Carbamate Formation in the System (2-Methylpiperidine + Carbon Dioxide) by Raman Spectroscopy and X-ray Diffraction

In aqueous solution, 2-methylpiperine (2-MP) has been proposed as a phase-separating amine for carbon-capture applications, whose carbamate is considered to be unstable due to steric hindrance. This paper demonstrates, for the first time, that the carbamate can be synthesized as the salt of the 2-methylpiperidinium cation (2-MPH⁺) and the 2-methylpiperidine- N-carboxylate anion (2-MPCOO^-) by adding carbon dioxide gas to anhydrous liquid 2-MP at CO₂/amine ratios α > 0.32 to yield well-defined prismatic crystals. Raman spectra have been measured for anhydrous liquid 2-MP and aqueous solutions of 2-MP and 2-MPH⁺Cl^- at 298 K. The spectra of anhydrous liquid 2-MP, containing dissolved CO₂ at lower CO₂/amine ratios,_. clearly showed the presence of 2-MPH⁺ and several unidentified bands that were attributed to the carbamate, 2-MPCOO^-. Quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory yielded simulated Raman spectra consistent with the experimental spectra. The spectra show the methyl group of both liquid and aqueous 2-MP and that of aqueous 2-MPH⁺Cl^- to be in the equatorial position. The crystal structure shows the same conformations in the solid state and confirms that Raman spectroscopy can be used to determine the conformation of 2-MP species in the liquid state and aqueous solutions.