Diamagnetic Ring Current Research Articles

1H NMR Study of Mixed Micellization of Sodium Dodecyl Sulfate and Triton X-100

The two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY), 1H NMR chemical shift, and spin–lattice and spin–spin relaxation measurements of aqueous solution of various molar ratios of sodium dodecyl sulfate (SDS) to Triton X-100 (TX-100) from 0.27 to 8.0 give some detailed structure of the mixed micelles formed. The appearance of cross peaks between the protons of SDS and those of TX-100 in the 2D NOESY map is direct evidence for the solubilization of the hydrocarbon chains of SDS inside the TX-100 micelles that have nonextended hydrocarbon chains. The effect of the aromatic ring currents of TX-100 on the protons of the methylene groups, which are nearest to the polar head of SDS, indicates the near vicinity of these groups. The existence of the cross peaks between the hydrophilic protons and the remote hydrophobic protons of TX-100 suggests that the hydrophilic chains are not extended in the mixed micelles. 1H spin–lattice relaxation time (T1) and spin–spin relaxation time (T2) measurements support the results obtained by the 2D NOESY experiments.

Unconventional ring currents in an `all-metal aromatic', Al 42−

The square planar anion Al 4 2− supports an unconventional diamagnetic ring current which originates in the σ system, has negligible contribution from the two-electron π system, and survives in pyramidal bimetallic clusters MAl 4 − (M=Li, Na, Cu).

Conjugated macrocycles related to the porphyrins. Part 18: Synthesis and spectroscopic characterization of electron-rich benzi- and oxybenziporphyrins: influence of steric and electronic factors on porphyrinoid aromaticity

Although 4,6-dihydroxyisophthalaldehydes failed to condense with tripyrranes to produce porphyrinoid products, the related dimethoxydialdehydes reacted with tripyrrane 9 to give, following oxidation with aqueous ferric chloride solution, nonaromatic dimethoxybenziporphyrins 20 in excellent yields. In the presence of TFA, dications were generated that showed weakly diatropic properties by proton NMR spectroscopy. This observation was attributed to the ability of the methoxy units to facilitate charge delocalization. Treatment of 20 with 50 equiv. of boron tribromide cleaved one of the methyl ethers to produce the related methoxyoxybenziporphyrins 24. These porphyrinoids showed strong diamagnetic ring currents due to the presence of 18 π-electron delocalization pathways within these structures. On treatment with TFA, the resulting dications show significantly reduced ring currents although their diatropic character is retained to a far greater extent than when the methoxy substituents are absent. For both 20 and 24, the methoxy groups are more effective at charge delocalization, which results in increased diatropicity, when there is no adjacent alkyl substituent. Treatment of 20a with HBr in refluxing acetic acid cleaved both methyl ethers to give the highly insoluble hydroxyoxybenziporphyrin 17. Although the spectroscopic properties of the free base could not be assessed, this compound was soluble in TFA–chloroform and the resulting dication proved to be highly aromatic as judged by NMR and UV–vis spectroscopy. Treatment of the 20b with HBr–acetic acid again cleaved both methyl ethers but also gave an unexpected oxidation to give a diketone bearing a 3° alcohol unit. This novel porphyrinoid retains an 18 π-delocalization pathway, shows porphyrin-like UV–vis spectra and is highly diatropic as the free base and in protonated form as judged by proton NMR spectroscopy.

The cyclooctatetraenoquinones, a computational study

Semiempirical AM1, ab initio HF/3-21G, and DFT pBP/DN∗ calculations were performed on 3,5,7-cyclooctatriene-1,2-dione (1,2-cyclooctatetraenoquinone) and 2,5,7-cyclooctatriene-1,4-dione (1,4-cyclooctatetraenoquinone). NICS (nucleus-independent chemical shift) calculations on the planar conformations (which are potential energy surface hilltops) showed that these conformations, and by implication the actual, nonplanar, conformations of the diones, are not aromatic (do not sustain a diamagnetic ring current). The diprotonated diones in their planar conformations (potential energy surface hilltops) are evidently slightly aromatic, leaving open the possibility that the diprotonated, nonplanar diones may sustain a weak diamagnetic ring current. The E,Z,E-configurational isomer of the 1,2-dione was calculated to lie 191 or 159kJmol−1 (Cs and C2 conformers, respectively; pBP/DN∗) above the all-cis isomer. The orbital symmetry accessible (Z)-bicyclo[4.2.1]octa-2,4-diene-7,8-dione was calculated (pBP/DN∗) to lie 18.4kJmol−1 below the monocyclic 1,2-dione, and the acyclic diketene (3Z,5Z)-1,3,5,7-octatetraene-1,8-dione 10.1kJmol−1 above. (E)-bicyclo[4.2.0]octa-3,7-diene-2,5-dione, formed by conrotatory ring closure of the 1,4-dione, lies (pBP/DN∗) 75.4kJmol−1 above the monocyclic dione. Syn-and anti-tricyclo[5.1.0.01,7]oct-5-ene-3,8-dione, potentially accessible from the 1,4-dione by intramolecular Diels-Alder reactions, lie more than 100kJmol−1 (pBP/DN∗) above the 1,4-dione.

The use of complexation induced proton NMR chemical shifts for structural analysis of host-guest complexes in solution

Proton shielding variations in supramolecular complexes contain a wealth of information on complex geometries in solution that has been until now mostly neglected. We describe herein ways for such analyses with five cyclophane and two cyclodextrin complexes in water, by using a program SHIFT which is based on and parametrized with the analyses of over 300 intramolecular proton shift variations in well defined molecular frameworks such as steroids or cyclophanes. The intermolecular shift changes in the host-guest complexes at 100% complexation (CIS values) are calculated as sum of anisotropy effects deltachi from aromatic ring currents and linear electric field effects LEF, based on force field generated geometries. The conformations with the best agreement between calculated and observed CIS values are at least for noncharged guest compounds close to those obtained from molecular mechanics and/or MD calculations and intermolecular NOEs (where available), noticeably without adjusting the complex geometries to the experimental CIS. Through-space electrostatic field effects LEF, which have been until now often neglected, can be sizeable also for noncharged systems; best agreement between experiment and calculation is observed with Gasteiger atomic charges.

Ring currents in six-membered heterocycles: the diazaborinines (CH)2B2N2

Six of the eleven possible cyclic isomers (CH)2B2N2 are found, by geometric optimization at a correlated level of theory, to have planar closed-shell ground states. Current density maps calculated with distributed-origin coupled Hartree–Fock theory wavefunctions show that all six of these have delocalized π-electron systems supporting diamagnetic ring currents. In comparison with benzene at the same level of theory, the isomer with lowest energy, 1,3,2,4 diazadiborinine, has a 5% greater centre of ring shielding, but 20–25% smaller mean magnetizability, mean polarizability and polarizability anisotropy.

Synthesis, Properties and Optical Resolution of rac-[n](1,6)Heptalenophanes

[n](1,6)Heptalenophanes, rac-4a–c, having a five, six or seven membered methylene bridge, respectively, across the 12 π-perimeter were synthesized from rac-1,6-dimethylheptalene (1) in two steps. Detailed NMR and UV/vis studies of the new phanes reveal their bridged heptalene moiety to be more twisted than that of the parent heptalene system. Furthermore, the reduction of the [5](1,6)heptalenophane (4a) with lithium to its aromatic dianion 10 with 14 π-electrons is presented. NMR spectra of the latter show that its perimeter, although remarkable bent out-of-plane, still exhibits a considerable aromatic ring current. Liquid chromatography with triacetyl cellulose as stationary phase resulted in the optical resolution of racemic [5](1,6)heptalenophane (4a).

Aromatic Ring Currents at a Protein Surface: Use of 1H-NMR Chemical Shifts to Refine the Structure of a Naked β Sheet

The naked β sheet, a newly recognized motif of protein structure, exhibits ordered surfaces in the absence of a conventional hydrophobic core. A model is provided by an archaeal Zn ribbon homologous to eukaryotic RNA polymerase II subunit 9 (RPB9). This subunit, which regulates transcriptional start-site selection and downstream pausing, contains Zn2+-binding motifs similar to those of general transcription factors TFIIB and TFIIS. Interestingly, distance-geometry yields two models of the archaeal Zn ribbon differing in the orientation of a conserved tyrosine side chain on the well-ordered surface of the naked β-sheet. The models are equally consistent with conventional restraints and otherwise contain indistinguishable structural features, including a tetrahedral Cys4 Zn2+-binding sites, four antiparallel β-strands, and disordered loop. Due to the change in tyrosine orientation and correlated changes in the configuration of neighboring side chains, the two models predict inequivalent patterns of aromatic ring-current shifts. The observed secondary shifts of adjoining resonances are shown to be consistent with one model but not the other. In the consistent model the surface of the β-sheet contains successive aromatic edge-to-face contacts in accord with semi-classical and ab initio potentials. We speculate that the aromatic-rich surface of the hyperthermophilic RPB9 domain contributes its thermodynamic stability and provides a nucleic-acid-binding site in the eukaryotic and archaeal transcriptional machinery. The present study demonstrates how the reduced dimensionality of a surface can lead to ambiguities in the interpretation of nuclear Overhauser enhancements. The results illustrate the utility of chemical shifts at such a surface in the cross-validation of a high-resolution solution structure.

Proton chemical shifts in NMR. Part 14. Proton chemical shifts, ring currents and π electron effects in condensed aromatic hydrocarbons and substituted benzenes

The proton resonance spectra of a variety of condensed aromatic compounds including benzene, naphthalene, anthracene, phenanthrene, pyrene, acenaphthylene and triphenylene were obtained in dilute CDCl3 solution. Comparison of the proton chemical shifts obtained with previous literature data for CCl4 solution shows small but significant differences. A previous model (CHARGE6) for calculating the proton chemical shifts of aliphatic compounds was extended to aromatic compounds. This was achieved by including an automatic identification of both five- and six-membered aromatic rings based on atomic connectivities plus a dipole calculation of the aromatic ring current. The ring current intensity in the molecules was calculated by two alternative methods. a) The ring current intensity in the individual benzenoid rings was a function of the number of adjoining rings and b) the molecular ring current was proportional to the molecular area divided by the molecular perimeter. This, plus the inclusion of deshielding steric effects for the crowded protons in these molecules, gave a good account of the observed chemical shifts. The model was also applied successfully to the non-alternant hydrocarbons of fulvene and acenaphthylene and to the aliphatic protons near to and above the benzene ring in tricyclophane and [10]cyclophane.The Huckel calculation of the π electron densities in CHARGE6 was used to calculate the π electron densities in substituted benzenes. The π-inductive effect was used to simulate the effect of CX3 groups (X = H, Me, F) on the benzene ring. These together with the long range effects of the substituent groups identified previously allowed a precise calculation of the SCS of a variety of substituents on all the benzene ring protons.The model gives the first accurate calculation of the proton chemical shifts of condensed aromatic compounds and of the proton SCS in the benzene ring. For the data set of 55 proton chemical shifts spanning 3 ppm the rms error of the observed vs. calculated shifts was ca. 0.1 ppm. The model also allows the interpretation of the shifts in terms of the separate interactions calculated in the programme, i.e. π electron densities and steric, anisotropic and electric field effects. Previous correlations of the proton SCS with π electron densities and substituent parameters are shown to be over simplified. The relative proportions of these different interactions are very different for each substituent and for each ring proton. p

Isolated Linear Alkanes in Aromatic Nanochannels

Novel channel-like inclusion compounds were formed by tris(o-phenylenedioxy)spirocyclotriphosphazene (TPP) with linear hydrocarbons. The molecular characterization was performed by 13C and 31P solid-state NMR under magic angle spinning conditions. The hexagonal symmetry of the host was verified in the 31P and 13C MAS spectra and by X-ray powder diffraction patterns. The new inclusion compounds present congruent meltings, and the temperatures increase with chain length. These compounds provide the opportunity to observe hydrocarbons confined as isolated chains in the solid state at temperatures 200 K above their melting points. Furthermore, the aliphatic chains are in the unusual state of being surrounded by aromatic rings displaced parallel to the channel. This geometry was proved by a through-space 1.4 ppm upfield shift on the 13C resonances of the guests, due to the aromatic ring current. The mobility at the chain ends and the propagation of the conformational defects within the guest molecules is discu...

Conjugated Macrocycles Related to the Porphyrins. 12.1 Oxybenzi- and Oxypyriporphyrins: Aromaticity and Conjugation in Highly Modified Porphyrinoid Structures

The “3 + 1” route for porphyrinoid synthesis is an excellent methodology for preparing aromatic porphyrin analogues with six-membered ring subunits. Condensation of 5-formylsalicylaldehyde with tripyrranes 15, 24, and 25 in the presence of 5% TFA-dichloromethane, followed by neutralization and oxidation with DDQ, afforded a series of semiquinone-containing porphyrinoids 12, 26, and 27 in 35−52% yield. In these novel systems, the macrocycle achieves aromatization by undergoing a keto−enol tautomerization whereby the phenolic subunit is transformed so that the inner three carbon atoms become part of the 18 π-electron aromatic core, whereas the outer carbons generate an enone unit. The aromatic nature of these “oxybenziporphyrins” is evident from their porphyrin-like electronic spectra and the presence of powerful diamagnetic ring currents in their proton NMR spectra, where the inner CH is shifted upfield to δ = −7 ppm, whereas the external meso-protons appear downfield between 8.8 and 10.3 ppm. The presence of a carbonyl unit is confirmed by IR and proton NMR spectroscopy. Addition of trace amounts of TFA give an aromatic monocation, but further protonation to the dication leads to the loss of macrocyclic aromaticity. Modified tripyrranes 15b, 26, and 27 were used to prepare oxybenziporphyrins with fused benzene, phenanthrene, and acenaphthylene ring systems, the former via a tetrahydrobenzo intermediate; and these compounds showed many of the same characteristics, including aromatic free bases and nonaromatic dications. The ring fusion gave rise to gradual bathochromic shifts from benzo- (23) to phenanthro- (26) to acenaphtho- (27) oxybenziporphyrins, which qualitatively followed the same trends observed for true porphyrins, although the influence of the acenaphthylene ring system was somewhat muted in this series. Condensation of isophthalaldehyde with tripyrrane 15a gave the nonaromatic analogue “benziporphyrin” in 28% yield, and this species was thoroughly characterized and contrasted to oxybenziporphyrin 12a. Reaction of 3-hydroxypyridine-2,6-dicarboxaldehyde with tripyrranes under the “3 + 1” conditions afforded exceptionally high yields of the corresponding azaoxybenziporphyrins (named “oxypyriporphyrins”; 35, 47−49), and these structures also exhibited porphyrinoid aromaticity. For this series, the dications retained their aromatic character as did the related nickel(II), copper(II), and zinc chelates. Ring fusion effects were investigated for the oxypyriporphyrins and their metal complexes, and again the major absorptions shifted to higher wavelength from benzo- to phenanthro- to acenaphthooxypyriporphyrins. However the effect of metalation on the oxypyriporphyrin chromophore differed considerably from the trends seen for metalloporphyrins. These results show that novel aromatic porphyrinoids are easily accessible via the “3 + 1” approach, and this work will facilitate in-depth studies on the chemical and physical properties of these exciting new bridged annulene structures.

π‐Electron Ring‐Current Effects in Multiple Adducts of 3He@C60 and 3He@C70: A 3He NMR Study

AbstractBis‐ to hexakisadducts of C60 (1–6) and mono‐ to tetrakisadducts of C70 (7–13) containing 3He atoms (endo‐hedral helium complexes) were prepared and studied by 3He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical shift of the 3He atom. In the series of C60 derivatives, which included the previously measured 6–6 monoadduct 3He@C61H2, the 3He resonance was shifted considerably upfield relative to that of 3He@C60 (δ = −6.36) up to the bisadduct 1 (δ = −11.45). The resonances of the higher adducts 3–6, however, were shifted only slightly further up‐field and all appeared in a rather narrow spectral range between δ = −11.84 and −12.26. The absence of further substantial upfield shifts was rationalized in terms of the compensation of deshielding due to the functionalization‐induced decrease in the diamagnetic π‐electron ring currents that extend around the fullerene sphere by the shielding that results from the weaker pentagonal‐ring paramagnetic currents and the increased number of localized benzenoid substructures in 3–6. In contrast, all 3He resonances of the C70 adducts are shifted downfield relative to the signal of 3He@C70 (δ = −28.81). A monotonic relationship exists between the chemical shift and the degree of functionalization, whereby the 3He chemical shifts steadily decrease from monoadduct 7 (δ = −27.53) to the isomeric tetrakisadducts 12 (δ = −21.09) and 13 (δ = −20.68). This deshielding was explained by the reduction of the strong diamagnetic ring currents extending along the polar corannulene perimeters as a result of the functionalization at α‐ and β‐type polar bonds in 7–13. In both series, the addition pattern was also found to have a distinct influence on the 3He chemical shift.

Synthesis, Structure, Spectroscopy, and Reactivity of a Metallabenzene1

A rare example of a stable metallabenzene complex has been synthesized in three high-yield steps from (Cl)Ir(PEt3)3. In the first step, (Cl)Ir(PEt3)3 is treated with potassium 2,4-dimethylpentadienide to produce the metallacyclohexadiene complex mer-CHC(Me)CHC(Me)CH2Ir(PEt3)3(H) (1b) via metal-centered CH bond activation. Treatment of 1b with methyl trifluoromethanesulfonate removes the hydride ligand, producing [CHC(Me)CHC(Me)CH2Ir(PEt3)3]+O3SCF3- (2). Finally, deprotonation of 2 with base yields the metallabenzene complex CHC(Me)CHC(Me)CHIr(PEt3)3 (3). The X-ray crystal structure of 3 shows the coordination geometry about iridium to be square pyramidal. The metallabenzene ring is nearly planar, and the ring π-bonding is delocalized. In the 1H NMR spectrum of 3, the ring protons (H1/H5 and H3) are shifted downfield, consistent with the presence of an aromatic ring current. Compound 3 reacts with a variety of small 2e- ligands under mild conditions to produce monosubstituted metallabenzenes, CHC(Me)CHC(Me...

Persistent oxidation dications of dialkyl- and tetraalkyl-perylenes and dibenzo[cd,lm]perylene; charge distribution mode, substituent effects and conformational aspects

Starting with appropriate alkylnaphthalenes, 3,10-dimethyl- 2, 3,10-diethyl- 3, 2,3,10,11-tetramethyl- 4 and 3,4,9,10-tetramethyl-perylene 5 were synthesized by a sequence of radical cation and radical anion coupling steps. Persistent oxidation dications of parent perylene 1, alkylperylenes 2–5 and dibenzo[cd,lm]perylene 6 were generated in SbF5–SO2ClF.Based on 1D- and 2D-NMR experiments combined with AMl calculations the charge delocalization mode in the dications was deduced. The total deshielding (ΣΔδ13C), substituent effects, conformational/geometrical changes and tropicity in the dications are examined.Whereas alkylperylene dications represent 18π electron Huckel dictations with a diamagnetic ring current (more diatropic than their neutral precursors), the 24π (20π periphery) dication of dibenzoperylene exhibits a paramagnetic ring current which greatly shields its protons (more paratropic than neutral hydrocarbon).

Stopping the aromatic ring current. Synthesis of the first bis-metal complexed benzannulene. On the way to a molecular conductor with a photo-switch

The mild ligand exchange reaction between (tricarbonylchromium)naphthalene and the dibenzannulene 4 permits synthesis of the first bis-complexed large ring benzannulene 5. Progressive reduction of the strong ring current of 1 occurs in going from 1–5, which in the latter is only about 6% of that in 1.

Complete account of the π-electron correlation in calculating ring currents in conjugated aromatic and antiaromatic systems

A semiempirical theory of ring π currents is developed, which accurately takes into account the electron correlation using the full configuration interaction (FCI) method. An elementary model is proposed, in which the calculation of the π-electron correlation is reduced to the model of generalized alternative Huckel systems. The FCI and SCF (self-consistent field) methods are used to calculate the diamagnetic susceptibilities and ring currents for some typical π systems. It is shown that, as a rule, the SCF method allows the qualitatively true description of the magnetic properties of condensed systems, although for antiaromatic molecules it often yields underestimated values of paramagnetic currents. The results of the FCI calculations suggest that electronic excitations can induce transitions from π paramagnetism to π diamagnetism (e.g., in fulvenes) and back (in quinones).

Structure and Dynamics of a Protein Assembly. 1H-NMR Studies of the 36 kDa R6Insulin Hexamer

The structure and dynamics of the R 6human insulin hexamer are investigated by two- and three-dimensional homonuclear 1H-NMR spectroscopy. The R 6hexamer, stabilized by Zn 2+and phenol, provides a model of an allosteric protein assembly and is proposed to mimic aspects of receptor recognition. Despite the large size of the assembly (36 kDa), its extreme thermal stability permits high-resolution spectra to be observed at 55°C. Each spin system is represented uniquely, implying either 6-fold symmetry or fast exchange among allowed protomeric conformations. Dramatic changes in chemical shifts and long-range nuclear Overhauser enhancements (NOEs) are observed relative to the spectra of insulin monomers. Complete sequential assignment is obtained and demonstrates native secondary structure with distinctive R-state N-terminal extension of the B-chain α-helix (residues B1 to B19). The distance-geometry structure of an R-state promoter is similar to those of R 6crystal structures. Specific long-range intra- and intersubunit NOEs, assigned by stepwise analysis of engineered insulin monomer and dimers, demonstrate that tertiary and quaternary contacts are also similar. Although the hexamer is well-ordered in solution, binding of phenol to an internal cavity occurs within milliseconds, implying the existence of “gatekeeper” residues whose flexibility provides a portal of entry and release. Changes in 1H-NMR chemical shifts on hexamer assembly are readily rationalized by analysis of aromatic ring-currents and provide sensitive probes for sites of protein – protein interaction and phenol binding. Our results provide a foundation for the studies of insulin analogues (such as “designed” insulins of therapeutic interest) under conditions of clinical formulation and for the investigation of the effects of protein assembly on the dynamics of individual protomers.

Triphenanthro‐anellierte [18]Annulene mit Alkoxy‐Seitenketten — Eine neue Klasse discotischer Flüssigkristalle

AbstractTriphenanthro‐Anellated [18]Annulenes with Alkoxy Side Chains ‐ A Novel Class of Discotic Liquid CrystalsTwo different types 6 and 16 of [18]annulenes condensed with phenanthrene systems were prepared by cyclocondensation reactions (Schemes 2 and 3). The central 18‐membered rings do not show a macrocyclic diamagnetic ring current. Conformational analyses and X‐ray analyses in the solid state reveal non‐planar structures in which aromatic “islands” are connected by (E)‐configurated olefinic bridges. The introduction of long alkoxy side chains yields a new class of discotic liquid crystals; either ND or hexagonal columnar phases are generated, which were characterized by DSC, microscopy with polarized light, and small‐angle scanning X‐ray.

3He NMR: A Powerful New Tool for Following Fullerene Chemistry

The fullerenes react readily with a variety of reagents. Following their reactions with NMR spectroscopy has not been easy. If several products are formed or, as is common, bis-adducts are produced, assigning the carbon peaks is difficult or impossible. Continuing our studies of endohedral fullerene compounds of noble gases, we have recently used high pressure and heating to introduce [sup 3]He into C[sub 60] and C[sub 70], increasing incorporation levels to around 0.15%. We have used this material to obtain the first [sup 3]He NMR spectra of helium compounds. Each helium-labeled fullerene gives a single sharp peak. The large chemical shifts (C[sub 60], [minus]6.3 ppm; C[sub 70], [minus]28.8 ppm from dissolved [sup 3]He as reference) show that there are substantial aromatic ring currents modifying the magnetic field experienced by the helium nucleus in the center of the fullerenes. We expected that altering the [pi]-bonding structure of the fullerene through reaction would produce substantial shifts in the [sup 3]He peak. We have now verified this expectation. We have subjected a [sup 3]He-labeled fullerene mixture (about 70% C[sub 60], 30% C[sub 70]) to the reaction conditions recently reported by Maggini, Scorrano, and Prato for the azomethine ylide addition to fullerenes to formmore » N-methylpyrrolidines. It is striking that addition to just one of the 30 double bonds of [sup 3]He C[sub 60]produced a 3 ppm shift of the helium upfield. At this point, even qualitative rationalization of the direction of these shifts would be speculative. It is interesting that the adducts of C[sub 70] shift downfield. 6 refs., 2 figs.« less

Probing the interior of fullerenes by 3He NMR spectroscopy of endohedral 3He@C60 and 3He@C70

FULLERENES have internal cavities large enough to encapsulate atoms1,2. Recently, noble-gas atoms were introduced into about one in a million fullerene molecules3. It is now possible to achieve far greater yields of these noble-gas endohedral compounds4. Because 3He has a spin of 1/2 and is an excellent NMR nucleus5,6, it can be used as a probe for the magnetic shielding environment inside the fullerene cavity. This environment should reflect possible ring currents and hence the aromaticity in fullerenes7–15, an issue that measurements of magnetic susceptibility16,17 have not completely resolved. Here we present 3He NMR spectra of the endohedral compounds 3He@C60 and 3He@C70 (the @ symbol denotes a compound that is endohedral). We find that the 3He nuclei encapsulated in C6o and C70 are shielded by 6 and 29 parts per million respectively, relative to free 3He. These shieldings are unexpectedly large, indicating significant diamagnetic ring currents in C60 and very large ones in C70. Our results also show that, because of its small size and inertness, helium can serve as a useful probe of magnetic molecular properties. In addition, our work represents the first 3He NMR spectra of stable helium compounds.