  Molecular clouds are essentially made up of atomic and molecular hydrogen,
which in spite of being the simplest molecule in the ISM plays a key role in
the chemical evolution of molecular clouds. Since its formation time is very
long, the H2 molecules can be transported by the turbulent motions within the
cloud toward low density and warm regions, where its enhanced abundance can
boost the abundances of molecules with high endothermicities. We present high
resolution simulations where we include the evolution of the molecular gas
under the effect of the dynamics, and we analyze its impact on the abundance of
CH+.
