Dynamics of bromohexadecane and bromodecane chains included in an incoherent quasielastic neutron scattering study an organic matrix

Abstract

The nitroxide C 9 H 16 NO 2 (tano) forms channel inclusion compounds with a large range of linear chains or slightly branched molecules. All of them present disorder phenomena of the guest chains in the channels of the structure, but also of the host matrix molecules from one chiral form to the other. The evolution of this disorder as a function of the temperature is accompanied by one or two phase transitions above 100 K. The incoherent quasielastic neutron scattering (IQNS) technique was used to study the motion of the tano molecules for the tano/1-bromohexadecane and the tano/1-bromodecane systems in their low, intermediate and high temperature phases. The experiments were carried out on a polycrystalline sample at the Institut Laue± Langevin in Grenoble using the time-focusing time-of-¯ight spectrometer IN6 with a temperature ranging from 103 K to 300 K. The motion of the guest chains also was shown. The results are examined in terms of the general model proposed in an earlier IQNS study of other tano/alkane compounds with shorter included chains and the original features associated with the chain length are discussed. IntroductionThe nitroxide radical 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl, C 9 H 16 NO 2 , (® gure 1), referred to as tano, forms stable crystalline inclusion compounds with linear n-alkanes C n H 2n+ 2 for n ranging from 7 to 44 at least [1± 5]. These compounds show analogies with urea or thiourea inclusion compounds [6]. However, the percentage of alkane to tano is only 5% by weight.At room temperature, X-ray analyses show that all the alkane inclusion compounds are isomorphous: space group C2/c, Z = 4 and their crystallographic parameters di er by less than 1%. Typically, for tano-heptane at T = 233 K, a = 36´0°A, b = 5´95°A, c = 35´5°A, b = 120 Â . The guest molecules are enclosed end-to-end into parallel channels, having a diameter of 5 A Ê and being 18 A Ê apart [1, 2]. X-ray rotation photographs show homogeneous di use layers perpendicular to the direction of the b axis, i.e., the long axis of channels. Their spacing is strictly related to repetition length L of the alkane chains along the channels. There is an incommensurability between L and b. The thickness of the di use planes supports the good periodicity of the