Gas–Water Interfacial Conversion of CO2-Derived Carbamates to Substituted Ureas

Abstract

Amines (RNH2) capture carbon dioxide (CO2) in water as carbamates (RNHCO2–), one C–N bond short of a urea (RNHCONHR′). Interfacial studies have so far produced only unsubstituted urea from inorganic nitrogen sources, whereas conversion of organic carbamates to substituted ureas at aqueous interfaces remains unexamined. Here we show that gas–water interfaces (GWIs) promote this downstream step under ambient aqueous conditions. When an aqueous solution of amines is pneumatically nebulized with CO2, some of the initially formed carbamates convert to substituted ureas, including cyclic products from diamines and intermolecular products from monoamines. 13CO2 labeling traces the urea carbonyl carbon to CO2, and offline chromatographic comparison against an authentic standard further identifies the product independent of the online mass spectrometry. Pure CO2 nebulization gives the strongest urea signals, and longer droplet flight times increase the apparent urea ion fraction. Structurally matched substrates and crossed-spray experiments link dehydration to ring-closure geometry, steric/electronic structure, and amine nucleophilicity. Density functional theory and exploratory ab initio metadynamics support a proton-assisted ionic dehydration pathway modulated by interfacial electric fields and water-mediated proton transfer. These results identify GWIs as reactive microenvironments that drive aqueous CO2–amine chemistry toward carbamate-to-urea conversion without added coupling reagents.