Piperazine is a saturated six-membered heterocyclic compound that is used for commercial CO₂ removal for carbon capture and storage. In this work, we have investigated piperazine conformers’ structureand orientation at an aqueous solution’s surface using surface-specific vibrational sum-frequency generation(VSFG) spectroscopy. It is found that though piperazine molecules exist majorly as centrosymmetric conformersin gas-phase and crystalline states, they can have a substantial population of non-centrosymmetricconformers on the air-water interface with net polar orientation. The VSFG signal varies slightly withconcentration, indicating some changes in the conformational distribution and orientation of the interfacialmolecules. In the presence of HCl, VSFG spectra of piperazine aqueous solution change considerably withthe extent of protonation. The VSFG signal in the C-H stretching region, even from completely protonatedpiperazine cations, and its similarity with the peak positions of unprotonated piperazine, establishes thepresence of non-centrosymmetric conformers at the air-water interface for piperazine and its cations. Fromthe O-H stretching VSFG spectra, it is found that on increasing the concentration of piperazine up to 800 mM,there is an increase in the orientational order of the interfacial water molecules, and disorderliness sets in athigher concentrations and in acidic conditions.

The structure and orientation of piperazine conformers at the surface of an aqueous solution are investigated. Piperazine molecules which exist majorly as centrosymmetric conformers in gas-phase and crystalline states are found to have a substantial population of non-centrosymmetric conformers on the air-water interface with net polar orientation. VSFG signal in the C-H stretching region even from completely protonated piperazine cations and its similarity with the peak positions of unprotonated piperazine establishes the presence of non-centrosymmetric conformers on air-water interface, for piperazine and its cations.