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Communications to t h e Editor
Comment on the Bending Mode of A 1 2 0 ’ 7 2 a Pub//cai/oncosts assfsted by the Offrce of Naval Research
Sv In an article entitled “Infrared Spectra of the Aluminum Family Suboxides”2b published in this journal, a weak absorption at 503 cm-I in the spectrum of what was assumed to be matrix-isolated AI20 was assigned to the bending mode vz. This was not observed in either the original investigation of Linevsky. White, and Mann3a nor in the subsequent studies3b of Snelson which focused exclusively on the missing bending fundamental v2. In both of these earlier studies it was suggested that this vibration C should be observable below 250 can-l, a conclusion based on the assumption of ionic bonding4 as well as what is observed in the analogous case, Li20.5 This unusually high frequency of 503 cm-1 for the bending mode of the sym, An481 I metric nonlinear molecule AI20 was justified by Carlson, et al.,Zb in terms of metal-metal bonding. This argument, although interesting. does not, however, seem plausible. We have therefore reexamined the spectrum of A120 with particular emphasis on impurities that on occasion give 1 I , ‘ + A I 1100 1000 900 600 550 490 430 rise to bands attributable to molecular aggregates formed on matrix deposition. FREQUENCY ( ~ 6 ’ ) In the three matrix isolation experiments discussed F i g u r e 1. C o m p o s i t e s p e c t r a of major absorption features obabove,2b,3 the molecular species A120 is formed by the s e r v e d at 4°K f o l l o w i n g the deposition of AI a t o m s , f r o m an reaction of liquid aluminum with solid A1203. The molecAl2O3 K n u d s e n cell, w i t h various isotopic o x y g e n m o l e c u i e s in ular beam effusing from a Knudsen cell at approximately a r g o n in t h e f r e q u e n c y r a n g e 1100-430 cm-’: ( A ) n o oxy e n (B) 5 % l 6 0 2 o n a r g o n ; (C) 5% l8O2 in argon; (D) 5 % 30 2 ,; 1300°K containing these materials consists, however, of ’ 8 0 1 6 0 , 1 6 0 2 in argon. several species. Mass spectrometric analysis of these vapors shows not only gaseous AlzO(g) but also Al(g). Thus deposition on the optically transmitting cold window prothe Knudsen cell, but rather a molecular aggregate duces a solid rare gas matrix containing atomic aluminum formed on deposition of the matrix. Finally, the lower as well as A120. 4 s we have observed in our lithium expanel D clearly illustrates that the oxygen-containing periments6 the spectrum of such matrices, i e , matrices molecule formed in the matrix contains two oxygen containing the free metallic atoms together with other imatoms. purities, can be quite complex, particularly if diffusion Although one cannot at this time assign the band to a occurs. The reason for this, as demonstrated by A n d r e ~ s , ~ , particular ~ molecule, in all probability it is due to an aluis the formation of peroxides and superoxides on the minum peroxide. From its location, intensity and l6O -lSO surface of the crystallizing matrix by the reaction of the shift, we are presently inclined to assign it to the Bzu metal atoms with molecular oxygen. These are not only mode of rhombic AlzOz. This will be discussed in greater characteristic of the alkali m e t a h a but also metals in detail elsewhere. other groups of the periodic table.g References and Notes In this brief report we examine the effect of the impuriThis work wa$ supported i? part by the Office of Naval Research ty, molecular oxygen, on the spectrum of the matrix-isounder Contract No. ONR-N00014-67-A-0216-0001. lated mixture of A1 and AlzO. The conditions here are (a) Editor’s Note. Professor Carlson has advised u s in corresponidentical with what has previously been reported in ref 2b dence that on reconsidering the concentrations involved in their A120 experiments, he and his associates no longer accept the band at 503 and 3, namely, effusion from a Knudsen cell containing c m - ’ as being the up mode of AIpO. They are not in a position, howliquid aluminum in contact with A1203. The results are ever, to conclude whether or not this absorption is the same as that studied by Marino and White. The energy difference amounts to 7 shown graphically in Figure 1 where we focus on two specc m - ‘ . I t is possible then that the two arise from different carriers, tral regions, namely, the v3 region of AlzO(g) a t approxiwith the 503-cm-‘ band perhaps being a dimer of AIzO, similar to mately 1000 cm-I and the 500-cm-l region where Carlthe so-calied up modes of the heavier metal suboxides [A. J. Hinchciiffe and J. S. Ogden, J. Phys. Chem.. 75, 3908 (1971)]. (b) D. son, et a1.,2breported the bending mode v2. The asymmetM. Makowiecki, D. A. Lynch, Jr., and K . D. Carlson. J. Phys. Chem., ric stretch, vg, of A120 is the most intense feature in the 7 5 , 1963 (1971). spectrum, when the mixture AI and AI20 is trapped in the (a) M. J. Linevsky, D. White, and D. E. Mann, J . Chem. Phys.. 41, 542 (1964): (b) A. Snelson, J. Phys. Chem., 74, 2574 (1970). argon matrix a t liquid helium temperatures (upper panel A. Buchler, ’W. Klemperer, and A . G. Emslie, J . Chem. Phys.. 36, A of Figure 1).There is no evidence for even a very weak 2499 (1962). D. White, K . S. Seshadri. D. F. Dever, D. E. Mann, and M . J. Linfeature in the vicinity of 500 cm-l. Introducing approxievsky. J. Chem. Phys., 39,2463 (1963). mately 3% l 6 0 ~into the argon stream produces the specK. S. Seshadri, D. White, and D . E. Mann, J . Chem. Phys.. 45. 4697 trum in panel €3. There is now a strong band at 496 cm-I ( 1966). L. Andrews. J. Chem. Phys.. 5 0 , 4288 (1969). which is directly attributable to the oxygen impurity that L. Andrews. J . Chem. Phys.. 54, 4935 (1971) codeposits with the AI and A120 on the cold window. In Unpublished results, this laboratory. panel C the isotope l 8 0 z is now introduced as the impuriDepartment o f Chemistry Charles P. M a r i n o ty. Whereas u3 of A120(g) is unaffected by the presence U n i v e r s i t y of Pennsylvania D a v i d White* of l8O2, the band a t lower frequency shifts to 481 cm-1. Philadelphia, Pennsylvania 19104 This clearly demonstrates tha? the absorption at 500 cm-I is not due to an oxygen-containing species effusing from Received August 27, 1973 T I ”
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The Journal of Physical Chemistry, Vol. 77, No. 2 4 , 1973
