Abstract
A recently discovered β polymorph of 1,3-diacetylpyrene has turned out to be a prominent negative thermal expansion material, with a linear thermal expansion coefficient of -199 (6) MK(-1) at room temperature. Its unique properties can be linked to anharmonic oscillations in the crystal structure that can be attributed to several weak C-H...O interactions. The onset and development of anharmonic effects have been successfully tracked over a wide (90-390 K) temperature range using single-crystal X-ray diffraction. Experimental data of sufficient quality combined with Hirshfeld atom refinement of the crystal structures enabled a quantitative analysis of elusive anharmonic effects within the Gram-Charlier formalism. This example highlights that quantum crystallography tools can and should be applied in structural analysis even for data collected at high temperatures as well as of standard (∼0.8 Å) resolution.