Daisuke Kuwahara

Ionic Liquids of Cationic Sandwich Complexes

Simple cationic sandwich complexes that contained alkyl- or halogen substituents provided ionic liquids (ILs) with the bis(perfluoroalkanesulfonyl)imide anion. Ferrocenium- and cobaltocenium ILs [M(C(5)H(4)R(1))(C(5)H(4)R(2))][Tf(2)N] (M=Fe, Co) and arene-ferrocenium ILs [Fe(C(5)H(4)R(1))(C(6)H(5)R(2))][Tf(2)N] were prepared and their physical properties were investigated. A detailed comparison of their thermal properties revealed the effects of molecular symmetry and substituents on their melting points. Their viscosity increased on increasing the length of the substituent on the cation and the perfluoroalkyl chain length on the anion. Upon cooling, ILs with low viscosities exhibited crystallization, whereas those with higher viscosities tended to exhibit glass transitions. Most of these salts showed phase transitions in the solid state. A magnetic-switching phenomenon was observed for the paramagnetic ferrocenium IL, which was associated with a liquid/solid transformation, based on the magnetic anisotropy of the ferrocenium cation. (57)Fe Mössbauer spectroscopy was applied to [Fe(C(5)H(4)nBu)(2)][Tf(2)N] to investigate the vibrational behavior of the iron atom in the crystal and glassy states of the ferrocenium IL.

Crystal Structures and Phase‐Transition Dynamics of Cobaltocenium Salts with Bis(perfluoroalkylsulfonyl)amide Anions: Remarkable Odd–Even Effect of the Fluorocarbon Chains in the Anion

Crystal structures and thermal properties of cobaltocenium salts with bis(perfluoroalkylsulfonyl)amide (C(n)F2(n+1)SO2)2N anions [n = 0 (1), 1 (1 a), 2 (1 b), 3 (1 c), and 4 (1 d)] and the 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonylamide anion (2) were investigated. In these solids, the cations are surrounded by four anions around their C5 axis, and stacking of these local structures forms two kinds of assembled structures. In the salts with even n (1, 1 b, and 1 d), the cation and anion are arranged alternately to form mixed-stack columns in the crystal. In contrast, in the salts with odd n (1 a and 1 c), the cations and anions independently form segregated-stack columns. An odd-even effect was also observed in the sum of the phase-change entropies from crystal to melt. All of the salts exhibited phase transitions in the solid state. The phase transitions to the lowest-temperature phase in 1, 1 a, and 2 are accompanied by order-disorder of the anions and symmetry lowering of the space group, which results in the formation of an ion pair. Solid-state (13)C NMR measurements on 1 a and 1 b revealed enhanced molecular motions of the cation in the higher-temperature phases.

High-Resolution Solid-State NMR Investigation of the Phase Transition in Decamethylferrocene−Acenaphthenequinone Charge-Transfer Complex

A charge-transfer complex composed of decamethylferrocene (D) and acenaphthenequinone (A) was prepared. The material was a 1:1 neutral complex with a mixed-stack structure and exhibited a phase transition at -16 degrees C. High-resolution (13)C and (1)H NMR spectroscopy revealed that an inclination of A with respect to D occurs below the phase-transition temperature. The (1)H spin-diffusion rates of the complex undergoing high-speed magic-angle spinning (MAS) were measured to determine the shortest (1)H-(1)H distance r between D and A. To analyze the experimental results, we derived the analytical expression of the spin-diffusion rate W(z) for a homonuclear multispin system undergoing MAS. It was found that W(z) for the complex is proportional only to 1/r(6) under high-speed MAS conditions. On the basis of this relationship and the crystal structure at 20 degrees C, it was determined that the shortest (1)H-(1)H distance r at -27.7 degrees C (below the phase transition temperature) is 0.4 A shorter than that at 20 degrees C. Given this information, a plausible model of the low-temperature structure is discussed.

SPINNING SIDEBAND SUMMATION FOR QUADRUPOLE MAS NMR SPECTRA USING A MAGIC-ANGLE TURNING TECHNIQUE

Abstract Two-dimensional (2D) magic-angle spinning (MAS) NMR measurements for unravelling the overlapping spinning sidebands (SSBs) for quadrupolar nuclei having half-integer spins were reported; the method utilizes a magic-angle turning technique recently exploited for chemical-shift spectra. The projection of the resultant 2D spectrum reconstructs the inherent MAS lineshapes without SSBs, which can be straightforwardly analyzed to yield quadrupolar parameters. The SSB-free 23 Na MAS spectrum for Na 2 SO 4 , which would otherwise be observed under an infinite spinning speed, was obtained.

Nanocomposite materials of alternately stacked C 60 monolayer and graphene

We synthesized the novel nanocomposite consisting alternately of a stacked single graphene sheet and a C60 monolayer by using the graphite intercalation technique in which alkylamine molecules help intercalate large C60 molecules into the graphite. Moreover, it is found that the intercalated C60 molecules can rotate in between single graphene sheets by using 13C NMR measurements. This preparation method provides a general way for intercalating huge fullerene molecules into graphite, which will lead to promising materials with novel mechanical, physical, and electrical properties.

Simulations of the 13C–14N REDOR NMR experiments performed with a frequency-sweep 14N overtone pulse

Abstract The author investigated the 13C–14N rotational echo double resonance (REDOR) NMR experiment performed with a frequency-sweep 14N overtone pulse. The computer simulations of the experiment for powdered N-acetyl- d,l -valine were carried out. It was found from the simulations that, in a slow spinning region, 13C diminished echo intensities remained invariant even when the center frequency of the frequency-sweep range was shifted a few tens of kHz. As a result, it became apparent that the REDOR experiment employing the frequency-sweep pulse had the adequate ability to determine the heteronuclear dipolar interaction for a 13C–14N spin system in solids.

Spinning-sideband suppression in DOR NMR spectra using a magic-angle-turning technique

Abstract Spinning sidebands (SSBs) in double rotation NMR spectra for quadrupolar nuclei having half-odd-integer spins were suppressed using a two-dimensional (2D) NMR method based on a magic-angle-turning technique. The resultant 2D spectrum showed substantially no SSBs along one frequency axis whereas, along the other axis, conventional SSBs exhibited themselves.

Simulation of homonuclear two spin-12 systems under sample spinning: Effects of jump motions on NMR lineshapes

Abstract A method for the calculation of NMR lineshapes is presented for homonuclear two spin- 1 2 systems performing jump motions in rotating solids. The NMR spectra of such spin systems were found to be useful for detecting the existence of conformational changes as well as that of molecular motions in the solid state spin systems.

Novel satellites in a two-dimensional spin-echo NMR experiment for homonuclear dipole-coupled spins in rotating solids

Abstract Additional resonance lines emerged along the F 1 axis on a two-dimensional (2D) NMR spectrum, when the 2D spin–echo pulse sequence was applied to a solid-state homonuclear two-spin system with a negligibly small J -coupling constant under magic-angle sample spinning. We found that the positions of the lines are determined by the sample spinning frequency and the difference of the isotropic chemical shifts.

Real figure of two-dimensional spin-echo NMR spectra for a homonuclear two-spin system in rotating solids

Two-dimensional (2D) spin-echo NMR experiments have been carried out on polycrystalline [2,3-13C2]-alanine under magic-angle sample spinning (MAS) conditions, so that two unusual resonance lines emerged along the F1 axis (Kuwahara, D., Nakai, T., Ashida, J., and Miyajima, S., 1999, Chem. Phys. Lett., 305, 35). To examine the spectral structure observed in the F1 direction more closely the 2D NMR experiment was undertaken using a sufficiently small tl increment, yielding many more resonance lines on a spectrum sliced along the F1 axis. The line distribution had a very unique and interesting structure. To elucidate the line positions theoretically, the signal for the 2D spin-echo experiment performed with any t1 increment was calculated analytically for a homonuclear two-spin-1/2 system undergoing MAS. Virtually six resonance lines (exactly 12 resonance lines) occurred on a spectrum sliced along the F1 axis. In addition, it was demonstrated that the intensities of some resonance lines were largely dependent on the dipolar interaction.