Infrared spectra of the two ortho-Mannich bases 2-(N,N-dimethylaminomethyl)-4-chlorophenol (I) and 2-(N,N-dimethylaminomethyl)-4,6-dichlorophenol (II) with a medium-strong intramolecular hydrogen bond, were measured in a wide frequency (4000-50 cm^-1) and temperature (-105°C to +120°C) range in liquid Xe, CCl₄ solutions and gas phase. The temperature studies of the first and second spectral moments of the ν_s(OH) band in liquid Xe within the temperature range from -105 to +30°C allow for the analysis of the ν_s(OH) mode coupled with the low-frequency vibrations (ν_l) of the non-linear hydrogen bond. Transitions located at ca. 150, 250 and 340 cm^-1 were selected as such ν_l modes. The effective coupling parameter between high-frequency ν_s(OH) and low-frequency ν_l vibrations of the hydrogen bridge was evaluated: b = α_ssl/ω_l {reversed tilde equals} 0.7. Model calculations accounting for the hot and combination transitions give satisfactory reproduction of the gas phase ν_s(OH) and ν_s(OD) bands shapes. Both temperature experiments and model calculations predict that even at 0 K the halfwidth of the ν_s(OH) band can be quite large, viz. ca. 200-400 cm^-1.