Nuclear Independent Chemical Shift Values Research Articles

The Antiaromatic Nucleophilic Substitution Reaction (SNAAr) in Cycloheptatrienyl-Anion Containing Zwitterions with a Möbius-Aromatic Intermediate.

Antiaromatic nucleophilic substitution reactions in cycloheptatrienide pyridinium and phosphonium zwitterions with initial formation of a cycloheptatetraene intermediate are explored. The mechanism was supported by quantum chemical calculations, first-order reaction kinetics, and high-resolution mass spectrometry. The pyridinium zwitterion exhibited weak antiaromaticity, whereas the intermediate displayed Möbius aromaticity, as evidenced by nuclear independent chemical shift values and the shape of its HOMO. This study represents the eighth confirmed instance of a Möbius-aromatic organic species in its ground state.

DFT study of acceleration of electrocyclization in photochromes under radical cationic conditions: Comparison with recent experimental data

Radical cation formation is proposed for the rapid cyclization of 1, 2-bis[5-phenyl-2-methylthien-3-yl]cyclopentene and oligothiophene functionalized dimethyldihydropyrenes (DMDHP). Density functional theory calculations have been performed to rationalize the effect of a radical cation on the activation barrier of different classes of electrocyclic photochromes (DHP, dithienylethene, dihydroazulene and fulgide). For exact comparative analysis, the activation barrier of neutral (singlet) analogues at the same level of theory are also calculated. In addition, the concerted nature and aromaticity of transition states were investigated with the help of synchronicity (Sy.) and nuclear independent chemical shift values NICS(0) calculations, respectively, for both the radical cation and neutral systems. In case of the radical cation, thermal return of CPD to DHP, the activation barrier is very low (ΔH = 3.13 kcal mol−1, ΔG = 4.01 kcal mol−1) as compared to the neutral analogue (ΔH = 20.6 kcal mol−1, ΔG = 20.98 kcal mol−1), which is consistent with experimental observations. Similarly for dithenylethenes, radical cation formation has a large impact on the activation barrier (ΔH = 19.44 kcal mol−1, ΔG = 22.29 kcal mol−1). However, radical cation formation has almost negligible impact on the activation barrier of VHF-DHA and fulgide isomerization. The significant difference has been observed for synchronicity and NICS(0) values of all types of photochromes under radical cation conditions as compared to the neutral system.

Synthesis, Optical Properties, and Electronic Structures of Tetrakis(pentafluorophenyl)tetrathiaisophlorin Dioxide.

The synthesis, structure, optical and redox properties, and electronic structure of tetrakis(pentafluorophenyl)tetrathiaisophlorin dioxide (12) are reported. Oxidation of tetrakis(pentafluorophenyl)tetrathiaisophlorin (11) with dimethyldioxirane afforded the oxidized product, which was the tetrathiaisophlorin with two thiophene 1-oxide moieties (12). More significant nonplanarity and greater bond length alternation in 12 than those of 11 were observed by X-ray structural analysis. The absorption spectrum of 12 contains two bands at λ=348 and 276 nm, with a weak tail that extends to λ≈650 nm. Analysis of the magnetic circular dichroism spectrum of 12, based on Michl's 4N-perimeter model and molecular orbital calculations, indicate that the broad band at λ=348 nm appears to contain N2 and P2 bands, and 12 is classified as a 4nπ system, similar to 11. The nuclear-independent chemical shift values and (1) H NMR spectroscopy data indicate that 12 has more antiaromatic character than 11.

Nucleus-independent chemical shift profiles along the intrinsic distortion path for Jahn-Teller active molecules. Study on cyclopentadienyl radical and cobaltocene

The aromatic/antiaromatic behavior of the cyclopentadienyl anion (Cp-), bis(?5-cyclopentadienyl)iron(II) (Fe(Cp)2), as well as of the Jahn-Teller (JT) active cyclopentadienyl radical (Cp?) and bis(?5-cyclopentadienyl)cobalt(II) (Co(Cp)2) has been investigated using Density Functional Theory (DFT) calculations of the Nuclear Independent Chemical Shifts (NICS). According to the NICS values, pentagon ring in Fe(Cp)2 is more aromatic than isolated Cp-. The NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for Cp? and Co(Cp)2 showing antiaromaticity, which decreases with increasing deviation from high symmetry D5h to low symmetry (LS) C2v. Changes in the NICS values along the IDP revealed that Co(Cp)2 in the LS nuclear arrangement has aromatic character, in contrast to the case of Cp?

A bond, ring and cage resolved Poincaré–Hopf relationship for isomerisation reaction pathways

We introduce a new tool for reaction pathway analysis based on a chemical bonding density descriptor obtained from the higher resolution Poincaré–Hopf relation. To remove the degeneracy of the Poincaré–Hopf relationship for critical points of the electron density, we introduce the concepts of bonding cardinality and characterise the topology of rings and cages. This allows us to relate the changes in the relative energetic stability along an isomerisation reaction pathway with changes in the topology of the electron density. To demonstrate the utility of this approach, we consider the complete circuit of the reaction pathway connecting the various Möbius and Hückel structures in Au and/or Pd bis-metallated octaphyrins. Where the topological feature ‘cages’ exist in adjacent topologies, the degeneracy in the Poincaré–Hopf relationship is completely removed even for differences in relative energies of only 0.1 kcal/mol. Metallation of the Möbius and Hückel structures was found to increase the topological complexity and bonding density. Combining Pd and Au nuclei in the octaphyrin complexes reduced the smoothness of the potential energy surface. The presence of long, weak C–H contacts correlates in all cases with the lowest energy enantiomers and the most negative nuclear-independent chemical shift values of the reaction pathways in Au and/or Pd bis-metallated octaphyrins.

Magnetic criteria of aromaticity in a benzene cation and anion: how does the Jahn–Teller effect influence the aromaticity?

The aromatic/antiaromatic behavior of the Jahn–Teller (JT) active benzene cation and anion has been investigated using Density Functional Theory (DFT) calculations of Nuclear Independent Chemical Shifts (NICS) and magnetic susceptibility. NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for the benzene cation showing antiaromaticity which decreases with increasing deviation from D6h to D2h symmetry. Changes in NICS values along the IDP from D6h to C2v in the benzene anion revealed non-aromatic character.

Theoretical study on platinabenzene and mono- and difluorinated platinabenzenes: Structure, properties, and aromaticity

The electronic structures and properties of the platinabenzene and mono- and difluorinated platinabenzenes isomers have been investigated using hybrid density functional B3LYP theory. Basic measures of aromatic character were derived from structure, molecular orbital, and nuclear independent chemical shift (NICS). An energetic criterion suggests that ortho isomer of monofluorinated and F15 isomer of difluorinated platinabenzenes enjoy conspicuous stabilization. The polarizability and molecular orbital analysis are compatible with this result. NICS values calculated at several points above the ring center fail to give the result consistent with that based on relative energy, polarizability, and molecular orbital analysis. The atoms in molecules analysis indicates a correlation between NICS (1.0) and the electron density of the ring critical point (ρrcp) in monofluorinated platinabenzenes. There is a similar correlation in difluorinated platinabenzenes (except for F12 and F24 isomers) between NICS (0.5) and ρrcp.

Electronic properties of tricoordinated phosphorus in hexagonal phosphininium compounds and molecular aromaticity

Calculations with B3LYP within quantum chemical density functional theory have been carried out for 1-H-phosphininium cation and a series of 1-R-phosphininium molecules (R=cyclopentadiene, alpha and beta pyrroles, alpha and beta phosphole, C5BH5-- and CH2--). The negative nuclear-independent chemical shift values and the positive aromatic stabilization energies confirm that they are aromatic compounds. In particular, the 1-H-phosphininium cation even exhibits stronger aromatic character than the well-known aromatic phosphinine. The aromatic substituents have strong capability to attract electrons. It is the conjugation and aromaticity that keeps the stability and conformations of the molecules investigated. Owing to the perturbation of the aromatic substituted groups, the predicted large T values and the enlarged HOMO-LUMO gap of the phosphininium cation indicate that these compounds are expected in experiment.

Polyacene Spacers in Intramolecular Magnetic Coupling

We predict the intramolecular magnetic exchange coupling constant (J) for eleven nitronyl nitroxide diradicals (NN) with different linear and angular polyacene couplers from broken-symmetry density functional treatment. For the linear acene couplers, J initially decreases with increase in the number of fused rings. But from anthracene coupler onward, the J value increases with the number of benzenoid rings due to an increasing diradical character of the coupler moiety. The J value for the diradical with a fused bent coupler is always found to be smaller than that for a diradical with a linear coupler of the same size. The nuclear independent chemical shift (NICS) is calculated, and it is observed that the average of the NICS values per benzenoid ring in the diradical is less than that in the normal polyacene molecule. An empirical formula for the magnetic exchange coupling constant of a NN diradical with an aromatic spacer is obtained by combining the Wiberg bond order (BO), the angle of twist (phi) of the monoradical (NN) plane from the plane of the coupler, and the NICS values. A comparison of the formula with the computed values reveals that, from tetracene onward, the diradical nature of the linear acene couplers becomes prominent thereby leading to an increase in the ferromagnetic coupling constant. Isotropic hyperfine coupling constants are calculated by using a polarized continuum model for the diradicals in different solvents and in vacuum.

(π+σ)‐double aromatic and π,σ‐mixed aromatic boron compounds with two electrons delocalized over three centers

AbstractUsing Schleyer's measure for aromaticity, computed nuclear independent chemical shifts (NICS), we review molecules with two electrons delocalized cyclically over three centers: one class with two π‐ as well as two σ‐electrons, i.e. (π + σ)‐double aromatics and a second class with two electrons intermediate between π and σ, i.e. π,σ‐mixed aromatics. These classes show distinctly different patterns of NICS values. On this basis, sila‐, germa‐, and stannaboriranes, recently presented by Jemmis et al., are shown to be the first π,σ‐mixed aromatics which are not homoaromatics. “Improvement of overlap by pyramidalization” a building principle introduced by Haddon in 1988, is the key to understanding the structure of π,σ‐mixed aromatics. New representatives with large aromatic stabilization energies (up to 60 kcal mol−1) and very strong pyramidalizations at tricoordinate B, C, and Si centers (up to 63°) are designed by computations. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:224–237, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20225

Density functional study of potential energy surfaces and relative stabilities of halonium cations of ethylene and cyclopentenes

Potential energy surfaces, hydride affinities and nuclear independent chemical shifts (NICS) of a variety of halonium cations of ethylene C 2H 4X +, cyclopentene C 5H 8X + and hydroxy substituted cyclopentenes C 5H 8− n (OH) n X + ( n=1, 2), where X=Cl and Br were computed at the Becke3LYP/6-311++G(d,p) level of theory. The potential energy surfaces of all molecules under investigation have been scanned and the equilibrium geometries and their harmonic vibrational frequencies have been calculated. The computed hydride affinities of all conformers, as well as the NICS values for the 1,2-bridged cations indicate that the bromo cations are more stable than the analogous chloro cations.