Kinetics and mechanism of the reversible dissociation of ammonium carbamate: Involvement of carbamic acid

The kinetics of the reversible decomposition of ammonium carbamate, NH4CO2NH2 (AC), into NH3 (A) and CO2 (C) has been studied between 278 and 312 K by recording the time dependence of the total pressure above an AC sample that, after being previously evacuated, is exposed to the headspace of an isoteniscope. At early times (<1 s), the data show an inflection, which is enhanced in first derivative transformations. Three mechanisms are discussed, each containing six rate constants. Two assume the presence of gas phase carbamic acid (CA) as an intermediate. These mechanisms are explored further through ab initio computational studies of CA. These results indicate that gas phase CA is a very stable species. No evidence could be found for its presence from Fourier transform TR spectra of the equilibrium vapor above AC. The mechanism that we suggest involves the reversible decomposition of AC into surface-bound CA, A, and C molecules, with the latter two reversibly desorbing from the surface. The experimental kinetic data are consistent with all three mechanisms. The rate data do not contain sufficient information to allow all rate constants to be uniquely determined. Simulation studies are used to demonstrate consistency of the mechanisms with the data.