Nucleus-independent Chemical Shift Values Research Articles

Novel Beltlike and Tubular Structures of Boron and Carbon Clusters Containing the Planar Tetracoordinate Carbon: A Theoretical Study of (C3B2)nH4 (n = 2−6) and (C3B2)n (n = 4−8)

By use of the most stable structure of C3B2H4 as a basic block, we construct two types of novel compounds containing the planar tetracoordinate carbon (ptC), beltlike compounds (C3B2)nH4 (n = 2−6) and tubular compounds (C3B2)n (n = 4−8). Equilibrium geometries, vibrational frequencies, and electronic spectra of both series of compounds have been determined by the density functional theory. The results reveal that the same direction arrangement of C2CB2 units with ptC will result in the most stable isomers of (C3B2)nH4 (n = 2−6). Predicted nucleus-independent chemical shift values show that the three-membered rings have aromaticity, whereas the six-membered rings have antiaromaticity. Calculations show that these compounds containing ptC atoms exhibit remarkable size dependence of their highest-occupied molecular orbital−lowest-unoccupied molecular orbital energy gaps and the first excitation energies.

Stability, Reactivity, and Aromaticity of Compounds of a Multivalent Superatom

In this article, we analyze the stability, reactivity, and possible aromatic behavior of two recently reported clusters (Reveles, J. U.; Khanna, S. N.; Roach, P. J.; Castleman, A. W., Jr. Proc. Natl. Acad. Sci. 2006, 103, 18405), viz., Al(7)C(-) and Al(7)O(-) in the light of the principles of the maximum hardness and minimum electrophilicity as well as the nucleus-independent chemical shift values. Stability of these clusters in the context of addition/removal of an electron or an Al atom is now clearly understood.

Is Nucleus-Independent Chemical Shift Scan a Reliable Aromaticity Index for Planar Heteroatomic Ring Systems?

Density functional theoretical investigation has been performed to explore the reliability of the nucleus-independent chemical shift (NICS) scheme in assessing aromatic behavior of some planar six-membered heteroatomic systems. It has been observed that the NICS scan and the diamagnetic and paramagnetic contributions of the in-plane and out-of-plane components are quite reliable in assessing any aromatic or antiaromatic behavior in borazine. However, for boraphosphabenzene, the aromatic stabilization energy is too small to consider it as an aromatic system but the NICS scans and the homodesmotic reactions suggest an opposite trend. Interestingly, in the case of alumazene, a very shallow minimum is observed for the out-of-plane component, which suggests the presence of weak diamagnetic ring current. However, the diamagnetic and paramagnetic contribution curves to the out-of-plane component for alumazene clearly reveal a net paramagnetic contribution. Thus we may surmise that apart from the single NICS value, the NICS scan also is not a very authentic tool for the assessment of aromaticity of planar six-membered heteroatomic systems.

DOUBLE FJORD MOTIF IN OVERCROWDED LARGE POLYCYCLIC AROMATIC HYDROCARBONS: THE CONFORMATIONAL SPACE OF HEXABENZO[a,cd,f,j,lm,o]PERYLENE

The overcrowded large polycyclic aromatic hydrocarbon (LPAH) hexabenzo[a,cd,f,j,lm,o]perylene (HBP) was subjected to a theoretical study. The ab-initio Hartree-Fock (HF) and the density functional theory (DFT) B3LYP methods were employed to calculate energies and geometries of the stationary point conformations of HBP. The global minimum was found to be the twisted t-D 2 ; the local minimum anti-folded a-C 2h was 20.5 kJ/mol higher in energy (B3LYP/6-31G(d)). Thus, only t-D 2 -HBP exists at room temperature in solution. The enantiomerization of HBP proceeds via a pathway that connects the chiral t-D 2 to the achiral a-C 2h through a chiral twisted-folded transition state, tf-C 1 . The energy barrier for enantiomerization was 134.4 kJ/mol. The central benzene ring F of t -D 2 is highly distorted from planarity (torsion angles of 15.4° and −30.5°), and has a remarkable resemblance to the twist angle of the central ene in the parent BAE bifluorenylidene (32°). Yet, nucleus-independent chemical shift (NICS) values, calculated at GIAO-B3LYP/6-31G(d)//B3LYP/6-31G(d) and the almost equal bond lengths (ca. 142 pm) of the central ring are indicative of the aromatic character of this ring. The NICS values are also indicative of the aromatic character of the peripheral rings and the central rings of the two minima and the transition state, consistent with the Clar picture.

Evidence for d-Orbital Aromaticity in Sn- and Pb-Based Clusters: Is Sn122- Aromatic?

The electronic structures and stabilities of pure M(12)- and M(12)(2-) were systematically investigated within density functional theory. The nucleus-independent chemical shifts (NICSs) of I(h) Sn(12)(2-) and Pb(12)(2-) are -5.0 and -20.7 ppm, respectively, based on B3LYP/aug-cc-pVDZ-PP predictions, whereas the NICS of Sn(12)(2-) is predicted to be 1.1 ppm by B3LYP/LanL2DZ. A startling conclusion is that the NICS4d of Sn(12)(2-) and NICS(5d) of Pb(12)(2-) are -5.0 and -7.5 ppm, respectively, suggesting the significant contribution of the inner d orbitals to the total NICS values. This provides the first quantitative evidence for the existence of "d-orbital aromaticity" in Sn- and Pb-based clusters with three-dimensional structures. The d orbitals also contribute to the total NICSs of the K-coordinated clusters. The NICS predictions suggest that larger basis sets including d-orbitals are needed to analyze the aromaticity of some main-group-metal-based clusters (e.g., Sn- and Pb-based clusters) to obtain accurate predictions.

Are Chelate Rings Aromatic? Calculations of Magnetic Properties of Acetylacetonato and o-Benzoquinonediimine Chelate Rings

The aromaticity of the chelate rings of acetylacetonato (acac) and o-benzoquinonediimine (bqdi) ligands was investigated theoretically by calculating nucleus-independent chemical shifts (NICS). The calculations were done for the complexes with various metals and various other ligands. The results show that acac chelate rings in none of the complexes satisfy this magnetic criterion for aromaticity. According to the results for bqdi chelate rings, there is only the Ru2+-bqdi chelate ring with large negative NICS values, indicating possible aromaticity by magnetic criterion.

Multidimensionality of delocalization indices and nucleus independent chemical shifts in polycyclic aromatic hydrocarbons

The aromaticity and local-aromaticity of a large set of polycyclic aromatic hydrocarbons (PAHs) is studied using multicenter delocalization indices from generalized population analysis and the popular nucleus independent chemical shift (NICS) index. A method for the fast computation of the NICS values is introduced, using the so-called pseudo-pi-method. A detailed examination is made of the multidimensional nature of aromaticity. The lack of a good correlation between the NICS and the multicenter delocalization indices is reported and the grounds discussed. It is shown through a thorough statistical analysis that the NICS values arise not only from local aromaticity of the benzenoid rings, but also from other circuits. It is shown that the NICS indices do not reveal the individual aromatic nature of a specific ring, contrary to the delocalization indices.

Experimental and Theoretical Investigations into the Paratropic Ring Current of a Porphyrin Sheet

Anomalous induced magnetic effects were observed in a directly fused square-planar porphyrin sheet 1, in that the protons above the center of the tetraporphyrin core were characteristically shifted downfield in the 1H NMR spectrum. These observations suggest a rare paratropic ring-current effect around the planar cyclooctatetraene (COT) core of 1. To examine the spatial distribution of the induced magnetic effect, face-to-face dimeric complexes of porphyrin sheet 1 with bipyridyl-type guest molecules (G1-G3) were prepared, which provided complexation-induced shifts (CISs) of the guest molecules as a neat experimental guide to the distance dependence of the induced magnetic effects in 1. Nucleus-independent chemical shift (NICS) values of 1 were calculated by varying the distance of the probe from the plane of 1. Whereas a simple bell-type profile was estimated for the complex (1)2-(G1)4, the distance profiles of the CIS became increasingly flat for (1)2-(G2)4 and (1)2-(G3)4. Finally, we investigated the paratropic ring-current effect just above the COT core of the complex 1-(G4)2, which agrees well with the theoretically estimated distance-dependent induced magnetic effect. Consequently, both experimental and theoretical studies on the complexes of porphyrin sheets with guest molecules revealed for the first time a unique distance dependence of the paratropic ring current.

Structures, Stabilities, and Electronic and Optical Properties of C58Fullerene Isomers, Ions, and Metallofullerenes

The 1205 classical isomers of fullerene C58, as well as one quasi-fullerene C58 isomer with a heptagonal ring (labeled as Cs:hept) have been investigated by the quantum chemical methods PM3, HCTH/3-21G, and B3LYP/6-31G(d). Isomer C3v:0001, which has the lowest number of adjacent pentagons, is predicted to be the most stable isomer, but the quasi-fullerene isomer Cs:hept is only 2.50 kcal mol-1 higher in energy. Systematic investigations of the electronic properties of C3v:0001 and Cs:hept find that the C3v:0001 isomer has high vertical electron affinity (3.19 eV). The nucleus-independent chemical shifts (NICS) value at the center of Cs:hept (-5.1 ppm) is more negative than that of C60 (-2.8 ppm). The NICS value at the center of the heptagonal ring in Cs:hept (-2.5 ppm) indicates weakly aromatic character. In contrast, the C58(6-) and C58(8-) ions of the C3v:0001 and Cs:hept geometries possess large aromatic character, with NICS values between -14.0 and -26.2 ppm. To clarify the thermodynamic stabilities of C58 isomers at different temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the B3LYP/6-31G(d) level. The C3v:0001 isomer prevails in a wide range of temperatures, and the Cs:hept isomer is also an important component around 2800 K. The IR spectra of C58 isomers are simulated to facilitate experimental identification of different isomers. In addition, the electronic spectra and the second-order hyperpolarizabilities are predicted by ZINDO and the sum-over-states model. The static second-order hyperpolarizability of the C3v:0001 isomer is 96.5 % larger than that of C60, and its second-order hyperpolarizabilities at external field frequencies are at least nine times larger than those of C60.

Aromaticity in Polyacene Analogues of Inorganic Ring Compounds

The aromaticity in the polyacene analogues of several inorganic ring compounds (BN-acenes, CN-acenes, AlN-acenes, BO-acenes, BS-acenes, and Na6-acenes) is reported here for the first time. Conceptual density functional theory-based reactivity descriptors and the nucleus-independent chemical shift (NICS) values are used in this analysis. The nature of the site selectivity is understood through the charges and the philicities.

Observable Azacyclobutenone Ylides with Antiaromatic Character from 2-Diazoacetyl-azaaromatics

Azacyclobutenone ylides 2 and 11 were generated in solution by laser flash photolysis of 2-diazoacetylpyridine (1) and 3-diazoacetylpyridazine (10), respectively, together with the corresponding ketenes. The ylides were identified by their characteristic IR and UV spectra: 2, nu (CH3CN) 1725 cm(-1), lambdamax 360 and 550 (br) nm; 11, nu (CH3CN) 1776 cm(-1), lambdamax 370 and 550 (br) nm. 2-Triisopropylsilyldiazoacetylpyridine 20 upon photolysis at 5 degrees C in CH3CN forms the ylide 21 as a rather persistent (T1/2 2 h at 25 degrees C) purple solution, nu (CH3CN) 1718 cm(-1), lambdamax 245, 378 and 546 (br) nm, but no ketene is observed. Quinolyl ylide 14 and pyridyl ylides 17 and 19 with Me and 2-pyridyl substituents, respectively, with characteristic IR and UV spectra were also generated. The 1H NMR spectrum of the pyridyl ring of 21 shows substantial upfield shifts relative to those of 20. Calculated nucleus-independent chemical shifts (NICS) for 2, 11, and 21 are comparable to those for benzocyclobutadiene (22) and benzocyclobutenone enolate (23), with substantial positive values for the 4-membered rings, while the NICS values for the 6-membered rings are significantly more positive than for benzene or pyridine. Significant bond alternation is also found in the calculated ylide structures, and these results suggest strong antiaromatic character for the 4-membered rings of 2, 11, 14, 17, 19, and 21, and greatly reduced aromatic character for the 6-membered rings.

Most stable structure of fullerene[20] and its novel activity toward addition of alkene: A theoretical study

Structures and stabilities of fullerene C20 and C20- have been investigated by the density functional theory and CCSD(T) calculations. In consideration of the Jahn-Teller distortion of Ih-symmetric C20, possible subgroup symmetries have been used in the full geometry optimization. On the basis of relative energetics, vibrational analyses, and electron affinities, fullerenes C20 and C20- have most stable D2h and Ci structures, respectively. The controversy on the relative stability of fullerene[20] arises from the use of different subgroups in calculation and the basis set dependence in vibrational analysis. Predicted nucleus-independent chemical shift values show that the most stable fullerene C20 and its derivatives C20(C2H2)n and C20(C2H4)n (n=1-3) exhibit remarkable aromaticity, while C20(C2H2)4 and C20(C2H4)4 have no spherical aromaticity. The C20 (D2h) cage has remarkable activity toward the addition of olefin, and such feasibility of the addition reaction is ascribed to strong bonding interactions among frontier molecular orbitals from C20 and olefin. Calculations indicate that both C20(C2H2)n and C20(C2H4)n have similar features in electronic spectra.

Theoretical study of Borazanaphthalene and itsmono-Fluorinated derivatives: structure and properties

The structure and properties of borazanaphthalene and mono-fluoroborazanaphthalene derivatives have been investigated using ab initio calculations. The aromaticity of the species has been studied by using nucleus independent chemical shift (NICS) val

Implications of Molecular Orbital Symmetries and Energies for the Electron Delocalization of Inorganic Clusters

Isostructural clusters exhibit contrasting magnetic properties when the number of electrons differs. Surprisingly, the same is true even for isoelectronic cages (e.g. O(h) B6H6(2-) is diatropic, whereas O(h) Si6(2-) is paratropic) or for those with different substitutents (e.g. T(d) B4H4 is paratropic, whereas T(d) B4F4 is diatropic). Indeed, the total nucleus-independent chemical shift (NICS) values, based on shieldings computed at cluster centers, may range considerably in magnitude and even change from diatropic (up-field shifted) to paratropic (down-field shifted). Similarly, individual dissected canonical molecular orbital contributions to the total NICS values computed at the "gauge-including atomic orbitals" (GIAO) level vary greatly. This contrasting behavior arises from molecular orbital energy differences, from the extent of orbital overlap, as well as from symmetry-based selection rules derived from group theory. Differences in magnetic properties may originate from the symmetry of the orbitals; specifically from the forbidden nature of the highest occupied molecular orbital --> lowest unoccupied molecular orbital (HOMO --> LUMO) electronic excitation weighted by the occupied-unoccupied orbital energy difference. Thus, HOMO-NICS values are generally highly paratropic if the HOMO --> LUMO rotational transition is allowed by symmetry selection rules.

Aromatic C20F20 cage and its endohedral complexes X@C20F20 (X = H−, F−, Cl−, Br−, H, He)

The structure and stability of endohedral X@C20F20 complexes (X = H-, F-, Cl-, Br-, H, He) have been computed at the B3LYP level of theory. All complexes in I (h) symmetry were found to be energy minimum structures. H-@C20F20 and F-@C20F20 complexes have negative inclusion energies, while other complexes have positive inclusion energies. Similarity between C20F20 and C20H20 has been found for X = H and He. On the basis of the computed nucleus independent chemical shift values at the cage center, both C20F20 and C20F20 are aromatic.

Theoretical Studies on Graphyne Substructures: Geometry, Aromaticity, and Electronic Properties of the Multiply Fused Dehydrobenzo[12]annulenes

The geometries of multiply fused dehydrobenzo[12]annulenes [12]DBAs 2-7 with various topologies, which are considered as graphyne fragments, have been optimized at the B3LYP/6-31G* level of theory. Most of the optimized geometries of fused DBAs have planar structures excluding a boomerang-shaped bisDBA 4, a trefoil-shaped trisDBA 6, and a wheel-shaped DBA 7. For the boomerang-shaped bisDBA 4 and the trefoil-shaped trisDBA 6, distortions originate from the steric repulsion between hydrogen atoms attached to adjacent benzene rings. The harmonic oscillator model of aromaticity (HOMA) values at the central benzene ring of multiply fused DBAs decrease as the number of fused 12-membered rings increases except for the closely related structures 4 and 5 and 6 and 7, because of bond length elongation due to conjugation with the phenylethynyl groups. Nucleus-independent chemical shifts (NICS) were computed at the individual ring centers of the fused DBAs. The fusion of the antiaromatic 12-membered rings results in increasing (more positive) NICS values at the central benzene ring, indicating the decrease of diatropic ring currents. Furthermore, HOMO-LUMO gaps of the DBAs 2-7 are strongly influenced by the molecular topology. The para-conjugation pathway of the bis(phenylethynyl)benzene unit plays a more important role in the determination of the electronic properties of multiply fused DBAs than the meta- and ortho-conjugation pathways.

Structure and properties of polycoordinate planar boron compounds

Polycoordinate planar B compounds BX(n) (X=B, Al, C, N and Si; n=3-8) are optimized at B3LYP/6-311++G (3df,p) theoretical level. For X=B, center B atom can coordinate three to eight atoms, while for X=Al, C, Si, and N, it can only coordinate three to five atoms. The natural bond orbital analysis shows that the center B atom does not violate the octet rule, though the numbers of coordinated atom even reach 8. According to molecular orbital analysis and nucleus independent chemical shift value calculation, it seems that these polycoordinate planar B compounds BX(n) (X=B, Al, C, N, and Si; n=3-8) hold twofold (alpha and pi) aromatic, which play an important role in their stability and keeping all atoms in one plane.

First-principles studies ofAlPbnandAlPbn+clusters(n=1–12): Search for Al-doped clusters with large stabilities

A first-principles density functional theory based B3LYP (Becke's hybrid three-parameter exchange functional with the LYP correlation functional) method with aug-cc-pVDZ(-PP) [Dunning's augumented correlation consistent valence double zeta with polarization function on Al and pseudopotential for Pb] basis set has been used to search the most stable structures and study the electronic properties of neutral $\mathrm{Al}{\mathrm{Pb}}_{n}$ and cationic $\mathrm{Al}\mathrm{Pb}_{n}{}^{+}$ clusters $(n=1--12)$. The evolution of energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, binding energies, and vertical ionization potentials of these clusters have been investigated. Several cationic clusters, such as $\mathrm{Al}\mathrm{Pb}_{9}{}^{+}$, $\mathrm{Al}\mathrm{Pb}_{10}{}^{+}$, and $\mathrm{Al}\mathrm{Pb}_{12}{}^{+}$ with enhanced stability have been identified. Molecular orbital analyses reveal that the special stability of ${D}_{4d}$ $\mathrm{Al}\mathrm{Pb}_{10}{}^{+}$ with 42 valence electrons may arise from the closed-shell nature of the $\ensuremath{\pi}$ subsystem, which is subjected to the $2{({N}_{\ensuremath{\pi}}+1)}^{2}$ rule with ${N}_{\ensuremath{\pi}}=1$. The large nucleus-independent chemical shift (NICS) values indeed demonstrate the high stability of the cationic $\mathrm{Al}\mathrm{Pb}_{10}{}^{+}$. Meanwhile, the big cavity and large NICS values of isoelectronic ${D}_{4d}$ $\mathrm{Pb}_{10}{}^{2\ensuremath{-}}$ suggest that it can encapsulate other appropriate metal atoms successfully. As to the ${C}_{3v}$ $\mathrm{Al}\mathrm{Pb}_{9}{}^{+}$ cluster, its large stability may also arise from the closed-shell nature of the $\ensuremath{\pi}$ subsystem with 8 $\ensuremath{\pi}$ electrons, further confirmed by the large NICS values. Additionally, the nine-atom Pb clusters $\mathrm{Pb}_{9}{}^{2\ensuremath{-}}$, $\mathrm{Pb}_{9}{}^{3\ensuremath{-}}$, and $\mathrm{Pb}_{9}{}^{4\ensuremath{-}}$ possess strong aromatic character. Especially, the ${D}_{3h}$ $\mathrm{Pb}_{9}{}^{4\ensuremath{-}}$ cluster exhibits double spherically aromatic character and the largest NICS value at the center of the cage. These pure Pb clusters with large cavities and their encapsulating clusters should be good candidates for making new cluster-assembled nanostructured materials.

Origin of the Nonplanarity of Tetrafluoro Cyclobutadiene, C4F4

Density functional theory as well as Møller-Plesset investigations has been carried out on tetrafluoro cyclobutadiene, C4F4, to explore the origin of its nonplanarity. Although Petersson et al. (Petersson, E. J.; Fanuele, J. C.; Nimlos, M. R.; Lemal, D. M.; Ellison, G. B.; Radziszewski, J. G. J. Am. Chem. Soc. 1997, 119, 11122-11123) had earlier predicted a nonplanar geometry of this compound on the basis of spectral and bond orbital analysis, the explanation of the same from a more fundamental point of view is still missing. In the present study, we provide a heuristic explanation for the origin of nonplanarity of C4F4. The two major driving forces behind this nonplanar geometry are the unusual aromaticity of this cyclic homoatomic 4pi electron system and the second-order Jahn-Teller effect (SOJTE). These driving forces can well be explained by various energy and density parameters and also by nucleus-independent chemical shift (NICS) values. Aromaticity of a cyclic homoatomic 4pi electron system is quite remarkable. The enhancement of pi- delocalization as evidenced from molecular orbital analysis may be attributed to s-ppi mixing in nonplanar C4F4.

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.