3922
LESTERANDREWS
Infrared Spectra and Bonding in the Sodium Superoxide and Sodium Peroxide Molecules by Lester Andrews Chemistry Department, Universitu of V$rgin&z, Charlottemille, V h g i n h 88901
(Received May 18, 1969)
Sodium superoxide has been produced for infrared spectral study by condensing sodium atoms and oxygen molecules at high dilution in argon on a CsI window at 15'K. Use of oxygen isotopic mixtures and comparison with lithium superoxide verify the molecular identity and indicatea symmetricaltriangular structure. Nine frequencies from three isotopic molecules determine the potential constants Fo-o = 5.46 0.05, FNa-O = 0.80 0.02, FNa-O,Na-O = -0.12 Z!Z 0.02 mdyn/A. The oxygen-oxygen stretching force constant for NaOz is near those for Li02 and 0 2 - which suggests that the NaOz molecule is highly ionic and may consist of a sodium cation bonded to a supersxide anion by coulombic forces. Further reaction of NaO2 with sodium atoms produces NaOzNa which has been identified and compared to lithium peroxide.
*
Introduction Recently, we have produced the lithium superoxidell2 and lithium peroxide2 molecules for infrared spectral study b y the reaction of lithium atoms and oxygen moleculw using the matrix isolation technique. Mixed oxygen isotopic experiments indicate that LiOz has Ctv symmetry, and the oxygen-oxygen stretching force constant agrees with that calculated for the isolated superoxide ion from Raman spectra. * This comparison suggests ionic bonding between a lithium cation and superoxide anion in the lithium superoxide molecule. Indeed, LiOs is a unique molecule, and the infrared spectra of further alkali metal analogs are of interest to determine the generality of the bonding and structure found for Li02. The symmetrical bonding of molecular oxygen to a metal atom has been observed in several of the more reactive complexes. The oxygen-oxygen distance and the tendency to bond oxygen increase as the ancillary ligands become more electron releeasing. Oxygenoxygen distances of 1.30 and 1.51 A have been reported*Jj which correspond, respectively, to the superoxide and peroxide internuclear distances. The catalytic" and possible biochemical4 usefulness of these compounds have been recently discussed. Reaction of sodium with excess oxygen gives mainly the peroxide. Further oxidation at 500" and 300 atm produces the superoxide, but the resulting product is a mixture of crystalline sodium superoxide and sodium peroxide.' X-Ray diffraction measurements on crystalline NaO2 yield an oxygen-oxygen internuclear d i e tance of 1.33 f 0.06 A in the superoxide anion,8 and in NaOzNa crystal an 0-0 distance of 1.50 f 0.03 A is found for the peroxide anion.@ Infrared spectra of crystalline NaOz and NaOzNa showed no absorption1" due to the homopolar nature of the superoxide and peroxide anions. The infrared spectra of molecular The Journal of Physical Chemistry
*
NaOa and NaO2Na are of interest for comparison to those of their analogous lithium compounds and for observation of sodium-oxygen vibrational frequencies which have heretofore not been reported. Accordingly, we have prepared the NaO2 and NaO2Na molecules for infrared spectra1 study using the matrix reaction of sodium atoms and oxygen molecules.
Experimental Section The 15°K refrigeration system, vacuum vessel, alkali metal atom source, and experimental technique have been described ear1ier.l1 Sodium metal (J. T. Baker, lump), oxygen gas (Linde, welding) and isotopically enriched oxygen 99.3%% (O.R.N.L.) were used without purification. The oxygen isotopic mixture le02-ls0 l80-'*02 was prepared by discharging equimolar amounts of 1 6 0 2 and lsOz for 6 hr in a 3-1. bulb using a tesla coil. A fresh 6-mm cube of sodium metal was cut under an argon atmosphere, washed with dry hexane, dried by evaporation, and placed in the Knudsen cell which was sealed with a tantalum gasket. The Knudsen cell was positioned in a resistance wire wound ceramic heater, and the vacuum vessel was immediately evacuated. (1) L. Andrews, J. Amer. Chem. Soc., 90,7368 (1968). (2) L.AndTews, J . Chem. Phys., 50, 4228 (1969). (3) J. Rolfe, W. Holzer, W. F. Murphy, and J. H. Bernstein, ibid., 49, 963 (1968).
(4) 5. J. La Placa and J. A. Ibers, J. Amer. Chem. Soc., 87, 2581 (1965). (5) I. A. McGinnety and J. A. Ibers, Chem. Commun., 235 (1968). (6) J. P. Callman, Accounts Chem. Res., 1,136 (1968). (7) 8. E. Stephanou, UT. H. Sohechter, W. J. Arsinger, Jr., and J. Kleinberg, J . Amer. Chem. Soc., 71, 1819 (1949). (8) H. H . Templetan and C. H. Dauben, ibid.,72, 2251 (1950). (9) H. Fioppl, 2.Anorg. A&. Chem., 291,12 (1957). (10) E. G. Brame, Jr., 8.Cohen, J. L. Margrave, and W. V. Meloche, J . Inorg. Nucl. Chem.,4, 90 (1957). (11) L. Andrews, J. Chem. Phys., 48,972 (1968).
3923
IRSPECTRA AND BONDING IN SODIUM SUPEROXIDE MOLECULES
100
BO
60
P
5 c
a
B 40
!O
I 1100
I
I
I
1050
1000
950
A
"
I
I
I
I
550
500
450
400
FREQUENCY,
I 350
I
I
300
250
0
(crn"1
Figure 1. Infrared spectra in the 210-560- and 920-1120-cm-l spectral regions for sodium atoms deposited with oxygen molecules in an argon matrix a t 15'K: spectrum a, Ar/1602 = 100, Ar/Na zs 200; spectrum b, Ar/l*02 = 150, Ar/Na = 200.
After cooling the sample window to 15"K, the deposition of oxygen in argon (M/R = 100: 1 to 300: 1) or pure oxygen a t 2.0 mmol/hr was started, and the Knudsen cell was warmed to 240" where the vapor pressure of sodium is near 1 p . l 2 Deposition times for the argon matrix experiments typically ranged from 20 to 30 hr. Infrared spectra were recorded during and after sample deposition on a Beckman IR-12 filter-grating spectrophotometer in the 200-2000-~m-~spectral region. I n certain experiments the deposited sample was warmed to near 40°K by bucking the refrigerator with electrical heaters after which it was allowed to recool to 15°K. During this operation the sample window temperature was monitored with a goId-2.1% cobalt os. copper thermocouple. Frequency accuracy is =t0.5 cm-' with spectral slit widths near 0.8 cm-I a t 1100, 900, and 700 cm-', 2.4 cm-' a t 500 cm-', and 4.0 cm-' a t 300 cm-I.
Results Argon Matrix. A sample of oxygen was deposited
Table I: Infrared Absorptions (cm-1) Produced by the Reaction of Sodium Atoms and Oxygen Molecules in an Argon Matrix"
+
Na 1eOz Ar/Oz = 200 1112.5 (0.04) 1080.0 (0.05) 1001.0 (0.43) 991.0 (0.61) 524.5 (0.53) 453 (0.08) 390.7 (0% T)' 350 (0.11) 332.8 (0.24) 295.5 (0.32) 262.0 (0.58) 254.0 (0.47) 239.0 (0.28) 232 (0.16)
+
Na 18Oz Ar/Oz = 160 1106.5 (0.04) 1019.1 (0.04) 946.0 (0.16) 936.8 (0.19) 507.4 (0.58) 451 (0.05) 381.5 (O%, T) 348 (0.18) 317.8 (0.16) 289.0 (0.16) 256.4 (0.29) 246.0 (0.68) 231.0 (0.18) 223 (0.06)
Conoentratiod DiffusionC behavior
effects
Na/Oa
Decd Con/con Con/dec Con/dec Dec/inc I Reference f
Con/con Con/con Dec/inc Dec/inc Dec/inc Dec/inc
Con Dec Inc Inc Dis Con Dec Inc Dec Inc Inc Dis Dis Con
Optical densities are shown parenthetically. Changes (decrease, increase, or constant) in relative intensities between experiments relative to data in first column of this table using 390.7cm-1 band as an internal reference. Bands increased, decreased, disappeared, or remained constant on diffusion. Not observed in appropriate experiment. e Quantitative measurement of broad bands not possible. 'The relative optical densities of the 390.7-, 332.8-, and 1080.0-cm-' bands measured before the 390.7-cm-1 band became completely absorbing were 40: 4: 1.
in argon (Ar/Oz = 100) without alkali metal, and no infrared absorptions were observed in addition to the usual traces of COZ and HzO. Figure 1 contrast spectra in the 210-560 and 920-1120-cm-' spectral regions for samples of oxygen (a) Ar/160z = 100 and (b) Ar/'802 = 150 deposited with sodium (Ar/Na = 200), which illustrates the oxygen isotopic shifts. The observed frequencies and optical densities are listed in Table I. (12) W. T. Hicks, J . Chem. Phys., 38,1873 (1963). The equilibrium vapor contained less than 0.4% Nas at 500°K as calculated from Figure 2 shows the spectrum recorded for the major thermodynamic data tabulated by W. H. Evans, R. Jacobson, T. R. absorptions using a 1:2 : 1 mixture of 1602-160180-1802 Munson, and D. D. Wagman, J. Res. Natl. Bur. Stand., 55, 83 (1965). Volume 78, Number 11
November 1969
3924
-(v'j (Vp LESTERANDREWS
I
l
o
00
z E! v)
5v,
60
z
z 40 I-
W
9 L1: W
n 20
r
V
I
I
A V
A
I
I A"
I
I V
I
IA
I
I
'
-
-
sI-
o
A
Na +
'"0,/ I6O "O/
"02
-
I
k
0
I
I
I
I
A
I
I
A
I
I I A
I
Figure 2. Infrared spectra showing oxygen isotopic splittings for sodium atoms deposited with a 1:2: 1 mixture of argon (Ar/Oz = 100, Ar/Na ~3 200) at 15°K.
Table 11: Major Infrared Absorptions Produced by the Reaction of Sodium Atoms with a 1:2: 1 Mixture of 160z-160180-180z in an Argon Matrix with Ar/Oz = 100 Absorptions, cm-1
1001.0
991 (sh) 986.5 975.0 963.5 (sh) 959 * 5 948.5 936.5
Absorptione, om-'
332.6 325.2 317.5 259.5
524.0 518.0 507.4
254.3 249.7 246.0
390.3 386.7 381.5
239.0 235.0 231.0
(Ar/Oz = 100) deposited with sodium (Ar/Na = 200), and the frequencies are listed in Table 11. Additional experiments were run with Ar/Oz = 100, Ar/Na = 800, Ar/Oz = 150, Ar/Na = 200, and Ar/Oz = 300, Ar/Na = 300 in order to study the effect of concentration of both sodium atoms and oxygen molecules on the relative intensities of the absorptions produced in these experiments. The behavior of the absorptions in these experiments is summarized under concentration effects in Table I by listing the change in band intensity relative to the 390.7-cm-l band when the sodium concentraThe Journal of Physical Chemistry
I
V
1602-1~0180-18O2
in
tion is decreased at constant oxygen concentration and when oxygen concentration is decreased a t constant sodium concentration. During all of the above experiments the bands a t 1080.0, 390.7, and 332.8 cm-' maintain approximately constant relative intensities as do the three absorptions at 524.5,254.0, and 239.0 cm-'. Oxygen Matrix. Two experiments were run trapping sodium vapor in an oxygen matrix using evaporation temperatures of 221 and 249". The observed frequencies and optical densities are listed in Table 111. Obviously, an oxygen matrix provides the upper limit on oxygen concentration. Several of the absorptions Table 111: Infrared Absorptions (cm-1) Produced by the Reaction of Sodium Atoms in an Oxygen Matrix" Knudsen cell 1, o c
Approximate Oa/Na Deposition, T,OK
249 iz 3
221
*2
500
100
15
15
Warm to 37
b
1109 (0.13) 1081.0 (0.085) 998 (0.18) 350 (0.15) 306.0 (0.90) 259.0 (0.32) 242.8 (0.11)
(0.08)e
1080.8 (0.09)d 998 (0.44) 351 (0.23) 306.0 ( l . O ) d 259.5 (0.42)d 242.6 (0.13)
(0.02) (0.03) (0.20) (0.15) (0.06) (0.00)
a Optical densities are shown parenthetically. * Not observed here. ' Optical densities recorded after warming sample to 37°K and recooling to l5OK. Assigned, respectively, to V I , V Z , and va of NaO?,in an oxygen matrix.
3925
IRSPECTRA AND BONDING IN SODIUM SUPEROXIDE MOLECULES produced in the argon matrix reaction are not observed in the oxygen matrix as comparison of Tables I and I11 shows. Diflusion Experiments. When the argon matrix samples are warmed to 38-40°K and recooled to 15"K, the absorptions listed in Table I remain approximately constant, increase, or decrease in intensity or disappear completely. The diffusion operation is accompanied by increased light scattering and poorer spectral operating conditions; however, certain trends are clear, and the diffusion behavior of the observed absorptions is summarized in Table I. After warming the oxygen matrix samples to 36-37°K and recooling to 15"K, most of the absorptions decrease in intensity as is shown in Table 11.
Discussion We are interested in identifying the molecular species responsible for the infrared absorptions reported here and determining their structures and vibrational potential constants. Identification of New Molecular Species. I n the previous study1r2 of the reaction of lithium atoms with oxygen molecules, mixed lithium isotopic experiments showed that the major product species contained a single lithium atom, and that the second most intense absorptions were due to a species containing two equivalent lithium atoms. Unfortunately sodium has only one stable isotope so we are left with concentration variation studies to determine the number of sodium atoms present in the species observed here. I n the lithium work, conclusions from mixed lithium isotopic experiments parallel those from concentration variation experiments. Na02. The 390.7-cm-' band is by far the most intense feature observed in the reaction of sodium atoms with oxygen molecules in an argon matrix, and we attribute it to the species NaO2 since the most intense band in lithium-oxygen experiments was due to Li02. The features at 1080.0 and 332.8 cm-' listed in Table I are the only absorptions which show constant concentration dependence and the same diffusion behavior as the 390.7-cm-' band. The mixed oxygen isotopic experiment illustrated in Figure 2 shows that the 390.7- and 332.8-cm-' bands become 1:2 : 1 relative intensity triplets indicating that two equivalent oxygen atoms are present in the species NaO2. Unfortunately, insufficient NaOz was produced in this experiment to observe the three isotopic components of the much weaker bands in the 1019- to 1080-cm-' region. Increased light scattering in this spectral region makes it more difficult to detect very weak bands after 24 hr of sample deposition. We conclude that the 1080.0-, 390.7-, and 332.8-cm-' absorptions are due to the NaOz molecule. NaO2Na. The absorptions a t 524.5, 254.0, and 239.0 cm-' decrease relative to the 390.7-cm-' band with a
decrease in sodium concentration and increase relative to the 390.7-cm-' absorption with a decrease in oxygen concentration. Counterparts of the above bands are not observed in the pure oxygen matrix experiments. These three bands disappear completely upon sample warming, as is shown in Table I. The triplet structures illustrated in Figure 2 for these absorptions indicates that two equivalent oxygen atoms are present in this molecular species. The intensity changes observed on concentration variation suggest that the molecule contains a t least one more sodium atom than NaO2. Failure to observe counterparts of these bands in the pure oxygen matrix experiments is consistent with the formation of this species by adding Na to NaO2, a reaction made improbable in the oxygen matrix experiments. The concentration and diffusion behavior of the intense 524.5- and 254.0-cm-' bands parallel earlier observations for Li02Li. We assign the 524.5- and 254.0-cm-' bands to Na02Na and tentatively associate the 239.0cm-' absorption with NaOzNa. Dimer. The intense doublet a t 1001 and 991 cm-I in the argon matrix experiments shows a decrease relative to the 390.7-cm-1 band with decreasing oxygen concentration but remains constant relative to the 390.7cm-' feature with decreasing sodium concentration in the range presently studied. In the oxygen matrix experiments, a decrease in sodium atom concentration produces an apparent decrease in the 998-cm-I band relative to the 306-cm-' absorption. However, both bands are broad and accurate quantitative comparison is difficult. Upon sample warming, the intense doublet shifts to 1008 and 1002 cm-l with a marked increase in intensity. The 1002-cm-' component is more intense than the 1008-cm-' peak after warmup.12& I n the experiment whose spectrum is illustrated in Figure lb, the presence of about one-sixth 1 6 0 2 is indicated by the observation of the 524.5- and 390.7-cm-' bands. For a species containing two equivalent oxygen molecules, we expect t,o see features due to (1602)z, (1602)(l*Oz), and ('*02)2 with 1: 10:25 statistical weights. The bands a t 968.8 and 959.0 cm-' could be due to a l6O2l802 containing sodium compound since the observed intensities are in excellent agreement with the statistical prediction for such a species. Figure 2 shows the spectrum for a 1:2 : 1 mixture of 1602-160180-'802. For this mixture six possible associations of two oxygen molecules exist with heavier statistical weights for the intermediate mixed isotopes. Owing to overlap of the doublet absorptions, we cannot make definite assignments to each of the six isotopic molecules from the spectrum in Figure 2, but we can conclude that the observed absorptions are likely due to (12a) NOTEADDEDIN PROOF.Experiments in progress reacting potassium atoms with oxygen molecules yield a single sharp, intense feature a t 993 om-' in an argon matrix. This observation suggests that the doubling of the absorption near 1000 om-' in the sodium experiments may be due t o an argon matrix site effect.
Volume 78,NUntbeT 11
November 1969
3926
LESTERANDREWS
a species containing two equivalent oxygen molecules, the same conclusion reached from the spectrum in Figure lb. Since we did not observe2 a molecule containing one lithium atom and two oxygen molecules, even in pure oxygen matrix experiments, we believe that the 1001and 991-cm-l bands observed here could be due to the dimer of NaO2. This tentative assignment is supported by the decrease of NaOz and growth of (NaOz)2 on diffusion in the argon matrix experiments. The 295.5- and 262.0-cm-1 absorptions may be associated with (NaOz)z since they also grew markedly upon sample warming. Our data are insufficient to make a definite assignment of the 295.5- and 262.0-cm-l bands; however, their marked growth on diffusion definitely associates them with an aggregate species. Vibrational Assignments. NaOz. The spectrum observed in the mixed oxygen isotopic experiment indicates that the two oxygen atoms are equivalent in NaO2 and that the molecule has the isosceles triangular structure. This species belongs to the CzVpoint group and has three normal modes, VI (a1, symmetric 0-0 stretch), vz (a1, symmetric Na-0 stretch), and v3 (bl, antisymmetric Na-0 stretch) all of which are infrared active. The 1080.0-cm-' band shifts to 1019.1 cm-l when l80z is substituted for l 6 0 2 in the argon matrix experiments and to 1081.0 cm-l in the pure l6O2 matrix. VI of 7Li160za t 1096.9 cm-' was observed2 a t 1035.2 cm-' with 1 8 0 2 and a t 1098.7 cm-l in an oxygen matrix. Therefore, the 1080.0-cm-1 band observed here is assigned to vl of NaOz. The larger oxygen isotopic shift found for the 332.8-cm-l band indicates2 its assignment as v3 while the smaller oxygen isotopic shift observed for the 390.7-cm-l absorption dictates its assignment as vZ of NaOz. The difference between oxygen and argon matrix shifts is a factor 4.0 k 0.3 greater for NaOz than for LiOz. Vibrational assignments for NaOz are listed in Table IV. Table IV : Frequencies (cm-1) Assigned to NaOe and NaOzNa in an Argon Matrix"
-
NaOz-
7
Isotope
Y1
Y2
Y8
23-16-16 23-16-18 23-18-18
1080.0 (1049. 6)b 1019.1
390.7 386.7 381.5
332.8 325.2 317.8
---Isotope
23-16-16-23 23-16-18-23 23-18-1 8-23
NaOzNa Yd
239.0 235.0 231 .O
__
7
YK
Yb
524.5 518.0 507.4
254.0 249.7 246.0
of NaOz and rt0.5 a Frequency accuracy is rt0.3 cm-l for cm-1 for all other frequencies. 'Estimated by averaging 23-1616 and 23-18-18 values. An analogous average is within the average frequency accuracy for both lithium isotopes of Li180180. The Journal of Physical Chemistry
NaOzNa. We have discussed2the possible structures containing two equivalent alkali metal and two equivalent oxygen atoms and concluded that the molecule likely is a planar rhombus with D2h symmetry. Our data suggest that NaOzNa is formed by the addition of a second sodium atom to KaOz. A rhombus structure has three Raman active and three infrared active vibrational modes. The two most intense absorptions assigned to NaO2Na are considered for assignment to vs(Bzu) and v6(Bau)using the normal mode description and notation of Berkowitz.l* By analogy with NaO2, the higher frequency vibration is vs(Bzu) which corresponds to the motion of sodium cations perpendicular to the peroxide anion, whereas the lower frequency vibration Vg(B3u) corresponds to the motion of sodium cations parallel to the peroxide anion. Since these two vibrations are alone in their respective symmetry classes, we can estimate the 0-Na-0 angle y using the calculation detailed by White and coworke r ~ .The ~ ~primary assumption in this calculation is that the stretch-stretch interaction force constants F I Zand F14 are equal, which should be a reasonably good approximation. For two sets of isotopic data, the frequency ratio VS/V6 is 2.065 which yields y = 51.7'. The weaker absorption a t 239.0 cm-l which may be associated with NaOzNa is considered for the remaining infrared-active fundamental v4(Blu), the out-of-plane bending motion. Since this mode usually has a lower frequency than vs and Vg, the 239.0-cm-1 band is a reasonable assignment to v4 of NaOzNa. Vibrational assignments for NaOzNa are given in Table IV. Force Constant Calculations. NaOz. To do normal coordinate calculations, we need estimates of structural parameters for NaO2. Since preliminary force constant calculations show the 0xygen;oxygen stretching force constant t," be near 5.5 mdyn/A which is close to the 5.6 mdyn/A calculated for 02- from Raman data, we assume the 9-0 internuclear distance in NaOz to be 1.33 & 0.06 A which was determined for crystalline NaO2. Product rule and normal coordinate calculations based on an abundance of isotopic data for LiOz showed that the Li-0 distance is likely near 1.77 f 0.07 A, a value bracketed by internuclear distances for the gas phase LiF (1.564 A)l6 and LiCl (2.021 molecules, Accordingly, we take the internuclear distances of the NaF (1.925 A)17and NaCl (2.360 A)18 diatomic molecules as defining a likely range for the Na-0 bond length in our calculations for NaOz. That this is a (13) J. Berkowits, J . Chen. Phys., 29,1386 (1968). (14) D.White, K. S. Seshadri, D. F. Dever, D. E. Mann, and M. J. Linevsky, ibid., 39,2463 (1963). (15) L.Wharton, W. Klemperer, L. P. Gold, R. Strauch, J. J. Gallagher, and V. E. Derr, ibid., 38, 1203 (1963). (16) D . R.Lide, Jr., P. Cahill, and L. P. Gold, ibid., 40, 156 (1964). (17) 8.E.Veasy and W. Gordy, Phys. Rew., 138,A1306 (1965). (18) A. Honig, M. Mandel, M. L. Stitch, and C. H. Townes, ibid., 96, 629 (1964).
IR SPECTRA AND BONDING IN SODIUM SUPEROXIDE MOLECULES reasonable range of Na-0 bond lengths if verified by an estimate of 2.07 A from the empirical correlations of Herschbach and Laurie, l9 using a preliminary Na-0 force constant of 0.8 mdyn/A. Force constant calculations were doneofor Na-0 bond lengths of 1.90, 2.04, 2.20, and 2.35 A using the nine isotopic frequencies listed in Table IV. The calculations were performed on a least-squares adjustment program FADJ written by J. H. Schachtschneider using the Wilson FG matrix method. Since the off-diagonoal constants Fl2 and F18 were very small (
