We present very high signal-to-noise ratio absorption-line observations of CN and CH ^ { + } along 13 lines of sight through diffuse molecular clouds .
The data are examined to extract precise isotopologic ratios of ^ { 12 } CN/ ^ { 13 } CN and ^ { 12 } CH ^ { + } / ^ { 13 } CH ^ { + } in order to assess predictions of diffuse cloud chemistry .
Our results on ^ { 12 } CH ^ { + } / ^ { 13 } CH ^ { + } confirm that this ratio does not deviate from the ambient ^ { 12 } C/ ^ { 13 } C ratio in local interstellar clouds , as expected if the formation of CH ^ { + } involves nonthermal processes .
We find that ^ { 12 } CN/ ^ { 13 } CN , however , can be significantly fractionated away from the ambient value .
The dispersion in our sample of ^ { 12 } CN/ ^ { 13 } CN ratios is similar to that found in recent surveys of ^ { 12 } CO/ ^ { 13 } CO. For sight lines where both ratios have been determined , the ^ { 12 } CN/ ^ { 13 } CN ratios are generally fractionated in the opposite sense compared to ^ { 12 } CO/ ^ { 13 } CO. Chemical fractionation in CO results from competition between selective photodissociation and isotopic charge exchange .
An inverse relationship between ^ { 12 } CN/ ^ { 13 } CN and ^ { 12 } CO/ ^ { 13 } CO follows from the coexistence of CN and CO in diffuse cloud cores .
However , an isotopic charge exchange reaction with CN may mitigate the enhancements in ^ { 12 } CN/ ^ { 13 } CN for lines of sight with low ^ { 12 } CO/ ^ { 13 } CO ratios .
For two sight lines with high values of ^ { 12 } CO/ ^ { 13 } CO , our results indicate that about 50 % of the carbon is locked up in CO , which is consistent with the notion that these sight lines probe molecular cloud envelopes where the transition from C ^ { + } to CO is expected to occur .
An analysis of CN rotational excitation yields a weighted mean value for T _ { 01 } ( ^ { 12 } CN ) of 2.754 \pm 0.002 K , which implies an excess over the temperature of the cosmic microwave background of only 29 \pm 3 mK .
This modest excess eliminates the need for a local excitation mechanism beyond electron and neutral collisions .
The rotational excitation temperatures in ^ { 13 } CN show no excess over the temperature of the CMB .