: Single crystals of tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid [CH3CH2CH2]4N (H2AsO4) (H3AsO4)2, a potential new nonlinear optical (NLO) material of interest were prepared by the slow evaporation technique and characterized by means of single-crystal X-ray diffraction, thermal analysis, FT-IR and Raman spectroscopy. The title compound belongs to the monoclinic space group Ia with the following unit cell dimensions: a=8.116(2) Ǻ, b=33.673(4) Ǻ, c=8.689(2) Ǻ, β=95.34(2)°. The structure consists of infinite parallel two-dimensional planes built of mutually [H2AsO4(-)] and [H3AsO4] tetrahedra connected by strong O-H⋯O hydrogen bonding giving birth to trimmers. The planes of inorganic groups are alternated with those of the organic cations. The geometry, first hyperpolarizability and harmonic vibrational wavenumbers were calculated by means of density functional theory DFT with the B3LYP/6-31G(d) level of theory. Good consistency was found between the calculated results and the experimental structure, IR, and Raman spectra. The detailed interpretation of the vibrational modes was carried out building on the proposed DFT calculations as primary source of assignment and by comparison with the spectroscopic studies of similar compounds. The first hyperpolarizability βtot of the title compound is about 14.6 times more than that of the reference crystal KDP, which may explain the importance of the compound under study.Read less <