Matrix Reactions of Cesium Atoms with Oxygen Molecules
1065
Matrix Reactions of Cesium Atoms with Oxygen Molecules. Infrared Spectrum nalysis of Csf02- Infrared Observation of C S + O ~ ~ - C S + oretical Structure Elucidation of M+04.es%erAndrews,* Jenn-Tai Hwang, and Carl Trindle Department of Chemistry, University of Virginia, Charlottesvilie, Virginia 22907 (Received December 29, 7972)
The simultaneous matrix deposition of cesium atoms and oxygen molecules at high dilution in argon produced infrared absorptions a t 1115, 268, and 236 cm-1 which are respectively assigned to VI, u3, and u2 of the Cs +O2- species. The use of isotopic mixtures confirmed these assignments and the isosceles triangular structure for Cs+02-. The most intense mode of Cs+022-Cs+ was observed at 357 cm-I. A strong band at 1002 cm-I showed isotopic splittings for a species containing two 0 2 molecules which is assigned to the cesium disuperoxide species Cs+Ol-. CNINDO calculations were done to investigate possible 0 4 geometries and M + position in the M + 0 4 - molecular system.
Introduction The products of reactions of alkali metal atoms with oxygen molecules have been studied in our laboratory over the past 5 years beginning with the infrared observation of Li02 and L i 0 2 L i . l ~Spectra ~ of the mixed oxygen isotopic species indicated equivalent oxygen atoms and the symmetrical structures CazUand D2hr respectively, for LiO2 and LiOzLi. The 8-0 mode for LiO2 occurred in the 0 2 frequency region suggesting a n electronic distribution Li+Oz- for the lithium superoxide molecule. Following this example, the lithium peroxide molecule was suggested to exist predominanl ly as Li+0z2-Li+. Analogous superoxide and peroxide species have been produced and studied for Na, K, and Rb.3,4In addition, a new molecule, the disuperoxide species of formula M 0 4 , was observed for the latter three alkali metal^.^ Spectra for the MOa species suggested two equivalent 0 2 units with equivalent oxygen atoms in each unit and a single alkali atom;4.5 accordingly, a Dzd structure for the alkali disuperoxide WELS proposed. In a very recent letter reporting absorptions €or cSo4, Jacox and Milligan6 suggested a trans-04- structure in a molecule Cs+04- without considering the cation position with respect to 04-. Also pertinent, a reecnt Raman investigation? of Cs-02 matrix reactions attributed intense bands a t 287 and 269 cm-I to the alkali disuperoxide species. Were follows ii detailed infrared study of cesium atomoxygen molecule matrix reactions. Absorptions of CsOZCs and Cs04 were d ~ s e r v e dalong with the complete vibrational spectrum of GsOz which allows a vibrational analysis to be done for the CsO2 molecule. CNINDO calculations were performed on the M+04- system searching for the structure which best fits the isotopic frequency and reaction mechani.im data. Experimental Sectico The cryogenic refrigeration system, vacuum vessel, and alkali metal atom source have been described earlier.4 Oxygen gas (Air Products, therapy), isotopic oxygen samples 55% and 99% '813-enriched (Miles Laboratories), and argon 99.999% (Air Products) were used without purification. A cesium atom beam was provided by the reaction of lithium metal (0.R.N L.) and cesium chloride (Fisher) in
the Knudsen cell maintained a t 320". Analogous rubidium experiments utilized rubidium chloride (Fairmont) heated with lithium to 290". Samples of oxygen in argon (Ar/O2 = M/R = 100) were simultaneously deposited at 15°K with an atomic beam of cesium for 12-24 hr. Infrared spectra were recorded during and after sample deposition on a Beckman IR-12 filtergrating infrared spectrophotomete; in the 200-2000-cm-1 spectral region. High-resolution spectra were recorded using 8 or 3.2 cm-I/min scanning speeds and 20 cm-l/in. scale expansions. Additional high-resolution spectra were recorded following temperature cycling of the sample to 36°K. Frequencies were measured to the nearest 0.1 cm-1; however, wave number accuracy was kO.5 em-I. Spectral slit widths were 2.7 cm-' at 250 cm-l, 2.3 cm-l a t 350 cm-l, and 0.9 cm-I at 1100 cm-l.
Results Cesium Atoms with Oxygen. Several experiments were run depositing cesium atoms from the lithium metal-CsC1 source with argon-oxygen samples in order to determine the best conditions to maximize the yield of product absorptions. Operation of the Knudsen cell containing the lithium metal-CsC1 charge at 315-320" gave product yields comparable to those in elemental K and Rb experiNo feature ments using a metal vapor pressure of 1 ,u.~ was observed at 699 cm-I where the most intense LiQz band was observed; the lithium vapor pressure in these experiments is a factor of 2000 lower than that used for lithium atom reactions.2 The spectrum from one of these runs using Ar/Oz = 100 is depicted a t the top of Figure 1; the frequencies are listed in Table I, Of foremost importance are five groups of bands near 1100, 1000, 350, 260, and 230 cm-l. In the first group, sharp, weak bands were observed at 1115.6 and 1103.8 cm-I. The second group contained a band at 1002.6 cm-1 (5 cm-I half-width), a weak intermediate feature at 976.5 cm-I (0.05 OD), and a L. Andrews, J. Amer. Chem. Soc., 90, 7368 (1968). L. Andrews, J. Chem. Phys., 50, 4288 (1969). L. Andrews, J. Phys. Chem., 73,3922 (1969). L. Andrews, J. Chem. Phys., 54,4935 (1971) R, R. Smardzewski and L. Andrews, J. Chem. Phys., 57, 1327 (1972). (6) M. E. Jacoxand D. E. Milligan, Chem. Phys. Lett., 1 4 , 518 (1972).
(1) (2) (3) (4) (5)
The Journal of Physical Chemistry, Vol. 77, No. 8, 7973
L. Andrews,J.-T.
1066 IOC,
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Hwang, and C. Trindle
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'%' l602
II
Infrared spectra of the products of cesium atom-oxygen molecule argon matrix reactions, Ar/C+ = 100, deposition um atoms from CsCl(s)/Li(l) reaction, window temperature 15°K. Figure 1.
very sharp band a t !154,5 em-1 (1.5 cm-I half-width). The third spectral feature of interest appeared a t 357.0 cm-l with a shoulder near 348 c m - l . A weak, sharp band appearied a t 268.6 cm which had a rubidium counterpart4 at 282 cm-l. The rnajor feature in the spectrum was observed as a very intense absorption a t 236.5 cm-I; this feature was totally absorbing a t the end of the sample deposition period. A similar e:iperiment using a sample with Ar/O2 = 200 produced identical bands, including the sharp, weak features a t 1115.8 arid 1103.8 crn--l; however, two small differences are worth noting. The very sharp 954.5-cm-1 band, (0.48 OD) was relatively more intense than the 1 1 0 2 . 3 - ~ r n -band ~ (0.14 OD). Instead of a shoulder on the low-frequency side, the 356.8-cm-l band (0.30 OD) had a well-resolved partner at 347.0 cm-1 (0.22 OD). This behavior i s due to environmental effects owing to different trapping sites in f,he matrix or perturbations due to nearby m'olecules. Similar effects were observed for the rubidium counterparts of' these absorptions.4 The second trace in Figure 1 and the second column in Table I illustrate the effect of "aOn substitution on these new cesium-oxygen species. The first two weak, sharp bands exhibit oxygen isotopic shifts of approximately 63 ern.-' to 1052.5 and 1041.5 cm-I. The next two intense features shifted 56 and 52 cm-l, respectively, to 946.5 and 902.5 cm-I. 'The third band of interest exhibited an 18 em-'. 1 8 0 2 shlft to 339.0 cm--l, while the weak sharp band The Jocimal of Physica! Chemistry, Vo!. 77, No. 8, 1973
of cesi-
a t 268.6 cm-1 shifted 14.6 cm-1 and the very intense 2 3 6 . 5 - ~ m -band ~ was displaced 10.5 crn -l to 226.0 cm-l. Isotopic mixtures are helpful for revealing the composition of the absorbing species. The third position in the figure and table depict the spectra for an equimolar mixture of 1 6 0 2 and I8O2. All bands were observed, in excellent agreement with measurements made in the jeOz and l8Qz isotopic experiments separately. The only new feature appeared as an intense band a t 970.2 cm having double the intensity of the isotopic counterparts at 1002.0 and 946.5 crn-l. In this experiment, the cesium atom concentration was reduced. Note that the bands near 1000 and 250 cm-1 were of comparable intensities whereas the 357-339-cm-I doublet was markedly reduced in intensity, relative to previous experiments. Finally, a scrambled isotopic sample containing 20% 1602, 50% 1 6 0 1 8 0 , and 30% I8O2 was reacted with cesium atoms. The product absorptions are contrasted in Figure 1 a n d Table I with the other isotopic samples. ?'he first group of absorptions was observed as a sharp triplet with bands at 1115.8, 1084.8, and 1052.6 e m - l . It should be noted that this spectral region was scanned at 3.2 om l/ min; this slow speed was necessary to produce the best spectrum. Counterparts to the 1 1 0 3 . 8 - ~ m -feature ~ were not detected. The feature near 1000 cm- was split into a sextet of bands; no counterpart of the sharp 954.5-cm-l band was observed. A partially resolved trip!et was observed a t 356.5, 354.5, and 338.8 cm Presumably the
Matrix Reactions of Cesium Atoms with Oxygen Molecuies
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TABLE I: Absorptions (cim - I ) Observed Following Cesium Atom Matrix Reactions with Four Isotopic Oxygen Samples. I___
’602
’802
’602
+ ’802
1 1 15.7(0.03)
l115,6(0.07) 1052.5(0.06)
1052.3(0.03)
+ +
’602
’60’80
’802
1115.8(0.02) 1084.8(0.05) 1052.6(0.03)
1103.8(0.O4) 1041.5(0.02) 7 002.5( 0.3 I 1
1002.0(0.17)
970.2(0.30) 946.5(0.72)
946.5(0.14)
1 002.3(0.04) 987.7(0.15) 975.2(0.19) 970.6(0.10) 960.0(0.22) 946.8(0.07)
954.5(0.53) 902.5(0.05) 357.0t0.32)
356.8(0.05) 339.0(0.28)
338.8(0.05)
356.5(0.12) 354.5(0.14) 338.8(0.13)
Sample warminga
Identification
dec dec dec con con inc inc inc inc inc inc dec dec dec dec dec
299(0.10) 283(0.04) 268.6(0.05)
268.6(0.02) 254.0(0.05)
236.5( 1 .0) 226.0(0.67)
253.7(0.02) 236.5(0.24) 226.0(0.21)
289 268.4(0.02) 262.8(0.05) 253.3(0.02) 236.4(0.24) 229.6(0.47) 225.9(0.32)
207.5(0.30)
con dec dec dec dec dec dec ins
CS’6Q2 CS’60180
C§’80;! CS’602
CS’60’80
CS’802 /CSi6o2)7
a Sarnple warrniig behavior decreases, dec constant, con increases inc
335-cm-l shoulder was due to a site effect analogous to that reported for the 357-cm-l band. The lower two bands of interest produced well resolved triplets with intense central components which were not observed in the other isotopic experiments These frequencies are recorded in Table 1. Sample warming operations were conducted on these isotopic samples. Spectra follswing temperature cycling to 36-37°K and recooling to 15°K were compared to spectra of the final sample ’before the warm-up. The changes in intensity due to this operation are noted in ‘Fable 1 for each group of bands. The behavior of the first group of sharp, weak bands %as the most difficult to discern since the warming operation increases light scattering causing poorer spectral rorid hons, particularly in the higher frequency spectral region. Nevertheless, the spectra showed a decrease in intensity for the 1 1 1 5 . 6 - ~ m -feature ~ and its isotopic counterparts, whereas the 1103.8-cm-l feature remained approximately constant. The sample annealing had markedly contr,isting effects on the 1002- and 955cm-I bands; the former grew strongly whereas the latter almost disappeared. Sample warming decreased the intensitres of the remaining bands of inierest; sample warming effects are noted in Table 1. Rubidium Atoms a11.th Oxygen In order to compare reactions of alkali metal atoms produced by chemical reaction with alkali metal atoms evaporated from the pure element, one experiment was performed using a lithium metal-RbCl charge in the Knudsen cell. The principal features of the spectrum agree with those reported earVery intense sharp bands were observed at 991.6 and 954.3 em-1, The antense 389 0-em-I band exhibited weak splittings at 385 and 381 em-l. The 254.8- and 219.8cm - I bands were prominent as in the earlier experiments.
The 282.3-cm-l feature appeared here as a sharp weak band. Two new features appeared as weak, sharp bands a t 1111.3 cm-I (0.04 OD) and 1101.5 cm-I (0.03 OD). Similar weak features were observed in the previous experiments, but these bands were not reported.4 A reexamination of the previous Rb + l8Oz experiment revealed weak bands a t 1040 and 267.5 cm-l. Likewise, the latter band appeared as a weak triplet at 283, 277, and 268 cm-I iii the previous Rb l e J 8 0 2 experiment.* Sample warming to 38°K had an analogous effect on the absorptions produced by rubidium atom reactions as those discussed above for cesium. The sharp 1 1 1 1 . 3 - ~ m -fea~ ture decreased while the 1 1 0 1 . 5 - ~ m -band ~ remained constant.
+
Discussion The first task at hand is to identify the new molecular species and characterize the vibrational modes which produce the observed spectrum. Vibrational analysis of the CsO2 species is presented followed by CNINDO calculations for a geometry search for the M-i.04- species. Cs+02-. The infrared spectrum of lithium superoxide2 showed three absorptions, a weak sharp feature near 1100 cm-’ which was assigned to the superoxide stretch and very strong bands near 700 and 500 cm-l, respectively, which were assigned to symmetric and antisymmetric interionic Li+-Oz- modes. Although the infrared spectra of Na+Oz- and K + 0 2 - failed to produce bands near 1100 cm-l in the infrared, Raman bands were observed a t 1094 and 1108 cm-1, respectively. Accordingly, the weak, sharp infrared bands a t 1115.6 and 1103.8 cm-1 must be considered for assignment to the superoxide fundamental in Cs+Oz-. The Journal of Physical Chemistry, Vol. 77, NO.8, 1973
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The large oxygen isotopic shifts clearly show that the 1115.6- and 1103.8-12m-~features are essentially pure 0-0 stretchiiig vibrations. The 1115.6-cm-l feature and its isotopic counterparts (are observed in two mixed oxygen isotopic experiments which indicate the presence of two equivalent oxygen atoms. Counterparts of the 1 1 0 3 . 8 - ~ m - ~ feature were not observed in the mixed isotopic runs suggesting that this absorption might arise from a dimeric species whose isotopic intensities would be correspon!dingly weaker. Although it is difficult to determine whether the fate of a weak sharp band upon sample warming is due l o chemical reaction or to poorer spectral conditions, the sharp 1 1 1 5 . 6 - ~ n i -band ~ does decrease upon sample warming while the 1103.8-cm-l feature remains, further suggesting that 111b.6 cm-I is due to monomeric Cs+Ozwhile 1103.6 cm-1 arises from the corresponding dimer. The Raman spectrum of Cs plus 0 2 argon matrix reactions provides the conclusive data.7 This spectral region showed one sharp Itaman band of moderate intensity a t 1114 f 1 cm-1 which is in agreement with the 1115.6 f 0.5-cm -1 infrared band reported here. Hence, the sharp, weak 1 1 1 5 . 6 - ~ m -hand ~ and its oxygen isotopic counterparts are assigned i o the superoxide fundamental VI, in monomeric Cs+Oa-; it is suggested that the 1103.6-cm-l band is due to dimeric Cs+02-. The rubidium atom-oxygen molecule reactions produce an analogous set of bands at 1111.3 and 1101.5 cm-I. Although the isotopic data are less complete, the sample warming operations suggest assignments analogous to those above for Gs"02-. The Raman spectrum7 of Rb and 0 2 reaction products trapped in solid argon produced a single feature at 1110 $: 1 cm-I. The agreement between the infrared and Raman spectra indicate the assignment of the 1111.3-cm-l infrared band to V I , the superoxide fundamental in R b i 0 2 - ; similarly, it is suggested that the 1 3 0 1 . 5 - ~ m band - ~ is due todimeric Rb+Oz-. Previous work with the alkali metals K and Rb produced very intense bands a t 307.5 and 255.0 cm-I which , symmetric interionic mode, in were assigned to ~ 2 the K + & - and Rb+02- respectively. The very intense band a t 236.5 em-1 showed a 1 0 . 5 - ~ m - ~ shift and a band pattern in mixed oxygen isotopic experiments consistent with two equivalent oxygen atoms. The intense well-resolved triplet at 236.4, 229.6, and 225.9 cm-l clearly indicates that this vibrational mode arises from a structure containing two equivalent oxygen atoms. Accordingly, the 236.5-cm - I feature 1s assigned to the symmetric interionic Antisymmetric intcirionic modes have been reported for Li+Q- and NafOz-; however, these modes were not assigned for K+O2- and Rb+Oz-. The sharp, weak feature in the clear spectral region at 268.6 cm-l presents itself as a candidate for this antisymmetric mode. The large 'SO? shift of 14.6 cm-l is consistent with the antisymmetric interionic motion. AS was discussed in detail for L i + O - , the antisymmetric interionic mode exhibits more 1 8 0 2 shift than the symmetric interionic mode.2 The mixed oxygen isotopic experiments clearly show that the 268.6 cm-l feature arises from a single 0 2 molecule absorption and that the oxygen atoms are equivalent, as evidenced by the triplet at 268 4, 262.8, and 253.2 cm -I. Furthermore, diffusion behavior associates these bands with the other Cs+02- absorptions. The assignment of the 268.6cm-1 band to v a , t h r antisymmetric interionic mode of Cs+Oa , clearly follows. The present data on Cs+Oz- prompted a closer look at The Journai of Physical Chemistry, Vol. 77, No. 8, 1973
L. Andrews, J.-T. Hwang, and C . Trindle
the previous Rb+Oz- spectra. The weak band reported at 282 cm-1 also showed a large W z shift of 15 cm-1, which, along with its appearance 14 cm-l higher than the analogous band for Cs+O2-, indicated the assignment of the 282-cm-l band to v a of Rbi-Oa-. Unfortunately, the analogous mode for K + O y was probably obscured by the intense 307-cm-l band. Table 11 lists the fundamental frequencies of all of the alkali metal superoxide molecules. The decrease in frequency of the interionic vibrations v2 and v3 involving the metal cation follows the increase in atomic weight of the alkali cation; notice that the symmetric mode metal dependence is greater than the antisymmetric mode metal dependence, which follows from the G-matrix elements for these modes.2 The interesting trend in increasing superoxide frequency is explained in terms of the increasing polarizability of the metal cation in a M+O2- ionic modeL8 Cs+022-C~+. The infrared spectrum of lithium plus oxygen reaction products showed two intense features a t 796 and 445 cm-l which were assigned to the v g and V 6 , antisymmetric interionic motions of cations perpendicular and parallel to the 0-0 bond, respectively, in Li+022-Li+. Isotopic mixtures indicated that two equivalent lithium atoms and two equivalent oxygen atoms were present in this lithium peroxide species. Symmetry coordinate arguments were advanced to suggest the D2h structure.2 Similar features were assigned to the peroxide molecules of Na, K, and Rb a t 524.5, 433.0, and 388.8 cm-l. Mixed alkali metal experiments Na and K or K and Rb simultaneously verified that these absorbers contained two equivalent alkali metal atoms. In the Na and Rb cases, well resolved triplets in the scrambled oxygen isotopic experiments indicated two equivalent oxygen atoms in this The 357.0-cm-l feature in the\present experiments is assigned to 85, the antisymmetric interionic mode perpendicular to the 0-0 axis, of C S + O ~ ~ - C SThe + . I6O2, experiment indicates the incorporation of a single 0 2 molecule into this species. Unfortunately, the scrambled oxygen isotopic experiment produced an incompletely resolved triplet a t 356.5, 354.5, and 338.8 em-l. Apparently, the unsymmetrical C S ~ ~ O ~ ~species O C S absorbs at a frequency almost accidentally degenerate with the Csp60&s species, A similar observation was reported4 for K160180K. The lower frequency mode 16 was not observed for the K, Rb, and Cs peroxide molecules; It likely absorbs below 200 cm -1, the low-frequency limit of these experiments. Cs+Oa-. In the study of K and Rb matrix reactions with 02, very intense bands were observed near 993 cmwhich were assigned to an antisymmetric oxygen stretching mode in a species of formula M o r . Oxygen isotopic experiments suggested two equivalent 0 2 molecules, each with equivalent atoms, in the new M04 molecule. Detailed concentration studies4 and mixed Na-K reactions with oxygen5 showed that the species contained a single alkali atom, An additional band a t 955 cm-l in the K and Rb work was attributed to a matrix site effect or isomer of the MO4 species absorbing at 993 cm-l. The intense band at 1002.5 cm--l in the present cesiumoxygen study showed a large oxygen-18 shift of 56.0 cm-l. The sharp triplet structure in the 1602, l 8 0 2 experiment ( 7 ) R. R. Smardzewski and L. Andrews, J. Phys. Chem., in press. (8) L. Andrews and R. R. Smardzewski, J . Chem. Phys., in press, (1973).
Matrix Reactions of Cesium Atoms with Oxygen Molecules
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indicates the presence of two equivalent 0 2 units. Furthermore, the sextet i n the scrambled isotopic experiment suggests that the atornic oxygen positions in each 0 2 unit are equivalent. The sharper, more intense band a t 946.5 cm-1 showed a 52.0-cm-l oxygen-18 shift; there was a marked change in the relative intensities of these two features between the first two traces in Figure 1. The intense 1002.5- and 945.5-cm -I bands are assigned to different structural isomers of the most intense mode of the Mod disuperoxide species. This is an antisymmetric mode involving out-of-phase stretching of the two 0 2 parts of the MOb species The M 0 4 molecule is relatively large and its incorporation in two different matrix sites or geometric orientations is reasonable. The change of frequency from 1001 cm-l for NaOs, 933 for K04, 992 for R b 0 4 to 1002 cm-1 for Cs04 may also be attributed to matrix site or molecular orientation effects as the size of the alkali cation increases Jacox and Milligaris have suggested a trans-04- structure in the M 0 4 species since molecular orbital calculations of Conwayg show the trans-04- isomer to be the most
