  We present search results of 22 high latitude (b > 25 deg.) sightlines for OH
18-cm emission using the 305-m radio telescope at the Arecibo Observatory.
These sightlines appear in neutral hydrogen emission at intermediate velocities
(V_lsr values ranging from -90 to -20 km/s) and are predicted to have a
sufficient molecular composition so as to be detectable in molecular emission.
Such objects, known as Intermediate-Velocity Molecular Clouds (IVMCs), have
historically been detected through 12CO emission. Recent studies indicate that
IVMCs may be widespread in the Galaxy and have important implications for
models of the interstellar medium and star formation. However, we report
non-detections of OH emission toward the 22 sightlines and provide stringent
upper limits on the OH column density. Using available HI and Av data in
combination with existing state-of-the-art PDR models, we estimate H2 column
densities and find that they are more than an order of magnitude lower than the
predicted values. We also find that the hydrogen volume density of these clouds
is less than roughly 25 per cubic centimeter. In addition, we discuss the known
IVMCs with previous 12CO detections in the context of the PDR models. Our
analysis of these clouds indicates that the structure of molecular material in
IVMCs is morphologically clumpy. These results motivate the need for future
sensitive, on-the-fly searches (rather than targeted searches) for CO emission
from IVMCs with of order roughly 1' resolution. High angular resolution (1') HI
and Av data will also be helpful to better constrain the structure and
composition of IVMCs.
