Abstract This paper reports on an ATR-FTIR spectroscopic investigation of the CO 2 absorption characteristics of a series of heterocyclic diamines: hexahydropyrimidine (HHPY), 2-methyl and 2, 2-dimethylhexahydropyrimidine (MHHPY and DMHHPY), hexahydropyridazine (HHPZ), piperazine (PZ) and 2, 5-and 2, 6-dimethylpiperazine (2, 6- DMPZ and 2, 5-DMPZ). By using in situ ATR-FTIR the structure–activity relationship of the ...