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We present Jansky Very Large Array observations of 20 - 37 GHz absorptionlines from nearby Galactic diffuse molecular gas seen against fourcosmologically-distant compact radio continuum sources. The main newobservational results are that \linearC3H\ and \methCN\ are ubiqitous in thelocal diffuse molecular interstellar medium at \AV\ $\la 1$ while HC$_3$N wasseen only toward B0415 at \AV\ $>$ 4 mag. The linear/cyclic ratio is muchlarger in C$_3$H than in C$_3$\HH\ and the ratio \methCN/HCN is enhancedcompared to TMC-1, although not as much as toward the Horsehead Nebula. Moreconsequentially, this work completes a long-term program assessing theabundances of small hydrocarbons (CH, \cch, linear and cyclic C$_3$H andC$_3$\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 arepresented in detail here. We also observed but did not strongly constrain theabundances of a few oxygen-bearing species, most prominently HNCO. We setlimits on the column density of C\HH CN, such that the anion C\HH CN\m\ is onlyviable as a carrier of diffuse interstellar bands if the N(C\HH CN)/N(C\HHCN\m) abundance ratio is much smaller in this species than in any others forwhich the anion has been observed. We argue that complex organic molecules arenot present in clouds meeting a reasonable definition of diffuse molecular gas,ie \AV\ $\la 1$ mag.

astrophysical journal/the astrophysical journal

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