3He NMR Research Articles

3He@C60 as a Concise Probe in Complex Systems: Diels‐Alder Reac‐tions of Fullerene with Different Bis Anthracene Compounds

AbstractThree bis‐anthracene (BA) compounds, characterized by varying alkyl chain lengths between the two anthracene groups, were synthesized and investigated using 3He NMR spectroscopy on He@C60 derivatives. The Diels‐Alder reaction of these BAs with fullerene was comprehensively explored. Findings disclosed that BAs with shorter linkers formed mono‐adducts with 3He@C60 at lower concentrations. Conversely, the BA compound with the longest linker (1 c) yielded both mono‐adducts and bis‐para‐adducts with C60, marked by a distinct 3He NMR signal peak for bis‐adducts at −12.84 ppm. Importantly, the versatility of the 3He@C60 probe was exemplified in characterizing intricate fullerene reactions that conventionally pose challenges. Theoretical analyses validated reaction products, with computational models substantiating experimental results.

Solid-State mathrm {^3He} NMR of the Superconducting Rubidium Endofulleride mathrm {Rb_3(^3He@C_{60})}

A new variant of the superconducting fulleride Rb_3C_{60} is presented, with ^3He atoms encapsulated in the C_{60} cages. The ^3He nuclei act as sensitive NMR probes embedded in the material. The superconducting and normal states are characterized by ^3He NMR. Evidence is found for coexisting vortex liquid and vortex solid phases below the superconducting transition temperature. A strong dependence of the spin–lattice relaxation time constant on spectral frequency is observed in the superconducting state, as revealed by two-dimensional NMR utilizing an inverse Laplace transform. Surprisingly, this phenomenon persists, in attenuated form, at temperatures well above the superconducting transition.

On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes

The problem of aromaticity in heterocyclic rings of uracil and its 5-halogenoderivatives (5XU) was analyzed theoretically by calculating modified harmonic oscillator model of aromaticity (HOMA) for Heterocycle Electron Delocalization (HOMHED), nucleus-independent chemical shift parameters (NICS) and the so-called scan experiments, using helium-3 atom as a magnetic probe. The impact of halogen electronegativity on C5 atom’s NBO charges was also investigated. Water, as a polar environment, has a negligible impact on 5XU aromaticity. The most stable diketo tautomer shows a very low aromaticity while the “rare” dihydroxy form (tautomer No 6) is aromatic and resembles benzene. This is in agreement with traditional drawing of chemical formula of uracil’s six-membered ring, directly showing three alternating single and double bonds in its tautomer No 6. No good correlation between magnetic and geometric indexes of aromaticity for the studied 5XU tautomers was found. Linear correlation between the magnitude of NICS minimum, as well as the distance of the minimum above uracil ring plane center from 3He NMR chemical shift scan plot with respect to halogen electronegativity were observed. A strong linear dependence of magnetic index of aromaticity and the electronegativity of 5X substituent was observed.

Cryogenic Differential Amplifier for NMR Applications

We have designed and characterized a cryogenic amplifier for use in ^3He NMR spectrometry. The amplifier, with a power consumption of sim 2.5 mW, works at temperatures down to 4 K. It has a high-impedance input for measuring a signal from NMR resonant circuit, and a 50 {Omega } differential input which can be used for pick-up compensation and gain calibration. At 4.2 K, the amplifier has a voltage gain of 45, output resistance 146 {Omega } and a 4.4 MHz bandwidth starting from DC. At 1 MHz, the voltage and current noise amount to 1.3 text{ nV }/sqrt{text{ Hz }} and 12 text{ fA }/sqrt{text{ Hz }}, respectively, which yields an optimal source impedance of sim 100 k{Omega }.

Reply to 'Comment on "Angstrom-scale probing of paramagnetic centers location in nanodiamonds by 3He NMR at low temperatures"' by A. Shames, V. Osipov and A. Panich, Phys. Chem. Chem. Phys. 2018, 20, DOI.

Shames et al. made a comment on our article (DOI: 10.1039/C7CP05898E) stating that their experience in EPR studies of detonation nanodiamonds suggests the existence of two main types of paramagnetic center in detonation nanodiamonds which questions our results. In this reply we provide insights into why there is only one main type of paramagnetic centers detected in nanodiamonds used in this work, which validates the correctness of the proposed original method to determine the distances between paramagnetic centers and nanoparticle surfaces by 3He NMR.

Comment on “Angstrom-scale probing of paramagnetic center location in nanodiamonds by 3He NMR at low temperatures” by V. Kuzmin, K. Safiullin, G. Dolgorukov, A. Stanislavovas, E. Alakshin, T. Safin, B. Yavkin, S. Orlinskii, A. Kiiamov, M. Presnyakov, A. Klochkov and M. Tagirov, Phys. Chem. Chem. Phys., 2018, 20, 1476

Kuzmin et al. reported on the application of the 3He NMR relaxometry technique to localization of intrinsic paramagnetic defects in detonation nanodiamond (DND) particles. We found some inconsistencies in the DND characterization, the model of paramagnetic defects' allocation used in that work as well as the estimation of distances between 3He NMR probes and paramagnetic defects, which question the results obtained.

The Calcium Carbonate Geological Samples Study by 3He NMR

Geological samples of calcium carbonates (CaCO3) were investigated by 3He NMR, nitrogen porosimetry, X-ray diffraction and electron microscopy methods. The integral porosity of the samples was obtained by 3He nuclear magnetic resonance method and compared with nitrogen adsorption isotherms data. The advantages of 3He porometry method are discussed.

3He NMR studies on helium-pyrrole, helium-indole, and helium-carbazole systems: a new tool for following chemistry of heterocyclic compounds.

The (3)He nuclear magnetic shieldings were calculated for free helium atom and He-pyrrole, He-indole, and He-carbazole complexes. Several levels of theory, including Hartree-Fock (HF), Second-order Møller-Plesset Perturbation Theory (MP2), and Density Functional Theory (DFT) (VSXC, M062X, APFD, BHandHLYP, and mPW1PW91), combined with polarization-consistent pcS-2 and aug-pcS-2 basis sets were employed. Gauge-including atomic orbital (GIAO) calculated (3)He nuclear magnetic shieldings reproduced accurately previously reported theoretical values for helium gas. (3)He nuclear magnetic shieldings and energy changes as result of single helium atom approaching to the five-membered ring of pyrrole, indole, and carbazole were tested. It was observed that (3)He NMR parameters of single helium atom, calculated at various levels of theory (HF, MP2, and DFT) are sensitive to the presence of heteroatomic rings. The helium atom was insensitive to the studied molecules at distances above 5 Å. Our results, obtained with BHandHLYP method, predicted fairly accurately the He-pyrrole plane separation of 3.15 Å (close to 3.24 Å, calculated by MP2) and yielded a sizable (3)He NMR chemical shift (about -1.5 ppm). The changes of calculated nucleus-independent chemical shifts (NICS) with the distance above the rings showed a very similar pattern to helium-3 NMR chemical shift. The ring currents above the five-membered rings were seen by helium magnetic probe to about 5 Å above the ring planes verified by the calculated NICS index.

A Computational Investigation of the Electronic Properties of Partially Hydrogenated Fullerenes C60Hn (n = 18, 20, 24, 36 and 48)

A DFT study is performed to investigate the electronic and magnetic properties of partially hydrogenated fullerenes C60Hn (n = 18, 20, 24, 36, and 48) based on NMR parameters, NICS, and NPA analysis. A few peaks appear in the 13C NMR pattern of each C60Hn, showing the electrostatic environments of C nuclei are divided into a few layers that are related to three local structures around C atoms. Since, hexagons and pentagons in the zone of hydrogenated carbons are puckered, the range of isotropic chemical shifts (δiso) for each group of carbons are caused by different curvatures at the corresponding sites. Partially hydrogenation of fullerenes results in isolated cyclohexatriene fragments with almost planarity surface, in some cases, which yields 13C δiso of 130.7–132.4 ppm. Moreover, cyclohexatriene pole ring with the calculated NICS value of −8.9 ppm at its center, similar to that of benzene (−8.0), can be considered as benzenoid fragment. Magnetic properties are also examined inside the compounds, indicating that NICS values are strongly negative in the middle of all the cages, except for C60H24 and C60H48. The predicted NICS values may be also useful for identification of the hydrogenated fullerenes through their endohedral 3He NMR chemical shifts.

Sensitivity of 3He NMR Parameters to the Proximity of Heterocyclic Rings. The Helium–Furan Dimer

The potential of the NMR technique in probing the interaction between a noble gas atom (used as a spectroscopic probe) and five-membered heterocyclic systems was theoretically studied. Selected 3He NMR parameters of the free noble gas atom and its dimer were calculated using density functional theory with three functionals (M06-2X, CAM-B3LYP, and APFD), combined with dedicated polarization-consistent basis sets, and critically compared with theoretical high level calculations from the literature. The change of isotropic nuclear magnetic shielding of the free noble gas atom in the presence of furan (3He placed above the middle of the ring and used as NMR probe) increased with its proximity to the heterocyclic ring.

Sensitivity of Noble Gas NMR Parameters to the Heterocyclic Ring Proximity. Density Functional Theory Studies of Ne–Furan and Ar–Furan Complexes

Theoretical modeling of noble gas interaction with furan as a simple heterocyclic ring was performed. The structures of neon–furan and argon–furan complexes were calculated at the MP2, M06-2X, CAM-B3LYP, APFD, and VSXC levels of theory using 6-311++G** basis set. The predicted 21Ne and 39Ar NMR chemical shifts for the Ne–furan and Ar–furan complexes calculated with pcS-3 and aug-pcS-3 basis sets were sensitive to the presence of the aromatic furan ring. Our results indicate a higher sensitivity of the neon and argon NMR probes than the previously reported 3He NMR spectroscopic parameters in studies of small heterocyclic rings containing the oxygen atom.

Nuclear Magnetic Shielding for Hydrogen in Selected Isolated Molecules

We present the results of gas-phase NMR measurements designed to yield a new experimental value for the absolute (1)H magnetic shielding for an isolated hydrogen molecule and its deuterium isotopomers. The results are based on the original method of direct shielding measurements (Jackowski et al., 2010) and the density dependence of (1)H, (2)H, and (3)He NMR frequencies for molecular hydrogen and atomic helium-3. The absolute isotropic magnetic shielding measured for molecular hydrogen, σ(0)(H(2)), is 26.293(5) ppm at 300 K, within experimental error of previous measurements based on spin-rotation data and quantum chemistry computations, 26.289(2) ppm (Sundholm and Gauss, 1997), and recent ab initio calculations. We also report isotope effects in shielding for H(2), HD, and D(2) molecules that are consistent with theoretical predictions. In addition, gas-phase (1)H chemical shifts extrapolated to zero density have been measured for numerous small molecules. Our results yield precise absolute shielding data that will be useful in establishing benchmark computational chemistry methods for calculating rovibrational averaged magnetic shielding.

A computational NICS and 13C NMR characterization of the polyfluorofullerenes C60Fn (n = 18, 20, 24, 36 and 48)

A density functional study has been performed to investigate electronic and magnetic properties of polyfluorofullerenes C60Fn (n=18, 20, 24, 36, and 48) based on NMR parameters and NICS indices. A few picks appear in the 13C NMR pattern of each polyfluorofullerene which show the electrostatic environments of carbon nuclei are divided into a few layers related to three local structures around C atoms. Since, hexagons and pentagons in the zone of fluorinated carbon atoms are puckered, the range of isotropic chemical shifts (δiso) related to each group of C atoms are caused by different curvatures at the corresponding sites. According to the calculated spherical excesses Φi, chemical shifts are strongly sensitive to the curvature at the corresponding sites in the fullerene cage, so slightly difference in the Φi of carbon atoms leads to change in their δiso values. Moreover, the magnetic properties are examined inside the compounds indicating that independent of the type of polyfluorofullerenes NICS values are strongly negative at the middle of the cage except for the C60F24 and C60F48. The predicted NICS values may be also useful for identification of polyfluorofullerenes through their endohedral 3He NMR chemical shifts.

A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C70−2x(BN)x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated 13C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. 13C Chemical shifts (δiso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from −25.29 to −8.80) demonstrate that all the substituted species are aromatic, but less than C70 (−27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral 3He NMR chemical shifts.

Exploring electronic structures for the most stable isomers of C12B6N6 and B6N6C12 heterofullerenes based on NMR, NICS and NBO analysis: A DFT study

DFT calculations are applied to evaluate the effects of atomic arrangements of dopant atoms on electronic features of the most stable structures of (BC2N)6 including both C24 fullerene doped with six BN units (C12B6N6; model 1–7) and B12N12 nanocage doped with six CC units (B6N6C12; model 8–15). Our study reveals a relation between local structures, Cα, Cβ, Cγ, and Cδ within the molecules and 13C NMR signals. This is also observed with natural charges, so that the increase of negative charge on carbon is along with more shielding of corresponding carbon. All BN-substituted species (model 1–7) with the negative calculated NICS at their cage center are predicted to be aromatic, in contrast to C24 (NICS=±37). The predicted NICS values may be useful for the identification of the heterofullerenes through their endohedral 3He NMR chemical shifts.

A computational NICS and 13C NMR characterization of BN-substituted 60C fullerenes

DFT calculations are applied to devise NMR parameters of heterofullerenes C 60−2 x (BN) x ( x=1, 2, 3, 6, 9, 12, 15, 18, 21, and 24). The calculated chemical shielding tensors at the sites of 13C nuclei in the first and second neighbor of the doped atoms are perturbed and other distant atoms are not influenced significantly. Chemical shielding of the second neighbor of nitrogen and boron nuclei is significantly shifted to upfield and downfield (the second neighboring effect), respectively. Nucleus independent chemical shifts (NICS) at the cage center of BN-substituted fullerenes demystifies that substituted species are more aromatic than 60C. The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral 3He NMR chemical shifts.

Precise determination of the 13C nuclear magnetic moment from 13C, 3He and 1H NMR measurements in the gas phase

NMR spectroscopy was applied for the precise determination of the 13C nuclear magnetic dipole moment on the basis of new results available for nuclear magnetic shielding in methane and helium-3. For this purpose small amounts of 13CH 4 and 3He as the solutes were observed in CO 2 or Xe as the gaseous solvents by 13C, 3He and 1H NMR spectra at the constant external field, B 0 = 12.7586 T. In every case the measured resonance frequencies ( ν C, ν He and ν H) were linearly dependent on the density of solvent. It allowed us the evaluation of the above frequencies free from intermolecular interactions after the extrapolation of experimental points to zero-density of gaseous solvents. Combining these measurements with the relevant value of 13C shielding in methane it was possible to determine the 13C nuclear magnetic moment with much better precision than before, μ( 13C) = 0.70236944(68) μ N. Application of new theoretical results for carbon shielding in the methane molecule permitted similar evaluation of the 13C nuclear magnetic moment, it shows that the experimental and theoretical results are consistent and the new 13C magnetic moment can be efficiently applied in the field of NMR spectroscopy.

Assignment of the He@C 84 isomers in experimental NMR spectra using density functional calculations

The 3He chemical shifts were calculated for He n @C 84 ( n = 1, 2) fullerenes to obtain characteristic NMR patterns for distinguishing their isomers in a mixture. The density functional methods were calibrated on experimental data. Accuracy within 1 ppm could be reached without further fitting of individual shifts. Such precision allows for a semi-quantitative assignment of 3He NMR spectra. Additional criteria in the identification are discussed, such as the relative energies of the isomers, positions of the satellite di-helium peaks, and the differential 3He shifts in the fullerenes reduced to anions. Endohedral 3He shifts are predicted for so far experimentally unknown He@C 84 and He @ C 84 6 - isomers.

NMR of Liquid 3He in Pores of a Clay Sample

In the present work a new method for studying porous media by nuclear magnetic resonance of liquid 3He has been proposed. This method has been demonstrated by an example of a clay mineral sample. For the first time the integral porosity of the clay sample has been measured. For investigated samples the value of the integral porosity is in the range of 10–30%. The inverse Laplace transform of the 3He longitudinal magnetization recovery curve has been carried out and the distribution of the relaxation times T 1 has been obtained.

Fast-exchange model visualized withH3econfined in aerogel: A Fermi liquid in contact with a ferromagnetic solid

3He confined in aerogel in the millikelvin temperature domain exemplifies a Fermi liquid in the presence of disorder. In confined 3He systems, a solid layer of 3He atoms forms on the confining medium. This system can then be viewed as a model system for the study of the (strongly interacting) Fermi liquid in contact with a (ferromagnetic) "2D-like" adsorbed solid. This interaction, studied experimentally through NMR T2 experiments, is described in the framework of the "fast exchange" model. A complete analytical descripion of the model is given, explaining our measurements as well as related normal-state confined 3He NMR literature.