Interpretation of temperature-dependent low frequency vibrational spectrum of solid-state benzoic acid dimer

Temperature-dependent low frequency vibrational absorption spectrum of solid-state benzoic acid dimer has been interpreted, using ab initio method at the MP2 level, density-functional-theory (DFT) calculations done by different DFT functionals (PBE, B3LYP, and MPWB1K) including the anharmonic potential, and DFT-based periodic wave function calculations. The assignment is successful by assuming that the vibrational absorption signals at room temperature are from dimer molecules under packing force by neighboring molecules in crystal and that those at low temperature are from intralayer tetramers connected by weak hydrogen bond and/or a periodic order form via hydrogen-bonded network.