  There is observational evidence that nuclear winds and X-rays can heavily
influence the physical conditions and chemical abundances of molecular gas in
the circumnuclear disks (CND) of Active Galactic Nuclei (AGN). In this paper we
probe the chemical status of molecular gas in the CND of NGC1068, a
prototypical Seyfert 2 galaxy. Precedent claims that the chemistry of molecular
gas in the nucleus of NGC1068 is abnormal by galactic standards were based on
the high HCN/CO luminosity ratio measured in the CND. Results from new
observations obtained in this survey have served to derive abundances of
molecular species such as SiO, CN, HCO+, HOC+, H13CO+ and HCO. These estimates
are complemented by a re-evaluation of molecular abundances for HCN, CS and CO,
based on previously published single-dish and interferometer observations of
NGC1068. We report on the first detection of SiO emission in the CND of
NGC1068. The estimated large abundance of SiO in the CND, X(SiO)~5-10x10^{-9},
cannot be attributed to shocks related to star formation, as there is little
evidence of a recent starburst in the nucleus of NGC1068. Alternatively, we
propose that silicon chemistry is driven by intense X-ray processing of
molecular gas. We also report on the first extragalactic detection of the
reactive ion HOC+. Most remarkably, the estimated HCO+/HOC+ abundance ratio in
the nucleus of NGC1068, ~30-80, is the smallest ever measured in molecular gas.
The abundances derived for all molecules that have been the subject of this
survey are compared with the predictions of models invoking either
oxygen-depletion or X-ray chemistry in molecular gas. Our conclusions favour an
overall scenario where the CND of NGC1068 has become a giant X-ray Dominated
Region (XDR).
