  The discovery of amino acids in meteorites and the detection of glycine in
samples returned from a comet to Earth suggest that the interstellar chemistry
is capable of producing such complex organic molecules. Our goal is to
investigate the degree of chemical complexity that can be reached in the ISM.
We performed an unbiased, spectral line survey toward Sgr B2(N) and (M) with
the IRAM 30m telescope in the 3mm window. The spectra were analyzed with a
simple radiative transfer model that assumes LTE but takes optical depth
effects into account. About 3675 and 945 spectral lines with a peak
signal-to-noise ratio higher than 4 are detected toward N and M, i.e. about 102
and 26 lines per GHz, respectively. This represents an increase by about a
factor of 2 over previous surveys of Sgr B2. About 70% and 47% of the lines
detected toward N and M are identified and assigned to 56 and 46 distinct
molecules as well as to 66 and 54 less abundant isotopologues of these
molecules, respectively. We also report the detection of transitions from 59
and 24 catalog entries corresponding to vibrationally or torsionally excited
states of some of these molecules, respectively. Excitation temperatures and
column densities were derived for each species but should be used with caution.
Among the detected molecules, aminoacetonitrile, n-propyl cyanide, and ethyl
formate were reported for the first time in space based on this survey, as were
5 rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We
also report the detection of transitions from within 12 new vib. or tors.
excited states of known molecules. Although the large number of unidentified
lines may still allow future identification of new molecules, we expect most of
these lines to belong to vib. or tors. excited states or to rare isotopologues
of known molecules for which spectroscopic predictions are currently missing.
(abridged)
