  Using forward models for representative exoplanet atmospheres and a
radiometric instrument model, we have generated synthetic observational data to
explore how well the major C- and O-bearing chemical species (CO, CO2, CH4, and
H2O), important for determining atmospheric opacity and radiation balance, can
be constrained by transit measurements as a function of spectral wavelength
coverage. This work features simulations for a notional transit spectroscopy
mission and compares two cases for instrument spectral coverage (wavelength
coverage from 0.5-2.5 ${\mu}$m and 0.5-5 ${\mu}$m. The simulation is conducted
on a grid with a range of stellar magnitudes and incorporates a full retrieval
of atmospheric model parameters. We consider a range of planets from
sub-Neptunes to hot Jupiters and include both low and high mean molecular
weight atmospheres. We find that including the 2.5-5 ${\mu}$m wavelength range
provides a significant improvement in the degree of constraint on the retrieved
molecular abundances: up to ~3 orders of magnitude for a low mean molecular
weight atmosphere (${\mu}$=2.3) and up to a factor of ~6 for a high mean
molecular weight atmosphere (${\mu}$=28). These decreased uncertainties imply
that broad spectral coverage between the visible and the mid-infrared is an
important tool for understanding the chemistry and composition of exoplanet
atmospheres. This analysis suggests that the JWST/NIRSpec 0.6-5 ${\mu}$m prism
spectroscopy mode, or similar wavelength coverage with possible future
missions, will be an important resource for exoplanet atmospheric
characterization.
