The occurrence of an ultrathin SiO2 oxide layer at the interface between silicon and high-k dielectrics in metal-oxide-semiconductor devices contributes to degrading the capacitance of the gate stack. In this work, we investigate the dielectric and infrared properties of atomically thin SiO2 layers on Si(100) through a fully quantum-mechanical description. For this purpose, we construct atomistic models of the Si(100)-SiO2 interface on the basis of available experimental data, by using both classical and first-pninciples simulation methods. Our model structures account for the experimental density of coordination defects, the distribution of partially oxidized Si atoms, the oxide mass density profile, and the lateral displacements of the Si atoms in the channel region. Our first principles calculations indicate that the permittivity of the SiO2 layer departs from the bulk value in the limit of atomically thin oxides. This departure is well described through the consideration of an interfacial suboxide layer with a thickness of about 0.5 nm and a dielectric constant of about 6-7. As a consequence, the equivalent oxide thickness of the interfacial layer is smaller than the corresponding physical thickness by 0.2-0.3 nm. Variations of the local dielectric screening occur on length scales corresponding to first-neighbor distances, indicating that the dielectric transition is governed by the chemical grading. We find that the enhanced ionic screening in the substoichiometric oxide results from Si-O bonds formed by Si atoms in the partial oxidation state Si+2. We also extend our investigation to the infrared absorption at the Si(100)-SiO2 interface. Our study allows us to shed light on the pronounced thickness-dependent red shift of the oxygen stretching modes, which has so far not found a definite interpretation. Indeed, our calculations clearly show that the red shift results from a softening of the Si-O stretching vibrations in the interfacial layer.