Formation Constants and Conformational Analysis of Carbamates in Aqueous Solutions of 2-Methylpiperidine and CO2 from 283 to 313 K by NMR Spectroscopy

Abstract

Quantitative ¹³C nuclear magnetic resonance (NMR) spectroscopy was used to investigate the speciation in (2-methylpiperidine + H₂O + CO₂) systems at 283.2-313.2 K. The carbamate of 2-methylpiperidine(2-methylpiperidine- N-carboxylate) was shown for the first time to be a stable species in aqueous solutions. The spectroscopic results were used to obtain temperature-dependant formation constants for the carbamate using a simplified model for the activity coefficients from which the standard molar enthalpy of reaction was estimated. The results were incorporated into a self-consistent chemical equilibrium model, which includes vapor-liquid equilibria and all aqueous species, including the formation of carbamate. The predominant conformation of the sterically hindered carbamate, which was determined using two-dimensional exchange spectroscopy NMR, has the methyl group in the axial orientation and is in agreement with the density functional theory quantum chemical calculations.