The gas phase IR spectrum of the O . . .H . . .O fragment of H5O2+ and its deuterated analogue are calculated using ab initio classical molecular dynamics based on a MP2 potential energy surface. The assignment of the bands is made in terms of the quantum four-dimensional model calculations of anharmonic frequencies and intensities. Comparing low and high kinetic temperature simulations the importance of anharmonicities of the potential energy surface for understanding the vibrational band structure is highlighted. It is shown that any reasonable simulation of IR spectra of systems with very strong hydrogen bonds has to account for the dipole moment function beyond the linear approximation.