A semi-empirical AMl study of hydrogen bonding and protonation in water has been carried out to illustrate the two different O-H vibrational frequencies present in the protonated water dimer, H5O2+. Using the MOPAC quantum mechanical program, it is shown that the optimized geometry for this species is C2h, with an O-H bond distance of 1.20Å, and the fifteen normal frequencies have been determined. Results predict the two O-H vibrational frequencies at 553 and 3300 cm−1, with the former being the H-bond stretching mode. A grid calculation illustrating how the potential energy surface of water varies with the H-O-H angle and the O-H bond distance is also presented.