Blue Shift of a Molecular Crystal Phonon at the Solid to Liquid Phase Transition*

We investigate the dielectric response from the optically active phonons of single-crystal fructose using terahertz (THz) spectroscopic techniques. All samples were indexed by x-ray diffraction with space group of P2₁2₁2₁, lattice constants a = 8.09 Å, b = 9.21 Å, c = 10.03 Å and α = β = γ = 90°. Anisotropic THz absorbance unambiguously assigns the measured resonances to specific lattice vibrations calculated by density functional theory. Using [101] face polished crystals, the lowest frequency resonance polarized along the b axis at 1.70 THz is found to continuously red shift with heating to 1.59 THz at the melting temperature 103°C. As the sample is maintained at 103°C the resonance continues to red shift, broaden and decrease in amplitude with melting. Using [010] face polished crystals, the lowest frequency resonance polarized along the c axis at 1.31 THz is found to red shift with heating to 1.25 THz at 103°C, but then blue shifts to 1.26 THz as the temperature is held at 103°C. The peak then remains at this frequency, broadens and decreases in amplitude as the sample enters the liquid phase. The result is inconsistent with expected softening of intermolecular forces during the phase transition.