Experimental and Theoretical Study of the Kinetics of Dimerization of Ammonia at Low Temperatures.

The kinetics of the dimerization of NH in helium and nitrogen bath gas within the supersonic flow of a Laval nozzle were investigated at very low temperatures. Experimentally, the fraction of the NH monomer, , remaining in the flow at 91 K in N and at 35 K in He for a total bath gas density [M]∼5 × 10 molecules cm was monitored using fluorescence from the electronically excited NH photofragment formed following NH photolysis at 213 nm. No dimerization was observed up to [NH] = 1 × 10 molecules cm for 160 mm downstream of the 91 K N nozzle, nor up to [NH] = 5 × 10 molecules cm for 150 mm downstream of the 35 K He nozzle. Dimerization was observed at higher [NH], being more pronounced at lower temperatures. For the C and C conformers of the NH dimer, calculations at the CCSD(T)/aug-cc-pVTZ level gave a zero-point vibrational-energy corrected binding energy of -7.52 and -7.33 kJ mol, respectively. Energy-grained master equation calculations based on statistical rate theory using the open-source MESMER package were used to calculate rate coefficients for dimerization, , over the temperature range = 25-300 K and [M] = 10-10 molecules cm for He and N. displayed a negative dependence and a positive [M] dependence and was found to be sensitive to changes in the low-lying vibrational frequencies of the NH dimer, for example, the inclusion of a hindered rotor potential for the internal twisting mode, which alters the density of states. Using the axial profiles of , [M], and velocity for the Laval nozzles, the calculated values of were used to calculate in the flow for comparison with the experiment. At higher [NH], when dimerization was observed, the calculations significantly underestimated the degree of dimerization taking place in the flow, with a significant increase in the calculated value of required to match the experiment. The reasons for the discrepancy are discussed, for example, errors in the calculation of the density of states for the NH dimer and the average energy removed per collision by the bath gas at very low temperatures.