An NMR investigation of naphthalene nanostructures

ISSN： 1362-3028

Source： Molecular Physics, Vol.102, Iss.19-20, 2004-01, pp. : 1997-2005

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Abstract

The molecular mobility of naphthalene molecules in porous silica has been studied over the temperature range 223 K to 363 K using NMR relaxation times T 2 , T 1 and T 1&rgr; . The investigations were conducted in silicas with nominal pore diameters of 4 nm, 6 nm, 10 nm, 20 nm and 50 nm. The confined solid behaved in a way that indicated it formed a dual phase system consisting of a solid core in the centre of the pores surrounded by a mobile surface layer. The core naphthalene had the same line width as the bulk. The surface layer exhibited a narrower line of a width that suggested the onset of motional narrowing. This behaviour was characteristic of a plastic crystal phase for naphthalene that does not exist in the bulk. The T 1 and T 1&rgr; results were dominated by surface interactions between the confined naphthalene and the pore wall. Magnetization transfer experiments showed that enhanced relaxation occurred throughout the confined material in a time long compared to T 2 but short compared to T 1 and T 1&rgr; . Since the line shape ruled out diffusional motion through the rigid lattice naphthalene core, the magnetization transfer must have occurred via spin diffusion.