  Using the APEX sub-millimeter telescope we have investigated the 12CO(3-2)
emission in five face-on nearby barred spiral galaxies, where three of them are
high surface brightness galaxies (HSBs) lying at the Freeman limit, and two are
low surface brightness galaxies (LSBs). We have positive detections for two of
three HSB spirals and non-detections for the LSBs. For the galaxies with
positive detection (NGC0521 and PGC070519), the emission is confined to their
bulges, with velocity dispersions of ~90 and ~73 km/s and integrated
intensities of 1.20 and 0.76 K/km/s, respectively. For the non-detections, the
estimated upper limit for the integrated intensity is ~0.54 K/km/s. With these
figures we estimate the H2 masses as well as the atomic-to-molecular mass
ratios. Although all the galaxies are barred, we observe 12CO(3-2) emission
only for galaxies with prominent bars. We speculate that bars could dynamically
favor the 12CO(3-2) emission, as a second parameter after surface brightness.
Therefore, secular evolution could play a major role in boosting collisional
transitions of molecular gas, such as 12CO(3-2), especially in LSBs.
