¹H NMR Study of Molecular Motion of Benzene and n-Decane Confined in the Nanocavities of Metal–Organic Frameworks

Abstract

The molecular motion of benzene and n-decane confined in the nanocavities of [Zn4O(O2CC6H4CO2)3] n (IRMOF-1) was investigated in terms of the temperature dependence of the proton spin–lattice relaxation time (1H T 1). Both substances exhibited two components of T 1, below 216 K for benzene and 181 K for n-decane, indicating that the guest molecules are localized separately in large and small cavities. For n-decane below 181 K, methyl reorientation is excited although the rate of reorientation in large and small cavities differed, whereas the overall motion was frozen. A thermal anomaly accompanies the translational diffusion of n-decane and is associated with melting of the confined n-decane. For benzene, translational diffusion was also excited at the thermal anomaly at 216 K, indicating melting of benzene molecular assemblies in IRMOF-1. Below 216 K, the molecular motions in the large and small cavities differed. In the large cavities, benzene undergoes isotropic reorientation, which is unaffected by the thermal anomaly at 150 K, whereas in small cavities in-plane C 6 reorientation occurs, and the isotropic reorientation and the intracavity diffusion are successively excited as the temperature increases. In particular, the acceleration of the isotropic reorientation of benzene accompanies the thermal anomaly at 150 K, corresponding to the partial melting of molecular assemblies in the small cavities.

Structure and Dynamics of Confined C-O-H Fluids Relevant to the Subsurface: Application of Magnetic Resonance, Neutron Scattering, and Molecular Dynamics Simulations

“…Although the confined molecules may not always form ordered structure in a micropore, micropores can be organized in an ordered pattern utilizing the periodicity of a crystal. This establishes long-range regular arrangement of the molecules under confinement and collaborative molecular assembly in microporous matrix systems (Ueda et al, 2012). Such a candidate of porous system with well-defined pore structure and high crystallinity is isoreticular metal-organic frameworks (IRMOFs), a series of porous compounds with regular and homogeneous pores (Eddaoudi et al, 2001).…”

“…Such a candidate of porous system with well-defined pore structure and high crystallinity is isoreticular metal-organic frameworks (IRMOFs), a series of porous compounds with regular and homogeneous pores (Eddaoudi et al, 2001). Among the IRMOFs, [Zn 4 O(BDC) 3 ]n (BDC = benzene-1,4-dicarboxylate) (IRMOF-1) is the prototype of a series of these compounds where zinc oxide and terephthalic acid moieties are three-dimensionally connected with each other establishing a porous framework resembling "a jungle gym" (Ueda et al, 2012).…”

Nuclear spin relaxation in liquids and gases

“…Ueda et al 329 studied the motion of benzene and n-decane as guests within micropores of a metalorganic framework (MOF) system. They measured temperature dependence of 1 H T 1 and were able to characterize the molecular dynamics of both guests.…”

Local structure and dynamics of benzene confined in the IRMOF-1 nanocavity as studied by molecular dynamics simulation

“…Recently, from our own focus on the properties of guest molecules confined in IRMOF-1, we found that several guest molecules adsorbed in IRMOF-1 exhibit thermal anomalies contracting with the melt and/or solid-solid phase transitions, and it was also discovered that these correspond to the phase transition derived from the guest molecules. [24][25][26] In the case where benzene is the guest, two thermal anomalies were observed at 216 K and 150 K. 25,26 Both of these peaks were detected at lower temperatures than the melting point of liquid benzene at 279 K, implying that the solid-solid transition takes place in benzene within the IRMOF-1 framework; this transition is not observed in the bulk phase. Our previous study revealed the dynamical structures of the guest molecules at various temperatures using solid-state 1 H NMR, and elucidated the correlation between the phase transition and the dynamic behaviour of benzene.…”

“…Our previous study revealed the dynamical structures of the guest molecules at various temperatures using solid-state 1 H NMR, and elucidated the correlation between the phase transition and the dynamic behaviour of benzene. 26 Based on the temperature dependence of 1 H T 1 , the local structure and the dynamic behaviour of benzene in IRMOF-1 could be clarified. Occurrence of the solid-solid phase transition at 150 K leads to acceleration of the isotropic reorientation, accompanying intra-cavity migration of benzene in the small cavities, but there is no modulation of the molecular motion of benzene in the large cavities.…”