  After the recombination of Hydrogen, primordial molecules such as H_2,
  $HD$ can be formed.The formation of structures in the post-recombination
gaseous medium is commonly explained by primordial density fluctuations. In a
self-gravitating collapsing cloud, temperature tends to increase with
contraction. In this context molecular lines emissions can play an important
role in the thermal balance of collapsing molecular clouds and in the dynamics
of collapsing proto-cloud.
  We calculate analytically the molecular cooling due to H_2 and $HD$. Our
analysis is done under the hypothesis we consider only the first two excited
levels of the H_2 and $HD$ molecules. The potentiality of fragmentation of
collapsing protoclouds through the thermal instability is discussed. Finally we
suggest that this study could also be extended to the $CO$ molecule.
