An experimental and theoretical study of the clustering reactions between Na+ ions and N2, O2 and CO2

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Abstract
The termolecular clustering reactions of Na + with N 2 , O 2 and CO 2 initiate the neutralisation of the metal ions in the earth's lower thermosphere, and have been proposed as a mechanism for forming sporadic sodium layers. This paper reports a kinetic study of these reactions in a low-temperature fast-flow reactor coupled to a quadrupole mass spectrometer. This yielded: k(Na+ + N2 + He, 93–255 K) = (1.20 ± 0.13) × 10−30(T/200 K)−(2.20±0.09) k(Na+ + O2 + He, 93–130 K) = (5.20 ± 2.62) × 10−31(T/200 K)−(2.64±0.74) k(Na+ + CO2 + He, 158–300 K) = (9.05 ± 1.38) × 10−30(T/200 K)−(2.84±0.48) k(Na+ + CO2 + Ar, 200–300 K) = (3.63 ± 0.41) × 10−29(T/200 K)−(4.41±0.15) where the units are cm 6 molecule −2 s −1 and the stated errors are a combination of the 2σ standard errors in the kinetic data and the systematic errors in the temperature, pressure and flow rates. The diffusion coefficient of Na + in He was also found to be D(Na + –He, 127–293 K) = (4.30 ± 1.70) × 10 16 T (1.03±0.06) cm 2 s −1 molecule cm −3 , in excellent agreement with previous measurements of the ion mobility. The recombination rate coefficients are shown to be within 1% of their respective low-pressure limits. The bond energies for the Na·N 2 + , Na·O 2 + and Na·CO 2 + clusters were then determined from ab initio quantum calculations to be only 37.8, 29.0 and 59.0 kJ mol −1 , respectively, so that the recombination reactions are surprisingly fast. This is satisfactorily explained by the semi-empirical formalisms of J. Troe and D. R. Bates if it is assumed that the ligands continue to behave as free rotors when initially clustered to the Na + , thereby permitting the exchange of orbital and rotational angular momentum as the ligand orbits about the ion. The lifetimes of the clusters are then greatly increased, facilitating stabilisation by collision with the third body. This point is examined in greater detail for the Na + –N 2 case by a classical trajectory study, which also produces an estimate of the recombination rate coefficient in good accord with the present experimental results.

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