AN EXPERIMENTAL AND THEORETICAL-STUDY OF THE CLUSTERING REACTIONS BETWEEN NA-2, O-2 AND CO2( IONS AND N)

The termolecular clustering reactions of Na+ with N-2, O-2 and CO2 ini tiate the neutralisation of the metal ions in the earth's lower thermo sphere, and have been proposed as a mechanism for forming sporadic sod ium layers. This paper reports a kinetic study of these reactions in a low-temperature fast-flow reactor coupled to a quadrupole mass spectr ometer. This yielded: k(Na+ + N-2 + He, 93-255 K) = (1.20 +/- 0.13) x 10(-30)(T/200 K)-((2.20 +/- 0.09)) k(Na+ + O-2 + He, 93-130 K) = (5.20 +/- 2.62) x 10(-31)(T/200 K)(-(2.64 +/- 0.74)) k(Na+ + CO2 + He, 158- 300 K) = (9.05 +/- 1.38) x 10(-30)(T/200 K)(-(2.84 +/- 0.48)) k(Na+ CO2 + Ar, 200-300 K) = (3.63 +/- 0.41) x 10(-29)(T/200 K)(-(4.41 +/- 0 .15)) where the units are cm(6) molecule(-2) s(-1) and the stated erro rs are a combination of the 2 sigma standard errors in the kinetic dat a and the systematic errors in the temperature, pressure and how rates . The diffusion coefficient of Na+ in He was also found to be D(Na+-He , 127-293 K) = (4.30 +/- 1.70) x 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 f or the Na . N-2(+), Na . O-2(+) and Na . CO2+ clusters were then deter mined 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 s urprisingly fast. This is satisfactorily explained by the semi-empiric al formalisms of J. Tree and D. R. Bates if it is assumed that the lig ands continue to behave as free rotors when initially clustered to the Na+, thereby permitting the exchange of orbital and rotational angula r momentum as the ligand orbits about the ion. The lifetimes of the cl usters are then greatly increased, facilitating stabilisation by colli sion 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.