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We present very high signal-to-noise ratio absorption-line observations of CNand CH+ along 13 lines of sight through diffuse molecular clouds. The data areexamined to extract precise isotopologic ratios of 12CN/13CN and 12CH+/13CH+ inorder to assess predictions of diffuse cloud chemistry. Our results on12CH+/13CH+ confirm that this ratio does not deviate from the ambient 12C/13Cratio in local interstellar clouds, as expected if the formation of CH+involves nonthermal processes. We find that 12CN/13CN, however, can besignificantly fractionated away from the ambient value. The dispersion in oursample of 12CN/13CN ratios is similar to that found in recent surveys of12CO/13CO. For sight lines where both ratios have been determined, the12CN/13CN ratios are generally fractionated in the opposite sense compared to12CO/13CO. Chemical fractionation in CO results from competition betweenselective photodissociation and isotopic charge exchange. An inverserelationship between 12CN/13CN and 12CO/13CO follows from the coexistence of CNand CO in diffuse cloud cores. However, an isotopic charge exchange reactionwith CN may mitigate the enhancements in 12CN/13CN for lines of sight with low12CO/13CO ratios. For two sight lines with high values of 12CO/13CO, ourresults indicate that about 50 percent of the carbon is locked up in CO, whichis consistent with the notion that these sight lines probe molecular cloudenvelopes where the transition from C+ to CO is expected to occur. An analysisof CN rotational excitation yields a weighted mean value for T_01(12CN) of2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmicmicrowave background of only 29 +/- 3 mK. This modest excess eliminates theneed for a local excitation mechanism beyond electron and neutral collisions.The rotational excitation temperatures in 13CN show no excess over thetemperature of the CMB.

Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation