Limits on chemical complexity in diffuse clouds: search for CH3OH and
  HC5N absorption
  Context: An unexpectedly complex polyatomic chemistry exists in diffuse
clouds, allowing detection of species such as C2H, C3H2, H2CO and NH3 which
have relative abundances that are strikingly similar to those inferred toward
the dark cloud TMC-1
  Aims: We probe the limits of complexity of diffuse cloud polyatomic
chemistry.
  Methods: We used the IRAM Plateau de Bure Interferometer to search for
galactic absorption from low-lying J=2-1 rotational transitions of A- and
E-CH3OH near 96.740 GHz and used the VLA to search for the J=8-7 transition of
HC5N at 21.3 GHz.
  Results: Neither CH3OH nor HC5N were detected at column densities well below
those of all polyatomics known in diffuse clouds and somewhat below the levels
expected from comparison with TMC-1. The HCN/HC5N ratio is at least 3-10 times
higher in diffuse gas than toward TMC-1.
  Conclusions: It is possible to go to the well once (or more) too often
