engleski

Refinement of TLS Matrices from Powder Diffraction Data: Application to Naphthalene

Analysis of thermal motion of molecules in crystals with the assumption of a rigid body can be approached in two ways: (1) TLS matrices can be refined from the independently refined ADPs of each atom and (2) ADP parameters can be a priori constrained to conform to the rigid body assumption. In the latter case the translational (T), librational (L) and the correlation (S) matrices contain the refineable parameters and the ADPs are than calculated from them [1, 2]. The approach (2) is, regarding the number of required parameters, a compromise between the usual practice with powder diffraction data which is to describe the thermal motion of all atoms by a common isotropic Debye-Waller factor and an independent anisotropic refinement of each atom customary with single crystal data sets. The flexible macro scripting language of TOPAS [3] has been used to implement the refinement of TLS matrices from the approach (2) for powder diffraction data and has been applied to crystalline naphthalene for data collected between 293 K and 343 K. The refinement of elements of TLS matrices is performed in the Cartesian coordinate system followed by calculation of each atom’s ADPs in the crystal coordinate system. Results are compared to previous studies [4, 5] on thermal motion in crystalline naphthalene and the feasibility of this approach for routinely collected laboratory powder diffraction data is estimated. The macro scripts are readily applied to other molecular compounds for which the results of refinement of TLS matrices are also presented.

powder diffraction; thermal motion; rigid body; TLS formalism; naphthalene

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