J. Phys. Chem. 1996, 100, 14609-14617
14609
Reactions of Laser-Ablated Iron Atoms with Nitrogen Atoms and Molecules. Matrix Infrared Spectra and Density Functional Calculations of Novel Iron Nitride Molecules George V. Chertihin,† Lester Andrews,*,† and Matthew Neurock‡ Departments of Chemistry and Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22901 ReceiVed: May 16, 1996X
Matrix infrared spectra of the Fe + N2 system show that laser-ablated Fe atoms react with nitrogen atoms and molecules to give the FeN and NFeN molecules and Fe(N2)x complexes. The iron nitride molecules FeN and NFeN were identified from nitrogen and iron isotopic shifts and splittings and density functional frequency calculations. Sharp 934.8 and 903.6 cm-1 bands are assigned to the 56FeN and N56FeN molecules in solid nitrogen. The NFeN molecule is bent with valence angle 115 ( 5° as determined from iron and nitrogen isotopic shifts. Nitrogen-to-argon matrix shifts for FeN and NFeN are small. The cyclic Fe2N molecule is observed at 779 and 719 cm-1 in solid nitrogen. Strong bands in the 2200-2000 cm-1 region are associated with end-bonded Fe(NN)x complexes; the FeNN molecule absorbs at 2017.8 cm-1 in solid argon. New absorptions at 1826.8 and 1683.7 cm-1 in argon matrix experiments, identified as side-bonded Fe(N2) and (Fe2)(N2), respectively, agree very well with earlier CASSCF frequency calculations and approach the frequency of N2 adsorbed on Fe(111). Structure and frequency calculations were done using density functional theory to support the identification of these new Fe(N2)x molecular complexes.
Introduction Iron is a very important metal in materials, chemistry, and biology. Recent investigation of the Fe + H2 and Fe + O2 systems revealed very high reactivity for laser-ablated iron atoms.1-3 The main products were the FeH, HFeH, FeO, and OFeO molecules, which require activation energy for formation; these products were not seen in ordinary thermal evaporation experiments.4,5 The next challenge is to react laser-ablated iron atoms with nitrogen molecules because the dissociation energy of N2 is much higher than that for O2 and this technique is expected to provide a source of N atoms for reaction with metal atoms. Reactions of laser-ablated U and Ti atoms with N2 have been studied in this laboratory.6,7 Furthermore, the reaction of iron with dinitrogen is of considerable practical and theoretical importance. An iron dinitrogen intermediate has been suggested in the conversion of dinitrogen to ammonia by the industrially important Haber process and in the enzyme nitrogenase.8,9 Molecularly adsorbed N2 on Fe(111) exhibits a 1550 cm-1 fundamental and has been characterized as π-bonded to the surface and proposed as an intermediate in the catalytic dissociation of N2.10,11 Model systems for end-on and side-on bonding of Fe and Fe2 to N2 have been investigated by CASSCF calculations.12,13 One experimental study of the Fe + N2 system has been reported using pure and krypton diluted nitrogen,14 and an argon matrix observation has been quoted in a review.15 Investigation of the Fe + O2 system found that FeO and OFeO readily form complexes with N2.3 This required iron experiments with nitrogen-doped gas mixtures as well as with pure nitrogen. These experiments revealed strong absorptions in the 2200-2000 cm-1 region and new bands around 935 and 904 cm-1, which did not produce oxygen isotopic shifts. The latter bands are due to Fe-N vibrations of new iron nitride molecules, which will be characterized here. Unlike metal oxides, there is very little information on metal nitrides in †
Department of Chemistry. Department of Chemical Engineering. X Abstract published in AdVance ACS Abstracts, August 1, 1996. ‡
S0022-3654(96)01423-2 CCC: $12.00
Figure 1. Infrared spectra in the 2250-1550 and 1000-700 cm-1 regions following laser-ablated Fe atom reactions with nitrogen during condensation in excess argon at 10 K: (a) Fe + Ar/14N2 ) 100/1 sample codeposited for 1 h; (b) after first annealing to 22 ( 2 K; (c) after second annealing to 27 ( 2 K; (d) after third annealing to 32 ( 2 K.
general, and the simple diatomic FeN molecule has not been observed experimentally. Thus, the new FeN and NFeN molecules are of considerable interest owing to the importance of iron. © 1996 American Chemical Society
14610 J. Phys. Chem., Vol. 100, No. 35, 1996
Chertihin et al. TABLE 1: Absorptions (cm-1) Observed in Reactions of Iron Atoms with Nitrogen on Condensation with Excess Argon at 10 K 14
N2
2214 2171.0 2143.1b 2100.7 2089.7 2070.7 2045b 2035.1 2017.8 2008.9 1826.8c 1720.4d 1683.7c 1635.5 960.7 945.7 938.0 933.1 928.2 903.4 887.2 872.8 868.6 840.8 812.9 811.2 798.1 771.0
15N
2
2140 2098.8 2072.1 2031.1 2020.2 2001.9 1977 1967.6 1951.1 1942.3 1766.2 1663.6 1628.2 1581.7 960.7 945.7 913.4 908.7 928.2 881.4 887.2 872.8 868.6 840.6 812.9 788.7 797.9 748.3
R(14/15) 1.0346 1.0344 1.0343 1.0343 1.0344 1.0344 1.0344 1.0343 1.0342 1.0343 1.0342 1.0341 1.0341 1.0340 1.0269 1.0269 1.0250
1.0285 1.0303
anneal + ++ ++ ++ + + + + + + + + + + + + + + ++ + ++
assignmenta Fe(NN)x Fe(NN)x Fe(NN)x Fe(NN)x Fe(NN)x Fe(NN)x Fe(NN)2 Fe(NN)2 site FeNN Fe(NN)x Fe(N2) (Fe2)(N2) isomer (Fe2)(N2) (Fe2)(N2) isomer XFeO2 ν3FeO2 FeN FeN site O2FeO NFeN NNFeO FeO Fe2O ? ? Fe2N NFeO (FeFeN)
a
Assignment of iron oxides from ref 3 and unpublished work. Region congested in mixed isotopic experiments but 2101 and 1997 cm-1 intermediate components observed with 14N2/15N2 and 2112 and 2015 cm-1 components observed with 14,15N2. c Produced triplet isotopic structures in experiments with 14,15N2: intermediate components are 1976.0, 1796.8, and 1655.5 cm-1, respectively. d This band was favored over the 1683.7 cm-1 band after annealing in experiments with low N2 concentration (
