Measurements of infrared polarized reflectance spectra of single-crystal have been made at room temperature in the wavelength range from 30 to for different orientations of the wave vector k and the electric field E of the incident radiation relative to the crystallographic axes. On the basis of Born and Huang's theory, relations between characteristics of TO modes of the monoclinic lattice and the reflectivity for the orientations used are established. Dipole momentum orientations, frequencies, strengths and linewidths of almost all of the IR-active modes predicted by the factor-group analysis for the were determined. Good agreement between experimentally measured and model spectra was achieved. It was shown that in order to obtain reliable values of parameters of modes three spectra for different orientations of E in the ac-plane should be fitted simultaneously. It was found that modes have significantly greater intensities than modes. The effect of the rotation of the principal dielectric axes within the ac-plane was studied.