Negative Nucleus-independent Chemical Shift Values Research Articles

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations.

SummaryMononuclear metal-peroxo species are invoked as the key intermediates in metalloenzymatic or synthetic catalysis. However, either transience or sluggishness reactivity of synthetic analogs of metal-peroxo species impedes our understanding of oxygen activation mechanism. Herein, we designed and characterized a dioxygen-derived mononuclear osmium-peroxo complex, in which the peroxo ligand is stabilized by internally aromatic metallacycle. We demonstrate that the osmium-peroxo species shows catalytic activity toward promoterless alcohol dehydrogenations. Furthermore, computational studies provide a new mechanism for the osmium-peroxo-mediated alcohol oxidation, starting with the concerted double-hydrogen transfer and followed by the generation of osmium-oxo species. Interestingly, the internally aromatic metallacycle also plays a vital role in catalysis, which mediates the hydrogen transfer from osmium center to the distal oxygen atom of Os–OOH moiety, thus facilitating the Os–OOH→Os=O conversion. We expect that these insights will advance the development of aromatic metallacycle toward aerobic oxidation catalysis.

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.

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.

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.

Unraveling a generic growth pattern in structure evolution of thiolate-protected gold nanoclusters.

Precise control of the growth of thiolate-protected gold nanoclusters is a prerequisite for their applications in catalysis and bioengineering. Here, we bring to bear a new series of thiolate-protected nanoclusters with a unique growth pattern, i.e., Au20(SR)16, Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32. These nanoclusters can be viewed as resulting from the stepwise addition of a common structural motif [Au8(SR)4]. The highly negative values of the nucleus-independent chemical shift (NICS) in the center of the tetrahedral Au4 units suggest that the overall stabilities of these clusters stem from the local stability of each tetrahedral Au4 unit. Generalization of this growth-pattern rule to large-sized nanoclusters allows us to identify the structures of three new thiolate-protected nanoclusters, namely, Au60(SR)36, Au68(SR)40, and Au76(SR)44. Remarkably, all three large-sized nanoclusters possess relatively large HOMO-LUMO gaps and negative NICS values, suggesting their high chemical stability. Further extension of the growth-pattern rule to the infinitely long nanowire limit results in a one-dimensional (1D) thiolate-protected gold nanowire (RS-AuNW) with a band gap of 0.78 eV. Such a unique growth-pattern rule offers a guide for precise synthesis of a new class of large-sized thiolate-protected gold nanoclusters or even RS-AuNW which, to our knowledge, has not been reported in the literature.

Is NICS a reliable aromaticity index for transition metal clusters?

In the present account the nature of aromaticity/antiaromaticity of fourteen metallic complexes/clusters are reexamined. These species were classified as aromatic by means of different nucleus independent chemical shift (NICS) based approaches, previously. Visualization of the current density and magnetizability of atomic basins reveals that none of the studied systems are magnetic aromatic, i.e. sustain diamagnetic ring current. It is demonstrated that negative NICS values near the ring plane of the studied molecules originates from remarkably strong local paramagnetic current around their transition metal atom nuclei. This phenomenon has been observed only for Sc3 − all-metal cluster but current study demonstrates that the influence of the local paramagnetic currents around transition metal atoms on NICS is a general phenomenon that must be carefully considered prior to classification of the metallic systems as aromatic. Furthermore, this study suggests that NICS is not a reliable aromaticity index for transition-metal clusters/molecules.

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.

Silaphenolates and Silaphenylthiolates: Two Unexplored Unsaturated Silicon Compound Classes Influenced by Aromaticity

Monosilicon analogs of phenolates and phenylthiolates are studied by quantum chemical calculations. Three different silaphenolates and three different silaphenylthiolates are possible; the ortho-, meta-, and para-isomers. For the silaphenolates, the meta-isomer is the thermodynamically most stable, regardless if the substituent R at Si is H, t-Bu or SiMe3. However, with R = H and SiMe3 the energy differences between the three isomers are small, whereas with R = t-Bu the meta-isomer is ~5 kcal/mol more stable than the ortho-isomer. For the silaphenylthiolates the ortho-isomer is of lowest energy, although with R = H the ortho- and meta-isomers are isoenergetic. The calculated nucleus independent chemical shifts (NICS) indicate that the silaphenolates and silaphenylthiolates are influenced by aromaticity, but they are less aromatic than the parent silabenzene. The geometries and charge distributions suggest that all silaphenolates and silaphenylthiolates to substantial degrees are described by resonance structures with an exocyclic C=O double bond and a silapentadienyl anionic segment. Indeed, they resemble the all-carbon phenolate and phenylthiolate. Silaphenylthiolates are less bond alternate and have slightly more negative NICS values than analogous silaphenolates, suggesting that this compound class is a bit more aromatic. Dimerization of the silaphenolates and silaphenylthiolates is hampered due to intramolecular Coulomb repulsion in the dimers, and silaphenolates with a moderately bulky SiMe3 group as substituent at Si should prefer the monomeric form.

Experimental and theoretical characterization of MSi16−, MGe16−,MSn16−, and MPb16− (M=Ti, Zr, and Hf): The role of cage aromaticity

Silicon (Si), germanium (Ge), tin (Sn), and lead (Pb) clusters mixed with a group-4 transition metal atom [M = titanium (Ti), zirconium (Zr), and hafnium (Hf)] were generated by a dual-laser vaporization method, and their properties were analyzed by means of time-of-flight mass spectroscopy and anion photoelectron spectroscopy together with theoretical calculations. In the mass spectra, mixed neutral clusters of MSi(16), MGe(16), and MSn(16) were produced specifically, but the yield of MPb(16) was low. The anion photoelectron spectra revealed that MSi(16), MGe(16), and MSn(16) neutrals have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of 1.5-1.9 eV compared to those of MPb(16) (0.8-0.9 eV), implying that MSi(16), MGe(16), and MSn(16) are evidently electronically stable clusters. Cage aromaticity appears to be an important determinant of the electronic stability of these clusters: Calculations of nucleus-independent chemical shifts (NICSs) show that Si(16)(4-), Ge(16)(4-), and Sn(16)(4-) have aromatic characters with negative NICS values, while Pb(16)(4-) has an antiaromatic character with a positive NICS value.

STRUCTURE AND STABILITY OF MONOCYCLIC $({\rm CH})_{4-n} ({\rm BL})_{n}^{2-}$(L = CO, N2, CS) DIANIONS AND THEIR DILITHIUM COMPLEXES

Structure and stability of monocyclic [Formula: see text] dianions ( L = CO , N 2, CS ) with 6π-electrons, which are isolobal with cyclobutadiene dianion [Formula: see text], have been investigated at the B3LYP and CCSD(T) levels of theory. ( CH )3( BL )2- have non-planar singlet ground states. [Formula: see text] have planar singlet ground states, while [Formula: see text] isomers have folded four-membered central rings. Both [Formula: see text] and [Formula: see text] have planar ground states, the ground state of [Formula: see text] has a six-membered ring with two bridging and one terminal CS. Both [Formula: see text] and [Formula: see text] have reduced aromaticity compared to [Formula: see text] as indicated by the less negative nucleus independent chemical shifts (NICS) values. The mono-, di- and trisubstituted ( CH )3( BL )2-, [Formula: see text], and [Formula: see text] also have reduced aromaticity, while the 1,3-substituted [Formula: see text] have positive NICS values due to the localization of the negative charge at the ring carbon centers. In addition, the electrostatic stabilization of Li + in favor of the singlet or triplet states depends on the nature of their structures.

Photophysical Properties of Core-Modified Expanded Porphyrins: Nature of Aromaticity and Enhancement of Ring Planarity

We have investigated the excited-state dynamics and nonlinear optical properties of representative core-modified expanded porphyrins, tetrathiarubyrin, tetraselenarubyrin, pentathiaheptaphyrin, tetrathiaoctaphyrin, and tetraselenaoctaphyrin, containing 26, 30, and 34 pi electrons using steady-state and time-resolved absorption and fluorescence spectroscopic measurements along with femtosecond Z-scan method, with a particular attention to the photophysical properties related to molecular planarity and aromaticity. Core-modification of macrocycles by sulfur and selenium leads to NIR-extended steady-state absorption and fluorescence spectra and short-lived excited-state due to the heavy-atom effect in time-resolved spectroscopic experiments. Large negative nucleus-independent chemical shift values ranging from -13 to -15 ppm indicate that all molecular systems are highly aromatic. The observed enhancement of two-photon absorption cross-section values over 10 (4) GM for core-modified hepta- and octaphyrins is mainly attributable to their rigid and planar structures as well as their aromaticity. Overall, the observed spectroscopic and theoretical results consistently demonstrate the enhanced molecular planarity of core-modified expanded porphyrins compared with their corresponding all-aza expanded porphyrins.

Structure and aromaticity of [formula omitted], Bi 5M (M = Li, Na, K) and Bi 5M + (M = Be, Mg, Ca) clusters

The equilibrium geometries, electronic structures, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICS) of Bi 5 - , Bi 5M (M = Li, Na, K), and Bi 5M + (M = Be, Mg, Ca) clusters are calculated by the density functional theory (DFT) method. Our calculation show that the ground states of Bi 5 - , Bi 5M, and Bi 5M + species are predicted to be the D 5 h , C 5 v , and C 5v structures, respectively. In addition, the Bi 5 - unit preserves its structural and electronic integrity in forming the Bi 5M and Bi 5M + complexes. Molecular orbital analysis and NICS show that the planar Bi 5 - ( D 5 h ) anion satisfies the Hükcel rule of 4 n + 2 π electrons and magnetic criteria for aromaticity with six delocalized π electrons and negative NICS values. The dissected NICS suggests that the aromaticity of Bi 5 - ( D 5 h ) arise primarily from the contributions of Bi–Bi σ bonds and Bi–Bi π bonds, while the aromaticity of Bi 5M and Bi 5M + mainly owe to the Bi–Bi σ bonds.

Structure and aromaticity of [formula omitted] and Sb 5M (M = Li, Na, and K) clusters

The equilibrium geometries, electronic structures, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICS) of Sb 5 - and Sb 5M (M = Li, Na, and K) clusters are calculated by the density functional theory (DFT) method. Our calculation show that the Sb 5 - species has the aromatic planar cyclic D 5 h structure and the Sb 5M species have the pyramidal C 5 v structures as the ground states, and the Sb 5 - unit preserves its structural and electronic integrity in forming the Sb 5M complexes. Molecular orbital analysis and NICS show that the planar Sb 5 - anion satisfies the Hükcel rule of 4 n + 2π electrons and magnetic criteria for aromaticity with six delocalized π electrons and negative NICS values. The dissected NICS suggests that the aromaticity of Sb 5 - ( D 5 h ) and Sb 5M ( C 5 v ) arise primarily from the contributions of Sb–Sb π bonds, Sb–Sb σ bonds, and the NICS distributions and contributions, and their changing tendencies of the Sb 5M ( C 5 v ) species are similar to the isolated Sb 5 - ( D 5 h ) anion.

Novel Superalkali Superhalogen Compounds (Li3)+(SH)−(SH=LiF2, BeF3, and BF4) with Aromaticity: New Electrides and Alkalides

Optimized structures, with all real frequencies, of superalkali superhalides (Li(3))(+)(SH)(-) (SH=LiF(2), BeF(3), and BF(4)), are obtained, for the first time, at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. These superalkali superhalides possess three characteristics that are significantly different from normal alkali halides. 1) They have a variety of structures, which come from five bonding mode types: edge-face, edge-edge, face-face, face-edge, and staggered face-edge. We find that the bonding mode type closely correlates with the Li(3)-SH bond energy. 2) The valence electrons on the Li(3) ring are pushed out by the (SH)(-) anion, and become excess electrons, conferring alkalide or electride characteristics on these Li(3)-SH species, depending on the bonding mode type. 3) The highest occupied molecular orbital of each Li(3)-SH species is a doubly occupied delocalized sigma bonding orbital on the Li(3) ring, which indicates its aromaticity. It is noticeable that the maximum negative nucleus-independent chemical shift value (about -10 ppm) moves out from the center of the Li(3) ring, owing to repulsion by the SH(-) anion. We find that these superalkali superhalides are not only complicated "supermolecules", but are also a new type of alkalide or electride, with aromaticity.

Octahedral and Tetrahedral Coinage Metal Clusters: Is Three-Dimensional d-Orbital Aromaticity Viable?

The first quantitative evidence for the viability of three-dimensional aromatic clusters involving d-orbitals in pseudo-octahedral coinage metal cages M(6)Li(2) (M = Cu, Ag, Au) as well as in tetrahedral coinage metal cages M'(4)Li(4) (M' = Cu, Ag) was obtained computationally. These cages exhibit many features similar to those of their square planar M(4)Li(2) analogues. The large negative nucleus-independent chemical shifts (NICS) at the cage centers indicate three-dimensional delocalization. This diatropic character arises mostly from d-orbital delocalization combined with substantial contributions from the lowest-valence orbitals. The bonding molecular orbitals of the pseudo-octahedral clusters M(6)Li(2) (M = Cu, Ag, Au) are analogous to those in similar octahedral clusters involving p-orbital delocalization (e.g., B(6)H(6)(2-)). The M'(4)Li(4) clusters exhibit two isomeric forms: metal tetrahedral cages tetracapped by lithium cations on the outside [(M'(4)).4Li] and lithium tetrahedra on the inside capped by coinage metal atoms on each of the four faces [(Li(4)).4M]. Whereas the (M'(4)).4Li type structure is preferred for copper, gold and silver favor the (Li(4)).4M arrangement. NBO-NICS analysis shows that the large diatropic character in (M'(4)).4Li structures is due to the favorable contribution from both s- and d-orbitals, whereas the small NICS values in the center of (Li(4)).4M are due only to the diatropic contributions from the s-orbitals.

Li 3 – O – Li 3 molecule: A metal-nonmetal-metal sandwichlike compound with a distending electron cloud

The D3d and D2d isomers of the Li3-O-Li3 molecule are metal-nonmetal-metal sandwichlike structures that contain two Li3 superalkali atoms. Their geometries and the real frequencies are obtained at the CCSD(T)/aug-cc-pVDZ level. They are different from the traditional types of the nonmetal-metal-nonmetal sandwich compounds. The natural bond orbital calculation and the topological property nabla2rho(r) calculation indicate that they are typical ionic compounds. In two isomers, the O2- anion is sandwiched in between two Li3+ cation rings. However, the different orientations of two Li3+ planes give the D3d isomer its own special characteristics. Under the action of the O2- anion in the center, the valence electrons of the D3d isomer are pushed out from two Li3+ triangle rings. This special interaction causes three phenomena. First, the valence electron clouds are distended. Second, the vertical ionization energy of the D3d isomer is considerably low, 4.39 eV, so that it may also be viewed as a superalkali atom. Third, we find that the D3d isomer owns the out-of-plane aromaticity and the largest negative nucleus-independent chemical shift value (-10.8 ppm) exists at 2.5 A above the center of the Li3+ ring, not at the center of the Li3+ ring like the isolated aromatic Li3+ cation.

Structure and Stability of Be5, Be5+, and Be5-Clusters

The electronic structure and stability of Be5, Be5+, and Be5- clusters have been investigated at the B3LYP, B3PW91, and MP2 levels of theory, along with the 6-311G* basis set for neutral and cationic clusters and the 6-311+G* basis set for anion clusters. Six Be5, six Be5+, and six Be5- isomers are identified. Of these eighteen species, twelve have not been reported previously. The trigonal bipyramid structure is found to be the global minimum for the neutral Be5 cluster, in agreement with the previous results. The most stable Be5+ and Be5- clusters have trigonal bipyramid structures similar to the neutral global minimum. Natural bond orbital (NBO) analysis and molecular orbital (MOs) investigation suggest a delocalized π bond in the three trigonal bipyramid structures, and the negative nucleus-independent chemical shift (NICS) values indicate aromatic character for them. For the planar pentagon structures, the NBO and MOs analyses show that there has no delocalized π bond, and the NICS values show that both the cationic and the neutral clusters are antiaromatic, while the anionic cluster is nonaromatic.

Spherical homoaromaticity.

The 2(N+1)2 electron-counting rule for spherical aromaticity can also be applied to spherical homoaromatic systems: The adamantane-, cubane-, and dodecahedrane-based species containing two or eight delocalized electrons were designed by bridging the sp2 carbon atoms of highly symmetrical (Td, Th) carbon polyhedrane units. Their aromatic character was demonstrated by the highly negative nucleus-independent chemical shift (NICS) values at the cage centers, for example C20H12 (see picture) has an NICS value of −36.4 ppm.