J. Phys. Chem. 1983, 87, 2004-2011
2004
Matrix Reactions of Molecular Oxygen and Ozone with Aluminum Atoms Susan M. Sonchlk,' DepaHment of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106
Lester Andrews, Department of Chemlstry, University of Virginia, Charlottesvllle, Virginla 2290 1
and K. Douglas Carlson' Chemistty Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: September 8, 1982)
Frozen matrices of Nz or Ar containing small concentrations of A1 atoms and 0, or O3 have been prepared by matrix isolation techniques and studied by infrared absorption spectroscopy. The spectra were analyzed on the basis of isotopic oxygen substitution, variations in the concentrations of reagents, and changes in band intensities after reagent diffusion and reaction in the matrix. These analyses have shown that the principal reaction product in Nz matrices with either 0 2 or 0 3 was an asymmetrical and highly bent aluminum dioxygen molecule, A100 (Cs). Very little of this species was produced in Ar matrices. The stretching modes of A 1 0 0 in Nz were identified as 1337 cm-l for q(0-0)and 1091 cm-l for ~ ~ ( A l - 0 )Metal . ozonides absorbing in the region of 850 cm-' also were produced by both oxygen reagents in both Nz and Ar matrices. Several other complex species were identified as additional reaction products. No bands could be identified as belonging to a metal superoxide species, which is the principal product in the matrix reactions of Ga, In, and T1 with 02.
Introduction In recent article^^*^ we have shown that oxygen molecules react with Ga, In, and T1 atoms to produce metal superoxide molecules and various aggregate species in matrices of frozen inert gases. In this article we describe similar matrix isolation experiments for the reactions of O2 with A1 atoms. The reaction products of these reagents are expected to be somewhat different from those of the heavier metals of the aluminum family because A1 is thermodynamically capable of abstracting an oxygen atoma4 In order to characterize the product species, we have also investigated and described here the matrix reactions of A1 and ozone. Molecules of the A1-0 system have been studied extensively by infrared spectroscopy of matrix-isolated species. Much of this research has been devoted to characterizing the oxide species, primarily AlzO, isolated from the vapors of the solid oxide."s Several investigations have been published dealing with the matrix reactions of aluminum with oxygen. Marino and Whiteg have identified a rhombic AZO2species produced in Ar matrices containing Al, AZO,and 02.Finn, Gruen, and Pagelo have identified the same rhombic species, an aluminum dioxygen species, and various other molecules in Ar matrices containing the sputtered products of alumina targets or aluminum cathodes flushed with 0,-Ar mixtures. A recent report" describes the existence of an AlO, superoxide (1)Present address: Versar, Inc., P. 0. Box 1549, Springfield, VA 22151. (2)Zehe, M. J.; Lynch, D. A., Jr.; Kelsall, B.J.; Carlson, K. D. J. Phys. Chem. 1979,83,656. (3)Kelsall, B.J.; Carlson, K. D. J. Phys. Chem. 1980,84,951. (4) Huber, K. P.; Herzberg, G. 'Molecular Spectra and Molecular Structure"; Van Nostrand-Reinhold: New York, 1979; Vol. 4,pp 28-31. (5)Linevesky, M. J.; White, D.; Mann, D. E. J. Chem. Phys. 1964,41, 542. (6)Snelson, A. J. Phys. Chem. 1970,74,2574. (7)Makowiecki, D. M.; Lynch, D. A., Jr.; Carlson, K. D. J. Phys. Chem. 1971,75, 1963. (8) Lynch, D. A., Jr.; Zehe, M. J.; Carlson, K. D. J . Phys. Chem. 1974, 78,236. (9)Marino, C. P.;White, D. J. Phys. Chem. 1973,77,2929. (10)Finn, P. A.; Gruen, D. M.; Page, D. L. Adv. Chem. Ser. 1976,No. 158,30. (11)Serebrennikov, L.V.; Osin, S. B.;Maltsev, A. A. J. Mol. Struct. 1982,81, 25. 0022-3654/83/2087-2004$01.50/0
species, analogous to the superoxides of Ga, In, and T1. Our experiments confirm some of these identifications but show that others are incorrect.
Isolation Experiments The matrix samples were prepared by simultaneously depositing Al atoms and O2or O3 mixed with a matrix gas on a CsI plate maintained at about 20 K with a closed-cycle helium refrigerator. The experiments employing O2were carried out a t Case Western Reserve University, and the experiments involving O3were performed at the University of Virginia. The A1 samples used in the O2 experiments consisted of powdered metal, free of metallic contaminants (Fisher Scientific Co.). This material proved to have a considerable concentration of surface oxide, which produced significant amounts of A120 in the matrices even after appreciable outgassing prior to deposition. For the O3 experiments, samples were prepared by using pea-sized chucks of metal cut from a spectroscopic-grade aluminum rod (Johnson, Mathey, and Co., supplied by Jarrell-Ash Co.). This material after outgassing was virtually free of oxide contaminant. The metal was contained in an outgassed Ta effusion cell and heated above temperature for about 1h prior to deposition on the CsI window. After outgassing, the metal sample was heated to a constant temperature of about 1350 K, and the vapor was deposited at a rate of 1pmol/h over a period of 10-30 h along with the 0,or 0,-doped matrix gas a t a rate of 0.5-3.0 mmol/h. The matrix gases (N, or Ar) were of high purity. These were doped with O2or 0, to concentrations of 0.2-5.0 mol %. Natural O,, 95%enriched 1802,and unscrambled and scrambled isotopic mixtures of O2 were employed. The ozone dopants, consisting of 1803,1803,and a scrambled isotopic mixture, were prepared by previously described methods.'2 Undiluted natural O2 also was used as a matrix gas to study the oxygen dependence of the absorption bands. The infrared spectra of matrix samples were recorded over a range of 200-2000 cm-' with Beckman IR-12 spectrophotometers. After the spectrum of a freshly prepared (12) Andrews, L.; Spiker, R. C., Jr. J. Phys. Chem. 1972, 76,3208.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 11, 1983
Matrix Reactions of 0,and O3 wlth Ai
-w-N 1350
1325 1100 1075
I
950
1
1
900
,
I
I
850
I
I
800
750
2005
F.-
500
475
FREQUENCY, cm-'
Figure 1. Infrared spectra for AI in an N, matrix containing 1 % matrix after prolonged diffusion.
matrix was recorded, the matrix was warmed to a temperature of 30-35 K for several minutes to induce the diffusion and aggregation of species. The matrix was then cooled to 20 K, and the spectrum was rescanned to determine changes in band intensities. The occurrence of spurious bands, vaporized oxide contaminants, and window impurities was explored by studying matrices prepared in the absence of O2 or O3 and matrices produced by vaporization of the solid oxide.
Spectra of A1 with O2 and O3 The infrared spectra of 02-doped matrices with A1 exhibited a much larger number of absorption bands than those with Ga, In, and T1, and the major absorption peaks were generally weaker in intensity. The spectrum of A1 + 1 6 0 2 in an N2matrix with a large concentration of metal atoms (1% O2 in N2,N2:A1= 600) is illustrated in Figure 1. Trace A shows the spectrum of the freshly prepared matrix, and trace B shows the spectrum of the same matrix after prolonged diffusion. The figure displays a prominent triplet band at 1091cm-', a weaker band at 1337 cm-' along with a shoulder absorption at 1332 cm-', and two broad bands of moderate intensity at 502 and 492 cm-'. The 1091-cm-' band was the most intense feature in the spectrum. All of the above-mentioned bands disappeared after warming the matrix to temperatures up to 33 K for a period of 20 m. Figure 1 shows a large number of bands in the region between 970 and 730 cm-l, and these bands either remained constant in intensity or grew in intensity after diffusion. The prominent bands in this region are the absorption peaks at 961, 938, and 809 cm-'. A group of initially weak absorptions near 850 cm-' and a weak band at 909 cm-l are shown in trace B with remarkably increased intensities after diffusion. In the region above 975 cm-l, the 992-cm-' band of A120was detected at relatively large intensities in all the matrix experiments with 02.This band originated primarily from oxide contamination in the vaporized A1 sample. With an N2 matrix containing dilute A1 atoms (1% O2 in N2, N2:A1 = 5000), the triplet band at 1091 cm-' remained as the most intense absorption in the spectrum. The band at 1337 cm-' appeared with the same weaker intensity relative to the 1091-cm-' system, and the band at 961 cm-' was again prominent in the spectrum. How-
with O,/Ai
N
6: trace A for the matrlx before diffusion; trace B for the
ever, the previous bands at 938 and 809 cm-' and the two bands near 500 cm-' were absent. Thus, these latter four bands apparently belong to species having more than one metal atom. With a matrix having a larger concentration of oxygen (5% O2in N2, N2:A1 = 7200), the band at 961 cm-I and a very weak absorpt,ion at 925 cm-' appeared as relatively more intense features in the spectrum. These bands, therefore, likely belong to species having more than two oxygen atoms. The infrared spectrum of Al 1603 in N2was generally very similar to that shown in Figure 1, for an 02-doped matrix, although smaller concentrations of both A1 and O3 were involved (0.5% O3 in N2, N2:A1 = 5000). Aside from the intense v3 and v2 modes of parent 03,12 the triplet band system near 1091 cm-' was the strongest absorption. The band at 1337 cm-' was again a weak absorption with the same intensity relative to the 1091-cm-' band as that with 02-dopedNP The two bands near 500 cm-l had intensities which were comparable to those obtained with the high concentration of Al in 02-dopedN P The bands at 961 and 938 cm-' were appreciably more intense than they were with 02,and the bands near 850 cm-', which were weak with 02-doped matrices, were approximately tripled in intensity with ozone in freshly prepared matrices. The 992-cm-' band of AlJ80appeared in the ozone experiments only as a very weak absorption ( A N 0.01) largely because of the reduction or absence of oxide contaminant in the vaporized metal. A summary of the bands detected with the 02-and 03-doped N2 matrices is given in Table I. Along with the frequencies, the diffusion behavior is indicated, and absorbance values are listed for the most intense bands and some of the more important weaker bands. Table I lists only bands which lie below 1350 cm-'. Some very weak and often broad absorptions were detected at higher frequencies, including water bands and absorptions possibly belonging to H20 and CO ~omp1exes.l~The region below 475 cm-' was generally devoid of structure except for occasional water bands. The spectra of matrices doped with 'So2 and lsO3 showed isotopic shifts in frequency for nearly all of the bands. The ISO bands are given in Table I in a parallel arrangement
+
(13) Hinchcliffe, A. J.; Ogden, J. S.;Oswald, D.D.Chem. Commun. 1972, 338.
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Sonchik et al.
The Journal of Physical Chemistry, Vol. 87, No. 11, 1983
Detected with A1 in 0,- and 0,-Doped N, Matricesa TABLE I: Infrared Absorption Bands (cm-') ' 6 0 ,
/ ' 6 0 ,b
492 502 543 592 644e 682 ( 0 3 ) 692 735 ( 0 , ) 740 757 ( 0 , ) 765 775 ( 0 , ) 789 ( 0 , ) 798 809
initial absorbanceC 0.05, 0.04 0.11,0.12 0, 0.05 0,0.02 0,O.Ol 0 0.05, 0.08
0.12, 0.07
818 ( 0 3 )
824 838 ( 0 , ) 846 ( 0 , ) 853 863 873g 879 ( 0 , ) 881 887 ( 0 3 ) 899 ( 0 3 ) 909 925 938 961 992' 1077 1088 1091 1098 1142 1148 1160 ( 0 . 3 ) 1168 1179 1189 1332 1337
0.06 0.04, 0.12 0.02,0.09
D D I I
468 478 523
752 781 786 795 791
844 ( 0 3 )
0.12, 0.16 0.13, 0.10 0.23, 0.25 0.14, 0.08
0.03, 0.03 0.08, 0.09
866 ( 0 3 ) 876 901 ( 0 , ) f 912 922h 931 94 7' 1048 1056 1059 1065 1108 1 1 1 3 (O,)f 1131 ( 0 , ) 1137 1146 1254 1262
absorbance
54 7
0.08
648 685 690
0.11 0.06 0.25
842
0.04
850, 8 5 3 866 873
0.04
881
0.22
926
0.04
C C
(0,) (0,)f (0,) 800 ( 0 , ) 808 819 8 2 3 (O,)f,g ( 0 3 )
undiluted 0,
I I
(O,)f
835
0.09, 0.15
I I D
698 ( 0 , ) 708 733 ( 0 3 ) 736 744 ( 0 2 )
831
0 . 0 4 , 0.05
diffusiond
lsOJ'80,b
C C I I I I I I I I
c C I C I I I
D C
1058.1062
D D D C
1128.1132
D D D D D D D
a Other weak absorptions were detected at the following frequencies (cm-l): 604, 612 (l6OS);577, 585, 596 (l8O,). The designations (0,)or (0,)after the band frequencies indicate that the band was detected only with 0, or 0,; otherwise bands The absorbance values are listed first for 0, and second for 0,. Bands without absorbwere detected with both reagents. ance values were generally weak and in the range of 0.02-0.04. Symbols I, C, D denote increased, approximately constant, or decreased intensities, respectively, after prolonged diffusion of the matrix. e This band grew into a matrix which was concentrated in 0, ( 5 % in N 2 ) . This band likely was undetected in the " 0 , matrix because of differences in concentration. P The 873-em-' band appeared as a weak, closely spaced, doublet system. The likely isotopic component a t 8 2 3 cm-' appeared with similar structure as a broad shoulder on the 819-cm.' band. See text for a discussion of the possible I6O component of this band. The 992- and 947-cm-' bands belong t o the u , mode of A1,O.
which implies a direct correlation between the isotopic components. The correlations were based on the deposition of relative intensities, similar diffusion behavior, and the apparent uniqueness of certain bands with respect to O2or O3 Ambiguities occurred in the correlation of several weak bands because of differences in concentration and thickness of matrix samples and the near coincidence or possible reversal in frequencies of adjacent bands. The band at 922 cm-', which is listed in Table I as a product of the 1 8 0 2 or leg3reactions, had no readily apparent oxygen-16 component. This band was weak ( A = 0.02) in freshly prepared matrices, but it more than doubled in intensity after diffusion. The 922-cm-' band possibly correlates with a weak absorption peak at -958 cm-' on the shoulder of the more intense band at 961 cm-' or more likely correlates with a band which is near-coincident with the 961-cm-' band. The correlation of the band at 1254 cm-' (1802/1803) with is uncertain. The the band at 1332 cm-l (1602/1603)
1332-cm-' band appeared as a weak, poorly resolved absorption on the shoulder of the 1337-cm-' band with both 1602 and 1 6 0 3 . These bands vanished on diffusion. The 1254-cm-' band was a similar absorption on the shoulder of the 1262-cm-' band with 1802. Two well-resolved bands of comparable intensity appeared at these frequencies with 1 8 0 3 matrices, and after diffusion the 1262-cm-' band vanished but the 1254-cm-' band was still visible with only slightly reduced intensity. With 1 8 0 2 samples, the A12160band was again observed as a moderately strong absorption, but the Al2I80 was absent. This confirmed that A120 in the O2 experiments was vaporized from the metal source. From previous investigation~~,~ of the A120 system, the v3 absorption of A12180 is expected to occur at 947 cm-'. In the lSO3experiments, the 947-cm-' band occurred as a weak absorption, indicating that AlzO in ozone samples was formed by a matrix reaction. The intense v3 band of 1803 near 983 cm-' along with some mixed isotopic ozone impurities
Matrix Reactions of
O2 and O3 with AI
The Journal of Physical Chemistry, Voi. 87, No. 11, 1983 2007
TABLE 11: Infrared Absorption Bands (cm-' ) Detected and 0,-Doped Ar Matrices'sb with A1 in 0,160,1160,
'*0,/'80,
498 544 644 682 687 775 786 809
482
840 846' 853 863 870 880
16O2/'60,
180,1180,
910 925c 94 2
901 923
964c 992 1083e logoe 109ge
782 785 795
1115
801
1168
1143 1174
808d
822
' All bands were very weak (absorbance = 0.02-0.05) except for the band at 498 cm" in metal-rich matrices Other weak absorptions were de(absorbance = 0.27). tected at the following frequencies (cm-'): 530, 596, 623, 1051 (I6O,). 1017, and a broad band a t -1057 (1803); The 808-cm-' band was This band was rather broad. associated with a weaker component a t 806 cm-'. e The three bands near 1 0 9 0 cm-' appeared only in the doped Ar matrix. Unless they are spurious absorptions, they likely belong t o oxygen-16 from isotopic contamination. The band a t 1090 cm-' was the most intense by a factor of -3. obscured detection of a possible vaporized Al,'60 contaminant. With a matrix containing A1 deposited in undiluted 02, the spectrum was considerably different. The bands and absorbance values for this matrix spectrum are included for comparison in Table I. The most intense band occurred at 690 cm-'. Moderately intense bands were observed at 881 and 648 cm-'. The prominent bands near 1091,961,809, and 500 cm-' and the weak absorption at 1337 cm-l were absent with the undiluted 0, matrix. The spectra of Ar matrices containing A1 with O2 or O3 were similar to those with N2, except for two notable diferences: (1)the prominent triplet system near 1091 cm-l was generally undetectable, and (2) all of the other bands, aside from an intense band at 498 cm-l, were very weak absorptions. The bands detected with Ar matrices are summarized in Table 11. A comparison with Table I shows generally small shifts in absorption frequencies but obvious correlations in most cases between the bands for Nz and Ar matrices.
Spectra of A1 with Isotopic Oxygen Mixtures Figure 2 shows the band systems near 1300 and 1100 cm-l for N2matrices containing 1 6 0 3 (trace A), I8O3(trace B), and a scrambled mixture of ozone (trace C)with approximately equal parts of l60and l80.Traces A and B in the region of 1300 cm-' show the band at 1337 cm-l, a weak satellite a t 1332 cm-', and the previously discussed pair of bands at 1262 and 1254 cm-'. Trace C in this region shows the appearance of a pair of centrally located bands, one being a relatively intense absorption at 1299 cm-' and the other a weak shoulder absorption at 1292 cm-'. All of these bands vanished on diffusion except for the 1254-cm-' band. The pattern of absorptions for the bands at 1337, 1299, and 1262 cm-l resembles an isotopic triplet system. The central component, however, appears as a broader absorption band with an intensity which is considerably less than double that of the terminal components. Therefore, this band system likely represents an isotopic quartet with unresolved central components. A similar but
-
'
L
h
r
1340 1300 1260
'
I100
1
I080
1
I060
FREQUENCY, c r r - '
Flgure 2. .ilgh-frequency infrared spectra for AI In N, matrices containing isotopic O3 mixture: trace A for 0.5 % "0,with O,/Al = 25; trace B for 0.4% "0,with O,/Al N 4; trace C for 0.4% isotopically scrambled O3 wlth O,/Al = 4. The bands at 1096 and 1066 cm-' in trace C are the v1 bands of O3 for the 16-16-18 and 16-18-18 constituents, respectively.
better resolved quartet has been observed for the HO, species.14 The spectrum for the 1091-cm-' band system with scrambled O3 was complicated by the superposition of bands arising from the intrinsic multiplet structure of the system. In traces A and B of Figure 2, the multiplet components are illustrated by symbolic line spectra to clarify the overlapping absorptions in trace C. The experimental spectrum in trace C shows two separated groups of bands with each group consisting of four closely spaced bands. The first group has absorptions at 1096, 1091,1088, and 1086 cm-', and the second group has absorptions at 1066, 1062, 1059, and 1056 cm-'. The first band in each group is dominated by the v1 vibrational mode for the 16-1618 and 1618-18 isotopic constituents of 03.12 These are the most intense absorptions of the v1 vibration because of the doubling in statistical weight and the drop in symmetry from C2"to C,. There is one new band in each of the two groups: the weak absorption at 1086 cm-' and the strong absorption at 1062 cm-'. This indicates that the basic pattern in trace C of Figure 2 is that of an isotopic quartet system with each isotopic component being associated with an intrinsic triplet system. This implies that the band carrier has two nonequivalent oxygen atoms and that the 1086-cm-' band belongs to the -W-'80 component and the 1062-cm-l band belongs to the -180-160 component. This assignment is illustrated in the figure by symbolic line spectra which represent two additional triplet systems, each having two bands which coincide with or fall close to bands of the original triplet systems or ozone. The superposition of the four isotopic constituents in a 1:l:l:l ratio of intensities and the inclusion of the ozone bands give a reasonably good representation of the experimental spectrum. For the most intense isotopic component of the intrinsic triplet systems, the band maxima are 1091, -1088,1062, and 1059 cm-'. (14)Smith, D.W.; Andrews, L. J. Chem. Phys. 1974,60, 81.
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The Journal of Physical Chemistry, Vol. 87,
No. 11, 1983
Sonchik et al. I
1
I
I
I
I
I
I
I
I
I
1
E
Al1,''O
809
899
/
960
940
920
900
880
860
840
820
800
780
FREQUENCY, cm"
Y 510
490
470
. -
525
500
I
Flgure 4. Infrared spectrum In the region of intermediate frequencies for AI in an N, matrix containlng 0.4% scrambled O3 with O,/Al = 4. Connected lines above the spectrum indicate a few of the band systems which may contribute to the absorptions. The dashed connected lines indicate that posslble central components are not identified.
475
FREQUENCY,crn-' Figure 3. Low-frequency infrared spectra for AI in N, or Ar matrices containing Isotopic mixtures: traces A-C are the same matrix samples as for Figure 2; trace D for 1% natural 0,in Ar with 02/Al = 95; trace E for 1 % scrambled 0,in Ar with 02/Al = 12.
The isotopic splitting patterns of the prominent bands in the region of 500 cm-l are illustrated in Figure 3. The isotopic multiplet for the 502-cm-' band of N2matrices is shown in trace C for a scrambled mixture of ozone. The multiplet pattern for the intense band at 498 cm-I with Ar matrices is shown in trace E for a scrambled isotopic mixture of oxygen. Both multiplets are triplet systems in the approximate intensity ratio of 1:2:1, and thus they belong to molecules having two equivalent oxygen atoms. The multiplet bands for the 502-cm-' system are broad and weak with absorptions at 502, 492, and 478 cm-'. The isotopic triplet system of the 498-cm-' band appears as a sharp multiplet with absorptions at 498,489, and 482 cm-'. The spectrum of absorptions in the region of 1000-500 cm-l with isotopic mixtures of oxygen was generally weak, and it was complicated by the existence of numerous overlapping bands. The spectrum of A1 with scrambled ozone in N2 is shown in Figure 4 for the region of 970-770 cm-'. Diagrammed above the spectrum in this figure are connected lines representing isotopic componentsof certain bands to illustrate the possible relationship of absorption peaks and the superposition of bands. In the region of 960 cm-', Figure 4 shows a series of nine relatively sharp absorption peaks at 961,957,951,947,941, 938,931,921, and 912 cm-'. The most intense peak at 947 cm-' can be assigned to A12180. From the pattern of relative intensities, we conclude that the remaining bands belong to a sextet multiplet for the 961-cm-' system and a quartet multiplet for the 938-cm-' system, with overlapping components at 938 and 931 cm-'. These multiplets are diagrammed in Figure 4 with intensity ratios of 1:2:1:1:2:1 for the sextet and 1:l:l:l for the quartet. The superposition of these two multiplets gives a reasonable representation of the experimental spectrum, although the bands at 961 and 921 cm-' appear to be more intense than predicted. This suggests that the previously described band at 922 cm-I ('802/1803) and its probable oxygen-16 analogue near to or coincident with the 961-cm-' band also are present in the spectrum. With an N2 matrix containing A1 and an unscrambled the spectrum contained all of the mixture of lSO2and 1802, bands from 961 through 912 cm-' except for the band at
957 cm-l. On the basis of their intensities, these bands could be grouped into a set of four bands at 961,951,941, and 931 cm-' belonging to the 961-cm-' system and the quartet bands belonging to the 938-cm-' system. The pattern for the 961-cm-' system indicates that an oxygen atom was abstracted by a metal atom and an oxygen molecule was added to form the final product species. The spectrum in Figure 4 for the region between 910 and 860 cm-' contains a series of very weak absorptions. We suggest that the weakness of the bands is exaggerated by the large number of absorptions and the overlapping of bands which obscure the base-line transmittance of the matrix. On diffusion, the absorptions in this region merged into a broad, nearly structureless band. Several of the possible band systems which absorb in this region are shown in the figure without identification of central components. The region below 860 cm-' contains a series of more intense absorptions, but the bands are broad and undoubtedly represent the overlapping absorptions of several multiplet systems. A few of the band systems which appear in thi region are shown in the figure, again without indications of central components except for a multiplet identified with a band at 853 cm-'. The 853-cm-' band with I6O2was a fairly prominent absorption, and it likely represents an ozonide specie^.'"'^ This band appeared at 808 cm-' with '803 In the spectrum of Figure 4, one observes bands at 853 and 808 cm-' and three intermediate bands at 843,832, and 822 cm-'. The ratios of frequencies compare very well with those for the isotopic components of the v3 mode of 0 3 , 1 2 except that a sixth band is apparently missing in a region near 815 cm-'. It is possible that the band at 822 cm-I represents the unresolved absorptions of an unrelated (and possibly shifted) band near 824 cm-' along with two other closely spaced bands. On the basis of the structure and width of the band after diffusion, we estimate frequencies of -825 and -818 cm-' for the possible two additional bands. The sextet pattern for the 853-cm-' system drawn above the experimental spectrum in Figure 4 includes the two estimated frequencies and represents bands in the intensity ratio of 1:2:1:1:2:1, which is typical for the strongest band of an ozonide. The distortion of intensity ratios in the experimental spectrum can be accounted for by the su(15) Spiker, R. C., Jr.; Andrews, L. J. Chem. Phys. 1973, 59, 1851. (16) Thomas, B. M.; Andrews, L. J. Mol. Spectrosc. 1974, 50, 220. (17) Andrews, L.; Prochaska, E. S.; Ault, B. S. J. Chem. Phys. 1978, 69, 556.
Matrix Reactions of
O2and O3 with AI
perposition of the unrelated bands at 824 and 809 cm-'. There are likely to be additional perturbations in the intensities from overlap with other band systems in this region, including the stronger components of bands belonging to other ozonide species.
Identification of Molecular Species A100. The prominent band system near 1091cm-' for N2 matrices occurred as an intrinsic triplet system with both pure and mixed oxygen isotopes of O2 and Os.This triplet had one intense band at 1091 cm-'and two weaker satellites a t higher and lower frequencies. All three components decreased and eventually disappeared after repeated diffusions, and all three belonged to a molecule having two nonequivalent oxygen atoms. Therefore, this intrinsic triplet represents a single molecular species with matrix-site perturbations. Because this band system consistently appeared as an intense absorption with all nitrogen matrices, the carrier molecule likely contained a single metal atom and can be identified as an 0-AlO or A100 species of C, or C,, symmetry. According to the ratio of frequencies for the isotopic components, the 1091-cm-' vibration is primarily a metal-oxygen stretching motion which is nearly independent of the second oxygen atom. The weaker band at 1337 cm-' also consistently appeared in the spectra of all N2 matrices with O2 and 03. The oxygen isotopic studies indicated that the carrier molecule had two nearly equivalent oxygen atoms, and it likely contained a single metal atom. The ratio of frequencies for the isotopic components indicates that the 1337-cm-l band is very nearly a pure 0-0 vibration. The relative intensities imply that the 1337- and 1091-cm-' bands belong to the same carrier species; these two bands appeared approximately in the intensity ratio of 0.35 from one to another matrix sample, and this ratio remained constant on successive diffusions of the matrix. Therefore, both the 1337- and 1091-cm-' bands are assigned as the stretching modes of the same asymmetrical aluminum dioxygen molecule, A100. The bending mode, which is expected to be a weak absorption, was not observed. The matrix site-splitting pattern of the 1091-cm-' band was not observed for the 1337-cm-l band. Although different modes could sustain different matrix perturbations, the splitting pattern of the 1337-cm-' band was probably masked by the weak intensity and relatively large bandwidth. With regard to the geometry of A100, it is significant that the 1337-cm-l band with mixed oxygen isotopies behaved as a nearly pure 0-0vibration while the 1091-cm-l band behaved basically as vibrations of Al-l60 and Al-lW. Isotopic substitutions in the terminal oxygen atom caused additional frequency shifts of only 3 cm-' in the 1091-cm-' band. These effects indicate that the vibrations are determined largely by the diagonal G-matrix elements18for 0-0 and A1-0 bond-stretching motions and that the off-diagonal elements are small quantities. On this basis, we can rule out a linear structure. In C,, symmetry, the two stretching modes (A, representation) are independent of the bending mode (El). The G-matrix element which connects the two stretching modes18 is a maximum value for a linear molecule, and it involves the reciprocal mass of the oxygen atom adjacent to Al. As a consequence, both stretching modes would reveal the inequivalency of oxygen atoms on isotopic substitution, whether or not the offdiagonal force constant is appreciable in value. We find, (18) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. C. 'Molecular Vibrations"; McGraw-Hill: New York, 1955; p 63.
The Journal of phvsical Chemktry, Vol. 87, No. 11, 1983 2009
in fact, that real diagonal force constants for C,, cannot be derived from the band frequencies unless the off-diagonal force constant is unreasonably large. Similar considerations of G-matrix elements rule out a conformation which resembles a cyclic molecule of slightly distorted CZusymmetry, similar to the metal superoxides. In this geometry, all three vibrational modes would exhibit a near-equivalency of the two oxygen atoms. We conclude, therefore, that AlOO is a highly bent molecule of C, symmetry which does not approach C2, symmetry. This conclusion can be supported by a normal-coordinate analysis based on reasonable approximations. We assume that the off-diagonal force constants are zero and that the bending mode is separable from the FG matrix. We then have as unknown quantities the two force constants for the stretching motions, FM and F w ,and the Al-0-0 angle. For an angle of 90°, the G-matrix element which connects the two stretching motions is zero, so that the effect of the terminal oxygen atoms in the A1-0 mode vanishes. For an angle >11l0, we cannot obtain real force constants from the experimental band frequencies. Thus, we select an angle of looo, and from the band frequencies for Al160-le0and A1-180-180, we obtain the averaged force constants, FA14= 7.136 mdyn/A and FM = 8.250 mdyn/A. With these force constants, we obtain for the isotopic molecules A1-160-160, A1-160-160-, A1-180-160-, and A1-180-180, respectively, the following frequencies: 1337,1303,1298, and 1262 cm-' for the 0-0mode; 1095, 1092,1058, and 1055 cm-' for the A1-0 mode. The absolute agreement with the experimental frequencies is not important in view of the approximations, but the patterns of frequencies is significant. We observe that the 0-0 stretching mode involves nearly equivalent oxygen atom while the A1-0 mode involves an Al-0 stretch with little differentiation from the isotope of the terminal oxygen atom. AlOdZ. The 498-cm-' band of Ar matrices with O2and O3 and the 502-cm-' band of N2matrices with both oxygen reagents belong to molecules having two equivalent oxygen atoms. The ratio of isotopic frequencies for the former band is 1.033 for v(160)/v(180), which is close to the ratio of 1.037 for a pure A1-0 vibration. This is no doubt the same band as that reported originally by Marino and White: and we agree that it is likely to be the BzUmode of a rhombic AIOzAl species. The 502-cm-' band of N2 matrices had an intensity which increased with increased concentration of metal atoms. This suggests that the carrier molecule also is an A102Al species. The frequency ratio of the isotopic components, however, k 1.050, so that symmetry cannot be Dul. Perhaps this molecule was produced by the addition of an A1 atom to the terminal oxygen atom of A100. There is no possible connection between the 502- and 1337-cm-' bands. The former band was of stronger intensity with concentrated A1 and of weaker intensity with diluted Al. A103. The band at 853 cm-' with N2matrices containing O2 and O3 appeared at greater intensity with ozone. The isotopic-shift ratio is 1.056, which is close to that for a pure 0-0stretching motion. This band grew in intensity on diffusion, and a similar band appeared weakly in the spectrum for a matrix of undiluted oxygen. With scrambled 03,the multiplet was plausibly interpreted as the v3 vibration of an ozone-like molecule with three oxygen atoms in a C2,,arrangement. Furthermore, the band appears in the frequency region of the strongest absorption for the ozonide anion of the alkali and alkaline-erth metals.l"l' Thus, the 853-cm-' band likely belongs to an aluminum ozonide species and represents the v3 mode of
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the 03-ion. By inference, the same band for Ar matrices represents an ozonide, as originally proposed by Finn et aLIO Site-positional isomers and structural isomers are common for metal ozonides. The bands adjacent to the 853cm-' band, therefore, are likely candidates for other ozonide species. The six neighboring bands listed in Table I ('602/1603) from 838 to 881 cm-' have isotopic-shift ratios within the range of 1.054-1.061, and all but two occur with matrices of undiluted oxygen. In addition, all of these bands increased in intensity on diffusion. It is interesting and perhaps significant that the weak band a t 881 cm-' occurred as a prominent absorption in the spectrum for a matrix of undiluted oxygen, but the 853-cm-' band remained weak. Furthermore, the 881-cm-' band grew more slowly on diffusion than other members of this group. These observations suggest that the band carrier was not an ozonide. OA1O2or A1002 and A1,OO. The band at 961 cm-' (or one of two bands) for Nzmatrices occurred as a moderately intense absorption which increased rapidly in intensity on diffusion with both Oz and Os. The band appeared prominently with matrices which were dilute in Al, so that the carrier likely contained a single metal atom. The oxygen isotopic studies indicated that the molecule contained three oxygen atoms, two of which were equivalent, and that the production of this species from Oz involved an abstraction of an oxygen atom. We therefore conclude that the band carrier is an OAIOz or A10OZspecies of C2"symmetry or similar structure which preserves the equivalency of two oxygen atoms. This species may consist of a complex between AlO and 02,similar to a species proposed in studies of the Mg + O3reaction." According to the ratio of isotopic frequencies, the vibrational mode is primarily an A1-0 stretching motion. The band a t 964 cm-' for Ar matrices probably represents the same carrier molecule. The band a t 938 cm-' also was a prominent absorption with both 02-and 0,-doped N2 matrices, and its intensity increased rapidly with diffusion. The band appeared to be favored by high metal concentration, and therefore the carrier molecule probably involved at least two metal atoms. The isotopic studies indicated that the molecule contained two nonequivalent oxygen atoms. These conclusions suggest that the carrier molecule is AlAl00. The ratio of isotopic frequencies indicates that the vibrational mode is an A1-0 stretching motion mixed with other coordinate displacements. The band a t 942 cm-' with Ar likely represents the same molecule. A120and A10. We have already identified and discussed the band at 992 cm-l, which is the v3 mode of A1,O. Since AlzlSOwas detected in the 1803-dopedmatrices, then AlzO was likely produced in the matrix by abstraction of an oxygen atom. Such abstraction reactions for O3 are to be expected on the basis of the weak 0-O2 bond. We obtained no evidence that A Z Owas produced in the 0,-doped matrices. However, the production of OMO2or MOO2 and A103 in O2-doped matrices implies that abstraction also occurred with 02.In this case the question arises as to the existence of A10 in the matrices, for surely this is a likely product of an abstraction reaction. ESR mea~urements'~ and analyses of the optical spectralOJQ of matrices likely to contain A10 apparently confirm the existence of isolated A10. Nevertheless, there is a problem in identifying the vibrational frequency in inert gas matrices. The gas-phase frequency has been established as 979 cm-'.* Knight and WeltnerIg have identified (19) Knight, L. B., Jr.; Weltner, W. J., Jr. J . Chem. Phys. 1971, 55, 5066.
Sonchik et al.
a relatively intense band a t 917 cm-' as A10 in an Ar matrix containing the species vaporized from Alz03.There were no isotopic studies to support this assignment. If this assignment is correct, then the matrix shift of 62 cm-' is unusually large for a molecule which is probably less ionic than, for example, BaO. The matrix shift for BaO is 54 cm-' in frozen N2.20 Finn, Gruen, and Pagelo did not observe a band at 917 cm-l, but they assigned a weak band a t 946.5 cm-' to A10 in Ar. The frequency ratio u ( ' ~ O ) / ~ ( ' ~was 0 )1.050, compared with 1.037 based on the ratio of G-matrix elements for a diatomic molecule. This frequency may be more acceptable, but the discrepancy in the isotopic ratio is too large for A10. In the present experiments there were two bands in the region of 900 cm-' with acceptable isotopic ratios for an A10 species. One was the band a t 899 cm-', which was observed only with O3in an Nzmatrix. This appeared as a very prominent, sharp band which remained constant in intensity on diffusion. The second band was a weak absorption at 909 cm-' which grew rapidly on mild diffusion with both O2and O3in N2. Another possible candidate for A10 is the apparent second band near to or coincident with the 961-cm-' band. This is the l60analogue of the band at 922 cm-' which appeared initially as a weak absorption with lS02 or lag3in N2 and increased in intensity with diffusion. There is no convincing evidence that any one of these bands represents an isolated A10 species. We detected no bands a t 917 or 946 cm-' with either Ar or Nzmatrices doped with 1 6 0 2 or 1603. In view of the several independent matrix isolation studies, we conclude that the extinction coefficient for the A10 vibration is quite small, so that the existence of this species in the matrix is not obvious on the basis of the vibrational intensity.
Discussion Finn et al.'O have assigned weak bands at 1178 and 1168 cm-' along with two lower-lying bands to an aluminum dioxygen species of C2"symmetry in an Ar matrix. Their isotopic data, however, are incompatible with such an assignment. In Tables I and I1 we have listed similar band frequencies in the region of 1100 cm-'. We were unable to identify these, but they are unlikely to belong to an NO2 or a second A 1 0 0 species. Serebrennikov, Osin, and Maltsev" have assigned a weak band at 1096 cm-' and a prominent band a t 496 cm-' as the u1 and vz modes, respectively, of an aluminum superoxide species of CZusymmetry in an Ar matrix. The assignment of the 1096-cm-' band had no supporting oxygen isotopic data. Although these assignments appear to be reasonable in comparison with the superoxide species of Ga, In, and T1, they are nevertheless incorrect. The 496-cm-' band no doubt corresponds to the band that we observed a t 498 cm-' and assigned as a mode of AIOzA1,in agreement with previous a n a l y s e ~ . ~There J ~ is no relationshiop of this band to those in the region of 1090 cm-'. The band a t 1096 cm-* likely represents the A100 molecule of which little is formed in matrices of Ar. In most of our own experiments with Ar matrices, absorptions were absent in the region of 1090 cm-'. Such a difference in product yield between Nzand Ar matrices is not unprecedented, and it likely occurs because Nz is a more efficient sink for excess energy."~~' Among the reaction species identified in this study, AlOO is the most interesting because it represents the first example of a bent metal dioxygen molecule which is not of CZusymmetry. The bent structure, which occurs for (20) Ault, B. S.;Andrews, L. J. Chem. Phys. 1975, 62, 2320. (21) Bos, A.; Ogden, J. S.; Orgee, L. J . Phys. Chem. 1974, 78, 1763.
Matrix Reactions of
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O2and O3 with AI
TABLE 111: Band Assignments for t h e Species Produced in the Matrix Reactions of A1 with 0, and O,a molecule
mode
A I 0 0 (C,)
u,(O-0)
~~(Al-0)
oxygen isotopes -16-16 -16-18 -18-16 -18-18 -16-16 -16-18 -18-16 -18-18
Al, 0 OAlO, or A100,
Al,oo A10,
-16 -18 u (Al-0) 16- -16-16 16- -16-18 16- -18-18 18- -16-16 18- -18-16 18- -18-18 v(A1-0) -16-16 -16-18 -18-16 -18-18 v 3 ( O 3 - , C Z v ) 16-16-16 v3
16-1 6-18 18-16-1 8 16-1 8-1 6 18-18-16 18-18-18 A10,AI AlO,Al(Ar)
u(B,,)
-16-16-16-18-18-18-16-16-16-18-18-18-
frequency, cm-' 1337 -1299' -1299' 1262 1091,1098, 1088 1088, 1.095, 1086 1062, -1068, 1059 1059,1065, 1056 992 94 7 96 1 957 951 94 1 938 931 93 8 931b 921' 91 2 853, 838, 846, 863, 873, 879 84 3 832 823 -818 808, 791, 800, 8 1 9 , 8 2 3 , 831 50 2 492 478 498 489 482
-
-
a All bands are those detected with N, matrices except as The assignments €or the central isotopic componnoted. ents are uncertain without more information on the structure.
molecules such as HO0,14 F00,22and C100,23implies that the bonding in A100 is predominantly covalent. This structure can be explained as arising from a p orbital or sp hybrid on A1 overlapping with the lobe of a p orbital in the A* antibonding orbital of 02.%In contrast to this, the triangular CZustructure of the Ga, In, and T1 dioxygen species is typical of the ionically bonded metal superoxides M+Og, in which an electron is largely transferred onto the A* orbital of 02.Indeed, the stretching force constants of the Ga, In, and T1 species are very similar to those of the alkali and alkaline-earth superoxide^.^ Both stretching force constants of AlOO are significantly larger, implying a decreased transfer of electronic charge into the A* orbital of O2and a substantial accumulation of charge in a covalent metal-oxygen bond. This difference in bonding characteristics within the Al family cannot be explained solely on the basis of ionization (22) Spratley, R. D.; Turner, J. J.; Pimentel, G. C. J. Chem. Phys. 1966,44,-2063. (23) Arkell, A.; Schwager, I. J. Am. Chem. SOC.1967,89, 5999. (24) Spratley, R. D.; Pimentel, G. C. J. Am. Chem. SOC.1966,88,2394.
No. 11, 1983 201 1
potentials. These potentials are close to 6 eV for all atoms of the Al family. This value is comparableto the ionization potential of Ca and significantly less than that of Mg (7.65 eV), which forms a superoxide species. Therefore, although the relatively low ionization potential must be important, we conclude that another important factor is the size of the metal atom core insofar as it relates to the metal-oxygen internuclear distance. This core size is smaller for A1 than for Mg or the heavier atoms in the Mg or A1 families. Among these two families, we would expect covalent bonding to be more favorable in molecules of the Al family because of the occupied p orbital in the initial electronic configuration of the metal atom (s2p). Thus, in the dioxygen species of the A1 metals, there is a subtle balance between the low ionization potential, which tends to stabilize the ionic structure, and the metal-oxygen internuclear distance, which at small values stabilizes the covalent structure due to favorable orbital overlap. Additional evidence for covalency in A1-0 bonds is provided by the probable low extinction coefficient for the vibration of diatomic A10 and the relatively high vibrational frequencies for the A103 species. A low extinction coefficient implies a small change in dipole moment. This suggests that the static dipole moment of A10 is small because of considerable covalent character. The 03-antisymmetric vibrations of A103occur at higher frequencies than those common to the ozonides of the alkali and alkaline-earth metals ( a 4 4 cm-l). This implies that there is a smaller degree of charge transfer into the antibonding orbital of O3 in A103 and hence more covalent character in the Al-03 bond. A summary of the bands identified in this study is given in Table 111. With the possible exception of A120,which we observed only with 03,these reaction products occurred with both O2and O3 The major difference in the reactions of the two oxygen reagents is that O3 increased the concentration of carriers containing more than two oxygen atoms, most notably the ozonides. The direct combination of reagents is expected to produce the greatest concentration of species, so that it is not surprising that A100 was the strongest absorber with the O2 reagent. It is interesting that AlOO rather than AlO, also was the strongest absorber with ozone matrices. The production of a dioxygen species instead of a superoxide molecule is one significant difference between the matrix reactions of A1 and the matrix reactions of Ga, In, and T1. The abstraction of oxygen is another significant (but predictable) difference. The reaction products of Ga, In, and T1 with O2 involved only the addition, or possible insertion, of reagents.
Acknowledgment. S.M.S. gratefully acknowledges financial support by the Standard Oil Co. (Ohio) under its Employee's Educational Plan. L.A. gratefully acknowledges financial support for aspects of this research by the National Science Foundation under grant CHE 79-10966. K.D.C. acknowledges support by the US.Department of Energy, Office of Basic Energy Sciences, Division of Materials Science, under contract W-31-109-ENG-38. We thank B. J. Kelsall for his assistance with the presentation of the data. Registry No. Al, 7429-90-5; 02, 7782-44-7; 03, 10028-15-6; A100, 11092-32-3; A120, 12004-36-3; OA102, 85479-72-7; AlO3, 85479-73-8; AlOZAl, 12252-63-0.
