Reactions of Laser-Ablated Iron Atoms with Oxygen Molecules in

Andersen, T.; Lykke, K. R.; Neumark, D. M.; Lineberger, W. C. J. Chem. Phys. 1987, 86, 1858. [Crossref], [CAS]. (7) . Autodetachment study of the elec...
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J. Phys. Chem. 1996, 100, 5261-5273

5261

Reactions of Laser-Ablated Iron Atoms with Oxygen Molecules in Condensing Argon. Infrared Spectra and Density Functional Calculations of Iron Oxide Product Molecules George V. Chertihin, Wendy Saffel, Jason T. Yustein, and Lester Andrews* Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22901

Matthew Neurock Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22901

Alessandra Ricca and Charles W. Bauschlicher, Jr. STC-230-3, NASA Ames Research Center, Moffett Field, California 94035 ReceiVed: October 30, 1995X

Reaction of laser-ablated Fe atoms with oxygen molecules in a condensing argon stream produced FeO, FeO2, FeO3, FeO4, Fe2O, Fe2O2, and Fe2O4 molecules, which are identified from oxygen isotopic shifts and multiplets in matrix infrared spectra. The Fe + O2 reaction gives symmetrical bent, symmetrical cyclic, and asymmetrical bent FeO2 isomeric products with triplet, triplet, and quartet isotopic absorptions, respectively, using statistically mixed 16,18O2 as the reagent. The major reaction product symmetrical bent OFeO iron dioxide molecule (150 ( 10°) is characterized by stretching fundamentals at 945.8 and 797.1 cm-1, and the asymmetric bent FeOO form exhibits a 1204.5 cm-1 absorption. The cyclic isomer Fe(O2) produced spontaneously during annealing in solid argon absorbs at 956.0 cm-1. Oxygen and iron isotopic absorptions show that FeOFe is a symmetrical bent (140 ( 10°) molecule. Rhombic Fe2O2 absorbs at 517.4 cm-1. Evidence is presented for isomers of FeO3, FeO4, and Fe2O4. Density functional theory was used to calculate energies, structures, and frequencies for product molecules to support their identification.

Introduction The interaction of iron with oxygen is of great chemical interest because it takes place in a wide range of processes ranging from corrosion of materials to oxygen transport in biological systems. However, in spite of almost 20 years of investigation, the assignment of reaction products and the mechanism of reactions is still open to question. Among iron oxides, the diatomic FeO molecule has been most thoroughly investigated. Its molecular constants have been obtained by matrix infrared, photoelectron, photoluminescence, microwave, and electronic spectroscopy,1-9 and ab initio and semiempirical calculations have been done.10-12 However, many disagreements are found in the literature for the triatomic FeO2 molecule. Abramowitz, Acquista, and Levin reacted thermally evaporated iron atoms with oxygen during condensation with excess argon and assigned the 945.9 and 517.1 cm-1 bands to the ν1(O-O) and ν2(Fe-O) vibrations of the cyclic Fe(O2) molecule;13 however, the 16/18 isotopic ratio for the first vibration is too low for this assignment. In the next work Chang, Blyholder, and Fernandez produced iron atoms by hollow-cathode discharge,14 and three isomers of FeO2 were proposed: linear (ν3 ) 969 cm-1), cyclic (ν1(O-O) ) 956 cm-1), and bent (ν3 ) 946 cm-1, ν2 ) 517 cm-1). Later, Serebrennikov15 studied infrared spectra of the Fe + O2 system using iron atoms from both thermal evaporation and hollow-cathode sputtering techniques, and the following assignments were proposed: FeOO (1385 cm-1), cyclic Fe(O2) (1117 cm-1), and bent OFeO (948/944 cm-1). A very recent investigation of the photooxidation of matrixisolated iron pentacarbonyl in the presence of 5% O2 in argon has proposed several Fe(CO)n(O2) species as well as Fe(O2) (956 cm-1) and FeO3 (945 cm-1) binary iron oxide species in X

Abstract published in AdVance ACS Abstracts, February 1, 1996.

0022-3654/96/20100-5261$12.00/0

this system.16 A subsequent density functional theory study of these intermediates has been reported.17 In contrast, two gas phase studies18,19 report that atomic Fe is unreactive toward O2 although larger iron clusters are reactive with molecular oxygen.18,20 However, Fe from laser photodissociation of ferrocene vapor does react with O2 in N2 carrier gas.21 The cation and anion of FeO2 have been studied by ICR, and the photoelectron spectrum of FeO2- has been reported.9,22 The question of how many and which isomers exist for FeO2 remains open, which kindles our interest in the Fe + O2 system. Semiempirical calculations11 suggested three stable structures, linear FeOO and OFeO and cyclic Fe(O2). DFT calculations found the dioxo form to be more stable than the peroxo form,17 but HF calculations predicted a superoxide form to be the most stable structure.21 Analyzing the experimental results suggests that thermal and more energetic iron atoms react to give different initial products. These different products could be due to reactions with ground state iron atoms in the matrix or to reactions with energized or excited metal atoms in the gas phase or near the matrix surface. For example, the FeO molecule can be produced only with excess energy as reaction 1 is endo-

Fe + O2 f FeO + O

∆E ) +21 kcal/mol

(1)

thermic by 21 kcal/mol based on bond energies.23 The laser ablation technique employed in this work produces energetic metal atoms, which can undergo reactions that require activation energy. Thus, it was decided to reinvestigate the Fe + O2 system as the previous assignments are incomplete and subject to question, and more theoretical calculations are needed to support the vibrational assignments. Experimental Section The technique for laser ablation and FTIR matrix spectroscopy employed here has been described earlier.24 Iron (Johnson Matthey, pieces, 99.98%) was used for the laser target. Oxygen © 1996 American Chemical Society

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Figure 1. Infrared spectra in the 1200-500 cm-1 region for pulsed laser-ablated iron target codeposited with argon at 10 ( 1 K in a blank experiment: (a) spectrum after deposition for 2 h, (b) after annealing to 25 K, and (c) after annealing to 30 ( 2 K.

samples (Matheson, Yeda 55% and 97% 18O) were used as received. Matrix dilution was 0.5-5.0% O2 in argon. Complementary experiments were done with pure oxygen, N2O (Matheson, Isomet), and ozone in argon prepared as described.24 Gas mixtures were codeposited 1-2 h on a cold CsI window (10 ( 1 K measured by Lake Shore Cryotronics diode DT-470-CU13 attached to cold stage) at rates of 2-4 mmol/h with ablated Fe atoms using 20-60 mJ/pulse of 1064 nm radiation. Annealing with the refrigerator off was monitored by sample vapor pressure, and photolysis was done using a medium-pressure mercury arc (Philips, 175 W) with globe removed. FTIR spectra were recorded with 0.5 cm-1 resolution and (0.1 cm-1 accuracy on a Nicolet 750 spectrophotometer with MCT detector. Results FTIR spectra of the Fe + O2 system will be presented. In order to support assignment of spectral bands, experiments with ablation of Fe and Fe2O3 targets as well as experiments with Ar, N2O/Ar, N2/O2/Ar, and O3/Ar mixtures were also performed. Over 60 experiments were done to get good statistics on behavior of bands under a variety of conditions. Blank Experiments. The first blank experiments involved condensation of Fe surface ablation products with argon. Even keeping all conditions constant, the intensities of infrared bands after deposition changed from one experiment to the next. First, laser ablation of the oxidized iron surface leads to iron oxide bands in matrix spectra. Second, the iron-oxygen system is very sensitive to the traces of nitrogen from the vacuum system, which leads to nitrogen complex species. To minimize the amount of these species, several blank experiments were done until the intensities of the bands after deposition were almost the same in two consecutive experiments. (During this period the target was kept under vacuum.) Then experiments were done with oxygen added to the argon matrix gas. First, blank experiments with a new iron target gave bands at 1204.5 1040 (O3), 1002.0, 956.0, 953.8 (O4-),25,26 945.8, 887.3, 872.8, 797.1, 660.6, and 517.4 cm-1. Annealing to 20

K increased the 1002.0, 956.0, 887.3, and 517.4 cm-1 absorptions and revealed new 1147.5, 968.9, and 928.1 cm-1 bands, the 1204.5 cm-1 band disappeared, and other bands remained almost unchanged. Annealing to 25 K and higher decreased the 945.8, 872.9, and 517.4 cm-1 bands. Generally, all bands listed in Table 1 were observed in experiments with a fresh target, and their intensities depended strongly on laser power, time of deposition, and cleanliness of target surface. After several experiments with a clean target under continuous vacuum, only weak 945.8 cm-1 and medium 872.8, 660.6, and 517.4 cm-1 bands remained in the spectra after deposition with low laser power. Slightly increased laser power produced these bands and increased a new 868.6 cm-1 band (Figure 1a). Annealing to 25 K (Figure 1b) increased the ozone 945.8 and 872.8 cm-1 bands and almost destroyed the new 868.6 cm-1 band; weak 1002.0, 887.3, and 670.2 cm-1 bands increased. Subsequent annealing to 30 K (Figure 1c) further increased the latter and decreased the former bands. An experiment doped with 1% N2 gave the same bands reported in Figure 1, but absorptions above 2000 cm-1 and 1002.0, 917.1, 887.3, 670.2, and 530.0 cm-1 bands were enhanced. Binary iron nitrides will be reported in a later publication. Fe + O2. Spectra of Fe + O2 system in argon are presented in Figure 2 and listed in Table 1. Band intensities depended strongly on oxygen concentration. With 1% O2 in argon spectra contained several strong bands after deposition; water contamination was minimal (I(1590) ) 0.002 au). The main differences from the blank experiments were an increased 945.8 cm-1 band intensity relative to the 872.8 cm-1 FeO band,3 increased ozone bands near 1040 cm-1, and an increased 1204.5 cm-1 band absorbance. Weak bands were also observed at 1118.5, 953.8, and 804.2 cm-1 due to the O4+, O4-, and O3- molecular ions.26 Annealing gave the above described results, but the 1147.6, 968.9, 956.0, and 928.1 cm-1 bands were much stronger after annealing in O2-doped than in the blank experiments. Annealing also sharpened the 872.8 cm-1 FeO band (0.8-0.9 cm-1 fwhm) and enabled the weak 54Fe counterpart to be measured at 876.30

Reactions of Fe with O2 in Ar

J. Phys. Chem., Vol. 100, No. 13, 1996 5263

Figure 2. Infrared spectra in the 1500-500 cm-1 region for pulsed laser-ablated Fe atoms codeposited with 1% O2 in argon at 10 ( 1 K: (a) spectrum after sample deposition for 2 h, (b) after broad-band photolysis for 30 min, (c) after annealing to 25 K, and (d) after annealing to 30 K.

cm-1 with 1/20 relative intensity to the strong 56Fe band at 872.80 ( 0.05 cm-1, in good agreement with natural abundance iron (54Fe, 5.82%; 56Fe, 91.66%). Likewise, the sharp 945.8 cm-1 band gave an iron isotopic satellite at 951.8 cm-1. Increasing laser power favored the 868.6, 747.5, 721.4, 705.1, and 585.6 cm-1 bands but decreased the 1204.5 cm-1 band and 1207.1 cm-1 shoulder. Increasing oxygen concentration enhanced ozone and broad bands at 1421, 1390, and 1095 cm-1 and sharp bands at 975.8, 968.9, 705.1, and 861.5 cm-1 but decreased the 868.6 cm-1 band yield. With 5% O2 the 975.8 and 968.9 cm-1 bands were stronger on deposition but still less intense than 956.0 and 945.8 cm-1 bands. Annealing markedly increased the 968.9 and 956.0 cm-1 bands and associated much weaker 1496.5 and 548.4 cm-1 bands with the 956.0 cm-1 band, while the 975.8 and 945.6 cm-1 bands decreased. Annealing favored the 1147.5, 968.9, 956.0, and 928.1 cm-1 bands. Finally, broad-band photolysis decreased the 1002.0 cm-1 band and almost destroyed the 1204.5 cm-1 band; after annealing, photolysis decreased the 956.0 cm-1 band by 10% and increased the 945.8 and 797.1 cm-1 bands by 20%. Oxygen isotopic substitution was employed to identify product bands. Unfortunately, isotopic structures of many bands overlapped, but annealing behavior made it possible to identify 18O isotopic counterparts for most bands as given in Table 1. Spectra with Fe + 18O2 are shown in Figure 3; annealing had the same effect described above. In one experiment, exposure of the 10 K sample to the infrared source destroyed the 1137.6 cm-1 band, and warming to 25 K in the dark regenerated this band. The details of isotopic multiplets in scrambled (16,18O2) experiments will be discussed with the assignment of spectral bands. Note that annealing behavior allows definitive assignment of 18O counterparts of the 956.0 (broader) and 945.8 cm-1 (sharper) bands (Figure 2) and their corresponding scrambled (16,18O2) isotopic triplets (Figure 4, Table 1). Mixed (16O2 + 18O ) isotopic experiments gave the pure isotopic counterparts 2 for these bands with weak intermediate components at 986.2 and 930.9 cm-1 that were strong in the scrambled (16,18O2)

isotopic experiments. Again sharper Fe18O bands gave 56Fe and components at 834.50 and 838.10 ( 0.05 cm-1. Complementary experiments were done with pure oxygen films, and the absorptions are reported in Table 2; strong ozone bands are not listed. The major new bands at 970.2, 954.5, and 945.2 cm-1 are within 2 cm-1 of argon matrix counterparts. Annealing increased the 954.5 cm-1 band and weaker associated counterparts at 1494.8 and 549.7 cm-1 and decreased the 945.2 cm-1 band, as observed in solid argon. The 1209.8 cm-1 product band also decreased on annealing. An oxygen-18 film experiment gave analogous results. Fe2O3. Spectra of hematite (Fe2O3) laser-ablated into a condensing argon stream were very similar to those for a fresh iron target. The major bands after deposition were 1204.5, 1040 (O3), 956.0, 953.8 (O4-), 945.8, 887.3, 872.8, 797.1, and 517.4 cm-1. Annealing to 25 K decreased FeO and increased 1095, 928.1, 917.1, and 887.3 cm-1 bands with little change in the 956.0 cm-1 band. When Fe2O3 was ablated into 1% O2-doped argon stream, the FeO 1002.0 and 670-660 cm-1 bands were weaker, but the 1204.5 and 975-900 cm-1 bands were stronger. Fe + N2O. In order to provide complementary reactions, spectra of the Fe + N2O system were also studied. A very strong FeO band at 872.8 cm-1 and the 660.6 and 517.4 cm-1 bands were observed after deposition together with medium 2271.3, 2262.6, 1002.1, 945.8, 887.3, 868.6, and 670.2 cm-1 bands and weak 928.1, 797.1, 747.5, and 721.4 cm-1 bands. Annealing to 25 K increased 1002.0, 928.1, and 670.2 cm-1 bands, the 887.3 cm-1 band became very strong, and the 945.8, 797.1, and 517.4 cm-1 bands remained almost unchanged. Annealing to 30 K decreased 2271.3, 1002.1, 945.8, 797.1, and 517.4 cm-1 bands, but other bands continued to grow. Sharper bands gave 56Fe and 54Fe components at 887.25 and 891.00 ( 0.05 cm-1. Two experiments were done with 15N2O samples and low laser power. The spectra (Figure 5) were dominated by the 872.8 cm-1 FeO band and 887.2 cm-1 satellite; weak sharp bands were observed unshifted at 1002.0, 945.8, 797.1, and 54Fe

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TABLE 1: Infrared Absorptions (cm-1) Observed for the Laser-Ablated Fe + O2 System in Solid Argon 16O 2

2307.5 2271.3b 2262.6b 2245.1b 2233.5b 1496.5 1494.6 1421 1390 1339.4 1204.5 1174.7 1147.5 1118.6 1095.4 1002.0 975.8 968.9 960.7 956.0 957.7 953.8 945.8 951.8 928.1 917.1 908, 904.5 887.3 890.9 872.8 876.2 868.6 869.6 861.5 813.8 804.2 797.1 747.5 721.4 705.1 670.2 661.5 sh 660.6 585.6 548.4 547.3 530.0 517.4 473.6 a

18O

16,18O

2

2271.1 2262.6 2245.1 2233.5 1424.8 1423.1 1342 1312 1265.6 1137.6 1109.8 1081.8 1056.3 1033.2 964.1 938.2 931.3 925.2 906.7 908.2 901.7 911.2 917.4 888.1 869.5 848.7 852.7 834.5 838.2 825.9 827.0 827.1 768.2 759.3 754.8 717.9 693.5 674.5 641.2 632.8 631.9 563.2 524.8 523.9 507.6 494.8 450.3

R(16O/18O)

2

2245.1 2233.5 1457 1418, 1382, 1350 1388, 1341, 1318 1204.5, 1173.0, 1170.4, 1137.6 1147.1, 1116.1, 1082.3 1119, 1102, 1088, (1085),c 1072, 1057 1095.4, 1091.5, 1089.7, 1061.4, 1062.0, 1033.2 1002.0, 986.2, 964.1 968.9, 954.2, 931.3 960.7, 925.2 956.0, 931.2, 906.7 957.7, 933.1, 908.2 (925.1)c 945.8, 930.9, 911.2 951.8, 936.7, 917.4 928.3, 926.2, 923.9, 898, 895, 888 917.1, 898.6, 869.5 887.3, 848.7 890.9, 852.7 872.8, 834.5 876.2, 838.2 868.6, 825.9 869.6, 827.0 861.5, 860.2, 860.9, 845.7, 844.0, 841.9, 830.3, 828.4, 827.1 804.2, 794.5, 768.2, 759.8 797.1, 773.7, 754.8 747, 736, 720 720, 710, 695 705.2, 704.4, 703.4, 698.8, 695.7, 694.6, 675.7, 675.0, 674.3 670, 648.0, 641.2 661.6, 639.5, 632.8, 681.1 660.6, 638.7, 631.9, 680.3 585.5, 583.8, 582.0, 577.1, 575.2, 573.3, 567.8, 565.5, 563.2 530.0, 521.4, 507.8 517.4, 508.1, 494.8

Annealing behavior up to 50 K.

b 15N-isotopic

1.0503 1.0502 1.060 1.059 1.0583 1.0588 1.0585 1.0607 1.00590 1.0602 1.0393 1.0401 1.0404 1.0384 1.0544 1.0545 1.0578 1.0380 1.0375 1.0450 1.0547 1.0455 1.0448 1.0459 1.0453 1.0517 1.0416 1.0594 1.0593 1.0560 1.0412 1.0402 1.0454 1.0452 1.0454 1.0454 1.0398 1.0450 1.0447 1.0437 1.0457 1.0517

anneala

assignment

+ + + ++ ++ ++

(N2)xFe(O2) N2FeO2 N2FeO ? ? (ν1 + ν2)Fe(O2) site (O2)FeOO (O2)FeOO FexOy ? FeOO (O2)FeOO (O2)FeO O4+ (O2)FeO2 N2FeO2 (FeO3) (O2)FeO2 (X)(FeO2) ν1 Fe(O2) site O4ν3 56FeO2 ν3 54FeO2 (O2)FeO (N2)xFe(O2) FexOy? N256FeO N254FeO 56FeO 54FeO 56 FeO56Fe 54 FeO56Fe O(FeO)2O O3- site O3ν1(FeO2) (OFeFeO2)

++ ++ ++ ++ ++ + ++ ++ ++ + + + + 0 0 + 0 0 0 ++ ++ ++ ++

O(FeO)2O (OFeFeO)(N2) OFeFeO OFeFeO (O2FeFeO2) ν2Fe(O2) (FeO)2(N2)2 ν5(FeO)2 ?

counterparts: 2195.6, 2187.3, 2170.3, and 2159.1 cm-1. c Major band in 16O2 + 18O2 experiments.

517.4 cm-1; however, 1204.5, 1147.5, and 956.0 cm-1 absorptions were not detected although the 945.8/797.1 cm-1 bands maintained the same 11/1 absorbance ratio from O2 experiments. The 945.8/517.4 cm-1 absorbance ratio changed from 20/8 with O2 to 1/3 with N2O. The upper region revealed a new 2187.3 cm-1 band that increased on annealing with the 887.2 cm-1 band. Fe + N2/O2. Analyzing spectra of the previous system suggested that some bands can be associated with nitrogen complexes of iron oxides. Therefore, spectra of Ar/N2/O2 mixtures and their isotopic modifications were investigated. Results are included in Table 1, and detailed discussion will be given in the next section. The main difference from the Fe + O2 system was increasing relative intensities of the 1002.0, 887.3, 917.1, 670.2, and 530.0 cm-1 bands (particularly after annealing) together with the bands above 2000 cm-1. Fe + O3. As a final contrast to the Fe + O2 system, several experiments were done with 1% ozone in argon. Spectra of this system contained many but not all bands from the Fe + O2

system. After deposition the 1204.5, 1147.5, 1095.4, 975.5, 968.9, 956.0, 945.8, 928.1, 872.8, 868.6, and 517.4 cm-1 bands were observed. Behavior of these bands on increasing laser power was similar to that described above for Fe + O2. The main differences were that the 872.8 cm-1 band was almost the same intensity as the 945.8 cm-1 band and that the 1147.5 and 928.1 cm-1 bands were stronger after deposition in these experiments. Annealing gave only slight increases in the 1095.4, 968.9, and 956.0 cm-1 absorptions and no growth of 1147.5 and 928.1 cm-1 bands, and broad bands were not observed near 1400 and 1175 cm-1. Oxygen isotopic substitution matched the results obtained for experiments with O2. Calculations. Density functional theory (DFT) quantum chemical calculations were performed on various FeOx species in an effort to help interpret the experimental infrared results as well as to evaluate the geometric and electronic structure. Two conceptually different DFT methods were employed to compute optimized structure, energies, frequencies, and corre-

Reactions of Fe with O2 in Ar

J. Phys. Chem., Vol. 100, No. 13, 1996 5265

Figure 3. Infrared spectra in the 1400-500 cm-1 region for pulsed laser-ablated Fe atoms codeposited with 1% 18O2 in argon at 10 ( 1 K: (a) spectrum after sample deposition for 2 h, (b) after annealing to 25 K, and (c) after annealing to 30 K.

Figure 4. Infrared spectra in the 1400-500 cm-1 region for Fe atoms codeposited with scrambled 16,18O2 (1%) in argon: (a) spectrum after sample deposition for 2 h at 10 ( 1 K, (b) after annealing to 25 K, (c) after annealing to 30 K, and (d) after annealing to 35 K.

sponding intensities for a series of different FeO2, FeO3, and FeO4 structures. The first DFT approach uses the local spin density with exchange and correlation gradient corrections; the second approach involves the more recent hybrid Hartree-Fock/ DFT method. The DGauss program developed by Cray Research Inc.27 was used to perform many of the nonhybrid DFT calculations. DGauss employs optimized Gaussian basis functions to selfconsistently solve the single-particle Kohn-Sham equations.28 The local spin-density exchange-correlation potential is repre-

sented by the Vosko-Wilk-Nusair potential.29 Nonlocal gradient corrections to the exchange and correlation are determined in situ to the SCF calculation via exchange and correlation potentials developed by Becke30,31 and Perdew,32 respectively; this functional is denoted BP. All electrons are explicitly accounted for in iron. DFT-optimized DZVP quality sets33 were used for both iron (63321/531/41) and oxygen (621/ 41/1). All 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,

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TABLE 2: Infrared Absorptions (cm-1) Observed for Pure Oxygen Codeposited with Laser-Ablated Iron Atomsa 16O

2

1495.6 w 1413.4 1368.4 1209.8 1166.6 1097.6 w 970.2 s 954.5 s 949.5 945.2 s 895.0 877 848.6 w 637 w 618 w 549.7 w

ratio

annealb

assignment

1424.0 1334.7 1291.0 1142.7 1101.3

1.0503 1.0590 1.0600 1.0587 1.0593

932.6 905.6 914.8 909.1 852.9 842 809.8 608 592 526.2

1.0403 1.0540 1.0379 1.0397 1.0494 1.0416 1.0479 1.0477 1.0440 1.0447

+ + + + + + + + + 0 + + +

(ν1 + ν2)Fe(O2) (O2)FeOO (O2)FeOO FeOO (O2)FeOO (O2)FeO2 (O2)FeO2 ν1(Fe(O2)) O2- -FeO2 ν3(FeO2) ? O(FeO)2O FeOFe ? ? ν2(Fe(O2))

18O

2

a w ) weak, s ) strong. b Anneal: + ) increase, 0 ) no change, - ) decrease.

and frequencies, however, are all determined numerically using the harmonic oscillator approximation. Gaussian 92/DFT calculations employed the more recent hybrid B3LYP functional,34,35 which is a modification of the original Becke hybrid functional.36 The Fe basis set used is a [8s4p3d] contraction of the (14s9p5d) primitive set developed by Wachters.37 The s and p spaces are contracted using contraction number 3, while the d space is contracted (311). To this basis set two diffuse p functions to describe the 4p orbital are added; these are the functions optimized by Wachters multiplied by 1.5. Diffuse s and p functions (R(s) ) 0.013 963 and (R(p) ) 0.020 92) are also added. A diffuse d function38 and an f polarization function (R ) 1.339) are added. The final Fe basis set is of the form (15s12p6d1f)/[9s7p4d1f]. The O basis set is the 6-311+G(2df) set of Pople and co-workers.39 Geometry optimization and harmonic frequencies were computed from analytical first and second derivatives. The find grid option was used for all Gaussian 92/DFT calculations.40 Primary reaction energy changes were calculated from B3LYP energies. While B3LYP has established itself for the near accurate treatment of organic systems, for transition metal systems the distinction between conventional DFT with BP corrections and B3LYP is less clear and still a topic for debate. Herein we examine and compare both methods for the small FeO2 clusters. The larger FeO3 and FeO4 systems were all analyzed using only the BP functional via DGauss. Results from both DGauss/BP and G92/B3LYP were similar for computed structures and measured frequencies when compared for the same spin state. Differences, however, are noted in the determination of which spin state is the ground state. DGauss/BP calculations tend to favor triplet spin states, while the G92/B3LYP results favor quintet spin states. To examine the cause of these differences, we repeated a series of calculations using the BP functional within the Gaussian 92/DFT framework. The results, which are also presented in Tables 3 and 4, indicate that the differences in spin state are directly attributed to exchange-correlation functional employed. In an effort to test which of these functionals (BP or B3LYP) predict the appropriate ground state structures, we performed for FeO2 and Fe(O2) a set of restricted coupled cluster singles and doubles calculations,41 which include a perturbational estimate of the connected triples42 [RCCSD(T)] using Molpro 94.43 These RCCSD(T) calculations found the triplet state lower than the quintet state by about 1 kcal/mol for both FeO2 and Fe(O2).

FeO. With the BP functional and DGauss, FeO in the ground state was calculated to have a 907 cm-1 fundamental and 1.611 Å bond length, in good agreement with experimental values;23 the next lowest triplet state was 19 kcal/mol higher in energy. The B3LYP functional gave a 903 cm-1 fundamental and 1.611 Å bond length for ground state FeO. FeO2. Three plausible FeO2 geometries were computed. Each structure was optimized using different spin multiplicities to find the optimal ground state structure. The structures and energies for each of these species are summarized in Table 3. The computed harmonic frequencies for each are reported in Table 4. The energetic ordering of these structures is as follows: O-Fe-O (bent) < Fe(O2) (cycle) < Fe-O-O (bent) < Fe + O2. Linear O-Fe-O and Fe-O-O isomers are higher energy and have imaginary frequencies; such unstable species will not be considered for matrix products. FeO3. Three unique FeO3 molecules were examined to establish their energies, stabilities, and corresponding frequencies. These structures were chosen as logic structures formed upon the addition of oxygen atoms to the FeO2 structures above. The results are summarized in Tables 5 and 6. The FeO3 (D3h) species is the energetically most favorable structure while the O-Fe-O-O structures are the least stable. FeO3 (D3h) and (O2)FeO (C2V) have singlet and quintet ground states, respectively; but OFeOO has a triplet ground state. FeO4. As with FeO3, a set of FeO4 structures built from FeO2 counterparts was established. Structures, energies, frequencies, and relative peak intensities were computed. The ground state structure was determined by optimizing each arrangement using different spin multiplicities. The structural and energetic results are given in Table 7, while the corresponding frequencies and intensities are reported in Table 8. The relative energy ordering is as follows: 5∆

(O2)FeO2 (C2V) < FeO4 (Td) < (O2)Fe(O2)(D2d) < (O2)Fe(O2) (D2h) < (O2)FeOO Discussion The Fe + O2 product absorptions will be identified from oxygen isotopic shifts and mixed isotopic multiplets. The ironoxygen system is unfortunately complicated by the absorptions of many product molecules in the 1000-870 cm-1 region, but different annealing behavior facilitated matching isotopic components. This system is very sensitive to the presence of nitrogen as many iron oxides react with N2 to form ternary complex products, which will also be identified. FeO. The band at 872.8 cm-1 is due to the diatomic FeO molecule based on previous matrix work and isotopic substitution.3 The 16/18 ratio 1.0459 is just below the harmonic diatomic 1.0463 value as is the 54/56 ratio 1.004 01 below the harmonic 1.004 10 value as expected for cubic contributions to anharmonicity. The four sharp isotopic bands observed here for 54Fe16O, 56Fe16O, 54Fe18O, and 56Fe18O are 0.2-0.3 cm-1 higher than bands measured by Green and Reedy using the cathode sputtering technique3 and support its use as an anchor for the ground state FeO fundamental. The gas phase fundamental, 871.3 cm-1, is only 1.5 cm-1 lower; thus, the argon matrix blue-shifts the FeO fundamental by 1.5 cm-1.4 The photodetachment spectrum of FeO- reaches a higher electronic state with a 970 ( 60 cm-1 fundamental.2 It was mentioned above that FeO cannot be formed in the reaction between “cold” iron atoms and oxygen molecules as this reaction is endothermic by 21 kcal/mol. The main mechanism of FeO formation is reaction between energetic Fe atoms produced by laser ablation19,44-47 and O2 molecules during

Reactions of Fe with O2 in Ar

J. Phys. Chem., Vol. 100, No. 13, 1996 5267

Figure 5. Infrared spectra in the 1400-500 cm-1 region for Fe atoms codeposited with 15N2O (0.5%) in argon: (a) spectrum after sample deposition for 2 h at 10 ( 1 K, (b) after annealing to 20 K, (c) after annealing to 25 K, and (d) after annealing to 30 K.

TABLE 3: Relative Energies (kcal/mol), Bond Lengths (Å), and Bond Angles (deg) Calculated for FeO2 Species Using DFT BP (DGauss) OFeO benta

T Q S Fe(O2) T cycle Q S FeOO T d Q bent

0.00, 1.594, 139.4b 6.89, 1.622, 114.1 45.1, 1.737, 111.1 49.0, 1.786, 1.435c 55.5, 1.84, 1.450 64.0, 1.99, 1.355 58.9, 1.777, 1.327c converts to cycle

B3LYP (G92)

BP (G92)

0.00, 1.582, 141.5 0.00, 1.581, 136.9 -2.42, 1.604, 117.7 2.35, 1.608, 117.1 43.2, 1.798, 1.407 37.1, 1.812, 1.491 35.6, 2.001, 1.334 46.6, 1.869, 1.306 48.8, 1.835, 1.300

60.4, 1.774, 1.415 58.7, 1.820, 1.466 65.3, 1.937, 1.372 73.7, 1.768, 1.334 converts to cycle

a Linear triplet OFeO is 3.8 kcal/mol higher in energy, and the Fe-O bond length is 1.614 Å using BP (DGauss). b Relative energies, Fe-O bond length, O-Fe-O angle. c Relative energies, Fe-O bond length, O-O bond length. d Linear triplet FeOO is 15.7 kcal/mol higher in energy, and the Fe-O2 bond length is 1.68 Å using BP (DGauss).

deposition. Furthermore, the relative yield of FeO from the N2O reaction (2) was substantially higher than from the O2 reaction (1). This is reasonable as reaction 2 is exothermic, in contrast to reaction 1, and reaction 2 may require little activation energy. The yield of FeO is favored in the ozone reaction (∆E ) -70 kcal/mol based on bond energies23) for the same reason.

N2O + Fe f FeO + N2

∆E ) -57 kcal/mol (2)

FeO2 Molecules. Bent OFeO. The sharp bands 945.8 and 797.1 cm-1 were observed in all experiments after deposition. These bands increased slightly in concert on annealing to 20 K and then decreased at higher annealing temperatures; furthermore, the bands increased by 20% on photolysis after annealing but increased only 5% on photolysis after deposition. Reaction with 16,18O2 produced sharp triplets for two equivalent O atoms with 16/18 isotopic ratios 1.0380 and 1.0560, which bracket the diatomic 16/18 ratio as expected for the ν3 and ν1 vibrations of the bent triatomic OFeO molecule (Figure 4). Association of these triplets with the same molecule is indicated not only by constant relative intensities in all experiments but also in the outward displacement of the central 16OFe18O components

due to interaction between stretching modes for the lower symmetry isotopic molecule. This displacement from the mean of pure isotopic values is 2.4 cm-1 up for the ν3 mode and 2.3 cm-1 down for the ν1 mode. The 945.8 cm-1 band was first assigned to the O-O vibration of the cyclic FeO2 molecule,13 but this assignment is clearly not correct due to the isotopic ratios. In subsequent work the 945.8 cm-1 band was assigned to the ν3 vibration of bent OFeO,14,15 which is in agreement with the present work. Sharp, weak bands observed here at 951.8, 936.7, and 917.4 cm-1 in 16O , 16O18O, and 18O experiments are due to the ν funda2 2 3 mentals of 54Fe16O2, 16O54Fe18O, and 54Fe18O2 as for all bands the intensity ratio 54Fe/56Fe was about 1/16 as is required for natural isotopic abundance. The observed iron isotopic splitting confirms the presence of a single Fe atom and the OFeO identification. The O-Fe-O angle upper limit calculated from oxygen isotopic ν3 fundamentals (158 ( 5°) is in a good agreement with previous results.14,15 The lower limit determined from iron isotopic frequencies is 144 ( 5°. Comparisons with similar calculations for TiO2 using both Ti and O isotopic ratios suggest at 150 ( 10° value for the OFeO angle.44 Furthermore, the bond dipole model of infrared intensities for the ν3 and ν1 modes48 can also be used to estimate the angle as ν1 intensity increases with departure from linearity. Such a calculation predicts a 140° angle from the 1/11 intensity ratio observed for ν1/ν3. A 945 cm-1 band observed as the final photolysis product in the Fe(CO)5 + O2 study16 shifted to 911 cm-1 with 18O2 and showed almost the same 16/18 ratio as the present sharp 945.8/ 911.2 cm-1 bands, which predicts a 158 ( 5° OFeO angle, as described above. In contrast to the strong sharp 930.9 cm-1 intermediate band observed here, the carbonyl study gave weaker intermediate bands at 935 and 923 cm-1, which were interpreted to identify the D3h FeO3 species.16 In view of agreement with cyclic Fe(O2) observations (see below) between the present pulsed laser and previous carbonyl photolysis

5268 J. Phys. Chem., Vol. 100, No. 13, 1996

Chertihin et al.

TABLE 4: Vibrational Frequencies (cm-1) and Infrared Intensities (km/mol) Calculated for FeO2 Species in Triplet and Quintet States Using DFT with Different Functionals BP (DGauss) OFeO

T

Fe(O2)

T

FeOO

T

B3LYP

BP (G92) a

1031.4 (146) 910.6 (26) 478.5 (16) 1041.2 (46) 680.4 (33) 658.9 (17) 1241.5 (237) 707.5 (11) 419.4 (5)

BP (DGauss)

1033.8 (177) 926.5 (27) 205.6 (18) 967.4 (62) 620.4 (9) 441.2 (18) 1046.8 (285) 536.7 (10) 95.0 (6)

958.3 (91) 891.3 (25) 194.3 (29) 985.7 (81)b,c 500.9 (0.1) 433.5 (17) 1159.8 (185)d 472.7 (21) 134.7 (2)

Q Q Q

951.5 (72) 886.3 (44) 387.2 (13) 903 (90) 440 (7) 292i (413) e

B3LYP (118)a

958.1 920.0 (49) 292.7 (17) 842.9 (132)b,c 509.7 (3) 462.1 (2) 1176.3 (115)d 476.8 (22) 147.1 (1)

BP (G92) 944.5 (96) 908.9 (43) 301.8 (12) 868.0 (92) 436.8 (0.2) 311.4 (71) e

a Fe18O frequencies 922.3, 844.2, 186.4 cm-1 for T and 919.8, 875.3, 279.6 cm-1 for Q. b Fe(18O ) frequencies 930.4, 482.2, 410.7 cm-1 for T 2 2 and 799.7, 488.2, 438.0 cm-1 for Q. c Frequencies calculated for the septet (7A1) state: 1157 (51) A1, 401 (26) A1, 398 (5) B2. d Fe18O18O frequencies 1093.3, 452.2, 128.3 cm-1 for T and 1108.9, 457.1, 139.8 cm-1 for Q. e Converts to Fe(O2).

TABLE 5: Relative Energies (kcal/mol) and Bond Lengths (Å) for FeO3 Species Using the BP (DGauss) Functional FeO3 (D3h) (O2)FeO (C2V) OFeOO (Cs)

S T S T Q T Q

energy

bond lengths

0.0 13.2 42.0 31.4 27.6 54.5 54.5

1.585 1.585 1.621, 1.796, 1.360a 1.611, 1.830, 1.345 1.620, 1.830, 1.354 1.617, 1.839, 1.297b 1.617, 1.839, 1.297

a Fe-O, Fe-O , O-O, respectively. b Fe-O, Fe-OO, O-O, re2 spectively.

TABLE 6: Vibrational Frequencies (cm-1) and Infrared Intensities (km/mol) Calculated for the Lowest Energy FeO3 Species Using the BP (DGauss) Functional FeO3, D3h, S

(O2)FeO, C2V, O

OOFeO, Cs, T

1031 (94) 1018 (91) 912 (0.4) 759 (12) 522 (0.1) 398 (0.3)

1215 (136) 953 (53) 675 (1) 536 (0.4) 464 (19) 368 (14)

1527 (312) 968 (133) 736 (6) 615 (15) 549 (1) 416 (4)

TABLE 7: Relative Energies (kcal/mol) and Bond Lengths (Å) and Angles (deg) Calculated for FeO4 Species Using the BP (DGauss) Functional (O2)FeO2 (C2V)

S T FeO4 (Td) S T (O2)Fe(O2) (D2d) S T (O2)FeOO S T

energy

bond lengths

angles

0.0 14.4 7.5 24.5 16.9 32.6 47.0 47.6

1.578, 1.777, 1.387 1.597, 1.880, 1.327 1.603 1.59, 1.687 1.78, 1.375 1.81, 1.383 1.780, 1.66, 1.277, 1.38 1.825, 1.70, 1.28

119, 66.8 117.2, 69.8 109.5 89, 112 45.4 67.8 167.8, 67.7 148.4, 68.2

experiments, the 945.8 and 945 cm-1 bands (with 911.2 and 911 cm-1 18O2 counterparts) could be due to similar open OFeO species with adjacent CO perturbations altering equivalence of the two oxygen atoms in the carbonyl work. In any case, the present 945.8 cm-1 band is clearly not due to FeO3 (isotopic triplet and shift predicting 158° angle), and the photochemical identification16 of D3h FeO3 is open to question (carbonyl environment and 16/18 ratio predicting a large ≈150° OFeO angle). We find no evidence for linear OFeO based on observed 16/ 18 ratios; the ν3 mode for a linear O56FeO species should exhibit the harmonic 16/18 ratio 1.0374. The 968.9 cm-1 band assigned14 earlier to linear OFeO exhibits a 16/18 ratio more appropriate for a 120° OFeO angle, and furthermore its marked growth on annealing is not appropriate for an OFeO insertion product. Finally, the present DFT calculations show that linear OFeO has imaginary frequencies and is thus unstable.

TABLE 8: Vibrational Frequencies (cm-1) and Infrared Intensities (km/mol) for the Lowest Energy FeO4 Species Using the BP (DGauss) Functional (O2)FeO2, C2V, S FeO4, Td, S (O2)Fe(O2), D2d, S (O2)FeOO, Cs, S 1253 (71) 1009 (125) 959 (44) 664 (2) 650 (4) 426 (0.1) 346 (4) 309 (4) 291 (2)

1097 (268) 1060 (216) 1001 (22) 891 (42) 704 (3) 636 (6) 581 (24) 545 (4) 528 (97)

1197 (128) 1162 (142) 756 (3) 735 (0.1) 672 (5) 647 (8) 538 (2) 374 (2) 189 (4)

1426 (237) 1139 (193) 848 (3) 721 (11) 667 (2) 540 (0.4) 512 (9) 320 (2) 262 (5)

Cyclic Fe(O2). The 956.0 cm-1 band was weak in the spectra after deposition, but it increased markedly on annealing. A scrambled isotopic triplet was observed at 956.0, 931.2, and 906.7 cm-1 with 16/18 ratio 1.0544. This band was earlier assigned to the ν1 (O-O stretch) vibration of the cyclic Fe(O2) molecule, and the present results confirm that assignment, but the low 1.0544 ratio indicates mode mixing. A similar 956 cm-1 band in the iron carbonyl/oxygen system was likewise assigned.16 In other work a 1117 cm-1 band was assigned to this vibration,15 but a weak band has been observed at 1118 cm-1 in this and other O2 experiments, and it is due to the O4+ cation.26 Note that the hollow cathode used for evaporation of iron15 caused the formation of the tetraoxygen cation. Two much weaker bands (2%) both exhibiting site-split doublet character track with the strong 956.0, 957.7 cm-1 band on annealing. The first at 548.4, 547.3 cm-1 exhibits a 16/18 ratio 1.0450, and the second at 1496.5, 1494.6 cm-1 gives a 1.0503 ratio. The latter is out of place in the O-O stretching region, which suggests a combination band. For the major Fe(16O2) site, 548.4 + 956.0 ) 1503.3 cm-1, which exceeds 1496.5 cm-1 by 7.9 cm-1 as is appropriate for anharmonicity in the combination mode. For the major Fe(18O2) site 524.8 + 906.7 ) 1431.5 cm-1, which exceeds 1424.8 cm-1 by 6.7 cm-1. Similar agreement was found for these bands in solid oxygen where Fe(O2) is trapped in surface sites. The low 1.0544 isotopic ratio for ν1(O-O) of Fe(O2) requires mixing with ν2(Fe-O2). The very weak 548.4 cm-1 band has a slightly higher 16/18 ratio (1.0450) than expected for a noninteracting ν2 mode, but this ratio is appropriately increased by mixing with the ν1 mode, which reduces the 16/18 ratio for the ν1 mode. Accordingly, the weak 548.4 cm-1 band is assigned to ν2(Fe-O2) for Fe(O2), and this assignment is confirmed by observation of the ν1 + ν2 combination band at 1496.5 cm-1 with the appropriate oxygen isotopic ratio. The 956.0 cm-1 band compares favorably with the 966.2 cm-1 band assigned to the ν1(O-O) mode of Ni(O2), which exhibits an oxygen isotopic triplet and 1.0576 ratio. In addition, a much weaker band near 505 cm-1 was identified as the ν2

Reactions of Fe with O2 in Ar

J. Phys. Chem., Vol. 100, No. 13, 1996 5269

symmetric Ni-O2 stretching fundamental,49 which is just below ν2 of Fe(O2) at 548 cm-1. The relative intensities of the ν1 and ν2 modes of these peroxo-transition metal species stand in marked contrast to the alkaline earth metal peroxide and alkali metal superoxide species50-52 where the ν2 mode is substantially stronger. However, the alkaline earth metal peroxide species have substantially lower (729-754 cm-1) ν1(O-O) stretching modes52 denoting more charge transfer than in the transition metal species. For comparison, DFT/B3LYP calculations for 5A Fe(O ) predict +0.69 on Fe and for 1A Ca(O ) +1.05 on 1 2 1 2 Ca; in addition for 3B1 OFeO, these calculations predict +0.80 on Fe and for 3B2 OCaO +1.21 on Ca.53 The bent OFeO molecule was observed as the major product after deposition in all experiments, and its intensity increased slightly (15%) upon first annealing (20 K) along with O3 and a slight decrease in FeO whereas the cyclic Fe(O2) molecule was only a minor deposition product but increased 4-5-fold on first annealing. Upon higher annealing (>25 K) bent OFeO decreased and cyclic FeO2 increased markedly. This means that reaction 3 requires an activation energy more than the diffusion barrier but that reaction 4 can proceed with activation energy matrix

Fe + O2 f [O-Fe-O]* 9 8 OFeO relax (DFT/B3LYP calc ∆E ) -64 kcal/mol) (3) matrix

8 Fe(O2) Fe + O2 f [Fe-O2]* 9 relax (DFT/B3LYP calc ∆E ) -27 kcal/mol) (4) less than the diffusion barrier. Furthermore, only a very weak 945.8 cm-1 band was observed after very long thermal deposition.13 However, when an electric discharge was used to produce iron atoms, this band was strong after deposition.14 Later, Serebrennikov15 observed OFeO in experiments with the hollow cathode and thermal evaporation (Knudsen cell heated by electron bombardment). Clearly, electric discharge and laser ablation produce high-kinetic energy Fe atoms, Fe resonance radiation, and excited Fe atoms which may form an insertion reaction product with oxygen. It should be noted that the 5P3 state of Fe with a 282 µs radiative lifetime can survive the short (3 cm) flight from the Fe target to matrix surface and can provide 50 kcal/mol of excess energy48,54 which may activate reactions 1 and 3. Furthermore, only high-energy atoms from the Boltzmann thermal distribution can form the insertion product. Accordingly, OFeO bands are stronger using high-energy Fe evaporation techniques. Another way to form OFeO is by reaction of FeO and O atoms. Reaction 5 then can account for

O + FeO f OFeO

(5)

the small growth of OFeO on first annealing: the observation of a small amount of 16OFe18O from the mixed (16O2 + 18O2) isotopic experiment supports this minor contribution to the reaction mechanism. Finally, it should be noted that little Fe(O2) is formed on deposition in the experiments with energetic iron atoms, which clearly favor the OFeO product. The observation of Fe(O2) growth on annealing and reaction of cold reagents appears to be at odds with the lack of gas phase reactivity of iron atoms with O2 near room temperature.18,19 Reaction 4 no doubt requires a third body to remove the reaction exothermicity, and the matrix cage provides a very effective third body, which is in accord with an earlier suggestion55 that Fe(O2) formation will require three-body collisional stabilization. DFT calculations provide support for the above assignments to OFeO and Fe(O2) and show that bent symmetrical OFeO is lower in energy than the cyclic species. What is the spin

TABLE 9: Comparison of Observed Oxygen 16/18 Isotopic Frequency Ratios and Ratios Calculated for OFeO and Fe(O2) Triplet and Quintet States Using BP and B3LYP Functionals and G92 calculated BP OFeO

T Q

B3LYP

observed

ν3

ν1

ν3

ν1

ν3

ν1

1.0395 1.0418

1.0550 1.0509

1.0390 1.0416

1.0558 1.0511

1.0380

1.0560

calculated BP Fe(O2)

T Q

B3LYP

observed

ν1

ν2

ν1

ν2

ν1

ν2

1.0585 1.0553

1.0395 1.0427

1.0594 1.0540

1.0388 1.0440

1.0544

1.0450

multiplicity of these two species? Only triplet states were considered in a previous DFT study,17 but the B3LYP functional suggests that quintet states are lower in energy. Comparison between calculated and observed frequencies and isotopic ratios can be used to identify the spin states for OFeO and Fe(O2). Table 4 shows that the calculated triplet state frequencies fit the strong ν3 (945.8 cm-1) and weak ν1 (797.1 cm-1) observed bands for OFeO better than the calculated quintet state frequencies. Both DFT functionals underestimate the OFeO angle, stretch-stretch interaction, and separation between ν3 and ν1. The calculated B3LYP 16/18 ratios for ν3 (1.0390) and ν1 (1.0558) are in reasonable agreement with the observed values considering the difference between calculated (141.5°) and observed (150 ( 10°) OFeO angle; similar 16/18 ratios are found for the BP frequencies as shown in Table 9. Clearly, OFeO is a bent triplet species. On the other hand, the B3LYP functional predicts the ν1 and ν2 relative intensities for Fe(O2) very well, but the calculated triplet state frequency (985.7 cm-1) is above the observed 956.0 cm-1 value and the calculated quintet state frequency (842.9 cm-1) is below. However, the calculated quintet 16/18 ratios for ν1 (1.0540) and ν2 (1.0440) fit the observed ratios extremely well, but the calculated triplet 16/18 ratios ν1 (1.0594) and ν2 (1.0388) do not correctly account for the observed mode mixing; again similar 16/18 ratios are found for the BP frequencies as shown in Table 9. Accordingly, the observed isotopic data for Fe(O2) support the quintet state for the cyclic peroxo isomer. The present DFT calculations show that OFeO does not have a septet ground state. For Fe(O2), the BP functional rules out a septet ground state, while the B3LYP yields the septet state to be slightly below the quintet state. However, a comparison of the computed and experimental results shows that ground state is quintet. Thus the conclusions of this work are in disagreement with the low-level HF calculations.21 The calculated frequencies (Table 4, footnote c) for the 7A1 state do correspond to a superoxide species, based on the strongest 1157 cm-1 absorption, but no such species is observed here. The 1147.5 cm-1 band that appears on annealing shows evidence of coupling with a third inequivalent O atom and will be assigned below to (O2)FeO. FeOO Molecule. The 1204.5 cm-1 band depends on O2 concentration and was strongest (A ) 0.4) with 5% O2 and threshold laser power. The band was almost destroyed on broad-band and on glower photolysis after deposition and reappeared by 15% on subsequent intermediate annealings. However, the band decreased on annealing after deposition unless the infrared source was shielded from the sample, which allowed for a small growth on annealing. The 1204.5, 1173.0, 1170.4, and 1137.6 cm-1 isotopic quartet requires two inequiva-

5270 J. Phys. Chem., Vol. 100, No. 13, 1996 lent oxygen atoms in the vibration, and the observed 16/18 frequency ratio 1.0588 identifies an O-O stretching mode. This is the region for asymmetric superoxo absorptions in iron(II) porphyrins and oxyhemoproteins.56,57 Accordingly, the 1204.5 cm-1 band is assigned to FeOO. The 1385 cm-1 band assigned earlier15 to this species is probably due to HO2 (1388, 1101 cm-1),58 which is observed in many of these experiments. Finally, the failure to observe an accompanying Fe-O stretching mode is a concern, but this mode is calculated to be much less intense. The identification of FeOO is supported by DFT B3LYP frequency calculations given in Table 4, which predict the spectrum to be dominated by a strong band at 1160 cm-1 for the ground 3A′′ state. Since the B3LYP functional favors higher spin states, the fact that the 3A′′ state is calculated lower than the 5A′ state means that 3A′′ is the ground state. The 16/18 frequency ratios for ν1 (1.0604) and ν2 (1.0453) show that these normal modes are almost pure O-O and Fe-O stretching coordinates. The FeOO molecule is formed during deposition of Fe and O2, reaction 6. However, FeOO can be formed on

Fe + O2 f FeOO (DFT/B3LYP calc ∆E ) -17 kcal/mol) (6) annealing and reaction of cold reagents, but with less effenciency than the more stable cyclic Fe(O2) species. One presumes that a steric effect of the matrix cage allows FeOO to be formed on annealing. We speculate that the availability of only one more (namely with sixth) coordination site in iron porphyrins may also favor -FeOO over the more stable -Fe(O2) structure. Finally, the FeO2 molecule observed in the gas phase59,60 is believed to be the open FeO2 dioxide on the basis of the sharp photoelectron spectrum of open FeO2-, but the cyclic Fe(O2) molecule is also believed to be formed by electron transfer to Fe(O2)+ in ICR studies.9,22 We, however, find little evidence for the photochemical isomerism of Fe(O2) to FeO2 attributed9,22 to the earlier matrix isolation work. FeO3 Molecules. FeO3. DFT calculations predict that singlet D3h is the lowest energy FeO3 species with a strong antisymmetric stretching fundamental at 1031 cm-1, and the 975.8 cm-1 band is tentatively assigned to such a product. The band decreases on annealing as expected for a reactive species and is favored relative to FeO2 with increasing O2 concentration. The 975.8 cm-1 band gives a weak doublet at 975.8-938.2 cm-1 with scrambled 16,18O2, and weaker intermediate components could not be observed in this congested region. Of more importance, the 16/18 ratio 1.0401 predicts a OFeO angle upper limit of 130°, which is an appropriate estimate for the D3h FeO3 molecule. Finally, the ν3 fundamental for MoO3 (922 cm-1) is higher than for MoO2 (899 cm-1),61 the analogous relationship found here for a possible FeO3 molecule and FeO2. (O2)FeO. Two bands at 1147.5 and 928.1 cm-1 were weak after deposition and grew markedly on annealing in the same proportion. The 1147.5 cm-1 band shifted to 1081.8 cm-1 on isotopic substitution with the pure O-O ratio 1.0607. Unfortunately, this band and scrambled isotopic counterparts were weak and isotopic structure was not completely resolved, but a triplet was observed with evidence of coupling to another oxygen subgroup. The 928.1 cm-1 band shifted to 888.1 cm-1 with the ratio 1.0450, which is very close to that for FeO diatomic. Scrambled isotopic oxygen gave a doublet of triplets for a motion of one O atom coupled to two equivalent oxygen atoms. Accordingly, these bands are assigned to (O2)FeO, a C2V species. The 1147.5 cm-1 O-O stretching mode is not as low as in Fe(O2), and the 928.1 cm-1 Fe-O mode is higher than in FeO itself. The most likely mechanism of formation is

Chertihin et al. reaction 7 since the above bands do not appear until the 956.0 cm-1 band is strong; however, the reaction of O2 with FeO can also contribute (O2)FeO.

O + Fe(O2) f (O2)FeO

(7)

O2 + FeO f (O2)FeO

(8)

The 1147.5 and 928.1 cm-1 bands were favored on deposition in ozone experiments, which produces (O2)FeO directly from insertion reaction 9 and this product is relaxed by the matrix. relax

Fe + O3 f [O2FeO]* 98 (O2)FeO

(9)

DFT calculations show that quintet C2V (O2)FeO is 28 kcal/ mol higher in energy than singlet D3h FeO3 and predict the strongest infrared bands at 1215 and 953 cm-1, in good agreement with the above assignments. FeO4 Molecules. Several structures are possible all built from the above FeO2 species. The DFT calculations predict the relative energies (Table 7) following the energies of the FeO2, Fe(O2), and FeOO building blocks. The lowest energy FeO4 species is singlet (O2)FeO2 with bridged and open subunits. DFT calculations predict the strongest band at 1009 cm-1 due to the FeO2 subunit and a weaker O-O stretch at 1253 cm-1. The band at 968.9 cm-1 was weak after deposition but increased 5-10-fold on annealing with OFeO present, particularly in the 20-30 K range where diffusion of O2 is expected. The oxygen-18 counterpart at 931.3 cm-1 defines a 1.0404 ratio, which is appropriate for the antisymmetric stretching mode of an O-Fe-O subgroup. Isotopic structure of this band was complicated by overlapping with the 956.0 cm-1 band, but after strong annealing it was found that the 954.2 cm-1 shoulder only in 16,18O2 experiments is the likely intermediate isotopic counterparts. A weaker sharp band at 1095.4 cm-1 on top of a broad band shifts to 1033.2 cm-1 and is appropriate for a pure O-O stretching mode; intermediate isotopic components are in accord with coupling between two different O2 subunits. The 968.9 and 1095.4 cm-1 bands are assigned to (O2)FeO2. The lack of higher order mixed isotopic splittings on the 968.9 cm-1 band is due to the absence of an antisymmetric mode in the same region involving the cyclic Fe(O2) subunit. The 16/18 ratio 968.9/931.3 ) 1.0404 predicts a OFeO angle upper limit of 128°, which is in good agreement with the 119° OFeO angle calculated by DFT. Note that bonding of a cyclic O2 to FeO2 shifts the strong mode from 945.8 to 968.9 cm-1 and closes the valence angle. In the case of (O2)FeO2, the central Fe atom is incapable of as strong a bonding interaction as it supported separately with a single dioxygen molecule in Fe(O2). The electron density transferred from Fe to the terminal dioxo atoms is not available to reduce the peroxo subunit, and the “peroxo” stretching frequency is reduced from the free O2 value (1552 cm-1) only to 1095 cm-1 and not all the way to 956 cm-1 as in Fe(O2). The (O2)FeO2 molecule is formed on diffusion from reaction of O2. Such a molecule has precedent in the (O2)WO2 species.62,63 In the ozone experiments (O2)FeO2 can also be formed by O atom addition to (O2)FeO.

O2 + FeO2 f (O2)FeO2

(10)

DFT calculations predict the Td FeO4 and the D2d and D2h (O2)Fe(O2) structures to be next lowest in energy. The latter two structures require scrambled isotopic sextets as observed49 for (O2)Ni(O2); no such bands were observed here. It is postulated that the formation of D2d (O2)Fe(O2) from O2 and

Reactions of Fe with O2 in Ar

J. Phys. Chem., Vol. 100, No. 13, 1996 5271

Fe(O2) may require activation energy. The Td structure requires an isotopic pentet and the 16/18 ratio for a tetrahedral angle; no such bands were detected here. Two broad bands at 1421 and 1175 cm-1 appeared in O2 experiments on annealing. The 16/18 ratios 1.059-1.060 define O-O vibrations. The broad triplet for the 1421 cm-1 band shows interaction with more oxygen but cannot rule out two inequivalent O atoms. The broad 1390 cm-1 band behaves similarly and is presumably a matrix environmental effect on the 1421 cm-1 band or another O2 complex species. The structure (O2)FeOO is thus proposed. DFT calculations predict 1426 and 1139 cm-1 bands for this structure, which are in accord with the observed bands. Singlet (O2)FeOO is 47 kcal/mol above singlet (O2)FeO2, which is also in accord with FeOO being a higher energy species than OFeO. Fe2O. The band 868.6 cm-1 was observed only after deposition and was favored with high laser power and low oxygen concentration, particularly in the blank and N2O experiments (Figures 1 and 5). The 868.6 cm-1 band decreased markedly on 20-30 K annealing. Scrambled isotopic substitution produced a doublet with counterpart 825.9 cm-1 and 16/ 18 isotopic ratio 1.0517. Note that these bands were always accompanied by weak 869.6 and 827.0 cm-1 satellites. The intensity ratios for each pair are 1/8, which is required for a molecule with two equivalent iron atoms if the weak bands are intermediate 54,56-components of iron isotopic structure (54,54-components are very weak and overlap with strong Fe16O and Fe18O bands). These observations identify the new molecule iron suboxide FeOFe. The ν3 16/18 ratio gave a FeO-Fe angle lower limit value 139°. The observed 54FeO56Fe splitting from 54FeO54Fe is approximately half of the calculated 56FeO56Fe splitting. The iron suboxide molecule is formed by reaction of FeO molecules with Fe atoms during deposition, reaction 11, and is relaxed by collisions with argon atoms.

FeO + Fe f FeOFe

(11)

Fe2O2 Molecules. Rhombic (FeO)2. The 517.4 cm-1 band was strong after deposition when the FeO band was strong. This band was intense in experiments with N2O which gave a large yield of FeO and a small yield of bent FeO2. Scrambled isotopic substitution produced a triplet with the diatomic isotopic ratio, 1.0457, which is typical for rhombic (MO)2 molecules. Thus, the 517.4 cm-1 band is reassigned here to the rhombic dimer (FeO)2; another mode is expected to lower wavenumbers owing to asymmetry in the (Fe16O)(Fe18O) triplet. OFeFeO. The site-split doublet 661.5/660.6 cm-1 appeared in the spectra after deposition with high laser power and did not change on annealing. This band obscures the weak CO2 doublet at 663.5, 661.9 cm-1. Scrambled isotopic substitution produced an asymmetric triplet with the isotopic ratio 1.0454, also very close to the diatomic value, and a new weaker doublet at 681.1/680.3 cm-1. Triplet isotopic structure and diatomic isotopic ratio indicate two equivalent oxygen atoms and terminal Fe-O, respectively, as the (FeO)2 rhombus absorbs lower at 517.4 cm-1. Note that the 660.6 and 517.4 cm-1 bands are not due to the same species as the asymmetry in the triplets for the 660.6 cm-1 band requires interaction with a higher mode, and the 680.3 cm-1 band is appropriate. The leading candidate is the OFeFeO molecule. If this molecule has D∞h or C2h symmetry, only one antiysymmetric Fe-O mode should be IR active. But for 16OFeFe18O due to formal lowering of symmetry, the symmetric vibration is allowed and may intensify by interaction with the antisymmetric mode of 16OFeFe18O, which displaces the latter mode below the median of pure isotopic values. The appearance of the 681.1/680.3 cm-1 site-

split doublet for the symmetric Fe-O stretch only in experiments with 16O18O is in good agreement with this assumption. Hence, stretch-stretch interaction through the two iron atoms in OFeFeO is small, and we believe that the iron-iron bonding in this molecule is weak. OFeFeO2. Two broader bands were observed at 747.5 and 721.4 cm-1 on deposition with high laser power and did not change upon annealing. The bands were relatively weaker with N2O, which suggests a higher oxide. Isotopic substitution revealed a superposition of triplet absorptions (Table 1) with the 16/18 ratios 1.0412 and 1.0402. In spite of overlapping, it is clear that at least three isotopic components are present for each band. The triplet absorptions indicate two equivalent O atoms and suggest a relationship to FeO2 and another dimer structure. The O2Fe-FeO structure with Fe-Fe bonding is a possibility, but this identification is tentative. FeO2 Dimers. Three sharp bands grow on annealing and could be due to FeO2 dimers. The first two bands at 861.5 and 705.1 cm-1 are weak on deposition and double on final annealing in all experiments. The 16/18 ratios 1.0416 and 1.0454 are appropriate for antisymmetric OFeO and rhombic ring motions, respectively. The scrambled isotopic experiment yielded a triplet of triplets for two equivalent O atoms coupled to two other equivalent O atoms for each band. A possible structure is a rhombic ring (FeO)2 with terminal O atoms on each iron, namely, O(FeO)2O. The 861.5 cm-1 band is the antisymmetric (b2u) terminal Fe-O stretching mode, and the 705.1 cm-1 band is the antisymmetric (b2u) ring stretching mode. The third band at 585.6 cm-1 appears on annealing to 25 K and increases on annealing to 30 K. The 16/18 ratio, 1.0398, is just above the FeO2 value and indicates a terminal antisymmetric Fe-O stretching mode. The 585.6 cm-1 band also yielded a triplet of triplets with 16,18O2, which indicates two equivalent terminal FeO2 groups. Thus, the O2Fe-FeO2 isomer involving Fe-Fe bonding is suggested. Again, the Fe-Fe bonding is expected to be weak, and this is a tentative identification. N2FeO. The relative intensity of the 887.3 cm-1 satellite band depended strongly on sample history. In argon blank and O2-doped experiments it was very weak after deposition; in experiments with N2O it was very strong (Figure 5). Annealing always increased the 887.3 cm-1 band absorbance. Isotopic substitution produced a doublet with the 16/18 ratio 1.045 45, which is slightly lower than the diatomic FeO value, 1.045 90. In contrast, the 54/56 ratio, 1.004 23, is slightly higher than the FeO value, 1.004 01. The 887.3 cm-1 band increased markedly relative to the 872.8 cm-1 FeO band when the argon sample was doped with N2, and the 887.3 cm-1 is assigned to N2FeO in agreement with earlier workers.64 In the upper region the 2262.6 cm-1 band behaved similarly. It did not shift upon oxygen isotopic substitution but shifted with 15N2O and produced a doublet in experiments using 14N2/15N2/O2 with 14/15 ratio 1.0345, which is very close to the diatomic N-N ratio. The latter band is assigned to the N-N stretching vibration in the end-on bonded NN-FeO complex species. N2FeO2. The doublet 1003.1/1002.0 cm-1 was observed after deposition when the 945.8 cm-1 FeO2 band was strong, and the 1002.0 cm-1 band increased on annealing and on N2 doping. It was difficult to separate isotopic structure from ozone, but on annealing ozone bands disappeared and three bands remained in this region, at 1002.0, 986.2, and 964.1 cm-1, for a vibration of two equivalent oxygen atoms. The isotopic ratio 1.0393 is close to the value for bent OFeO. Again, in the upper region the 2271.3 cm-1 band showed the same annealing behavior. It did not produce an oxygen isotopic shift but gave a doublet in

5272 J. Phys. Chem., Vol. 100, No. 13, 1996 isotopic nitrogen-doped experiments with the ratio 1.0345. Strong correlation with intensity of the FeO2 band, isotopic substitution, annealing behavior, and N2 doping suggest that these two bands are due to OFeO and N-N stretching vibrations of the end-on bonded NNFeO2 complex, although it is difficult to be certain. In a nitrogen matrix FeO2 gives a strong band at 932.5 cm-1, which shifts to 919.3 cm-1 with 16O18O and 897.6 cm-1 with 18O2. The FeO2 molecule in solid nitrogen is complexed as (N2)xFeO2.65 (N2)xFe(O2). The 917 cm-1 band was the most intense in nitrogen-doped experiments and grew after annealing. Oxygen isotopic substitution gave a triplet with the ratio 1.0547, which is very close to the value for cyclic Fe(O2). In the upper region a broad 2308 cm-1 band correlated with the 917.1 cm-1 band, but its nitrogen isotopic counterparts were not resolved. Broadness of both bands and their dependence on N2 concentration suggest that this molecule has more than one nitrogen subgroup. Triplet oxygen isotopic structure and the ratio suggest the Fe(O2) fragment subunit. Thus, both bands are assigned to O-O and N-N stretching vibrations of (N2)xFe(O2) molecule. Such (N2)xNi(O2) complexes have been reported.66 (FeO)2(N2)2. In nitrogen-doped experiments the 530 cm-1 band was observed only after annealing. Its intensity correlated with the 887.3 cm-1 band (N2FeO). The oxygen isotopic triplet, position of this band, and annealing behavior suggest the (N2)(FeO)2(N2) molecule, which is the dimer of N2FeO. In a nitrogen matrix (FeO)2 was observed at 535.5 cm-1.65 (OFeFeO)(N2). The 670.2 cm-1 band was enhanced in nitrogen-doped experiments together with the strong doublet at 661.5/660.6 cm-1. Isotopic substitution produced at triplet with the 16/18 ratio 1.0452, which is very close to that for OFeFeO molecule. Accordingly, the 670.2 cm-1 band is assigned to an Fe-O vibration of the (OFeFeO)(N2) molecule. In a nitrogen matrix (OFeFeO) was observed at 657.2 cm-1.65 Conclusions Pulsed laser-evaporated iron atoms react with O2 to produce FeO, Fe2O, FeO2, and Fe2O2 molecules during condensation with excess argon as identified from isotopic infrared spectra. Annealing reactions lead to formation of many secondary products which can exist in different isomeric forms; annealing to near 20 K allowed diffusion and reaction of O atoms whereas annealing to near 30 K revealed growth of products due to O2 addition. Three isomeric forms were found for Fe + O2 products: asymmetric bent FeOO, symmetric Fe(O2) cycle, and symmetric bent FeO2. The behavior of bands after deposition and annealing indicates that the insertion reaction requires activation energy, but the addition products can be spontaneously formed on 20 K annealing. The 1204.5 cm-1 O-O stretching mode for FeOO reveals two inequivalent oxygen atoms and compares favorably with this mode for oxyhemoproteins. The 945.8 cm-1 antisymmetric stretching mode for bent FeO2 gives 18O and 54Fe isotopic shifts that define a 150 ( 10° valence angle; the weaker 797.1 cm-1 symmetric mode intensity is compatible with this angle. Cyclic Fe(O2) is characterized by a strong predominantly O-O stretching mode at 956.0 cm-1 and a weak predominately Fe-O2 stretching mode at 548.4 cm-1 and their combination band at 1496.5 cm-1. The iron suboxide molecule Fe2O is characterized here for the first time from 18O and 54Fe isotopic data and found to have a 140 ( 10° valence angle. Two isomeric forms are proposed for (FeO)2, the rhombus and linear OFeFeO. Evidence is presented for two FeO3 isomers formed from O + FeO2 and Fe(O2) and two FeO4 isomers formed from O2 plus FeOO and FeO2.

Chertihin et al. The main difference from previous work is the observation of a large number of products, which are formed due to the ablation source of energetic metal atoms for oxidative insertion reactions that require activation energy. Their identification is possible from the sharp FTIR isotopic spectra. Another interesting and complicating feature of the iron-oxygen system is strong sensitivity to the presence of nitrogen as iron oxides react with nitrogen to form different complexes. Density functional theory has been used to calculate structures and spectra for FeO2, FeO3, and FeO4 isomers. It was found that the B3LYP functional favored higher spin states relative to the BP functional and that a match between calculated and observed isotopic frequency ratios was required to assign the ground spin states as triplet FeO2 and quintet Fe(O2). Frequency calculations for FeO3 and FeO4 isomers using the BP functional assisted with vibrational assignments. Acknowledgment. We gratefully acknowledge financial support from NSF Grant CHE 91-22556 and the assistance of G. D. Brabson with several experiments. A.R. acknowledges an NRC Postdoctoral Fellowship. M.N. acknowledges DuPont Chemical Company for computational resources. References and Notes (1) West, S. J. B.; Broida, H. P. J. Chem. Phys. 1975, 62, 2566. (2) Engelking, P. C.; Lineberger, W. C. J. Chem. Phys. 1977, 66, 5054. (3) Green, D. W.; Reedy, G. T. J. Mol. Spectrosc. 1979, 78, 257 and references therein. (4) Cheung, A. S.-C.; Lee, N.; Lyyra, A. M.; Merer, A. J.; Taylor, A. W. J. Mol. Spectrosc. 1982, 95, 213. Cheung, A. S.-C.; Lyyra, A. M.; Merer, A. J.; Taylor, A. W. J. Mol. Spectrosc. 1983, 102, 224. (5) Endo, Y.; Saito, S.; Hirota, E. Appl. J. 1984, 278, L131. (6) Kroeckertskothen, T.; Knockel, H.; Tiemann, E. Chem. Phys. 1986, 103, 335; Mol. Phys. 1987, 62, 1031. (7) Andersen, T.; Lykke, K. R.; Neumark, D. M.; Lineberger, W. C. J. Chem. Phys. 1987, 86, 1858. (8) Steimle, T. C.; Nachman, D. F.; Shirley, J. E. J. Chem. Phys. 1989, 90, 5360. (9) Fan, J.; Wang, L.-S. J. Chem. Phys. 1995, 102, 8714. (10) Bagus, P. S.; Preston, H. J. T. J. Chem. Phys. 1973, 59, 2986. (11) Blyholder, G.; Head, J.; Ruette, F. Inorg. Chem. 1982, 21, 1539. (12) Krauss, M.; Stevens, W. J. J. Chem. Phys. 1985, 82, 5584. (13) Abramowitz, S.; Acquista, N.; Levin, I. W. Chem. Phys. Lett. 1977, 50, 423. (14) Chang, S.; Blyholder, G.; Fernandez, J. Inorg. Chem. 1981, 20, 2813. (15) Serebrennikov, L. V. Vestn. Mosk. UniV., Ser. 2: Khim. 1988, 29, 451. Serebrennikov, L. V. Sc.D. Thesis, Moscow State University, Moscow, 1990. (16) 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. (17) Lyne, P. D.; Mingos, D. M. P.; Ziegler, T.; Downs, A. J. Inorg. Chem. 1993, 32, 4785. (18) Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A. J. Phys. Chem. 1984, 89, 566. (19) Mitchell, S. A.; Hackett, P. A. J. Chem. Phys. 1990, 93, 7822. (20) Riley, S. J.; Parks, E. K.; Nieman, G. C.; Polo, L. G.; Wexler, S. J. Chem. Phys. 1984, 80, 1360. (21) Helmer, M.; Plane, J. M. C. J. Chem. Soc., Faraday Trans. 1994, 90, 395. (22) Schroder, D.; Fiedler, A.; Schwarz, J.; Schwarz, H. Inorg. Chem. 1994, 33, 5094. (23) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (24) Andrews, L.; Burkholder, T. R.; Yustein, T. J. Phys. Chem. 1992, 96, 10182. (25) Andrews, L.; Spiker, R. C., Jr. J. Phys. Chem. 1972, 76, 3208. (26) (a) Thompson, W. E.; Jacox, M. E. J. Chem. Phys. 1989, 91, 3826. Hacalogu, J.; Andrews, L. To be published. (b) O4-: Andrews, L. J. Chem. Phys. 1971, 54, 4935. (c) O3-: 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. (27) DGauss, UniChem 2.3, Cray Research Inc., Mendota Heights, MN. (28) Andzelm, J.; Wimmer, E. J. Chem. Phys. 1991, 96, 1280. (29) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (30) Becke, A. D. Phys. ReV. A 1988, 38, 3098.

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