The Molecular Chemistry of Diffuse and Translucent Clouds in the
  Line-of-Sight to Sgr B2
  The 1-50 GHz GBT PRIMOS data contains ~50 molecular absorption lines observed
in diffuse and translucent clouds located in the Galactic Center, Bar, and
spiral arms in the line-of-sight to Sgr B2(N). We measure the column densities
and estimate abundances, relative to H2, of 11 molecules and additional
isotopologues. We use absorption by optically thin transitions of c-C3H2 to
estimate the N(H2), and argue that this method is preferable to more commonly
used methods. We discuss the kinematic structure and abundance patterns of
small molecules including the sulfur-bearing species CS, SO, CCS, H2CS, and
HCS+; oxygen-bearing molecules OH, SiO, and H2CO; and simple hydrocarbon
molecules c-C3H2, l-C3H, and l-C3H+. We discuss the implications of the
observed chemistry for the structure of the gas and dust in the ISM.
  Highlighted results include the following. First, whereas gas in the disk has
a molecular hydrogen fraction of 0.65, clouds on the outer edge of the Galactic
Bar and in or near the Galactic Center have molecular fractions of 0.85 and
>0.9, respectively. Second, we observe trends in isotope ratios with
Galactocentric distance; while carbon and silicon show enhancement of the rare
isotopes at low Galactocentric distances, sulfur exhibits no trend with
Galactocentric distance; the ratio of c-C3H2/c-H13CCCH provides a good estimate
of the 12C:13C ratio, whereas H2CO/H2^13CO exhibits fractionation. Third, we
report the presence of l-C3H+ in diffuse clouds for the first time. Finally, we
suggest that CS has an enhanced abundance within higher density clumps of
material in the disk, and therefore may be diagnostic of cloud conditions. If
this holds, the diffuse clouds in the Galactic disk contain multiple embedded
hyperdensities in a clumpy structure, and the density profile is not a simple
function of A_V.
