Abstract
The harmonic model of atomic nuclear motions is usually enough for multipole modelling of high-resolution X-ray diffraction data; however, in some molecular crystals, such as 1-(2'-aminophenyl)-2-methyl-4-nitro-1H-imidazole [Paul, Kubicki, Jelsch et al. (2011 ▶). Acta Cryst. B67, 365-378], it may not be sufficient for a correct description of the charge-density distribution. Multipole refinement using harmonic atom vibrations does not lead to the best electron density model in this case and the so-called 'shashlik-like' pattern of positive and negative residual electron density peaks is observed in the vicinity of some atoms. This slight disorder, which cannot be modelled by split atoms, was solved using third-order anharmonic nuclear motion (ANM) parameters. Multipole refinement of the experimental high-resolution X-ray diffraction data of 1-(2'-aminophenyl)-2-methyl-4-nitro-1H-imidazole at three different temperatures (10, 35 and 70 K) and a series of powder diffraction experiments (20 ≤ T ≤ 300 K) were performed to relate this anharmonicity observed for several light atoms (N atoms of amino and nitro groups, and O atoms of nitro groups) to an isomorphic phase transition reflected by a change in the b cell parameter around 65 K. The observed disorder may result from the coexistence of domains of two phases over a large temperature range, as shown by low-temperature powder diffraction.
Highlights
When using accurate ultra-high-resolution X-ray diffraction data, the most commonly used harmonic model of the atomic nuclear motions may not be sufficient for some molecular crystals, even for lighter atoms
The aim of this paper is an attempt to relate this anharmonic refinement to the isomorphic phase transition by analysing several additional X-ray single-crystal diffraction experiments performed for 1, including a high-resolution full data collection at 10 K on an Agilent Technologies SuperNova diffractometer, accurate full data collections at 35 and 70 K using the homemade mini-goniometer system implemented on an Orange top-loading cryostat on the CRM2 Bruker AXS APEX II diffractometer (Fertey et al, 2007), and temperaturedependent unit-cell parameter determination from powder diffraction patterns collected on a Panalytical X’Pert Pro diffractometer
The aim of this study was to show the link between anharmonicity and isomorphic phase transition of a molecular crystal
Summary
When using accurate ultra-high-resolution X-ray diffraction data, the most commonly used harmonic model of the atomic nuclear motions may not be sufficient for some molecular crystals, even for lighter atoms. Iversen et al (1999) distinguished anharmonic nuclear motions from static electron density features in a thorium complex structure using extremely high-resolution (1.7 A À1) data from two very lowtemperature experiments (at 9 and 27 K), Henn et al (2010) were able to separate both contributions for lighter atoms (namely P atoms) at lower resolution (1.15 A À1) at 100 K. 1,3,5,7-tetranitro-1,3,5,7tetraazacyclooctane (HMX), which has a slightly more compact crystal structure, the refined ANM parameters were statistically significant, their effect on the resulting charge-density deformation and Laplacian maps was rather negligible
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