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

The kinetics of the dimerization of NH₃ 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 NH₃ monomer, f_monomer, remaining in the flow at 91 K in N₂ and at 35 K in He for a total bath gas density [M]∼5 × 10¹⁶ molecules cm^-3 was monitored using fluorescence from the electronically excited NH₂ photofragment formed following NH₃ photolysis at 213 nm. No dimerization was observed up to [NH₃] = 1 × 10¹⁵ molecules cm^-3 for 160 mm downstream of the 91 K N₂ nozzle, nor up to [NH₃] = 5 × 10¹⁴ molecules cm^-3 for 150 mm downstream of the 35 K He nozzle. Dimerization was observed at higher [NH₃], being more pronounced at lower temperatures. For the C_s and C_2h conformers of the NH₃ 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^-1, 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, k_dimer, over the temperature range T = 25-300 K and [M] = 10¹³-10²² molecules cm^-3 for He and N₂. k_dimer displayed a negative T dependence and a positive [M] dependence and was found to be sensitive to changes in the low-lying vibrational frequencies of the NH₃ 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 T, [M], and velocity for the Laval nozzles, the calculated values of k_dimer were used to calculate f_monomer in the flow for comparison with the experiment. At higher [NH₃], 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 k_dimer 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 NH₃ dimer and the average energy removed per collision by the bath gas at very low temperatures.