Nuclear magnetic resonance spectroscopy of annulenes

Haddon, R. C.; Haddon, Virginia R.; Jackman, L. M.

doi:10.1007/bfb0051390

Cited by 73 publications

(33 citation statements)

References 208 publications

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“…[36] The magnetic interpretation of aromaticity relates to the fact that an external magnetic field induces diatropic p ring currents resulting in a local shielding of the field. [37] The simple connection between ring currents and aromaticity was first suggested by Elvidge and Jackman in the early 1960s. [38,39] This relationship was extensively studied in the seventies and eighties.…”

Section: Introductionmentioning

confidence: 99%

Aromaticity of Giant Polycyclic Aromatic Hydrocarbons with Hollow Sites: Super Ring Currents in Super‐Rings

Hajgató

Deleuze

Ohno

2006

Chemistry A European J

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We present a systematic theoretical study based on semi-empirical, Hartree-Fock (HF), and density functional theory (DFT) models of a series of polycyclic aromatic hydrocarbons (PAHs) that exhibit hollow sites. In this study we focus particularly on the magnetic criteria of aromaticity, namely (1)H NMR and nucleus-independent chemical shifts (NICS), and on their relationships with other electronic properties. The computed shifts and NICS indices indicate that an external magnetic field induces exceptionally strong ring currents in even-layered PAH doughnuts, in particular in the layer directly adjacent to the central hole of double-layered compounds. These exceptionally strong ring currents also correlate with particularly small HOMO-LUMO gaps and electronic excitation energies and to abnormally high polarizabilities, indicating in turn that these compounds have a more pronounced metallic character. Comparison is made with further depictions of aromaticity in these systems and in [18]-[66]annulene rings by employing topological, structural, and energetic criteria.

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Section: Introductionmentioning

confidence: 99%

Aromaticity of Giant Polycyclic Aromatic Hydrocarbons with Hollow Sites: Super Ring Currents in Super‐Rings

Hajgató

Deleuze

Ohno

2006

Chemistry A European J

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“…[9] Much work has been carried out to correlate the scheme of ring currents with the notion of aromaticity, [7,8] and the fundamental concepts that have emerged are now so widely accepted that they are found in most textbooks that deal with organic chemistry or NMR spectroscopy. [10,11] Whereas most aromatic compounds possess cyclic hydrocarbon frameworks and electron delocalization takes place through p-p p-orbital interactions, recently there have been efforts to examine species that contain noncarbon atoms to gain additional insight into the nature of aromaticity. For instance, a group of cyclic M 4 2À (M = Al, and so forth) metal clusters has been reported to be aromatic due to delocalized s and p electrons in the square core.…”

Section: Introductionmentioning

confidence: 99%

Experimental and Theoretical Evidence of Aromatic Behavior in Heterobenzene‐Like Molecules with Metal–Metal Multiple Bonds

Fang

Tan

et al. 2011

Chemistry A European J

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Binding two quadruply bonded dimolybdenum units [Mo(2)(DAniF)(3)](+) (DAniF=N,N'-di-p-anisylformamidinate) with two chalcogen atoms generated two molecules with a central core composed of a cyclic six-membered [Mo(2)](2)(μ-EH)(2) species (E=S in 1 and O in 3, and [Mo(2)] is a quadruple-bonded [Mo(2)(formamidinate)(3)] unit). Aerobic oxidation of 1 and 3 followed by concomitant deprotonation gave rise to the corresponding [Mo(2)](2)(μ-E)(2) compounds 2 and 4. The latter show a striking coplanarity and near-bond equalization of the Mo/E cluster. The oxidized species 2 and 4 are diamagnetic in the measured temperature range of 5 to 300 K, which is somewhat unexpected for molecules that have dimetal units with a σ(2)π(4)δ(1) electronic configuration. This suggests there are strong interactions between the dimolybdenum units through the E atoms. The large electronic delocalization of the δ electrons over the entire Mo/E core is supported by the exceptionally large potential separation for the two successive one-electron reductions of the linked Mo(2)(5+) units from the oxidized species (ΔE(½)=1.7 V for the sulfur analogue). This large electronic delocalization has an important effect on the NMR spectroscopic signals for the two sets of methine (N-(CH)-N) protons from the DAniF ligands. Those essentially parallel to the core, H(∥), and those essentially perpendicular to the core, H(⊥), exhibit downfield and upfield chemical shifts, respectively, that are separated by δ=1.32 ppm. The structural, electronic, magnetic, and chemical behaviors for 2 and 4 are consistent with aromaticity, with the [Mo(2)E(2)Mo(2)] cores that resemble the prototypical benzene molecule. Theoretical studies, including DFT calculations, natural bond orbital (NBO) analyses, and gauge-independent atomic orbital (GIAO) NMR spectroscopic calculations, are also consistent with the aromaticity of the [Mo(2)](2)(μ-E)(2) units being promoted by d(δ)(Mo(2))-p(π)(E) π conjugation. The cyclic π conjugation of the central moiety in 2 and 4 involves a total of six electrons with 2e from δ(Mo(2)) and 4e from p(π)(E) orbitals, thereby conforming to Hückel's rule when electrons in the MOs with δ character are considered part of the delocalized system.

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“…The concept allows for both even-and odd-numbered cycles. A great number of reviews or lecture reports on annul~n~ in their diversity of forms are available; here we give reference to only some of them: Sondheimer (1963;1971); Haddon et al (1971); Sondheimer (1972;1974); Mullen (1984).…”

Section: Annul~n~mentioning

confidence: 99%