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

Molecular orbital calculations were carried out for the neutral and ionic s pecies that occur in the gas-phase chemistry of magnesium in the presence o f oxygen and hydrogen including Mg+, MgO, MgOH, MgOH+, MgOH2+, Mg(OH)(2), H OMgOH2+, and the hydrate structure MgOH+...(OH2). Standard enthalpies of fo rmation for these species were obtained from single-point calculations at t he QCISD(T)(full)/6-311 + + G(2df,p) and CCSD(T)(full)/6-311 + + G(2df,p) l evels of theory using geometrical parameters obtained from MP2(full)/6-311 + + G(d,p) optimizations. These Delta H-f(0) values provide a recommended a nd 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 IE00 of MgOH, bond dissociation energies D-0(0) of M g-O, Mg-OH, MgO-H, HOMg-OH, Mg-OH+ H2O-Mg+, and H2O-MgOH+; the latter two a re hydration energies. Values in the literature, both experimental and theo retical, for many of these quantities show considerable scatter and a detai led 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 nozzl e into a mass spectrometer. It was shown that the interconversion of Mg+ an d MgOH+ proceeds by way of the three-body reaction of Mg+ with OH, in suppo rt 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 MgHmOn+ molecular ions of 2 x 10(-7) cm(3) molecule(-1) s(-1), and for Mg+ of 4 x 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 m agnesium via the reactions of Mg + OH and MgO + H to give MgOH+ were estima ted to be 4.832 x 10(-9) exp(-55 700/T) and 3.341 x 10-9 exp(-32 970/T) cm( 3) molecule(-1) s(-1), respectively. (C) 1999 Elsevier Science B.V.