  We present calculations of rotational absorption spectra of the molecules HCN
and DCN in superfluid helium-4, using a combination of the Diffusion Monte
Carlo method for ground state properties and an analytic many-body method
(Correlated Basis Function theory) for the excited states. Our results agree
with the experimentally determined effective moment of inertia which has been
obtained from the $J=0\to 1$ spectral transition. The correlated basis function
analysis shows that, unlike heavy rotors such as OCS, the J=2 and higher
rotational excitations of HCN and DCN have high enough energy to strongly
couple to rotons, leading to large shifts of the lines and accordingly to
anomalous large spectroscopic distortion constants, to the emergence of
roton-maxon bands, and to secondary peaks in the absorption spectra for J=2 and
J=3.
