Identifying Ordered OH/F Anions in Hydroxyfluorides by a Terahertz Spectroscopic Approach

Distinguishing the OH–/F– ligands has been one of the vital challenges in X-ray crystallography. Fluorine and oxygen atoms have similar scattering cross sections of X-rays, and the hydrogen atoms are invisible to X-rays, rendering the two functional groups F– and OH– indistinguishable from each other with the X-ray diffraction (XRD) method. We demonstrated that terahertz (THz) spectroscopy is promising for solving this problem. THz spectroscopy probes the optical phonon modes given rise by intermolecular interactions. Different F–/OH– substitutions usually lead to remarkable changes in the intermolecular interactions, which can be reflected in THz spectroscopy. Guanidinium fluorooxoborates represent new-generation nonlinear optical materials used in the short-wavelength ultraviolet region. For such materials, a dispute remains regarding the OH–/F– substitution in the B3O3(F, OH)4 anion units, which suggests 16 possible configurations of the anion units. Employing solid-state density functional theory, we showed that all 16 configurations display fingerprint THz peaks. Against the experimental criterion of high-resolution THz spectroscopy, we proved that only one configuration exists, implying that the anion structure is well-ordered. THz spectroscopy provides an essential complementary tool to the XRD method for retrieving detailed structural information. Furthermore, the terahertz vibrational information reveals that the cationic unit of guanidinium is a rigid building block unit with little external and internal vibration mixing, whereas the anionic unit is not rigid and produces vigorous mixing. This finding leads to a deeper understanding of the role of the constituent units in producing the excellent optical properties of the guanidinium fluorooxoborate compounds.