The effect of spatial confinement on the properties of isoelectronic molecules HF, H2O, NH3, and CH4 has been studied by encapsulating them in a C-60 fullerene cage. Second-order Moller-Plesset perturbation theoretical (MP2) calculations suggest that all the guest species are stable inside the fullerene cage. This stabilization arises from the dispersion interaction between the guest and the host. It is shown that the excitation energy (E-o* - E-sigma) for the X-H bond increases and that there is a blue shift in the stretching frequencies due to confinement.