We study the infrared absorption spectra of ultrathin SiO2, films on Si(100) using a first-principles approach, and adopting a model Si(100)-SiO2 interface with a realistic transition structure. We calculate both the transverse-optical and the longitudinal-optical infrared absorption spectra across the interface, and show that the red shift of the high-frequency peaks observed experimentally with decreasing oxide thickness originates from the softer vibrational frequencies of the substoichiometric interfacial layer. From the calculated infrared properties, we are able to assess the effect of the frequency softening on the corresponding static permittivity.