Spin-Probe Study on the Dynamics and Distribution of Solution Molecules in the Nano-Channel of MCM-41

Abstract

The ESR spectrum of a spin-probe in 2-propanol, which was condensed into the nano-channel of MCM-41 from its vapor, showed three well-separated nitrogen hyperfine lines. This observation indicates that the system exists in the liquid state in the nano-channel, though the rotational diffusion of the spin probe molecule is considerably suppressed and anisotropic. In a mixed solvent composed of 2-propanol and cyclohexane, on the other hand, the ESR spectrum for the system prepared by the above procedure comprised two components, indicating that the solution is phase-separated in the nano-channel. When all of the space in the MCM-41 layer, including that among the granules, is filled with the solution, the molecules obtain a higher mobility and phase separation is suppressed even in the nano-channel. The distribution and dynamics of the solution molecules are discussed.

ESR study of molecular dynamics and orientation of TEMPO included in organic 1-D nanochannel

“…In fact, the dynamics and orientation of TEMPO adsorbed in typical alumino-silicates zeolites and mesoporous silicas have been reported: cation-exchanged X-type faujasite zeolite 32 and MCM-41. 33 In the former, TEMPO is accommodated in an isotropic spherical void of 1.1-1.3 nm, where TEMPO was undergoing isotropic rotation with activation energies comparable to those used for this study: 6.6 kJ mol À1 in Li-X and 11.0 kJ mol À1 in Na-X. In the latter case, TEMPO was confined in a homogeneous cylinder-type mesopore with ca.…”

“…Therefore, the electron spin resonance (ESR) spin probe technique is expected to be useful for examining the molecular dynamics and orientation of TEMPO in TPP nanochannels in TPP/TEMPO IC. In fact, ESR is known as an excellent procedure to investigate the structure, dynamics, and orientation of the probe-molecule surroundings and spin probe molecules themselves; it has been applied extensively to polymers, [27][28][29] liquid crystals, 30,31 zeolites, 32,33 nano-scale magnetic materials, 34,35 biomaterials, 36,37 and so forth. In solvents, TEMPO molecules typically give an isotropic triplet absorption line with the same peak intensities and isotropic hyperfine coupling constants as those obtained with 1.515 mT in benzene and 1.520 mT in acetone; 17 in polycrystalline samples, they give powder spectra characterized by axially symmetric g tensor and hyperfine (A) tensors.…”

Electron spin resonance (some chemical applications)

“…When all of the available space in the MCM-41 layer, including that among the granules, is filled with the solution, the absorbed molecules attain a yet higher mobility, and phase-separation is suppressed, even in the nanochannel. 117 In a related investigation, the molecular ''packing'' of long-chain n-alkanes in the MCM-41 nanochannel was examined. In this study, g-irradiation at 77 K of the n-alkanes, n-decane, n-undecane, n-dodecane and n-tridecane, as sorbed into MCM-41, was observed to produce radicals that could be detected by ESR.…”

Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy

“…The use of nitroxide-based paramagnetic probe molecules is advantageous owing to their stability, small size (∼0.7–0.9 Å) of the nitroxide moiety, and rather localized spin density of the EPR active unpaired electron . Continuous wave (CW) EPR spectroscopy in combination with paramagnetically labeled guest molecules has been applied − for the investigation of dynamics of the guest molecules inside MCM-41 by Okazaki et al Also, CW EPR has been applied for investigation of dynamics of spin labeled guest molecules inside UKON2a showing that the host–guest interactions in UKON materials strongly depend on the interplay of the polarities of solvent, guest molecules and the surface functionalization. However, CW EPR does not provide direct access to the spatial distribution of the guest molecules inside the pores.…”