Nucleus-independent Chemical Shift Values Research Articles

An insight on the aromatic changes in closed shell icosagen, tetrel, and pnictogen phenalenyl derivatives

A computational study of the aromatic and antiaromatic characteristics of closed shell charged phenalenyl (PLY+1 and PLY−1) upon replacement of the central carbon atom by icosagen (B, Al and Ga), tetrel (Si and Ge) and pnictogen (N, P and As) atoms comprising systems in which the icosagen and pnictogen derivatives considered are neutral while the tetrel ones are anions or cations, has been carried out at the B3LYP/6–311++G(d,p) computational level. By substitution, two different kinds of structures have been obtained, one planar (N and B) and another one bowl-shaped depending on the size of the central atom. In terms of aromaticity, the substitution of the central C atom causes a loss of the aromatic character in all cases as indicated by nucleus-independent chemical shifts (NICS) profiles and NICS values on the 0.001 au isosurface. Regarding the charge, PLY+1 presents larger electron delocalisation than PLY−1, phenomenon associated with aromaticity. Furthermore, the current density maps for those planar systems corroborate NICS findings, showing anticlockwise currents in PLY+1 (like in benzene) but clockwise in PLY-N0 and PLY-B0, indicating aromatic and antiaromatic behaviour, respectively.

Effect of Curvature on Carbon Chemical Shielding in Extended Carbon Systems.

The effect of curvature on the chemical shielding of carbons in curved polycyclic aromatic hydrocarbons has been systematically studied by examining structures analogous to the circumcoronene molecule with different degrees of curvature. We attempt to eliminate effects from Knight shifts in carbon nanotubes, differing ring currents from five-membered rings, and edge effects in finite nanotube models in order to separate out the change in shielding that is due to curvature alone. Using curved structures derived from geometry-optimized structures of carbon nanotubes, we calculate the carbon chemical shielding tensor for carbons in the central aromatic ring as well as the Nucleus Independent Chemical Shift (NICS) on the convex and concave side of each structure. All three tensor components become less shielded with increasing curvature of the system, with the σ33 component radial to the curve experiencing the greatest change. The NICS values are influenced by both the decrease in aromaticity as the structure is curved as well as geometric effects that bring the outside rings closer to the central aromatic ring.

Effect of Mono- and Poly-CH/P Exchange(s) on the Aromaticity of the Tropylium Ion

In view of the fact that the phosphorus atom in its low co-ordination state (coordination numbers 1 and 2) has been termed as the carbon copy, there have been attempts to investigate, theoretically as well as experimentally, the effect of the exchange(s) of CH- moiety with phosphorus atom(s) (CH/P) on the structural and other aspects of the classical carbocyclic and heterocyclic systems. Tropylium ion is a well-known non-benzenoid aromatic system and has been studied extensively for its aromatic character. We have now investigated the effect of mono- and poly-CH/P exchange(s) on the aromaticity of the tropylium ion. For this purpose, the parameters based on the geometry and magnetic properties, namely bond equalization, aromatic stabilization energies (ASE), Nucleus-Independent Chemical Shift (NICS) values, (NICS(0), NICS(1), NICS(1)zz), proton nucleus magnetic resonance (1H-NMR) chemical shifts, magnetic susceptibility exaltation and magnetic anisotropic values of mono-, di-, tri- and tetra-phosphatropylium ions have been determined at the Density Functional Theory (DFT) (B3LYP/6-31+G(d)) level. Geometry optimization reveals bond length equalization. ASEs range from −46.3 kcal/mol to −6.2 kcal/mol in mono- and diphospha-analogues which are planar. However, the ions having three and four phosphorus atoms lose planarity and their ASE values approach the values typical for non-aromatic structures. Of the three NICS values, the NICS(1)zz is consistently negative showing aromatic character of all the systems studied. It is also supported by the magnetic susceptibility exaltations and magnetic anisotropic values. Furthermore, 1H-NMR chemical shifts also fall in the aromatic region. The conclusion that mono-, di-, tri- and tetra-phosphatropylium ions are aromatic in nature has been further corroborated by determining the energy gap between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) (HOMO − LUMO gap), which falls in the range, ca. 3 × 10−19–9 × 10−19 J. The systems having more than four phosphorus atoms are not able to sustain their monocyclic structure.

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.

Aromatic changes in isoelectronic derivatives of phenalenyl radicals by central carbon replacement

A computational study of the aromatic characteristics of phenalenyl (PLY) upon replacement of the central carbon atom by Si, Ge, N, P, As, B, Al and Ga atoms comprising isoelectronic series with different charges (0, +1, −1) has been performed at the B3LYP/6-311++G(d,p) level. Two different geometries have been obtained, one planar and another one bowl-shaped. PLY derivatives exhibit a loss of the aromatic character in all cases indicated by NICS (Nucleus-Independent Chemical Shifts) values. These aromatic features are also in agreement with the pyramidalisation undergone by the central atom. The current density maps for those planar systems corroborate NICS findings. These variations in the aromaticity have been rationalised in terms of charge localisation. Harmonic oscillator model of aromaticity (HOMA) and para-delocalisation indices (PDI) have been used to evaluate the electron density delocalisation. HOMA values do not correlate with NICS but PDI indices follow the same trend than NICS.

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.

A DFT Insight on L n Pu + (L = C 4 H 4 , C 5 H 5 , C 6 H 6 ; n = 1–3) Complexes: Nature of Interactions and Electronic Spectroscopy

The structures, bonding nature, and electronic spectra of Pu+–hydrocarbon complexes were investigated by using DFT. Our results indicate that electron promotion from 7s to 6d orbitals occurs when the Pu+ ion is involved in chemical bonding. Nucleus-independent chemical shift (NICS) values and electron localization function (ELF) analyses suggest that Pu+C5H5 is aromatic and shows a large reinforcement of the ring delocalization. The weakly covalent characters of Pu+–C bonds were revealed by quantum theory of atoms in molecules (QTAIM) topological analyses. Simulated electronic spectra show that the absorption bands are localized at 200–500 nm and are mainly attributed to the Pu(f)→Pu(d) transition or LMCT.

A new aromatic probe — The ring stretching vibration Raman spectroscopy frequency

A new aromatic criterion is presented to determine the aromatic degree of the high symmetric molecules. Group theory is used to explain the correlation between the aromatic degree and the value of Ring Stretching Vibration Raman Spectroscopic Frequency (RSVRSF). The calculations of the geometrical optimization, nucleus-independent chemical shifts (NICS) and values of the Raman Spectroscopy for the aromatic molecules—LnHn (L=C, Si, Ge, n=3, 5–8) were performed using the Density Functional Theory (DFT) Method, as well as the correlations between the values of their RSVRSF and NICS values by Statistic Package for Social Science (SPSS17.0). There are high positive correlations between the theoretical calculated the NICS values and the value of the RSVRSF (A1g/A1′) of the LnHn (L=C, Si, Ge, n=3, 5–8). The bigger the aromatic degree, the bigger the RSVRSF is. The value of the RSVRSF is a new probe of aromaticity. Expectedly, it is predicted that the experimental determination of the aromatic degree can be achieved by the determination of the ring stretching vibration (A1g/A1′) Raman spectrum frequencies for the aromatic target molecules.

Synthesis of Pyrrole-Fused C,N-Cyclic Azomethine Imines and Pyrazolopyrrolopyrazines: Analysis of Their Aromaticity Using Nucleus-Independent Chemical Shifts Values

The AgOTf-catalyzed reaction of C-2 substituted pyrrole hydrazones having an N-propargyl group was studied. The selective 6-endo-dig mode of cyclization was observed, giving rise to the formation of pyrrole-fused C,N-cyclic azomethine imine derivatives. The reaction of one azomethine imine derivative with various dipolarophiles resulted in the formation of cycloadducts having a pyrazolopyrrolopyrazine skeleton. The aromaticity of C,N-cyclic azomethine imines as well as that of pyrazolopyrrolopyrazines was determined by calculating of nucleus-independent chemical shifts values.

The effect of Li doping on the nonlinear optical properties of [2.2]paracyclophane.

The similar molecules [2.2]paracyclophane (22PCP) and 1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane (8F22PCP) have both generated considerable synthetic interest since they were first prepared. In this work, the nonlinear optical properties of 22PCP, 8F22PCP, and the related Li-doped systems 22PCP-Li and 8F22PCP-Li (which have a Li atom above 22PCP and 8F22PCP, respectively) were investigated. An analysis of natural bond orbital charges showed that there is greater charge transfer from the Li atom to the benzene rings in 8F22PCP-Li than in 22PCP-Li. The variation in the calculated nucleus independent chemical shift (NICS) value as a function of the distance from the lower benzene ring towards the upper benzene ring was found to be W-shaped for both 22PCP and 22PCP-Li. Moreover, whereas all of the NICS values of 22PCP and 22PCP-Li were markedly negative, all of the NICS values of 8F22PCP and 8F22PCP-Li were either positive or only moderately negative. Calculations of the electro-optical properties of these systems showed that the first hyperpolarizability of 22PCP-Li was noticeably larger than that of 8F22PCP-Li. According to the two-level model, the larger first hyperpolarizability of 22PCP-Li is due to its smaller transition energy.

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.

A Study of π-π Stacking Interactions and Aromaticity in Polycyclic Aromatic Hydrocarbon/Nucleobase Complexes.

We analysed the interactions and aromaticity electron-density delocalisation observed in π-π complexes between the phenalenyl radical and acenaphthylene, and the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine and uracil). Interaction energies are obtained at the M06-2X/6-311++G(2df,p) computational level for gas phase and PCM-water conditions. For both the phenalenyl radical and acenaphthylene, the complexes formed with guanine are the most stable ones. Atoms in molecules and natural bond orbital results reveal weak π-π interactions between both interacting moieties, characterized by bond critical points between C⋅⋅⋅C and C⋅⋅⋅N atoms. Nucleus independent chemical shifts (NICS) indicate the retention of the aromatic character of the monomers in the outer region of the complex. The fluctuation indexes reveal a loss of electron delocalisation upon complexation for all cases except guanine complexes. Nevertheless, the interface region shows large negative NICS values, which is not associated with an increase of the aromaticity or electron-density delocalisation, but with magnetic couplings of both molecules, leading to an unrealistic description of the aromatic behaviour in that region.

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.

Structure, energetics, and bonding of novel potential high energy density materials Rh2(N5)4: A DFT study

Theoretical studies examining a series of binuclear transition metal pentazolides Rh2(N5)4 predict paddle wheel type structures with very short metal–metal distances. Natural bonding orbital analysis indicated that the bonding between the metal atom and the five-membered ring is predominantly ionic for Rh2(N5)4 species, and a high-order metal–metal multiple bonding exists between the two metal atoms. In addition, the presence of the delocalized π orbital plays an important role in the stabilization of Rh2(N5)4. Nucleus independent chemical shift values confirm that the planar N5− exhibits aromaticity. The dissociation energies into mononuclear fragments are predicted for Rh2(N5)4.

Computational study of gas-phase molecular structure and substitution effects in para-substituted nickelabenzenes (p-XC5H4)Ni(CO)2F

The structure and properties of nickelabenzene complex were examined by the Modified Perdew-Wang Exchange and Correlation method (mpw1pw91). The para-substitutions effect on the structure, frontier orbital energies, aromaticity and electronic spectra has been studied. Nucleus independent chemical shift (NICS) values show that these species are aromatic. Time dependent density functional theory (TD-DFT) was used to calculate the energy, oscillator strength and absorption maxima wavelength (λmax) of various electronic transitions and their nature within molecules.

Synthesis, aromaticity and photophysical behaviour of ferrocene- and ruthenocene-appended semisynthetic chlorin derivatives.

Two novel synthetic strategies to covalently link a metallocene electron-donor unit to a chlorin ring are presented. In one approach, pyropheophorbide a is readily converted into its 13(1) -ferrocenyl dehydro derivative by nucleophilic addition of the ferrocenyl anion to the 13(1) -carbonyl group. In another approach, the corresponding 13(1) -pentamethylruthenocenyl derivative is synthesised from 13(1) -fulvenylchlorin by a facile ligand exchange/deprotonation reaction with the [RuCp*(cod)Cl] (Cp*=pentamethylcyclopentadienyl; cod=1,5-cyclooctadiene) complex. The resulting metallocene-chlorins exhibit reduced aromaticity, which was unequivocally supported by ring-current calculations based on the gauge-including magnetically induced current (GIMIC) method and by calculated nucleus-independent chemical shift (NICS) values. The negative ring current in the isocyclic E ring suggests the antiaromatic character of this moiety and also clarifies the spontaneous reactivity of the complexes with oxygen. The oxidation products were isolated and their electrochemical and photophysical properties were studied. The ruthenocene derivatives turned out to be stable under light irradiation and showed photoinduced charge transfer with charge-separation lifetimes of 152-1029 ps.

Computational study on aromaticity and resonance structures of substituted BODIPY derivatives

We have developed a systematic study for the investigation of resonance structures of the substituted BODIPYs. Aromaticity of two rings of BODIPY were calculated by means of NICS (Nucleus independent chemical shift) values and these values gave us an insight to predict dominant resonance structure between two different resonance forms of BODIPYs. Furthermore, bond order comparisons were utilized to specify dominant forms of the resonance structures. BODIPYs substituted with electron donating (EDG) or withdrawing groups (EWG) at different positions were analyzed and it was observed that character of substituents affected the aromaticity as well as dominant resonance structure. Substituent positions were also investigated and some important points were yielded from these calculations. We further extended the study to aromaticity of pyridine or benzene[b]-fused BODIPYs.

Nucleus-Independent Chemical Shift Criterion for Aromaticity in Some of the Corannulene Derivatives as Carbon Nanostructure: Effect of Substituent Groups on Aromaticity

Substitution of the peripheral H atoms in the corannulene molecule as a carbon nanostructure by OH, CH3, NH2, and NO2 groups was done and the obtained structures were optimized employing MP2/6-31G(d) level of theory. Calculations of the nucleus-independent chemical shift (NICS) were performed to analyze the aromaticity of the corannulene rings and its derivatives upon substitution at B3LYP/6-311+G(d) level of theory. Comparison of NICS(1)zz for concave and convex sides of corannulene derivatives indicated that substitution of H atoms in the corannulene molecule with OH, CH3, NO2, and NH2 groups leads to decreasing of the aromaticity character of corannulene. We discussed about two efficient factors on the aromaticity and antiaromaticity of polycyclic aromatic compounds: electron donating or withdrawing and resonance effects. We showed that NICS values in all structures are strongly dependent on the type, number, and position of the substituted groups.

A series of novel sandwich complexes: MQ(η4-E4)2 (M = Be or Mg; Q = C or Si; E = P, As, Sb or Bi) with donor–acceptor bonds

A series of sandwich-like compounds MQ(η4-E4)2 (M=Be or Mg; Q=C or Si; E=P, As, Sb, or Bi) with centered donor–acceptor bond Q–M are investigated at the B3LYP/6-311+G(d)/SDD and M06-2x/6-311+G(d)/SDD levels of theory. The C4v staggered conformations are predicted to be more stable than the eclipsed ones. Wiberg bond index, electron density and Laplacian of electron density were computed, which can provide an indication of the strength and the character of the bonds. The results indicate that the donor–acceptor Q–M bonds in these complexes (with Wiberg bond index values 0.153–0.203, small electron density values less than 0.1 au and positive Laplacian of electron density) are all ionic, and the C–Be bonds with relatively larger electron density are stronger than the Si–Mg bonds. The bonds between M and E42− are ionic, whereas the Q-ligand bonds are covalent. The dissociation reactions show that these complexes are stable, and BeC(η4-E4)2 with C–Be bond dissociation energies (DE) close to the Zn–Zn bond DE in the synthesized Zn2(η5-C5Me5)2, may be synthesized in future experiments. Nucleus-independent chemical shift values suggest that the rings bonded to the donor atom have strong π aromaticity.

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.