J. Phys. Chem. 1996, 100, 11235-11241
11235
Reactions of Laser-Ablated Iron Atoms with N2O, NO, and O2 in Condensing Nitrogen. Infrared Spectra and Density Functional Calculations of Ternary Iron Nitride Oxide Molecules Lester Andrews,*,† George V. Chertihin,† Angelo Citra,† and Matthew Neurock‡ Departments of Chemistry and Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22901 ReceiVed: March 5, 1996; In Final Form: May 3, 1996X
Reactions of laser-ablated Fe atoms with N2O, NO, and O2 in excess N2 during condensation at 10 K produced new absorptions with 15N and 18O shifts that are assigned to the new ternary iron nitride oxide molecules NFeO, NFeO2, and N2FeO. DFT calculations of structure and isotopic frequencies support these assignments.
Introduction The reaction of iron with oxygen dominates corrosion processes owing to the stability of iron oxides, and several stable iron oxide molecules have been characterized by matrix infrared spectroscopy and DFT calculations.1-7 However, the reaction of iron with nitrogen is less favorable as iron nitrides and dinitrogen complexes are not as stable relative to the highly stable N2 molecule.8 Nevertheless, iron nitrides have been prepared by the reaction of laser-ablated iron atoms with nitrogen atoms and molecules.9 This work also provided evidence for ternary iron nitride oxide species, a group of molecules not yet explored. The relative fundamental frequencies of FeN (938 cm-1) and FeO (873 cm-1) show that the ironnitrogen chemical bond is viable and predict that N will compete favorably with O in ternary Fe species.5-7,9 Here follows a study of reactions of laser-ablated Fe atoms with N2O, NO, and O2 in excess N2 matrix carrier gas. Complementary DFT calculations were also done to predict vibrational frequencies and isotopic shifts for potential product molecules. Experimental Section The laser-ablation matrix-isolation experiment was identical to that employed for earlier iron studies.6,7,10 Nitrogen oxides (N2O, NO, Matheson; 15N2O, Isomet, 15N18O, Isotec) were used as received; the typical sample dilution was 1% in nitrogen (Airco, 99.99%). Gas mixtures were codeposited for 1-2 h on a 10 K CsI window at 2-3 mmol/h with Fe atoms ablated using 10-40 mJ/pulse of focussed 1064 nm radiation. Sample photolysis (Philips, 175 W, globe removed) and several annealing cycles (monitoring system vapor pressure, refrigerator cold stage temperature and matrix spectra) were also done. First, second, and third annealing cycles attained matrix temperatures of approximately 20, 25, and 30 ( 2 K, respectively. FTIR spectra were recorded at 0.5 cm-1 resolution and (0.1 cm-1 accuracy on a Nicolet 750 spectrometer with MCT detector. Results Matrix infrared spectra will be presented for Fe atom reactions with N2O, NO, and O2 in excess nitrogen along with DFT calculations for possible FeNxOy product molecules. Fe + N2O. Infrared spectra for the N2/N2O/Fe system are shown in Figure 1. Bands in the 2200-2000 cm-1 region have been observed in pure nitrogen and are due to binary FeN2x †
Department of Chemistry. Department of Chemical Engineering. X Abstract published in AdVance ACS Abstracts, June 15, 1996. ‡
S0022-3654(96)00674-0 CCC: $12.00
Figure 1. Infrared spectra in the 2200-1600 and 1000-600 cm-1 regions following reactions of laser-ablated Fe atoms with N2O in excess N2. (a) 1% N2O in N2 codeposited with Fe atoms at 10 K for 2 h, (b) after annealing to 20 K, (c) after annealing to 25 K, and (d) after annealing to 30 K.
species.9 A weak new absorption was observed at 2258.3 cm-1 (not shown). Several new bands in the NO stretching region are listed in Table 1 and include 1812.0, 1793.1, 1746.8, and 1731.0 cm-1; five additional weak features at 1379.2, 1347.2, 1337.4, 1302.8, and 1208.6 cm-1 are not shown. The sharp bands at 934.8 and 903.6 cm-1 and the broader band at 779 cm-1 have been observed in pure nitrogen and assigned to FeN, NFeN, and Fe2N, respectively.9 Several oxide features can be identified from their argon matrix counterparts and oxygen 16/ 18 isotopic ratios: 878.1 cm-1 (Fe2O), 657.3 cm-1 (OFeFeO), and 535.5 cm-1 ((FeO)2, not shown).6,7 Sharp new absorptions can be grouped on annealing behavior: the strongest new product band at 796.4 cm-1 (labeled NFeO) increased (to 2×) © 1996 American Chemical Society
11236 J. Phys. Chem., Vol. 100, No. 27, 1996
Andrews et al.
TABLE 1: Infrared Absorptions (cm-1) Produced by Codepositing Laser-Ablated Fe Atoms with N2O, O2, and NO in Excess Nitrogen at 10 K 14N 2
matrix N2O, 14N16O, 16O2 2258.3 2147 2095 2078 1874.9 1866.3 1812.0 1793.1 1780.0 1746.8 1731.0 1664.0 1657.6 1616.0 1379.2 1347.2 1337.4 1302.4 1236.8 1208.6 1204.2 1166.3 980.5 964.2 br 970 971.1 959.1 953.2 949.0 934.8 932.5 928.0 925.0 br 918.5 903.6 895.2 878.1 874.7 872.8 853.9 842.4 837.2 796.4 792.4 779 775.5 750.1 657.3 535.5
a
15N 2
matrix N2O
14N
2
matrix 18O2
2183.4 2099 2026 2009 1841.9
2258.5 2147 2095 2078 1826.3
1775.2 1756.6
1774.7
1711.4 1695.5 1633.3 1603.2 1581.2
1710.8 1695.7 1623.6 1657.6 1586.4
1324.6
1311.2
1211.6
1211.1
14N
2
matrix 15N18O 2147 2095 2078 1792.3 1784.1 1730.1 1718.4 1701.2 1674.1 1695.3 1592.0 1657.6 1550.8 1324.1 1287.7 1278.5 1240.9 1185.5 1157.6
1135.6 1130.9 980.1
956.6 921.5 947 943.3 938.2 (932) 949.0 910.2 932.5 927.9 924.2
in pure N2 exptb in pure N2 exptb in pure N2 exptb
inc ann, no int iso inc + ann, no int iso inc ann, no int iso 1646.6 in pure N2 exptb int iso inc ann dec ann inc ann int iso, dec ann no int iso, inc + ann broad 1174 inc + ann inc ann
950.8 934.8
956.7 950.8 912.6 934.8 897.6
893.0 869.8 903.6 853.4 834.9
881.6 895.2 878.1 874.7 872.6 849.3
818.2 800.6 795.9 760.8 748.1 779
837.2 796.2 792.4 758 775.5 740.9 657.3 534.8
Ratio given is oxygen 16/18; frequency given is
commentsa
? 628.8 512.7 16,18O
2
903.6 834.9 818.2 760.8 748.1 779 739.3 ? 628.8 512.7
1.0398, 929 in pure N2 exptb 1.0390, 919.3 1.0392, 914.3 inc + ann 1.0560 in pure N2 exptb 1.0490 1.0517 1.0436 1.0522, 817.7 1.0519. 810.0 1.0468 inc ann sextet in pure N2 expt inc ann weak 636.3, inc ann 1.0445, 526.3
assignment (NN)xFeO Fe(NN)x Fe(NN)x Fe(NN)2 NO (NO)2 NxFeNO ? (NO)2 FeNO FeNO Fe(NO)2 N3 NO2 ? (Fe+)(NO)(Fe+)(NO)A (N2O3) NO2NNO2FeOO ? NFeO2 ? ? NFeO N2FeO site N2FeO (NN)xFeO2 FeN N2xFeO2 NFeO2 FexNyOz N2xFe(O2) NFeN (Fe(O2))2 FeOFe FeOFe site (NN)xFeO N2FeO N2xFeO2 NFeO2 NFeO O3Fe2N A(N2O3) ? OFeFeO (FeO)2
intermediate component. b Reference 9.
on first annealing, decreased (to 1.5×) on second annealing, and decreased (to 0.2×) on third annealing whereas the second strongest new band at 932.4 cm-1 (labeled N2xFeO2) decreased progressively. In contrast sharp new bands at 980.5, 953.2, 928.0, and 853.9 cm-1 (labeled NFeO2 and N2FeO) increased on first annealing (to 3×), increased on second annealing (to 5×), and were almost destroyed on third annealing. Infrared spectra from a similar N2O/Fe experiment done in condensing 15N2 are shown in Figure 2. The bands above 2000 cm-1 are shifted as is appropriate for pure 14N-14N to 15N15N vibrations; the 2183.4 cm-1 counterpart to 2258.3 cm-1 appears to track with the 872.6 cm-1 band and is appropriate for the nitrogen matrix counterpart of NNFeO discussed earlier.7 Product bands in the 1900-1200 cm-1 region shift as is appropriate for 15N substitution (Table 1); note the 14NO and 15NO bands at 1874.9 and 1841.9 cm-1, which reveal the origin of N atoms in product species as being about 15% from the N2O reagent and 85% from the 15N2 matrix. Reaction with matrix atoms is also indicated by the Fe15N and 15NFe15N bands
at 910.2 and 881.6 cm-1.9 Nitrogen-15 shifts are also evident in sharp new bands at 956.6, (932 shoulder), 927.9, and 849.3 cm-1, which exhibit the same annealing behavior as their nitrogen-14 counterparts. Fe + NO. Infrared spectra for the N2/NO/Fe system are identical, as expected, above 2000 cm-1. In the 1700-1800 cm-1 region, the same bands were observed with NO and N2O; however, annealing increased the 1746.8 cm-1 band more with NO, as shown in Figure 3. In the lower region, bands due to dioxo species were much weaker. The sharp 796.4 cm-1 band (labeled NFeO) and the 953.2, 853.9 cm-1 pair (labeled N2FeO) behaved as with N2O. The weak sharp band at 775.5 cm-1 (labeled A) increased on annealing and is due to N2O3.11 Infrared spectra for the 14N2/15N18O/Fe sample show 15N18O isotopic shifts in the 1800-1200 cm-1 region but only oxygen18 shifts in the 1000-500 cm-1 region (Figure 4). The Fe14N and 14NFe14N bands were observed at 934.8 and 903.6 cm-1 as before. Part of the 878 cm-1 Fe2O band must arise due to reaction of Fe16O from target surface oxide contamination as
Ternary Iron Nitride Oxide Molecules
J. Phys. Chem., Vol. 100, No. 27, 1996 11237
Figure 2. Infrared spectra in the 2200-1600 and 1000-600 cm-1 regions following reactions of laser-ablated Fe atoms with N2O in excess 15N . (a) 1% N O in 15N codeposited at 10 K with Fe atoms for 2 h, 2 2 2 (b) after annealing to 20 K, (c) after photolysis for 30 min, (d) after annealing to 25 K, and (e) after annealing to 30 K.
Figure 3. Infrared spectra in the 1900-1100 and 1000-600 cm-1 regions following reactions of laser-ablated Fe atoms with NO in excess N2. (a) 1% NO in N2 codeposited with Fe atoms at 10 K for 2 h, (b) after annealing to 20 K, (c) after annealing to 25 K, and (d) after annealing to 30 K.
on annealing this band exceeds the new oxygen-18 counterpart at 834.9 cm-1. Note also the weak 796.4 cm-1 NFe16O band and stronger 760.8 cm-1 NFe18O counterpart. Bands labeled N2FeO at 950.8 and 818.2 cm-1 showed small and large oxygen18 shifts, respectively. One experiment was done with a mixed 14N2/14N16O/15N18O sample to produce mixed isotopic species. In the region above 1000 cm-1, mixed oxygen isotopic absorptions were observed only for the new 1236.8, 1208.6, and 1198.7 cm-1 product bands denoting the participation of two oxygen atoms. In the region below 1000 cm-1, the doublet character of band pairs at 953.2950.8 cm-1, 878.1-834.9 cm-1, 853.9-818.1 cm-1, and 796.4760.8 cm-1 shows that these are single O atom species. Fe + O2. Infrared spectra for Fe+O2 in excess nitrogen showed all of the bands described above, but the NO species were weaker as NO had to be formed (in low yields) during deposition, whereas the dioxo products were stronger. The strongest band in the oxide region at 932.5 cm-1 (Figure 5) shifted to 897.6 cm-1 with 18O2, exhibited a strong 919.3 cm-1 intermediate component with 16,18O2, and associated with a weaker 842.4 cm-1 band on annealing. The sharp 928.0 cm-1 band (labeled NFeO2), which grew strongly on annealing shifted to 893.0 cm-1 with 18O2 and revealed a strong 914.3 cm-1 intermediate component in 16,18O2 experiments. Weaker sharp 980.5 and 837.2 cm-1 bands tracked with the 928.0 cm-1 band on three annealing cycles. Major bands that appeared on annealing at 918.5 and 895.2 cm-1 and exhibited 18O2 counterparts at 869.8 and 853.4 cm-1; however, the former intermediate component was masked but the latter revealed a 1/4/ 4/2/4/1 sextet at 895.2, 883.1, 874.3, 866.9, 861.4, 853.4 cm-1.
A sharp 949.0 cm-1 band that decreased during the annealing sequence exhibited an 18O2 counterpart at 912.6 cm-1 and an intermediate component at 929 cm-1. Oxygen-18 counterparts for other product bands are given in Table 1. Calculations The DGauss program developed by Cray Research Inc.12 was used to perform DFT calculations. DGauss employs optimized Gaussian basis functions to self-consistently solve the single particle Kohn-Sham equations.13 The local spin-density exchange-correlation potential is represented by the Vosko-WilkNusair potential.14 Nonlocal gradient corrections to the exchange and correlation are determined in-situ to the SCF calculation via exchange and correlation potentials developed by Becke and Perdew, respectively.15,16 All electrons are explicitly accounted for in iron. DFT-optimized DZVP2 quality basis sets17 were used for both iron (63321/5211/41) as well as nitrogen and oxygen (721/51/1). The SCF calculations reported were converged to within 1 × 10-6 au on the SCF energy, and to within 1 × 10-3 au/Å for structural optimization. Second derivatives, force constants, and frequencies, however, were all determined numerically using the harmonic oscillator approximation. The first ternary species explored, iron nitride oxide, NFeO, was 3 kcal/mol more stable in the doublet state shown in Figure 6 than in a quartet state with 0.01 Å longer bond lengths and a 1° smaller bond angle. The pyramidal N2FeO and NFeO2 species had triplet and doublet lower states, respectively. Figure 6 illustrates the calculated ternary iron nitride oxide structures.
11238 J. Phys. Chem., Vol. 100, No. 27, 1996
Figure 4. Infrared spectra in the 1900-1100 and 1000-600 cm-1 regions following reactions of laser-ablated Fe atoms with 15N18O in excess N2. (a) 1% 15N18O in N2 codeposited with Fe atoms at 10 K for 2 h, (b) after annealing to 20 K, (c) after photolysis, (d) after annealing to 25 K, and (e) after annealing to 30 K.
Andrews et al.
Figure 5. Infrared spectra in the 1000-500 cm-1 region following reactions of laser-ablated Fe atoms with O2 in excess N2. (a) 1% 16O2 in N2 codeposited with Fe atoms at 10 K for 2 h, (b) after annealing to 20 K, (c) after annealing to 25 K, (d) after annealing to 30 K, and (e) after annealing to almost 35 K.
Table 2 summarizes the DFT computed frequencies for iron nitride oxide species and several dinitrogen iron oxide complexes. Discussion The new ternary FeNxOy product molecules will be identified with the help of DFT isotopic frequency calculations. NFeO. The first molecule sought in this study was NFeO, iron nitride oxide. Recall that OFeO was made by insertion of laser-ablated Fe into O2 and NFeN was prepared by adding N to FeN.6,7,9 The sharp 796.4 cm-1 band shifted to 760.8 cm-1 with 18O2, and the 16/18 ratio (1.0468) is just above the harmonic Fe-O value (1.0463), which characterizes an Fe-O stretching mode. The 760.8 cm-1 band was unshifted with 15N18O in 14N , but the 796.4 cm-1 band shifted 0.2 cm-1 to 2 796.2 cm-1 with 14N2O in a 15N2 matrix. Thus, the N atom in the product comes from the matrix and not the reagent as implied by the formation of this molecule with O2 in a nitrogen matrix. The 796.4-760.8 cm-1 bands comprise a doublet with mixed isotopic oxygen denoting the vibration of a single O atom. Clearly this molecule is due to XFeO and X must contain passive nitrogen owing to the small 0.2 cm-1 15N shift. The two possibilities are NFeO and N2FeO (iron dinitride oxide). Table 2 shows that each has a strong Fe-O stretching mode, and based on similar DFT calculations for OFeO and NFeN, these calculated values must be scaled (×0.90 ( 0.02) to near 800 cm-1. Note, however, that the 15N shift calculated for the Fe-O mode of NFeO is 0.8 cm-1 and that for N2FeO is 6.1 cm-1. Clearly, the small observed 0.2 cm-1 shift fits the former better than the latter. The DFT calculations also predict an
Figure 6. Density functional theory optimized structures for iron oxide nitride molecular species.
Fe-N stretching mode in the 930-970 region and the weak sharp 971.0 cm-1 band on top of a broader unidentified absorption, which shifts to 943.3 cm-1 using 15N2, is appropriate although the intensity is about 10% of the 796.4 cm-1 band intensity. Finally, annealing behavior shows that the 796.4 cm-1 band is due to an N atom addition product formed early rather
Ternary Iron Nitride Oxide Molecules
J. Phys. Chem., Vol. 100, No. 27, 1996 11239
TABLE 2: Frequencies (cm-1) and Intensities (km/mol) Calculated by DFT for FeNxOy Molecular Species isotopes
OFeO (T)a
NFeN (T)
NFeO (D)
N2FeO (T)
NFeO2 (D)
NFe(O2) (D)
NNFeO2 (T)
(N2)FeO2 (T)
NNFeO (Q)
16,14
1010.8 (764) 887.0 (21) 217.7 (16)
1019.0 (4) 996.9 (106) 397.4 (1)
1049.8 (56) 909.4 (77) 294.1 (6) 1049.7 869.2 288.2 1022.3 908.6 289.9
998.5 (41) 897.9 (38) 742.9 (2)b 995.9 860.0 742.8 975.8 891.8 724.3
1077.1 (44) 960.8 (88) 875.8 (29)c 1076.5 923.7 832.2 1049.9 960.6 874.0
1154.3 (125) 1111.6 (44) 589.8 (1)
941.2 (132) 898.0 (11) 455.7 (12)d
954.8 (125) 912.3 (11) 464.5 (7)e
886.7 (81) 363.9 (21) 285.6 (2)f
18,14 16,15
a Spin state: T ) triplet, D ) doublet. b Deformation modes at 340.7 (0), 314.8 (0), 230.2 cm-1 (15 km/mol). c Deformation modes at 400.1 (0), 350.2 (1), 215.8 cm-1 (24 km/mol). d End-on N-N stretching mode calculated at 2211.7 cm-1 (261 km/mol). e Side-on N-N stretching mode calculated at 1974.3 cm-1 (145 km/mol). f End-on N-N stretching mode calculated at 2141.8 cm-1 (458 km/mol).
than late in the reaction sequence. The 796.4 cm-1 band increases on first annealing along with NFeN but decreases on second annealing while two tetraatomic products increase. Accordingly, the strong 796.4 cm-1 and weak 971.0 cm-1 bands are assigned to the NFeO molecule, which is formed by addition of N to FeO, reaction 1. This reaction is calculated by DFT to be exothermic by 96 kcal/mol.
N + FeO f NFeO
∆E ) -96 kcal/mol
(1)
N2FeO. The pyramidal iron dinitride oxide molecule N2FeO is calculated to have two strong stretching modes, the antisymmetric Fe-N stretch at 998.5 cm-1 and the Fe-O stretch at 897.9 cm-1; the symmetric Fe-N stretch calculated at 742.9 cm-1 is too weak to observe here (Table 2). The bands at 953.2 and 853.9 cm-1 increase on the second annealing cycle along with NFeN at the expense of NFeO, and they fall below (×0.95) the calculated values. The 853.9 cm-1 band and its 18O counterpart at 818.2 cm-1 form a doublet with mixed isotopic oxygen, denoting the vibration of a single oxygen atom. Unfortunately, no mixed isotopic nitrogen matrix data are available. Of more importance, the calculated and observed isotopic shifts match: the upper band has a calculated 2.6 cm-1 and observed 2.4 cm-1 18O shift whereas the lower band has a calculated 37.9 cm-1 and observed 35.7 cm-1 18O shift (a better comparison is the calculated (1.0441) and observed (1.0436) 16/18 isotopic frequency ratio). The lower band has a calculated 6.1 cm-1 and an observed 4.6 cm-1 15N shift whereas the upper band has a calculated 22.7 cm-1 and observed 21 cm-1 (shoulder and 20.9 cm-1 site) 15N shift. This agreement between observed and DFT calculated isotopic frequencies substantiates the identification of the pyramidal N2FeO molecule, which is formed by the exothermic reaction 2 of N and NFeO.
N + NFeO f N2FeO
∆E ) -78 kcal/mol
(2)
Although reaction 3 to produce the dinitrogen-iron oxide complex is more exothermic, absorptions due to N2FeO grow more on annealing than the band assigned to (NN)xFeO complex.
N + NFeO f NN-FeO
∆E ) -155 kcal/mol (3)
Apparently, nitrogen atom reaction with the electropositive iron center in NFeO is faster than addition to the terminal nitrogen atom. NFeO2. The pyramidal iron nitride dioxide molecule NFeO2 has three calculated frequencies with observable intensities, and this molecule can be formed on annealing by the N atom + open FeO2 molecule reaction 4 or the O atom + NFeO molecule reaction 5. The sharp 980.5, 928.0 and 837.2 cm-1 bands track together on three annealing cycles, and exhibit relative intensities and isotopic frequencies in accord with calculated values (Tables
N + FeO2 f NFeO2
∆E ) -91 kcal/mol
(4)
O + NFeO f NFeO2
4E ) -101 kcal/mol
(5)
1 and 2) except that the Fe-N mode intensity is about 10% rather than 50% of the strongest Fe-O mode. Note that the sharp 980.5 cm-1 band shifts 0.4 cm-1 with 18O and 23.9 cm-1 with 15N as is appropriate for the Fe-N stretching mode. In contrast the 928.0 and 837.2 cm-1 bands show no 15N shift and 16/18 ratios 1.0392 and 1.0519, which are appropriate for antisymmetric and symmetric Fe-O2 modes. Similar 16/18 ratios were observed for FeO2 in solid argon,6,7 but note the effect of slightly different valence angles on the ratios. Both bands exhibited triplets using 16,18O2 with central components at 914.3 and 810.0 cm-1 which show the asymmetry expected for interaction in the mixed 16,18 isotopic molecule. Again, agreement between observed and DFT calculated isotopic frequencies strongly supports this assignment. N2xFeO2. The major iron oxide absorption in N2/O2 experiments appeared at 932.5 cm-1, which decreased stepwise on annealing cycles, its behavior matched by a sharp but much weaker 842.4 band. Both absorptions gave triplets using 16,18O2 with matching 16,18 component asymmetries (4.2 ( 0.1 cm-1). The 16/18 isotopic ratios, 1.0390 and 1.0522, again denote antisymmetric and symmetric Fe-O2 stretching modes. Note that these Fe-O2 stretching modes are 5 cm-1 higher than the same modes for NFeO2 and that both fall inside of the 945.8 and 797.1 cm-1 stretching modes for FeO2 in solid argon.6,7 The 932.5 and 842.4 cm-1 bands are assigned to the N2xFeO2 molecule, i.e. open OFeO complexed by end-on bonded NN molecule(s). Finally, a straightforward assignment for the 949.0 cm-1 band, which exhibits a similar 1.0398 isotopic ratio for an open FeO2 subunit and broadens on annealing, is to a sideways coordinated (N2)FeO2 species. DFT calculations predict NNFeO2 to be more stable than (N2)FeO2 by 9 kcal/ mol and to exhibit the strong Fe-O2 mode 13 cm-1 lower. On annealing atomic N can replace complexed N2 as the former is much more strongly bound to Fe than the latter.
NNFeO2 + N f NFeO2 + N2
∆E ) -76 kcal/mol (6)
(NN)xFe(O2). The fate of cyclic Fe(O2) in the nitrogen matrix experiments is also of interest. Recall that cyclic Fe(O2) was formed, for the most part, on annealing and not on deposition in excess argon. The 918.5 cm-1 band that appears on annealing and remains on final annealing shows the proper 16/18 ratio 1.0559 for the predominantly O-O stretching mode found for Fe(O2) in solid argon.6,7 On doping with N2 in argon, a 917.1 cm-1 band grew on annealing, exhibited a 1.0547 oxygen 16/ 18 ratio, and was assigned to the above subject molecule.
11240 J. Phys. Chem., Vol. 100, No. 27, 1996 Clearly, the 918.5 cm-1 band is due to the dinitrogen complexed Fe(O2) molecule in solid nitrogen. Other Iron Oxides in Solid Nitrogen. The chemical trapping of FeO in solid nitrogen is expected in view of the ready formation of NN-FeO on annealing in solid argon.7 However, the anticipated region contained a strong 878.1 cm-1 band that shifted to 834.9 cm-1 and gave a mixed isotopic doublet with 1.0517 isotopic ratio. This ratio is appropriate for bent FeOFe (142° valence angle lower limit) and not for FeO; the FeOFe molecule was observed in solid argon at 868.6 cm-1 and exhibited a similar isotopic ratio.7 The weaker 872.8 cm-1 satellite feature is, however, appropriate for FeO in the (NN)xFeO complex, but the Fe18O counterpart is masked by the Fe18OFe band at 834.9 cm-1. A N-N stretching mode is also observed here at 2258 cm-1 and at 2262 cm-1 in solid argon for NN-FeO.7 Nitrogen matrix absorptions at 657.3 and 535.3 cm-1 exhibit analogous oxygen isotopic multiplets as did 661 and 517 cm-1 bands in solid argon, and they are assigned to the analogous OFeFeO and (FeO)2 species, of course, coordinated by N2. In the latter case, the 16/18 ratio is just lower than the harmonic FeO value which suggests distortion of the rhombic ring. Finally, the major new band at 895.2 cm-1 that survives final annealing (Figure 5) is unique in the 16/18 ratio, 1.0490, and mixed isotopic sextet. This denotes a species with two equivalent O2 subgroups, but the ratio is too high for an antisymmetric Fe-O2 mode. All of this information suggests a cyclic (Fe(O2))2 dimer with equivalent subunits. A similar band was observed in pure oxygen experiments.7 The fate of FeO2 and Fe(O2) in solid nitrogen has been determined, but the FeOO species remains to be considered. A weak band appeared at 1204.2 cm-1 in N2/O2 experiments, near the 1204.5 cm-1 argon matrix value,7 and the 18O2 band at 1135.6 cm-1 defines the 1204.2/1135.6 ) 1.0604 isotopic ratio expected for FeOO; a broad 1174 cm-1 central component was observed. Apparently, complexation with N2 has little effect on FeOO. NO Reaction Products. The major products in the NO region, 1793.1, 1746.8, and 1731.1 cm-1, exhibited almost identical isotopic ratios 14-16/15-16 ) 1.0208 ( 0.0001, 1416/14-18 ) 1.0209 ( 0.0001, and 14-16/15-18 ) 1.0434 ( 0.0002, which are slightly different from 14NO/15NO ) 1.0179, N16O/N18O ) 1.02661, and 14N16O/15N18O ) 1.0461. The new bands show more nitrogen and less oxygen isotopic dependence than the diatomic molecule and increase on annealing but show no evidence of mixed isotopic components. The latter two bands correspond to major products in the reaction of thermal iron atoms with NO in excess argon, which have been assigned to FeNO and Fe(NO)2, respectively.18 The absence of a mixed isotopic counterpart suggests that the 1731.1 cm-1 band is also due to FeNO in a different matrix site arrangement. Argon matrix experiments with laser-ablated Fe and NO also gave analogous 1742.5 and 1731.5 cm-1 bands and the same annealing behavior reported above. Similar work with N2/O2/ Ni in this lab produced analogous bands at 1749.6 and 1723.1 cm-1 with similar 14-16/14-18 ratios, which are due to analogous NiNO complex species. Finally, the very weak 1812.0 cm-1 band which appeared on annealing also exhibits NO isotopic shifts; it is tentatively assigned to a NO complex with iron nitride, possibly NFe-NO. The sharp 1664.0, 1662.0 cm-1 doublet, which increased markedly on final annealing shows almost the same isotopic ratios as the FeNO bands, and intermediate components at 1646.6, 1642.5 cm-1 using 16,18O2; these observations suggest the Fe(NO)2 complex. An analogous band was observed at
Andrews et al. 1666.9 cm-1 in solid argon. DFT calculations done by Ball18 predict that the strongest Fe(NO)2 mode should fall 40 cm-1 lower than FeNO in accord with this possible assignment. Minor products in the lower region include new 1347.2 and 1337.4 cm-1 bands (favored in O2 and NO experiments, respectively) and 1236.8, 1208.6, 1198.7 cm-1 bands. The latter three bands show mixed oxygen isotopic components and similar isotopic ratios with argon matrix bands at 1243.7 and 1205.5 cm-1 assigned earlier to NO2- and NNO2-.19 The latter bands, and the 792.4 cm-1 band due to isolated O3-,20 show that electrons are produced in these experiments. However, note that the NO2 band at 1616.0 cm-1 is 1 order of magnitude stronger than the NO2- band at 1236.8 cm-1, which suggests that electron capture processes play only a minor role in these experiments. The sharp 1337.4 cm-1 band shifts to 1278.5 cm-1 with 15N18O, and a 14N18O counterpart is also found at 1302.2 cm-1 in the 15N18O/14N2 experiment. The 14-16/14-18 ratio, 1.02703, and 14-16/15-18 ratio, 1.04607, are in excellent agreement with harmonic diatomic NO values, 1.02661 and 1.04609. The bands show no mixed isotopic counterpart and are due to the vibration of a single N-O oscillator in the region expected for (NO)-.21 The alkali metal species vary from 1374 cm-1 for (Cs+)(NO)- to 1352 cm-1 for (Li+)(NO)-.22,23 The 1347.2 cm-1 band is favored in N2O and O2 experiments, exhibits isotopic ratios in excellent agreement with diatomic N-O, and shows no mixed isotopic counterparts. Both 1347.2 and 1337.4 cm-1 bands are due to (NO)- species; it is probable that iron cations are involved directly as (Fe+)(NO)-. Conclusions The reactions of laser-ablated Fe atoms with N2O, NO, and O2 in excess nitrogen produce the simple iron oxides FeO and FeO2 and N atoms which react further to give the iron nitride oxide species NFeO, N2FeO, and NFeO2. The latter new molecules are identified by 15N and 18O isotopic shifts and agreement with isotopic spectra calculated by DFT. The observation of FeN (938 cm-1) higher than FeO (873 cm-1) is followed in the ternary species which have higher Fe-N than Fe-O stretching frequencies. Note also that Fe-N bonds are calculated to be slightly shorter than Fe-O bonds (Figure 6). These observations show that atomic nitrogen forms a strong bond to iron. Acknowledgment. We appreciate NSF support from Grant CHE 92-22556. A.C. is a visiting graduate student from the University of Southampton. References and Notes (1) Abramowitz, S.; Acquista, N.; Levin, I. W. Chem. Phys. Lett. 1977, 50, 423. (2) Chang, S.; Blyholder, G.; Fernandez, J. Inorg. Chem. 1981, 20, 2813. (3) Serebrennikov, L. V. Vestn. Mosk. UniV. Ser. 2, Khim. 1988, 29, 451. Serebrennikov, L. V., Sc.D. thesis, Moscow State University, 1990, Moscow. (4) Fanfarillo, M.; Cribb, M. E.; Downs, A. J.; Greene, T. M.; Almond, M. J. Inorg. Chem. 1992, 31, 2962. Fanfarillo, M.; Downs, A. J.; Green, T. M.; Almond, M. J. Inorg. Chem. 1992, 31, 2973. (5) Green, D. W.; Reedy, G. T. J. Mol. Spectrosc. 1979, 78, 257 and references therein. (6) Andrews, L.; Chertihin, G. V.; Ricca, A.; Bauschlicher, C. W., Jr. J. Am. Chem. Soc. 1996, 118, 467. (7) Chertihin, G. V.; Saffel, W.; Yustein, J. T.; Andrews, L.; Neurock, M.; Ricca, A.; Bauschlicher, C. W., Jr. J. Phys. Chem. 1996, 100, 5261. (8) Doeff, M. M.; Parker, S. F.; Barrett, P. H.; Pearson, R. G. Inorg. Chem. 1984, 23, 4108. (9) Chertihin, G. V.; Andrews, L.; Neurock, M. J. Phys. Chem., in press.
Ternary Iron Nitride Oxide Molecules (10) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 12131. (11) Varetti, E. L.; Pimentel, G. C. J. Chem. Phys. 1971, 55, 3813. (12) DGauss, UniChem 2.3, Cray Research Inc., Mendota Heights, MN. (13) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1991, 96, 1280. (14) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (15) Becke, A. D. Phys. ReV. A. 1988, 38, 3098. Becke, A. D. J. Chem. Phys. 1988, 88, 2537. (16) Perdew, J. P. Phys. ReV. B. 1986, 33, 8822. (17) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560. (18) Ball, D. W.; Chiarelli, J. A. J. Mol. Struct., in press, and personal communication.
J. Phys. Chem., Vol. 100, No. 27, 1996 11241 (19) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1971, 55, 3404. (20) Spiker, R. C., Jr.; Andrews, L. J. Chem. Phys. 1973, 59, 1851. Andrews. L.; Ault, B. S.; Grzybowski, J. M.; Allen, R. O. J. Chem. Phys. 1975, 62, 2461. In a nitrogen matrix, isolated O3- is red-shifted 12 cm-1 from the argon matrix value. (21) Spence, D.; Schulz, G. J. Phys. ReV. A. 1971, 3, 1968. (22) Andrews, W. L. S.; Pimentel, G. C. J. Chem. Phys. 1966, 44, 2361. (23) Tevault, D. E.; Andrews, L. J. Phys. Chem. 1973, 77, 1640, 1646.
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