Negative Nucleus-independent Chemical Shift Research Articles

Aromaticity and Antiaromaticity Reversals between the Electronic Ground State and the Two Lowest Triplet States of Thiophene.

It is shown, by examining the variations in off-nucleus isotropic magnetic shielding around a molecule, that thiophene which is aromatic in its electronic ground state (S0) becomes antiaromatic in its lowest triplet state (T1) and then reverts to being aromatic in T2. Geometry relaxation has an opposite effect on the aromaticities of the ππ* vertical T1 and T2: The antiaromaticity of T1 is reduced whereas the aromaticity of T2 is enhanced. The shielding picture around T2 is found to closely resemble those around certain second singlet ππ* excited states (S2), for example, those of benzene and cyclooctatetraene, thought to be "strongly aromatic" because of their very negative nucleus-independent chemical shift (NICS) values. It is argued that while NICS values correctly follow the changes in aromaticity along the potential energy surface of a single electronic state, the use of NICS values for the purpose of quantitative comparisons between the aromaticities of different electronic states cannot be justified theoretically and should be avoided. "Strongly aromatic" S2 and T2 states should be referred to simply as "aromatic" because detailed comparisons between the properties of these states and those of the corresponding S0 states do not suggest higher levels of aromaticity.

Engineering magic number Au19 and Au20 cage structures using electron withdrawing atoms.

Gold cages are a subset of gold nanoparticles and these structures are of major interest due to their favourable physiochemical properties. In order for these structures to be useful in applications, they must be chemically stable. The objective of this research is to transform non-magic number cage structures into magic number cage structures by the addition of electron-withdrawing groups on the cages. The electronic properties for Au19X and Au20X2 (X = F, Cl, Br, I) are calculated and observed. It is expected that the electron-withdrawing groups will remove the electron density from the gold cages and leave them positively charged. We first optimize the geometries of the initial gold cages and verify the structures are a local minima. From there, we attach our halogens to the gold cages and optimize the structures to determine the NICS values and HOMO-LUMO gaps. NICS values were found to be more negative when a more electronegative halogen was used. Calculations have found that Au19F and Au20F2 have the most negative NICS values, indicating greater spherical aromaticity. Iodine, being the least electronegative atom, had the most positive NICS value and smallest HOMO-LUMO gap. All calculations are compared to the magic cluster Au18 which satisfies Hirsh's 2(N + 1)2 rule for n = 2.

An aromatic Ca2B8 complex for reversible hydrogen storage

Ca2B8 cluster which shows high stability is predicted to be a symmetrical 3-dimension complex at coupled-cluster method. Charge analysis shows that the great charge transfers from Ca to B8 ring result in the formation of (Ca)22+(B8)4- complex. The aromaticity of this complex is identified through the negative nucleus-independent chemical shifts (NICS) and delocalized σ and π molecular orbitals. Importantly, the aromatic Ca2B8 is found to be a promising hydrogen storage material with moderate adsorption energy and high gravimetric density (10.6%). The adsorbed H2 molecules on Ca2B8 can be efficiently released at ambient temperature revealed by ab initio molecular dynamics (AIMD) simulation.

Binuclear ethylenedithiolate iron carbonyls: A density functional theory study

The structures and energetics of the binuclear ethylenedithiolate iron carbonyls H2C2S2Fe2(CO)n (n = 8, 7, 6, 5, 4) have been studied using density functional theory. The experimental H2C2S2Fe2(CO)6 structure with the ethylenedithiolate ligand bridging the central Fe–Fe bond through both of its sulfur atoms but with an uncomplexed CC double bond is predicted to be the lowest energy structure by a margin of ∼10 kcal/mol. The lowest energy singlet pentacarbonyl H2C2S2Fe2(CO)5 structure can be derived from this hexacarbonyl structure by displacement of one of its CO groups through coordination of the ethylenedithiolate CC double bond. For the tetracarbonyl H2C2S2Fe2(CO)4 the lowest energy singlet structure has a central distorted bicapped tetrahedral Fe2C2S2 unit shown to be aromatic by a highly negative diatropic nucleus independent chemical shift (NICS) value. Low-energy triplet and quintet structures are also found for the pentacarbonyl and the tetracarbonyl, respectively. The calculated ΔE and ΔG values for CO dissociation of the carbonyl richer octacarbonyl and heptacarbonyl H2C2S2Fe2(CO)n (n = 8, 7) indicate them to be disfavored relative to formation of the hexacarbonyl H2C2S2Fe2(CO)6 by CO loss.

Au42N20: A Stable Aromatic Deltoidal Hexecontahedron Molecule

A stable hollow cage, which contains 42 gold atoms and 20 nitrogen atoms, has been investigated using first-principles studies. This Au42N20 cage has an Ih group symmetry and a shape like Catalan deltoidal hexecontahedron. Molecular dynamics simulations demonstrate that the Au42N20 cage retains its geometric topology up to about 1317 K. Vibrational frequency analysis with no imaginary frequencies also emphasizes the stability of the Au42N20 molecule. The analysis for the bonding characteristic and electronic structure show that the p–d hybridization near the Fermi level is the key to the stability of the Au42N20 cage. The large negative nucleus-independent chemical shift value demonstrates its strong aromaticity. This stable cage-like Au42N20 structure with high surface area may have potential applications in catalysis.

Probing Reaction Mechanism of [1,5]-Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier.

The [1,5]-migration reaction has attracted considerable attention from experimentalists and theoreticians for decades. Although it has been extensively investigated in various systems, studies on pyrrolium derivatives are underdeveloped. Herein, a theoretical study on the reaction mechanism of [1,5]-migration in both pyrrolium and pyrrole derivatives is presented. The results reveal lower activation barriers in [1,5]-migration of electropositive groups (AuPMe3 and SnH3 ) in pyrrolium derivatives, although the bond dissociation energies of the Au-N bond (98.8 kcal mol-1 ) and Sn-N bond (81.7 kcal mol-1 ) are larger than that of the N-F bond (57.6 kcal mol-1 ). The unexpectedly lower activation barriers (4.5 and 4.9 kcal mol-1 for AuPMe3 and SnH3 , respectively) for [1,5]-migration of electropositive groups, in comparison with the [1,5]-fluorine shift, can be attributed to aromaticity stabilizing the transition states, as revealed by significantly negative nucleus-independent chemical shift (NICS) values. Further studies indicate that charge distribution and frontier molecular orbitals also play some roles in [1,5]-migration of pyrrolium derivatives.

Insights of the role of shell closing model and NICS in the stability of NbGen (n = 7–18) clusters: a first-principles investigation

In the present report, the structures, energetics and electronic properties of neutral and cationic Nb-doped Gen (n = 7–18) clusters are systematically investigated under the first-principles density functional theory approach. The isomers in which the Nb atom is encapsulated inside a germanium cage are relatively stable compared to the exohedral surface doping. The thermodynamic stability and chemical activity of the ground-state isomers are analyzed through various energetic parameters. The results highlight the enhanced stability of the neutral NbGe12 hexagonal prism-like structure with D6h symmetry and cationic NbGe16 fullerene isomers. The negative nucleus-independent chemical shift can explain the enhanced stability of neutral NbGe12. However, the enhanced stability of cationic NbGe16 is explained by shell closing model associated with the quasi-spherical geometry with a sequence 1S21P61D101F61G122S22P6IF8IG62D10 following Hund’s rule. To understand the effect of hybridization on stability, we have calculated density of states (DOS) and projected DOS (PDOS). From PDOS, it is clear that Nb-p and Ge-s and p orbitals are mainly take part in hybridization; however, near below Fermi level, the dominating contribution comes from Nb-d orbitals. In addition, IR and Raman spectra of clusters are also calculated to explain their vibrational properties of the isomers. Specifically, IR spectrum of the clusters in the range of 12–16 shows the possible application of these clusters in the IR sensing device.

The nature of structure and bonding between transition metal and mixed Si-Ge tetramers: A 20-electron superatom system.

A novel superatom species with 20-electron system, Six Gey M(+) (x + y = 4; M = Nb, Ta), was properly proposed. The trigonal bipyramid structures for the studied systems were identified as the putative global minimum by means of the density functional theory calculations. The high chemical stability can be explained by the strong p-d hybridization between transition metal and mixed Si-Ge tetramers, and closed-shell valence electron configuration [1S(2) 1P(6) 2S(2) 1D(10) ]. Meanwhile, the chemical bondings between metal atom and the tetramers can be recognized by three localized two-center two-electron (2c-2e) and delocalized 3c-2e σ-bonds. For all the doped structures studied here, it was found that the π- and σ-electrons satisfy the 2(N + 1)(2) counting rule, and thus these clusters possess spherically double (π and σ) aromaticity, which is also confirmed by the negative nucleus-independent chemical shifts values. Consequently, all the calculated results provide a further understanding for structural stabilities and electronic properties of transition metal-doped semiconductor clusters. © 2016 Wiley Periodicals, Inc.

Computational investigation on MB n (M = Li-Cs, Be-Ba, Sc-La and Ti; n = 28 and 38).

Differing from the weakly antiaromatic B80 buckyball, the medium-sized C 1-B28 and D 2h -B38, as well as their mono- to tetra-anions, are highly aromatic, as indicated by the negative nucleus-independent chemical shifts (NICSs) at their cage centers. The interior cavities and high aromaticity of the B28 and B38 cages render them very promising hosts to accommodate diverse metal atoms. Accordingly, we carried out systematic density functional theory (DFT) computations on the structures, stabilities and electronic properties of metalloborofullerenes MB n (M = Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Y, La and Ti; n = 28 and 38). Among them, besides the recently reported M@B38(M = Sc, Y and Ti) [Lu et al. (2015) Phys Chem Chem Phys 17:20897-20902], Ti@B28 and M@B38 (M = Ca and La) also favor endohedral structures with large binding energies, and are suggested promising targets for experimental applications. Note that Ti@B28 is the first endohedral derivative based on the new B28 fullerene, and La@B38 features the largest metal size inside a B38 cage thus far. These endohedral derivatives, as exemplified by Ca@B38, may exhibit σ and π double aromaticity over the whole cage surface, indicating their considerable stability. In contrast, the other metals prefer to reside at the exterior cage surface, due mainly to the mismatch of their sizes with the boron cages, though the size match is not the only factor to determine their doping form. Furthermore, the infrared absorption spectra and (11)B nuclear magnetic resonance spectra of the three new M@B n complexes were computed to assist future experimental characterization. Graphical Abstract Putting more metals into medium-sized boron cages!

Molecular Structures of N,N'-Dimethylbenzimidazoline-2-germylene and -stannylene in Solution and in Solid State by Means of Optical (Raman and UV-vis) Spectroscopy and Quantum Chemistry Methods.

X-ray data obtained for germylene 1 evidence its monomeric structure, unlike that of stannylene 2, which had been shown previously to form a coordination dimer. Raman spectra of solid and liquid 1 are identical, whereas the Raman spectra of solid 2 and its solution 2a differ significantly. The spectrum of 2 is complicated and contains the lines corresponding to N → Sn coordination bonds forming a dimer. The spectrum of 2a is simpler and close to that of monomeric 1, thus pointing to the rupture of the dimer in solution. The UV-vis spectrum of solid 2 exhibits a band corresponding to a transition involving the N → Sn coordination bonds. Quantum theory of atoms in molecules data estimate the energy of this bond as ∼19 kcal/mol. The aromaticity of 1 and 2 with their 10 π-electron systems including divalent Ge or Sn atoms is confirmed by negative nucleus-independent chemical shift values.

CBe5Hn((n-4)) (n = 2-5): Hydrogen-Stabilized CBe5 Pentagons Containing Planar or Quasi-Planar Pentacoordinate Carbons.

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design a new series ppC- or quasi-ppC-containing species C5v CBe5H5(+), Cs CBe5H4, C2v CBe5H3(-), and C2v CBe5H2(2-) by replacing the Al atoms in previously reported global minima planar pentacoordinate carbon (ppC) species D5h CAl5(+), C2v CAl4Be, C2v CAl3Be2(-), and C2v CAl2Be3(2-) with BeH units. The three-center two-electron (3c-2e) bonds formed between Be and bridging H atoms were crucial for the stabilization of these ppC species. The natural bond orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses revealed that the central ppCs or quasi-ppCs possess the stable eight electron-shell structures. The AdNDP analyses also disclosed that these species are all 6σ+2π double-aromatic in nature. The aromaticity was proved by the calculated negative nucleus-independent chemical shifts (NICS) values. DFT and high-level CCSD(T) calculations revealed that these ppC- or quasi-ppC species are the global minimum or competitive low-lying local minimum (Cs CBe5H4) on their potential energy surfaces. The Born-Oppenheimer molecular dynamic (BOMD) simulations revealed that the H atoms in C2v CBe5H3(-) and C2v CBe5H2(2-) can easily rotate around the CBe5 cores and the structure of quasi-planar C5v CBe5H5(+) will become the planar structure at room temperature; however, these interesting dynamic behaviors did not indicate the kinetic instability as the basic ppC structures were maintained during the simulations. Therefore, it would be potentially possible to realize these interesting ppC- or quasi-ppc-species in future experiments.

BN Belts: From Small Fullerenes to Nanocapsules

We have performed a density functional study to investigate the structural and magnetic properties of full equatorial BN-substituted fullerenes and nanocapsules C20(BN)m, m = 15, 25, 35, 45 and 55, in comparison to their parent cages Cn, n = 50, 70, 90, 110, 130, with D5h symmetry. Binding energies for the BN substituted nanocages C20(BN)m are lower than those of their parents, generally increasing as the size of tubular belt increases. According to the computed nucleus independent chemical shifts (NICS) along principal axes, different behaviors of magnetic field are detected inside the tubular belts of nanocages. First, the substitution of BN units for CC ones in fullerenes (n = 50, 70, 90) results in less negative NICS at the cage centers, less aromaticity, in comparison to their parent fullerene; however NICS at the cage centers of the BN-substituted nanocapsules (n = 110, 130) are more negative than pure carbon nanocapsules, more aromaticity. Second, the full equatorial BN substitution of nanocapsules leads to a homogeneous magnetic field, while dramatic changes in local ring currents cause a sharp maximum in the NICS plots of the pure carbon compounds. Finally, the presence of the caps bonded to BN tubular parts significantly affects electron delocalization and consequently induces aromaticity to these compounds.

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.

Density functional studies of the stepwise substitution of pyridine, pyridazine, pyrimidine, pyrazine, and 1,3,5-triazine with BCO

The structures, stabilities, and aromaticities of a series of (BCO) n (CH)5-n N (n = 0-5), (BCO) n (CH)4-n N2 (n = 0-4), and 1,3,5-(BCO) n (CH)3-n N3 (n = 0-3) clusters were investigated at the B3LYP density functional level of theory. The most stable positional isomers of individual clusters were obtained. All of the calculated CO binding energies were positive, suggesting that the BCO-substituted species are stable. It was found that the BCO-substituted structures are much less strained than their carbocation counterparts. The negative nucleus-independent chemical shifts (NICSs) obtained show that all of the BCO-substituted species possess three-dimensional aromaticity, in good accord with the aromaticities of the corresponding hydrocarbon species.

Density functional studies of the stepwise substitution of pyrrole, furan, and thiophene with BCO

The structures, stabilities, and aromaticities of a series of (BCO) n (CH)₄-n NH (n = 0-4), (BCO) n (CH)₄-n O (n = 0-4), and (BCO) n (CH)₄-n S (n = 0-4) clusters were investigated at the B3LYP density functional level of theory. The most stable positional isomers of the individual clusters were obtained. All of the calculated CO binding energies were exothermic, suggesting that these BCO-substituted species are stable. Calculated differences in strain energy between the BCO-substituted structures and their corresponding hydrocarbon clusters were all exothermic, indicating that the BCO-substituted structures are less strained. The negative nucleus-independent chemical shift (NICS) values obtained show that these BCO-substituted clusters are aromatic compounds, in good agreement with the aromaticities of the corresponding hydrocarbon species. To aid further experimental investigations, CO-stretching frequencies were also computed.

Exohedrally stabilized C70 isomer with adjacent pentagons characterized by crystallography

All C70 isomers other than the well-known D5h-symmetric C70 (#8149) are highly elusive due to their defiance of the isolated pentagon rule (IPR), and, in turn, have long been underrated for years. Here we show a non-IPR hollow C70 (#8064) with C2-symmetry stabilized by exohedral chlorination as #8064C70Cl10. Its connectivity with two pairs of fused pentagons was determined unambiguously by single crystal X-ray diffraction analysis. Based on the geometric criterion in terms of the pyramidalization angle as well as theoretical computations on the Kekule structure and the negative nucleus independent chemical shift, the exohedral stabilization of the labile #8064C70 is rationalized by releasing the local strain relevant to the fused pentagons and achieving the aromaticity of the remaining sp2-hybridized carbon framework in the exohedral derivative of #8064C70. This new entrant in the C70 family, in addition to two previously reported non-IPR isomers (by Yang, Dunsch et al. for Sc3N@#7854C70 and by Echegoyen et al. for Sc2S@#7892C70), provides valuable opportunities for experimental and theoretical research involving non-IPR isomers of C70 beyond the common D5h-C70.

Isomers of C12N12 as potential hydrogen storage materials and the effect of the electric field therein

The relative stability, aromaticity and hydrogen storage ability of different C12N12 isomers are computationally assessed at B3LYP/6-31G(d), DFT-D-B3LYP/6-31G(d) and M052X/6-311+G(d,p) levels of theory. A negative nucleus independent chemical shift at the cage center [NICS(0)] and a high value for the gas phase heat of formation justify the stability as well as efficacy of these C12N12 isomers as promising high-energy density materials (HEDMs). Each nitrogen site is found to adsorb one hydrogen molecule which corresponds to a gravimetric density of 7.2 wt%. The temperature–pressure plot of the Gibbs free energy change separates the thermodynamically spontaneous regions of the hydrogen adsorption and desorption processes. The application of an external electric field increases the binding energy of hydrogen adsorption by ∼0.50 kcal mol−1. One of the C12N12 isomers is the basic building block involved in the formation of two different kinds of one-dimensional cluster assembled materials.

Theoretical Study on Au-Doped Ge Semiconductor Clusters

Structure, electronic property, aromaticity and vibrational frequency of medium-sized Au-doped germanium clusters were systematically explored using the density-functional theory (DFT) in conjunction with the LanL2DZ basis set. Our results show that the endohedrally Au-doped cagelike structures are energetically preferred. The p- and d-states in endohedral Au atom mainly contribute to the chemical bonding at around −6.5 and −10.6 eV for the AuGe10 and AuGe12 clusters. Moreover, the cage aromaticity appears to be an important determination of the electronic stability of the two clusters, reflected by negative nucleus-independent chemical shifts (NICS) values. The theoretical work will be useful and helpful for the understanding in the further application, i.e., cluster-assembled optoelectronic nanomaterials.

Computational Insights on the Structure and Properties of C24, C18B3N3, C12B6N6 and Their Endohedral Complexes with Alkaline and Earth Alkaline Metals

The structural and electronic properties of C24-2nBnNn and M@ C24-2nBnNn (M = alkaline and earth alkaline metals; n = 3 and 6) molecules are studied using density functional theory. It was found that the most interaction is in M@ C24-2nBnNn complexes (M = Be, Mg; n = 3 and 6). The negative nucleus-independent chemical shifts confirm that C24-2nBnNn (n = 3 and 6) cages exhibit aromatic characteristics. The molecular analysis reveals a linear correlation between isotropic polarizability values and HOMO-LUMO gaps energies in stable complexes.

Chiral Interconversions of Pd and/or Au Bis-Metalated [32]Octaphyrins(1,0,1,0,1,0,1,0) Involving Hückel and Möbius Macrocyclic Topologies: A Theoretical Prediction

Several Pd and/or Au bis-metalated [32]octaphyrins(1,0,1,0,1,0,1,0) were theoretically designed with rich conformations of Hückel or Möbius topology. The conformations and hence properties of macrocycles were tuned by twisting the active pyrrolic ring either clockwise (through multistep reactions with several Hückel and Möbius macrocyclic intermediates) or anticlockwise (via a direct Hückel-Hückel chiral interconversion). The encapsulated metal atoms, M(1), M(2) = Pd, Au, give different impacts on these two reaction processes. Facile occurrences of chiral interconversions between two enantiomers of bis-metalated octaphyrins were predicted with the largest activation barrier less than 40 kcal/mol. Some Au-coordinated octaphyrins (M(1) = Au) were demonstrated to be thermodynamically stable with large negative nucleus-independent chemical shift (NICS) values, which are comparable to those of the synthetic Pd-coordinated complexes. The free-base [32]octaphyrins(1,0,1,0,1,0,1,0) display the characteristic absorption spectra with distinct sharp Soret-like bands. After metalation, the Soret-like bands are red-shifted in different degrees along with the appearance of rather weak Q-like band. The heterometal-coordinated complexes (i.e., M(1) ≠ M(2)) show stronger and more splitting absorptions than the homometal-coordinated ones with M(1) = M(2). The hyperpolarizabilities sharply augment with the metalation in Hückel systems due to the destruction of the centrosymmetry and the increase in polarizability by coordinated metal atoms.