  Recent studies have provided new experimental information on the very
neutron-rich nucleus 135Sb. We have performed a shell-model calculation for
this nucleus using a realistic effective interaction derived from the CD-Bonn
nucleon-nucleon potential. This gives a very good description of the observed
properties of 135Sb. We show that the anomalously low position of the first
excited state, J=5/2+, and the strongly hindered M1 transition to the ground
state have their origin in the effective neutron-proton interaction.
