The low frequency of the interstitial molecule can be thought of as due to the leaking of charge from the surrounding bonds screening the intra-molecular proton-electron attraction. If the size of the lattice cage were to be increased, for example, by a defect in the crystal, then the charge density at the centre of the cage due to the surrounding Si would fall. The molecular stretch frequency would then rise towards that of isolated hydrogen molecules.
Several types of extended defects in silicon are known to exist, with associated vibrational bands around 2100 cm-1, so the behaviour of H2 trapped at such sites becomes of interest in understanding the effects of plasma treatment on silicon.