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Low-frequency vibrational modes of benzoic acid investigated by terahertz time-domain spectroscopy and theoretical simulations

In this paper, the low-frequency vibrational modes of crystalline benzoic acid (BA) have been investigated by terahertz time-domain spectroscopy (THz-TDS) and theoretical simulations based on the linearity combination of atomic orbital within the Density Functional Theory (DFT) as well as ab initio molecular orbital method at second-order Moller-Plesset Perturbation Theory (MP2) level for single molecule and dimer. Experimentally, a series of prominent absorption features of pure benzoic acid relevant to intra- and inter-molecular vibrational modes have been obtained below 4 THz at room temperature. For the theoretical simulations, geometry-optimization results of bond lengths and dihedral angles in both BA monomer and dimer are very close to experimental neutron diffraction measurements. Furthermore, the simulation results demonstrate absorption profile centered at 1.89 THz contains low-frequency modes of Ph-COOH twisting due to intramolecular motion and cogwheel owing to intermolecular motion. All the intra- and inter-molecular vibrational modes measured have also been assigned.