  We present a transmission spectrum for the warm (500-600K) sub-Neptune
HD3167c obtained using the Hubble Space Telescope Wide Field Camera 3 infrared
spectrograph. We combine these data, which span the 1.125-1.643 micron
wavelength range, with broadband transit measurements made using Kepler/K2
(0.6-0.9 micron) and Spitzer/IRAC (4-5 micron). We find evidence for absorption
by at least one of H2O, HCN, CO2, and CH4 (Bayes factor 7.4; 2.5-sigma
significance), although the data precision does not allow us to unambiguously
discriminate between these molecules. The transmission spectrum rules out
cloud-free hydrogen-dominated atmospheres with metallicities <100x solar at
>5.8-sigma confidence. In contrast, good agreement with the data is obtained
for cloud-free models assuming metallicities >700x solar. However, for
retrieval analyses that include the effect of clouds, a much broader range of
metallicities (including subsolar) is consistent with the data, due to the
degeneracy with cloud-top pressure. Self-consistent chemistry models that
account for photochemistry and vertical mixing are presented for the atmosphere
of HD3167c. The predictions of these models are broadly consistent with our
abundance constraints, although this is primarily due to the large
uncertainties on the latter. Interior structure models suggest the core mass
fraction is >40%, independent of a rock or water core composition, and
independent of atmospheric envelope metallicity up to 1000x solar. We also
report abundance measurements for fifteen elements in the host star, showing
that it has a very nearly solar composition.
