  To study the chemical evolution during the formation of molecular clouds, we
model three types of clouds with different density structures: collapsing
spherical, collapsing ellipsoidal, and static spherical profiles. The
collapsing models are better than the static models in matching the
observational characteristics in typical molecular clouds. This is mainly
because the gravity can speed up the formation of some important molecules
(e.g., H$_2$, CO, OH) by increasing the number density during collapse. The
different morphologies of prolate, oblate, and spherical clouds lead to
differences in chemical evolution, which are mainly due to their different
evolution of number density. We also study the effect of initial chemical
compositions on chemical evolution, and find that H atoms can accelerate OH
formation by two major reactions: O + H $\rightarrow$ OH in gas phase and on
dust grain surfaces, leading to the models in which hydrogen is mainly atomic
initially better match observations than the models in which hydrogen is mainly
molecular initially. Namely, to match observations, initially hydrogen must be
mostly atomic. The CO molecules are able to form even without the pre-existence
of H$_2$. We also study the influence of gas temperature, dust temperature,
intensity of interstellar radiation field and cosmic-ray ionization rate on
chemical evolution in static clouds. The static CO clouds with high dust
temperature, strong radiation field, and intensive cosmic rays are transient
due to rapid CO destruction.

Astrophysics of Galaxies