  Previous works have demonstrated that the generation of secondary CMB
anisotropies due to the molecular optical depth is likely too small to be
observed. In this paper, we examine additional ways in which primordial
chemistry and the dark ages might influence the CMB. We present a detailed and
updated chemical network and give an overview of the interactions of molecules
with the CMB. We consider the optical depth due to line absorption,
photoionization, photodissociation and free-free processes, and estimate the
resulting changes in the CMB temperature and its power spectrum. The most
promising results are obtained for the negative hydrogen ion \HM and the \HeHII
molecule. The free-free process of \HM yields a relative change in the CMB
temperature of up to $2\times10^{-11}$, and leads to a frequency-dependent
change in the power spectrum of the order $10^{-7}$ at 30 GHz. With a change of
the order $10^{-10}$ in the power spectrum, our result for the bound-free
process of \HM is significantly below a previous suggestion. \HeHII efficiently
scatters CMB photons and smears out primordial fluctuations, leading to a
change in the power spectrum of the order $10^{-8}$. Improvements in the
accuracy of future CMB experiments may thus help to constrain and finally
detect these interesting signals from the dark ages of the universe.
