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.