  Aims. We investigate the physical and chemical conditions of molecular gas in
the circumnuclear disk (CND) region of NGC 1068. Methods. We carried out a
spectral line survey with the IRAM 30m telescope toward the center of NGC 1068
and mainly focused on the 2 mm band with a frequency coverage of 160.7-168.6
GHz and 176.5-184.3 GHz. Results. Fifteen lines are detected in NGC 1068, eight
of which are new detections for this galaxy. We derive the rotation
temperatures and column densities of fourteen molecular species. Conclusions.
Based on the [HCO+ (2 - 1)]/[HOC+ (2 - 1)] ratio, we obtain a high ionization
degree in the CND of NGC 1068. It is found that HC3N is concentrated in the
east knot, while 13CCH, CH3CN, SO, HOC+, CS, CH3CCH, and H2CO are concentrated
in the west knot. Compared to the star-forming galaxies M 82 and NGC 253, the
chemistry of NGC 1068 might be less strongly affected by the UV radiation
field, and its kinetic temperature might be lower.
