relatively sensitive.