  Using the Plateau de Bure interferometer, we observed the 3mm absorption
lines of CN, HCN and HNC from some of the diffuse clouds which lie toward our
well-studied sample of compact extragalactic mm-wave continuum sources. The
column densities of these species all vary by a factor of about fifty and are
prominent in only a limited subset of the clouds seen in the most ubiquitous
species such as OH, HCO+, C2H and C3H2. The CN, HCN, and HNC column densities
vary in fixed proportion to each other, with <N(HNC)/N(HCN)> = 0.21+/-0.05 and
<N(CN)/N(HCN)> = 6.8+/-1. We searched unsuccessfully for CH3CN and N2H+, which
are underabundant compared to dark clouds, by factors of at least 10 and 100,
respectively.
