Chemical complexity in local diffuse and translucent clouds: ubiquitous
  l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of
  CH2CN
  We present Jansky Very Large Array observations of 20 - 37 GHz absorption
lines from nearby Galactic diffuse molecular gas seen against four
cosmologically-distant compact radio continuum sources. The main new
observational results are that \linearC3H\ and \methCN\ are ubiqitous in the
local diffuse molecular interstellar medium at \AV\ $\la 1$ while HC$_3$N was
seen only toward B0415 at \AV\ $>$ 4 mag. The linear/cyclic ratio is much
larger in C$_3$H than in C$_3$\HH\ and the ratio \methCN/HCN is enhanced
compared to TMC-1, although not as much as toward the Horsehead Nebula. More
consequentially, this work completes a long-term program assessing the
abundances of small hydrocarbons (CH, \cch, linear and cyclic C$_3$H and
C$_3$\HH, and \cfh\ and \cfhm) and the CN-bearing species (CN, HCN, HNC,
HC$_3$N, HC$_5$N and CH$_3$CN): their systematics in diffuse molecular gas are
presented in detail here. We also observed but did not strongly constrain the
abundances of a few oxygen-bearing species, most prominently HNCO. We set
limits on the column density of C\HH CN, such that the anion C\HH CN\m\ is only
viable as a carrier of diffuse interstellar bands if the N(C\HH CN)/N(C\HH
CN\m) abundance ratio is much smaller in this species than in any others for
which the anion has been observed. We argue that complex organic molecules are
not present in clouds meeting a reasonable definition of diffuse molecular gas,
ie \AV\ $\la 1$ mag.
