  The large scale structure and physics of molecular gas in the Galactic center
region is discussed based on the analysis of a 9' resolution survey of the
Galactic center region in the J=1->0 line of C^18O. Emphasis is placed on the
comparison with ^12CO(1--0) data. The line shapes of C^18O(1--0) and
^12CO(1--0) differ significantly. The ratio of the intensities of the two
isotopomers in the Galactic center region is generally higher than the value of
~15 expected from the ``Standard Conversion Factor'' (SCF) of ^12CO integrated
line intensity to H_2 column density. In our data, this ratio is in the range
from 30 to 200, mostly ~60 to 80. From LVG calculations, we estimate that the
large scale ^12CO(1--0) emission in the Galactic center region is of moderate
or low optical depth. Higher optical depths are restricted to very limited
regions. We estimate H_2 densities and kinetic temperatures for different
ranges of intensity ratios. A considerable amount of molecular mass is in a
widespread molecular gas component with low densities and high kinetic
temperatures. The total molecular mass is found to be (3^+2_-1) 10^7 M_{sun}.
We show that the SCF is not valid toward the Galactic bulge. It overestimates
the H_2 column density by an order of magnitude because the assumptions
required for this factor are not fulfilled for a significant fraction of the
molecular gas. Therefore, also one cannot apply a modified conversion factor to
the Galactic center region since the N(H_2)/I(^12CO) ratio is highly variable
and cannot be represented by a universal constant. Results from external
galaxies indicate that the ^12CO emission is generally not a suitable tracer of
H_2 masses in galactic bulges.
