Magnesium chemistry in the gas phase: calculated thermodynamic properties and experimental ion chemistry in H 2–O 2–N 2 flames

Abstract

Molecular orbital calculations were carried out for the neutral and ionic species that occur in the gas-phase chemistry of magnesium in the presence of oxygen and hydrogen including Mg +, MgO, MgOH, MgOH +, MgOH 2 +, Mg(OH) 2, HOMgOH 2 +, and the hydrate structure MgOH + … (OH 2). Standard enthalpies of formation for these species were obtained from single-point calculations at the QCISD(T)(full)/6-311++G(2df,p) and CCSD(T)(full)/6-311++G(2df,p) levels of theory using geometrical parameters obtained from MP2(full)/6-311++G(d,p) optimizations. These Δ H f 0 values provide a recommended and self-consistent set with uncertainties as small as ±12.6 kJ mol −1 (±3 kcal mol −1) for deriving thermodynamic properties. The properties of interest include the proton affinities PA 298 0 of MgO, MgOH, and Mg(OH) 2, ionization energy IE 0 0 of MgOH, bond dissociation energies D 0 0 of Mg–O, Mg–OH, MgO–H, HOMg–OH, Mg–OH +, H 2O–Mg +, and H 2O–MgOH +; the latter two are hydration energies. Values in the literature, both experimental and theoretical, for many of these quantities show considerable scatter and a detailed comparison is made. Magnesium ions in fuel-rich, H 2–O 2–N 2 flames at atmospheric pressure in the temperature range 1820–2400 K were investigated experimentally by sampling the flames doped with magnesium through a nozzle into a mass spectrometer. It was shown that the interconversion of Mg + and MgOH + proceeds by way of the three-body reaction of Mg + with OH, in support of Sugden’s criterion that this occurs in flames if the weak HO–Mg + bond has a dissociation energy less than 335 kJ mol −1. The proton affinity PA 298 0(MgO) was measured to be 1056 ± 29 kJ mol −1 (252 ± 7 kcal mol −1) along with approximate values for PA 298 0(MgOH) = 919 kJ mol −1 (220 kcal mol −1) and PA 298 0[Mg(OH) 2] = 878 kJ mol −1 (210 kcal mol −1). Values were estimated for the electron–ion recombination coefficient for MgH m O n + molecular ions of 2 × 10 −7 cm 3 molecule −1 s −1, and for Mg + of 4 × 10 −24 T −1 cm 6 molecule −2 s −1 similar to those for alkali metal ions. Finally, small rate coefficients for the chemi-ionization of magnesium via the reactions of Mg + OH and MgO + H to give MgOH + were estimated to be 4.832 × 10 −9 exp(−55 700/ T) and 3.341 × 10 −9 exp(−32 970/ T) cm 3 molecule −1 s −1, respectively.