Advanced quantum mechanical calculations within ab initio molecular orbital theory and density functional theory were performed using GAUSSIAN98 programs in quantitative determination of hydrogen bond (H-bond) energies between various model compound pairs. Model compounds studied contained functional groups or segments that were similar to those in segmented polyurethanes and polyureas. These model compounds included urea, 1,3-dimethylurea, 1,3-dimethylcarbamate, diethyl ether, methyl acetate and ethyl alcohol. Optimized conformations, H-bond energies and H-bond lengths of the complexes were determined. Quantum mechanical calculations indicated that based on relative magnitudes of H-bond energies, appreciable amount of phase mixing between hard and soft segments in polyether or polyester based polyurethanes and polyureas should be expected. Vibrational spectra of individual compounds and their hydrogen-bonded complexes (with themselves and other compounds) were determined. Correlation between theoretical and experimental spectra was found to be very good.