Chemical reactivity of iron atoms near room temperature

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.

Theoretical Study of the Reaction Mechanism of Fe Atoms with H2O, H2S, O2 and H+

“…This conclusion agrees with the experimental finding by Mitchell and Hackett. 10 Further decomposition of q-HFeOH to FeO + H 2 is not likely to occur due to the high barrier at q-TS2. The reverse reaction, FeO + H 2 , is found to have a barrier of 12-13 kcal/mol, which can explain the fact that no reaction was observed between FeO and molecular hydrogen at 320 K in the recent experimental study by Rollason and Plane.…”

“…16 Annealing to 30 and 50 K gives cyclic FeO 2 . The gas-phase reaction of Fe atoms with water at room temperature was studied by Mitchell and Hackett, 10 who found that the corresponding rate constant is low. However, little information is available about the mechanism of this fundamental reaction.…”

“…3 Among others, FeO may undergo a reaction with molecular hydrogen, which is the reverse of Fe + H 2 O. This reaction was recently studied experimentally 4 and was found to be very slow at 320 K. While the simple and fundamental reaction Fe + O 2 has been examined in argon matrices, [5][6][7][8] in the gas phase, 4,[9][10][11][12] and by theoretical calculations, 8,[11][12][13][14][15] the reactions in the Fe/H 2 O/O 2 system are still far from being understood. Ground-state iron atoms are not reactive with O 2 in the gas phase near room temperature.…”

“…10 Of all reaction mechanisms considered in this study, we believe that the FeH + + O 2 reaction may be most relevant to the first step of rusting, and the initial protonation of free iron atoms is crucial. It should be mentioned that Fe clusters were found experimentally to be much more reactive toward small molecules than the free atoms, 9,10 so the reactions of the clusters with water and oxygen can also contribute to the rusting process.…”

Bonding of acetylene to copper atom, dimer, and trimer

“…On the other hand, there has been a recent surge of activity in gas-phase experimental work on the reactivity of molecules such as H2, CO, NH3 and small alkenes and alkynes toward transition-metal atoms [7,8], dimers [7,9, lo], and clusters [l 11 and on the determination of the binding energies and properties of the complexes formed [ 12-141. Full-geometry optimization of clusters and molecule-cluster complexes are desirable in order to make more meaningful comparisons between theory and experiment.…”