In Rutherford experiment, correct angle of scattering of $\alpha$ particle for impact parameter equal to zero is  

Impact parameter is the closest distance the alpha particle would have been to the nucleus had there been no Coulomb repulsion.
Zero impact parameter means that it is directed towards the nucleus and would, thus, simply bounce that due to force of repulsion acting along the line of motion. Thus, the scattering angle is 180^∘.