KINETICS AND MECHANISM OF THE REVERSIBLE DISSOCIATION OF AMMONIUM CARBAMATE - INVOLVEMENT OF CARBAMIC ACID

The kinetics of the reversible decomposition of ammonium carbamate, NH 4CO2NH2 (AC), into NH3 (A) and CO2 (C) has been studied between 278 an d 312 K by recording the time dependence of the total pressure above a n 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. Thr ee mechanisms are discussed, each containing six rate constants. Two a ssume 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 sta ble 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 surfa ce-bound CA, A, and C molecules, with the latter two reversibly desorb ing from the surface. The experimental kinetic data are consistent wit h all three mechanisms. The rate data do not contain sufficient inform ation to allow all rate constants to be uniquely determined. Simulatio n studies are used to demonstrate consistency of the mechanisms with t he data.