  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 12CN/13CN and 12CH+/13CH+ in
order to assess predictions of diffuse cloud chemistry. Our results on
12CH+/13CH+ confirm that this ratio does not deviate from the ambient 12C/13C
ratio in local interstellar clouds, as expected if the formation of CH+
involves nonthermal processes. We find that 12CN/13CN, however, can be
significantly fractionated away from the ambient value. The dispersion in our
sample of 12CN/13CN ratios is similar to that found in recent surveys of
12CO/13CO. For sight lines where both ratios have been determined, the
12CN/13CN ratios are generally fractionated in the opposite sense compared to
12CO/13CO. Chemical fractionation in CO results from competition between
selective photodissociation and isotopic charge exchange. An inverse
relationship between 12CN/13CN and 12CO/13CO 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 12CN/13CN for lines of sight with low
12CO/13CO ratios. For two sight lines with high values of 12CO/13CO, our
results indicate that about 50 percent 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(12CN) of
2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmic
microwave background of only 29 +/- 3 mK. This modest excess eliminates the
need for a local excitation mechanism beyond electron and neutral collisions.
The rotational excitation temperatures in 13CN show no excess over the
temperature of the CMB.
