  A theoretical description of IR spectroscopy data for a hydrogen bond (HB) is
constructed on the base of trigonometric double-well potential for which an
exact analytic solution of the one-dimensional Schr\"odinger equation (SE) is
available. The wave functions (full orthogonal basis) are expressed via the
spheroidal function while its spectrum of eigenvalues yields the corresponding
energy levels (both special functions are implemented in {\sl {Mathematica}}).
Then an approximate solution of two-dimensional SE taking into account the
excitation state of heavy atoms stretching mode in HB is obtained. It is
constructed by decomposing over the above mentioned basis within the framework
of standard adiabatic separating the proton motion from that of the heavy
atoms. We exemplify the general theory by calculating the IR relative
absorption intensities for HB in the Zundel ion ${\rm{H_5O_2^{+}}}$ (oxonium
hydrate).
