UPS and DFT investigation of the electronic structure of gas-phase trimesic acid

•In the current study outer valence band electronic structure of benzene-1,3,5-tricarboxylic acid was interpreted.•Experimental and calculated trimesic acid (TMA) spectrum were compared to ones of benzene and benzoic acid.•It is shown that similarities between MO energies and shapes for benzene and TMA exists.•Addition of carboxyl groups to the benzene ring clearly correlates with increasing binding energy of HOMO.