An external charge when brought close to a conductor induces bound charges on the nearer side and free charges on the further side so that the field induced in the conductor can counterbalance the field due to the external charge,right? Now in case of 2 concentric spheres, if the outer one is charged & the inner one is grounded, then the field inside the outer sphere is zero everywhere,then no charge should be induced on the inner sphere as well,then by Gauss' Law,field in between the spheres remains zero causing zero potential difference and thus no question of capacitance comes,right? How is that possible???
Assuming your spheres are conducting. How did you prove that between the spheres electric field would be zero ? All we can say is that inside the inner sphere potential will be zero. Lets say the outer sphere originalQy had charge Q and had potential of kQ/R now after grounding we can say that inner sphere gets a charge q and gives potential inside itself as kq/r, The sum of these two potentials inside the conductor will be zero. But we have not prove that field is zero between the two spheres.