Context :

Aims : The comparative study of several molecular species at the origin of the gas phase chemistry in the diffuse interstellar medium ( ISM ) is a key input in unraveling the coupled chemical and dynamical evolution of the ISM .

Methods : The lowest rotational lines of HCO ^ { + } , HCN , HNC , and CN were observed at the IRAM-30m telescope in absorption against the \lambda 3 mm and \lambda 1.3 mm continuum emission of massive star-forming regions in the Galactic plane .
The absorption lines probe the gas over kiloparsecs along these lines of sight .
The excitation temperatures of HCO ^ { + } are inferred from the comparison of the absorptions in the two lowest transitions .
The spectra of all molecular species on the same line of sight are decomposed into Gaussian velocity components .
Most appear in all the spectra of a given line of sight .
For each component , we derived the central opacity , the velocity dispersion , and computed the molecular column density .
We compared our results to the predictions of UV-dominated chemical models of photodissociation regions ( PDR models ) and to those of non-equilibrium models in which the chemistry is driven by the dissipation of turbulent energy ( TDR models ) .

Results : The molecular column densities of all the velocity components span up to two orders of magnitude .
Those of CN , HCN , and HNC are linearly correlated with each other with mean ratios N ( { HCN } ) / N ( { HNC } ) = 4.8 \pm 1.3 and N ( { CN } ) / N ( { HNC } ) = 34 \pm 12 , and more loosely correlated with those of HCO ^ { + } , N ( { HNC } ) / N ( { HCO } ^ { + } ) = 0.5 \pm 0.3 , N ( { HCN } ) / N ( { HCO } ^ { + } ) = 1.9 \pm 0.9 , and N ( { CN } ) / N ( { HCO } ^ { + } ) = 18 \pm 9 .
These ratios are similar to those inferred from observations of high Galactic latitude lines of sight , suggesting that the gas sampled by absorption lines in the Galactic plane has the same chemical properties as that in the Solar neighbourhood .
The FWHM of the Gaussian velocity components span the range 0.3 to 3 km s ^ { -1 } and those of the HCO ^ { + } lines are found to be 30 % broader than those of CN-bearing molecules .
The PDR models fail to reproduce simultaneously the observed abundances of the CN-bearing species and HCO ^ { + } , even for high-density material ( 100 cm ^ { -3 } < n _ { H } < 10 ^ { 4 } cm ^ { -3 } ) .
The TDR models , in turn , are able to reproduce the observed abundances and abundance ratios of all the analysed molecules for the moderate gas densities ( 30 cm ^ { -3 } < n _ { H } < 200 cm ^ { -3 } ) and the turbulent energy observed in the diffuse interstellar medium .

Conclusions : Intermittent turbulent dissipation appears to be a promising driver of the gas phase chemistry of the diffuse and translucent gas throughout the Galaxy .
The details of the dissipation mechanisms still need to be investigated .