Intermolecular anharmonicity in molecular crystals: interplay between experimental low-frequency dynamics and quantum
Michael T Ruggiero1, J Axel Zeitler1, Alessandro Erba2
1Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS, UK. mtr34@cam.ac.uk.
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.
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