  For triatomic chemical reactions under single-collision conditions, we
propose a new quasi-classical trajectory (QCT) approach to rotational-state
distributions of particular interest in the quantum regime where only a few
rotational states are available to the products. Our method is directly
inspired from the amendments to be introduced in classical phase space theory
(PST) in order to make it in exact agreement with quantum PST. The method is
applied to the D$^+$ + H$_2$ and H$^+$ + D$_2$ reactions and the population of
the rotational ground state is found to be in much closer agreement with the
exact quantum one than the same population obtained by means of standard QCT
calculations. The impact on the whole distribution is all the stronger as the
number of available states is small. Last but not least, the shape of the
distribution appears to be controlled to a large extent by three factors,
respectively called parity, edge and rotational shift factors.
