The influence of acetonitrile adsorption on the infrared spectrum of an acidic OH group inside a zeolite is studied by theoretical calculations. The zeolite is modeled by a cluster molecule. Potential energy and dipole surfacers of the stretch and two bending coordinates of the acidic H atom, and for the complex with acetonitrile, of an additional acetonitrile stretch coordinate, are computed employing Hartree-Fock as well as density functional methods. Infrared frequencies as well as absorption intensities are computed taking into account mechanical as well as electric anharmonicities up to fourth order. Fermi resonance proposed as the cause of the splitting of the OH stretch absorption bands in infrared spectra is explicitly considered.