SINGLE CRYSTAL AND POWDER X­RAY DIFFRACTION STUDY OF THE NONINTERDIGITATED VIRGIN STRUCTURE OF n­OCTADECYLAMMONIUM CHLORIDE

E.C. Reynhardt¹, G.J. Kruger¹ and R. Scholtz²

¹Department of Chemistry and Biochemistry, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa.
²Department of Physics, University of South Africa, P.O. Box 392, Pretoria 0003, South Africa.

Keywords: n­octadecylammonium chloride; single crystal x­ray diffraction; powder x­ray diffraction

Depending on the temperature at which crystallization takes place, n­octadecylammonium chloride crystallizes as either an interdigitated or a noninterdigitated structure. X­ray diffraction intensities of a single crystal with the noninterdigitated structure were measured on a Siemens SMART system with a two­dimensional CCD detector. The structure was solved by direct methods (orthorhombic, space group Pna21, a = 70.905(6), b = 5.4505(5), c = 5.3639(5) A, Z = 4). A difference Fourier synthesis and refinement by full­matrix least­squares (1) with anisotropic displacement parameters for all non­H atoms yielded a conventional R = 0.084. The H atoms were placed in calculated positions and refined as riding on their connected atoms with isotropic displacement parameters 1.2 times that of the equivalent value of the heavy atom. The refinement of the absolute structure parameter according to the method of Flack (2) gave a value of 0.4(3). This indicates merohedral twinning, a distinct possibility in a symmetric structure such as this, where spontaneous resolution into race mates is unlikely. The chains are packed in a herringbone structure (Fig. 1) and are not fully extended, but have a kink between the second and third carbon atoms (Fig. 2).

	Fig. 1
	Fig. 2

The average C­C distance and C­C­C angle are 1.507(17) A and 115.2(1.5)°, respectively. The average torsion angle around the C­C bonds for C(4) to (17) is 179.1(8)°. The quaternary N atoms and the chloride ions are hydrogen­bonded through the H atoms in such a way that each N is surrounded by three chlorine atoms.

A powder diffraction pattern of the noninterdigitated compound was recorded on a Siemens D5000 diffractometer. Using the atomic coordinates of the single crystal refinement, a Rietveld refinement (3) was performed successfully. Although the fit between measured and calculated patterns, is not perfect, it illustrates that, by employing sound experimental techniques, a laboratory powder diffractometer can produce data which are good enough to refine a crystal structure, such as the present one, to a point where the resulting atomic coordinates are chemically reasonable.

1. G.M. Sheldrick, SHELX­97, Program for the Refinement of Crystal Structures, University of Göttingen, Germany (1997).
2. H.D. Flack, Acta Cryst. A39 (1983) 876.
3. R.A. Young, J. Appl. Cryst. 28 (1995) 366.