Chemical reactivity of iron atoms near room temperature

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Abstract
The reactivity of ground state iron atoms with respect to atom transfer and adduct formation reactions with a variety of simple molecules in Ar buffer gas near room temperature has been investigated. Iron atoms are produced by visible multiphoton dissociation of iron pentacarbonyl or ferrocene, and their removal by added gases under pseudo-first-order conditions is monitored by resonance fluorescence excitation at variable time delay following the photolysis pulse. Upper limits for second-order rate constants for reaction of ground state iron atoms with O2, CO, H2O, (CH3)2O, C2H2, C2H4, N2O, C2H4O, and CF3Cl at room temperature and 100 Torr total pressure are estimated to be in the range (2–10)× 10−15 cm3 molecule−1 s−1, which corresponds to reaction probabilities of less than ≂10−5 per hard sphere collision. Pseudo-second-order rate constants in the range (2–60)× 10−14 cm3 molecule−1 s−1 are found for 1:1 adduct formation reactions of ground state iron atoms with C6H6, 1,3-butadiene, NH3, and NO. The formation of such complexes at room temperature implies that the binding energies are greater than ≂7 kcal mol−1. For the complex FeNH3, the kinetic data allow measurements of dissociation equilibrium constants at 278.5 and 295 K, from which a binding energy ΔH°0 = 7.5 ± 1 kcal mol−1 is derived. Measurements of the dependence of the pseudo-second-order rate constant for formation of FeNO on the pressure of Ar buffer gas at 296 K allow estimates to be made of the limiting low- and high-pressure recombination rate constants: k0 =2.3×10−32 cm6 molecule−2 s−1 and k∞=1×10−12 cm3 molecule−1 s−1. Comparisons are made between the present results which indicate a remarkably low reactivity for iron atoms, and previously reported results for reactions of iron atoms in low temperature matrices and gas phase reactions of other transition metal atoms. An interpretation of our results is given in terms of schematic potential energy curves which are based on the results of previous ab initio calculations for the complex FeCO. The qualitative features of these curves are consistent with low reactivity of ground state iron atoms with respect to adduct formation due to repulsive ground state interactions and low probabilities for transitions from repulsive to attractive potential surfaces.