  We study the effect of Coulomb interactions on the low energy band structure
of single-layer transition metal dichalcogenide semiconductors using an
effective low energy model. We show how a large conduction band spin splitting
and a spin dependent Fermi velocity are generated in MoS$_2$, as a consequence
of the difference between the gaps of the two spin projections induced by the
spin-orbit interaction. The conduction band and Fermi velocity spin splittings
found are in agreement with the optical absorption energies of the excitonic
peaks A, B measured in the experiments.
