  The formation of CH+ in the interstellar medium has long been an outstanding
problem in chemical models. In order to probe the physical conditions of the
ISM in which CH+ forms, we propose the use of CH3+ observations. The pathway to
forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the
reaction intermediary CH2+ results in a relationship between the CH+ and CH3+
abundances. This relationship depends on the molecular hydrogen fraction, f_H2,
and gas temperature, T, so observations of CH+ and CH3+ can be used to infer
the properties of the gas in which both species reside. We present observations
of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12.
Using our computed column densities and upper limits, we put constraints on the
f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average,
static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded
by our analysis. However, current theory suggests that non-equilibrium effects
drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider
a higher effective temperature due to collisions between neutrals and
accelerated ions, the CH3+ partition function predicts that the overall
population will be spread out into several excited rotational levels. As a
result, observations of more CH3+ transitions with higher signal-to-noise
ratios are necessary to place any constraints on models where magnetic
acceleration of ions drives the formation of CH+.
