Synthesis and characterization of bispyrrolidine derivatives of H2@C60: differentiation of isomers using 1H NMR spectroscopy of endohedral H2

Bisadduct isomers of a H(2)@C(60) derivative with nitroxide addends have been synthesized, isolated and characterized. The (1)H NMRs of endohedral H(2) of the major isomers show well-separated chemical shifts, which could be useful for structural assignment and identification of the purity of the C(60) bisadduct isomers.     

Paramagnet enhanced nuclear relaxation of H2 in organic solvents and in H2@C60

We have measured the bimolecular contribution (relaxivity) R1 (M(-1) s(-1)) to the spin-lattice relaxation rate for the protons of H2 and H2@C60 dissolved in organic solvents in the presence of paramagnet nitroxide radicals. It is found that the relaxation effect of the paramagnets is enhanced 5-fold in H2@C60 compared to H2 under the same conditions. 13C relaxivity in C60 induced by nitroxide has also been measured. The resulting value of R1 for 13C is substantially smaller relative to the 1H relaxation in H2@C60 than expected solely on the basis of the smaller magnetic moment of 13C. The observed values of R1 have been analyzed quantitatively using an outer-sphere model for bimolecular spin relaxation to extract an encounter distance, d, as the dependent variable. The resulting values of d for H2 and (13)C60 are similar to the sum of the van der Waals radii for the radical and the corresponding molecule. The value of d for (1)H2@C60 is substantially smaller than the corresponding van der Waals estimates, corresponding to larger than expected values of R1. A possible explanation for the enhanced relaxivity is a contribution from hyperfine coupling. Based on the results reported here, it seems that not only is the hydrogen molecule in H2@C60 not insulated from magnetic contact with the outside world but also the interaction with paramagnets is even stronger than expected based on distance alone.     

Synthesis, isomer count, and nuclear spin relaxation of h(2)o@open-c(60) nitroxide derivatives

H(2)O@C(60) derivatives covalently linked to a nitroxide radical were synthesized. The (1)H NMR of the guest H(2)O revealed the formation of many isomers with broad signals. Reduction to the diamagnetic hydroxylamines sharpened the (1)H NMR signals considerably and allowed for an "isomer count" based on the number of observed distinct signals. For H(2)O@K-8, 17 positional isomeric nitroxides are predicted, not including additional numbers of regioisomers; indeed, 17 signals are observed in the (1)H NMR spectrum.     

Nuclear relaxation of H2 and H2@C60 in organic solvents

The 1H nuclear spin-lattice relaxation time (T1) of H2 and H2@C60 in organic solvents varies with solvent, and it varies proportionally for H2 and for H2@C60. Since intermolecular magnetic interactions are ruled out, the solvent must influence the modulating processes of the relaxation mechanisms of H2 both in the solvent cage and inside C60. The temperature dependence of T1 also is very similar for H2 and H2@C60, T1 going through a maximum by varying the temperature in solvents which allow a wide range of temperatures to be explored. This behavior is attributed to the presence of dipolar and spin-rotation mechanisms which have an opposite dependence on temperature.     

NMR and nuclear spin relaxation of cement and concrete materials

Recent highlights of NMR works are reviewed for various cement-based materials. The emphasis is on nuclear magnetic relaxation methods at variable magnetic fields for continuous characterization of the evolving microstructure of various cementitious materials. Also, the recent applications of 2-dimensional T₁–T₂ and T₂–store–T₂ correlation experiments for characterization of water exchange in connected micropores of cement pastes are reported.     

1H NMR relaxation in glycerol solutions of nitroxide radicals: effects of translational and rotational dynamics

(1)H spin-lattice relaxation rates in glycerol solutions of selected nitroxide radicals at temperatures between 200 K and 400 K were measured at 15 MHz and 25 MHz. The frequency and temperature conditions were chosen in such a way that the relaxation rates go through their maximum values and are affected by neither the electron spin relaxation nor the electron-nitrogen nucleus hyperfine coupling, so that the focus could be put on the mechanisms of motion. By comparison with (1)H spin-lattice relaxation results for pure glycerol, it has been demonstrated that the inter-molecular electron spin-proton spin dipole-dipole interactions are affected not only by relative translational motion of the solvent and solute molecules, but also by their rotational dynamics as the interacting spins are displaced from the molecular centers; the eccentricity effects are usually not taken into account. The (1)H relaxation data have been decomposed into translational and rotational contributions and their relative importance as a function of frequency and temperature discussed in detail. It has been demonstrated that neglecting the rotational effects on the inter-molecular interactions leads to non-realistic conclusions regarding the translational dynamics of the paramagnetic molecules.     

Nitroxide paramagnet-induced para-ortho conversion and nuclear spin relaxation of H2 in organic solvents

The kinetics of para-ortho conversion and nuclear spin relaxation of H 2 in chloroform- d 1 were investigated in the presence of nitroxides as paramagnetic catalysts. The back conversion from para-hydrogen ( p-H 2) to ortho-hydrogen ( o-H 2) was followed by NMR by recording the increase in the intensity of the signal of o-H 2 at regular intervals of time. The nitroxides proved to be hundreds of times more effective at inducing relaxation among the spin levels of o-H 2 than they are in bringing about transitions between p-H 2 and the levels of o-H 2. The value of the encounter distance d between H 2 and the paramagnetic molecule, calculated from the experimental bimolecular conversion rate constant k 0, using the Wigner theory of para-ortho conversion, agrees perfectly with that calculated from the experimental relaxivity R 1 using the force free diffusion theory of spin-lattice relaxation.     

Electron spin density distribution of a series of nitroxide radicals with five-member ring as studied by solid-state 1H, 2H and 13C NMR

Stable nitroxide radicals are useful to construct molecular magnetic systems. Particularly, radicals substituted by –COOH and –CONH₂ can be coordinated to magnetic metal ions and may be used as cladding reagents of gold nano-particles for modifying magnetism. Nitroxide molecules with unsaturated five-member ring have almost planner structure and electron spin delocalization may be expected. We determined the hyperfine coupling constants (hfcc) of ¹H, ²H and ¹³C of a series of nitroxide radicals with five-member ring. Experimental values of hfcc were compared with those deduced from calculations based on density functional theory. The electron spin density distribution at β position of ring was sensitive to the ring structure, although the electron spin density at β position is small compared with N–O site. Magnetic susceptibility and UV–Vis absorption spectra were also measured and discussed.     

A suite of 2H NMR spin relaxation experiments for the measurement of RNA dynamics

A suite of (2)H-based spin relaxation NMR experiments is presented for the measurement of molecular dynamics in a site-specific manner in uniformly (13)C, randomly fractionally deuterated ( approximately 50%) RNA molecules. The experiments quantify (2)H R(1) and R(2) relaxation rates that can subsequently be analyzed to obtain information about dynamics on a pico- to nanosecond time scale. Sensitivity permitting, the consistency of the data can be evaluated by measuring all five rates that are accessible for a spin 1 particle and establishing that the rates obey relations that are predicted from theory. The utility of the methodology is demonstrated with studies of the dynamics of a 14-mer RNA containing the UUCG tetraloop at temperatures of 25 and 5 degrees C. The high quality of the data, even at 5 degrees C, suggests that the experiments will be of use for the study of RNA molecules that are as large as 30 nucleotides.     

Separation of N2@C60 and N@C60

We describe the HPLC separation and identification of N@C60 and N2@C60. These species were observed after eleven sequential HPLC separations. Their retention times are in the same range as those of the other noninteractive endohedral species of C60, such as noble gas endohedral C60. The separation factors of these endohedrals were evaluated by using a mixture of hexane/toluene as eluent. We note that this is the first evidence for the N2@C60 molecule existing in the form of endohedral C60 complex.     

Interaction of H2@C60 and Nitroxide through Conformationally Constrained Peptide Bridges

We synthesized two molecular systems, in which an endofullerene C₆₀, incarcerating one hydrogen molecule (H₂@C₆₀) and a nitroxide radical are connected by a folded 3₁₀‐helical peptide. The difference between the two molecules is the direction of the peptide orientation. The nuclear spin relaxation rates and the para → ortho conversion rate of the incarcerated hydrogen molecule were determined by ¹H NMR spectroscopy. The experimental results were analyzed using DFT‐optimized molecular models. The relaxation rates and the conversion rates of the two peptides fall in the expected distance range. One of the two peptides is particularly rigid and thus ideal to keep the H₂@C₆₀/nitroxide separation, r, as large and controlled as possible, which results in particularly low relaxation and conversion rates. Despite the very similar optimized distance, however, the rates measured with the other peptide are considerably higher and thus are compatible with a shorter effective distance. The results strengthen the outcome of previous investigations that while the para → ortho conversion rates satisfactorily obey the Wigner's theory, the nuclear spin relaxation rates are in excellent agreement with the Solomon–Bloembergen equation predicting a 1/r⁶ dependence.     

Electron spin relaxation of N@C60 in CS2 in CS2

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S> or =1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60 , indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170 K.     

Synthesis, NMR characterization and reactivity of 1-silacyclohex-2-ene derivatives

The chloro functionality of allyldichlorosilane (HSiCl₂(C₃H₅)) and allyldichloromethylsilane (MeSiCl₂(C₃H₅)) were replaced by alkynyl groups and new compounds, allyldialkynylsilane 1 and allyldialkynylmethylsilane 2, were obtained. These silanes, which served as starting materials for the onward reactions, were purified by fractional distillation. They were further subjected to hydroboration with 9-BBN (9-borabicyclo[3.3.1]nonane) and were converted into 1-silacyclohex-2-ene derivatives 5 and 6. The competition between C≡C and C=C in the reaction was studied. The hydroborating reagent 9-BBN was expected to prefer terminal C=C bonds and to leave C≡C bond untouched. This hypothesis of preferable hydroboration was experimentally proved, and hence, 1-silacyclohex-2-ene derivatives were obtained in reasonably pure form. The reaction of allyldialkynylsilane 2 with one equivalent of 9-BBN affords 1-silacyclohex-2-ene bearing Si-C≡C-function, ready to be hydroborated further with one equivalent of 9-BBN. The obtained compound bears two C-B bonds, which are attractive synthones for further transformations. This study aims to highlight the chemistry of C-B and Si-H functional groups. All new compounds obtained were colorless air and moisture sensitive oils, and they were studied by multinuclear magnetic resonance spectroscopy (¹H, ¹³C, ¹¹B, ²⁹Si NMR) in solution.      

Crystallographic characterization of isomer 2 of Er2@C82 and comparison with isomer 1 of Er2@C82

The X-ray crystal structure of (Isomer 2 of Er2@C82). NiII(OEP).2(benzene) shows that the fullerene cage in Isomer 2 of Er2@C82 is the C3v isomer (82:8) and that the erbium ions are distributed over 23 interior sites with occupancies ranging from 0.25 to 0.03.