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Context. Hot molecular cores correspond to the phase of star formation during which many molecules, in particular complex organic molecules (COMs), thermally desorb from the surface of dust grains. Sophisticated kinetic models of interstellar chemistry describe the processes that lead to the formation and subsequent evolution of COMs in star-forming regions. Aims. Our goal is to derive the chemical composition of hot cores in order to improve our understanding of interstellar chemistry. In particular, we want to test the models by comparing their predictions to the observed composition of the gas phase of hot cores. Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to perform an imaging spectral line survey of the high mass star-forming region Sagittarius B2(N) at 3 mm, called Re-exploring Molecular Complexity with ALMA (ReMoCA). We modeled under the assumption of local thermodynamic equilibrium the spectra obtained with this survey toward the sources embedded in the secondary hot core Sgr B2(N2). We compared the chemical composition of these sources to that of sources from the literature and to predictions of the chemical kinetics model MAGICKAL. Results. We detected up to 58 molecules toward Sgr B2(N2)’s hot cores, including up to 24 COMs, as well as many less abundant isotopologs. The compositions of some pairs of sources are well correlated, but differences also exist, in particular for HNCO and NH₂CHO. The abundances of series of homologous molecules drop by about one order of magnitude at each further step in complexity. The nondetection of radicals yields stringent constraints on the models. The comparison to the chemical models confirms previous evidence of a high cosmic-ray ionization rate in Sgr B2(N). The comparison to sources from the literature gives a new insight into chemical differentiation. The composition of most hot cores of Sgr B2(N2) is tightly correlated to that of the hot core G31.41+0.31 and the hot corino IRAS 16293–2422 B after normalizing the abundances by classes of molecules (O-bearing, N-bearing, O+N-bearing, and S-bearing). There is no overall correlation between Sgr B2(N2) and the shocked region G+0.693−0.027 also located in Sgr B2, and even less with the cold starless core TMC-1. The class of N-bearing species reveals the largest variance among the four classes of molecules. The S-bearing class shows in contrast the smallest variance. Conclusions. These results imply that the class of N-bearing molecules reacts more sensitively to shocks, low-temperature gas phase chemistry after nonthermal desorption, or density. The overall abundance shifts observed between the N-bearing and O-bearing molecules may indicate how violently and completely the ice mantles are desorbed. 

Context.Complex organic molecules (COMs) have been detected ubiquitously in protostellar systems. However, at shorter wavelengths (~0.8 mm), it is generally more difficult to detect larger molecules than at longer wavelengths (~3 mm) because of the increase in millimeter dust opacity, line confusion, and unfavorable partition function. Aims.We aim to search for large molecules (more than eight atoms) in the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 spectrum of IRAS 16293-2422 B. In particular, the goal is to quantify the usability of ALMA Band 3 for molecular line surveys in comparison to similar studies at shorter wavelengths. Methods.We used deep ALMA Band 3 observations of IRAS 16293-2422 B to search for more than 70 molecules and identified as many lines as possible in the spectrum. The spectral settings were set to specifically target three-carbon species such as i- and n-propanol and glycerol, the next step after glycolaldehyde and ethylene glycol in the hydrogenation of CO. We then derived the column densities and excitation temperatures of the detected species and compared the ratios with respect to methanol between Band 3 (~3 mm) and Band 7 (~1 mm, Protostellar Interferometric Line Survey) observations of this source to examine the effect of the dust optical depth. Results.We identified lines of 31 molecules including many oxygen-bearing COMs such as CH₃OH, CH₂OHCHO, CH₃CH₂OH, and c-C₂H₄O and a few nitrogen- and sulfur-bearing ones such as HOCH₂CN and CH₃SH. The largest detected molecules are gGg-(CH₂OH)₂and CH₃COCH₃. We did not detect glycerol or i- and n-propanol, but we do 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 ≳ 6σ 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. Conclusions.The effect of the 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 (~3 mm) are not only crowded and complex, but they also take significantly longer integration times than shorter wavelength observations (~0.8 mm) to reach the same sensitivity limit. The 3 mm search has not yet resulted in the detection of larger and more complex molecules in warm sources. A full deep ALMA Band 2–3 (i.e., ~3–4 mm wavelengths) survey is needed to assess whether low frequency data have the potential to reveal more complex molecules in warm sources.