Dynamics of n-hexane inside silicalite, as studied by 2H NMR

Perdeuterated n-hexane, adsorbed in silicalite, has been found to exhibit a H-2 NMR spectrum that represents a superposition of three Pake-powder patterns. Two Pake-powder patterns, with the spectral parameters C-Q = 23.5 kHz and eta = 0 and C-Q = 30.1 kHz and eta = 0 (C-Q represents quadrupole constants and eta is the asymmetry parameter), belong to different methylene groups and one powder pattern, with C-Q = 7.0 kHz and eta = 0.7, arises from the methyl groups. The observed line shapes have been concluded to arise from the fast anisotropic translational and rotational motions of the alkane in the silicalite channels. The reduction of the quadrupole constants, in comparison to that in rigid n-hexane, and the appearance of an asymmetry parameter for the methyl groups have been interpreted in terms of three modes of motion exhibited by n-hexane inside the channel system: (a) rotation of the molecule about the channel axis; (b) trans/gauche conformational isomerization around nonterminal C-C bonds and rotation of the methyl groups around terminal C-C bonds; and (c) fast interchange of the adsorbed molecules between both straight and zigzag channels and along zigzag channels with an effective angle of similar to90degrees between adjacent sites of the molecule location during successive jumps. The proportion percentage of trans/gauche conformations is estimated to be 78/22, which exceeds that in gas phase or liquids. This demonstrates that constraining effects of the narrow silicalite pore walls retain the molecules in a more elongated conformation, compared to that in a gas phase or liquid. The rotation of a methyl group around a terminal C-C bond has been estimated to occur with a characteristic time of similar to40 ps at 300 K and an activation energy of 9.4 kJ/mol. The reorientation of methylene groups, corresponding either to conformational isomerization or to translational diffusion, occurs I order of magnitude slower, with a characteristic time of similar to300 ps and an activation energy of 7.5 kJ/mol.