  Complex organic molecules (COMs) have been detected ubiquitously in
protostellar systems. However, at shorter wavelengths (~0.8mm) it is more
difficult to detect larger molecules than at longer wavelengths (~3mm) because
of the increase of millimeter dust opacity, line confusion, and unfavorable
partition function. We aim to search for large molecules (>8 atoms) in the ALMA
Band 3 spectrum of IRAS 16293-2422 B. We search for more than 70 molecules and
identify as many lines as possible in the spectrum. The spectral settings were
set to specifically target three-carbon species such as propanol and glycerol.
We identify lines of 31 molecules including many oxygen-bearing COMs such as
CH3OH and c-C2H4O and a few nitrogen- and sulfur-bearing ones such as HOCH2CN
and CH3SH. The largest detected molecules are gGg-(CH2OH)2 and CH3COCH3. We do
not detect glycerol or propanol but provide upper limits for them which are in
line with previous laboratory and observational studies. The line density in
Band 3 is only ~2.5 times lower in frequency space than in Band 7. From the
detected lines in Band 3 at a $\gtrsim 6\sigma$ level, ~25-30% of them could
not be identified indicating the need for more laboratory data of rotational
spectra. We find similar column densities and column density ratios of COMs
(within a factor ~2) between Band 3 and Band 7. The effect of dust optical
depth for IRAS 16293-2422 B at an off-source location on column densities and
column density ratios is minimal. Moreover, for warm protostars, long
wavelength spectra are not only crowded, but also take longer integration times
to reach the same sensitivity limit. The 3mm search has not yet resulted in
detection of larger and more complex molecules in warm sources. A full deep
ALMA Band 2-3 (i.e., 3-4 mm) survey is needed to assess whether low frequency
data have the potential to reveal more complex molecules in warm sources.
