  The study of primordial chemistry of molecules adresses a number of
interesting questions pertaining to the thermal balance of collapsing molecular
protoclouds. In numerous astrophysical cases molecular cooling and heating
influence dynamical evolution of the medium. The existence of a significant
abundance of molecules can be crucial on the dynamical evolution of collapsing
objects. Because the cloud temperature increases with contraction, a cooling
mechanism can be important for the structure formation, by lowering pressure
opposing gravity, i.e. by allowing continued collapse of Jeans unstable
protoclouds. This is particularly true for the first generation of objects. It
has been suggested that a finite amount of molecules such as $H_2$, $HD$ and
$LiH$ can be formed immediately after the recombination of cosmological
hydrogen (Lepp and Shull 1984, Puy et al 1993). The cosmological fractional
abundance $HD/H_2$ is small. Nevertheless, the presence of a non-zero permanent
electric dipole moment makes $HD$ a potentially more important coolant than
$H_2$ at modest temperatures, although $HD$ is much less abundant than $H_2$.
Recently Puy and Signore (1997) showed that during the early stages of
gravitational collapse, for some collapsing masses, $HD$ molecules were the
main cooling agent when the collapsing protostructure had a temperature of
about 200 Kelvins. Thus, we analytically estimate the molecular cooling during
the collapse of protoclouds Then, the possibility of thermal instability during
the early phase of gravitational collapse is discussed.
