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Cold gas-phase water has recently been detected in a cold dark cloud, Barnard5 located in the Perseus complex, by targeting methanol peaks as signposts forice mantle evaporation. Observed morphology and abundances of methanol andwater are consistent with a transient non-thermal evaporation process onlyaffecting the outermost ice mantle layers, possibly triggering a more complexchemistry. We present the detection of the Complex Organic Molecules (COMs)acetaldehyde and methyl formate as well as formic acid and ketene, and thetentative detection of di-methyl ether towards the methanol hotspot of Barnard5 located between two dense cores using the single dish OSO 20m, IRAM 30m, andNRO 45m telescopes. The high energy cis- conformer of formic acid is detected,suggesting that formic acid is mostly formed at the surface of interstellargrains and then evaporated. The detection of multiple transitions for eachspecies allows us to constrain their abundances through LTE and non-LTEmethods. All the considered COMs show similar abundances between $\sim 1$ and$\sim 10$ % relative to methanol depending on the assumed excitationtemperature. The non-detection of glycolaldehyde, an isomer of methyl formate,with a [glycolaldehyde]/[methyl formate] abundance ratio lower than 6 %,favours gas phase formation pathways triggered by methanol evaporation.According to their excitation temperatures derived in massive hot cores, formicacid, ketene, and acetaldehyde have been designated as "lukewarm" COMs whereasmethyl formate and di-methyl ether were defined as "warm" species. Comparisonwith previous observations of other types of sources confirms that "lukewarm"and "warm" COMs show similar abundances in low-density cold gas whereas the"warm" COMs tend to be more abundant than the "lukewarm" species in warmprotostellar cores.

Chemical complexity induced by efficient ice evaporation in the Barnard 5 molecular cloud