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 l -C _ { 3 } H and CH _ { 3 } CN are ubiqitous in the local diffuse molecular interstellar medium at A _ { V } \la 1 while HC _ { 3 } N was seen only toward B0415 at A _ { V } > 4 mag .
The linear/cyclic ratio is much larger in C _ { 3 } H than in C _ { 3 } \mathrm { H _ { 2 } } and the ratio CH _ { 3 } CN/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 , C _ { 2 } H , linear and cyclic C _ { 3 } H and C _ { 3 } \mathrm { H _ { 2 } } , and C _ { 4 } H and C _ { 4 } H ^ { - } ) 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 \mathrm { H _ { 2 } } CN , such that the anion C \mathrm { H _ { 2 } } CN ^ { - } is only viable as a carrier of diffuse interstellar bands if the N ( C \mathrm { H _ { 2 } } CN ) /N ( C \mathrm { H _ { 2 } } CN ^ { - } ) 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 A _ { V } \la 1 mag .