title: Intermolecular anharmonicity in molecular crystals: interplay between experimental low-frequency dynamics and quantum quasi-harmonic simulations of solid purine.
abstract: The intermolecular anharmonic potential of crystalline purine is probed by means of temperature-dependent terahertz time-domain spectroscopy, low-frequency Raman scattering, X-ray diffraction, and ab initio quasi-harmonic quantum-chemical simulations. As temperature increases, anharmonicity in the intermolecular interactions results in strongly anisotropic thermal expansion - with a negative thermal expansion along the b crystallographic axis - yielding corresponding bulk structural modifications. The observed thermally-induced shifts of most vibrational bands in the terahertz region of the spectra are shown to arise from volume-dependent thermal changes of the hydrogen-bond pattern along the a and b crystallographic axes.
authors.affiliation: Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS, UK. mtr34@cam.ac.uk.
authors.forename: Michael T
authors.identifier: http://orcid.org/0000-0003-1848-2565
authors.initials: MT
authors.lastname: Ruggiero
authors.affiliation: Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS, UK. mtr34@cam.ac.uk.
authors.forename: J Axel
authors.identifier: http://orcid.org/0000-0002-4958-0582
authors.initials: JA
authors.lastname: Zeitler
authors.affiliation: Dipartimento di Chimica, Universitá di Torino, via Giuria 5, Torino, Italy.
authors.forename: Alessandro
authors.identifier: http://orcid.org/0000-0002-2986-4254
authors.initials: A
authors.lastname: Erba