The Properties of Oxygen Investigated with Easily ... · For larger audiences, a video camera and...

40 Journal of Chemical Education

_Vol. 87 No. 1 January 2010

_pubs.acs.org/jchemeduc

_r2009 American Chemical Society and Division of Chemical Education, Inc.

10.1021/ed800008g Published on Web 12/18/2009

In the Classroom

The Properties of Oxygen Investigated with EasilyAccessible Instrumentation

The “One-Photon-Two-Molecule” Mechanism RevisitedManfred Adelhelm, Natasha Aristov, and Achim Habekost*Department of Chemistry, Padagogische Hochschule Ludwigsburg, Reuteallee 46, D-71634Ludwigsburg, Germany*[email protected]

Oxygen has spectacular and unusual properties (1). Studentsare generally familiar with this gas as making up about 20% of theatmosphere, being required for combustion, participating inmanyoxidation reactions, and being a colorless gas. The revelation that itis blue as a liquid and paramagnetic is surprising. The observationthat, under other conditions (as the product of a chemicalreaction), oxygen emits red light, provides further amazement.Over the years, several methods of demonstrating the paramag-netic and optical properties of oxygen have been published in thisJournal (2-6). Using these demonstrations is an elegant way ofintroducing or reinforcing the concepts ofmolecular orbital (MO)theory. Most recently, a demonstration using a small, strong,neodynium magnet was published (7). The availability of thesemagnets allows the demonstration of “magnetic” oxygen in mostclassrooms. The experiments described here show that the ob-servation of the optical properties of oxygen is also easily possiblewith instruments that are commonly accessible, for example, hand-held spectrophotometers and digital cameras.

A review of the literature led us to consider the usualdidactical discussion associated with the presentation of theparamagnetism and optical phenomena of oxygen. For example,often the blue color of liquid oxygen and its paramagnetism arepresented in such a way to suggest that these two properties areintimately connected to one another. In fact, the structure ofliquid oxygen is not simple, given that its open valence shell givesrise to intermolecular interactions stronger than van der Waalsforces (8). The earliest measurements of oxygen's magneticproperties implied that some interactions among oxygen mole-cules must be occurring (9). That is, part of the liquefied oxygenis paramagnetic and consists of oxygen molecules and clusters ofoxygen molecules with parallel aligned spins, but it is not a priorigiven that this is also the “blue” part. On the contrary, the bluepart of the oxygen is most likely to consist of diamagnetic oxygendimers (10). Thus, it is not the paramagnetism per se that isconsistent with the observation of a blue color, but rather thedeviation of the observed paramagnetism from the theoreticallypredicted value, which is usually not shown in classrooms.

While using the paramagnetism of oxygen to demonstratethe reliability of MO theory for predicting electronic structure islegitimate, understanding the blue color (due to photonabsorptions) of the liquid and the red chemiluminescence goesbeyond the predictions of MO theory for O2 molecules. Thestrongest absorption and emission bands near 630 and 570 nm(in the red and the yellow-green) correspond to transitions

between the ground state and first excited state of oxygen, butonly if two ground-state molecules are promoted and relaxed totwo excited-state molecules. This has been nicely presented as anexercise in spectral interpretation for the general chemistrylaboratory in this Journal (11). A key point to understandingthe absorption spectrumwas the observation of the same featuresboth for gaseous oxygen at high pressures and for large oxygen-layer thicknesses, on the order of the thickness of the earth'satmosphere.1 Inter-molecular interactions, more likely in con-densed or high-pressure phases and more likely to be seen in a gasreservoir as huge as the atmosphere, were proposed, and laterconfirmed, to be the origin of the observed absorption bands(12-15). In fact, the earliest spectroscopists considered theexistence of an O4 molecule to be likely (16). Nevertheless, inlater interpretations, a “one-photon-two-molecule” mechanismwas invoked, where the two molecules are interacting transientlyas a “collision pair” (6, 17, 18).

Much work has been done to understand the properties ofO2 dimers and the likelihood of the formation of a tetraoxygenmolecule in both the gas and various condensed phases of oxygen.Interest remains, not just because of the theoretical challenge ofisolating and characterizing this interesting molecule, but also forpractical reasons. It had been thought that oxygen polymersmight serve as highly energetic materials (19). Theoreticalcalculations showed, however, no such long-lived O4 species tobe available, as the barrier to dissociation to two O2 moleculeswas predicted to be low, about 20 kJ/mol (20). Even octaoxygen,O8, has been observed in solid oxygen (21). The tetraoxygenmolecule, (O2)2/O4, continues to be the object of attention, forexample, as a key metastable component in atmospheric pro-cesses such as the production of ozone, the de-excitation ofvibrationally hot O2 (22, 23), and in the absorption of solarradiation (15). It also plays a role in chemiluminescent processesnow commonly used in analytical methods (24, 25). In view ofthe results of scattering experiments, sophisticated spectroscopy,and extensive theoretical calculations, we argue in this article thatthe one-photon-two-molecule terminology for the absorptionand the chemiluminescence spectra of oxygen be abandoned.

Demonstrations

Paramagnetism

Our method of preparing liquid oxygen follows that ofShakhashiri (4, 26). We collect about 20-50 mL of liquid

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_Journal of Chemical Education 41

In the Classroom

oxygen and direct it in a stream past a 0.25 T permanent magnet.To remove ice crystals, we pour the oxygen through a cone linedwith filter paper that has been pierced in the center with a needle.For larger audiences, a video camera and monitor are used toenhance viewing ability.

Optical Properties

It is not difficult to assign the observed absorption andemission lines (Table 1) to well-documented transitions (18)among the lower electronic states of oxygen (Figure 1). Thespectra were calibrated against a spectrum of Nd(NO3)3 (notshown here).

Blue Color

The blue color of liquid oxygen can be plainly seen, but canbe more precisely investigated with spectrophotometers or, byeach student individually, with handheld spectroscopes. Notethat the spectra can be measured only to 350 nm, since Duran“test tube” glass is not transparent to UV.

Recording the Spectrum with a Spectrophotometer

A small transparent Dewar, ∼10 cm length and 4 cmdiameter, is precooled with liquid nitrogen, filled with liquidoxygen prepared as above, and placed into the beam path of aspectrophotometer (for example, a double-beam UV-vis Per-kin-Elmer 555) in place of the usual cuvette.2 The oxygenremains liquefied for about 30 min, which is sufficient time totake a visible spectrum (Figure 2, panels A and B).

Recording the Spectrum with a HandHeld Spectroscopeand Camera

A small volume, 2-3 cm3, of liquefied oxygen is pouredinto a tall test tube (D50 Duran glass). Alternatively, a trans-parent (unsilvered) Dewar can be used in place of the test tube.The test tube is held up to a strong light source (generally, brightsunlight is sufficient) and observed through a handheld spectro-scope (A. Kruss Optronic HS 1504, available through Leybolddidactic, catalog no. 667-339). Ice buildup is removed as neededwith a paper towel saturated with ethanol. The lens openingof a digital camera is placed directly behind the ocular of thespectroscope to take a photograph of the spectrum (Figure 2A).We used a Nikon CoolPix S4 because the diameter of its lensopening is exactly the same as that of the ocular on thespectroscope, saving us the labor of shielding the camera lensfrom stray light. As can be seen from Figure 2A, only lowresolution is possible at the wide lens openings necessary to seethe black absorption lines.

This is a powerful demonstration of the complementarybehavior of absorbed and scattered light. Viewing what isperceived by the naked eye as a blue liquid, one sees through aspectroscope red, (yellow-)green, and blue. Students' attentionneeds to be drawn to the black lines breaking up the continuity ofthe spectrum, that is, the light wavelengths that have beenabsorbed by the liquid oxygen. Their absence from the spectrumcauses the color to appear blue. This concept is reinforced byexamining the reverse process of emission of the red and yellow-green photons in the chemiluminescence process discussed below.

An elegant alternative is to use a Leybold didactic 337 47VideoCom CCD camera to record the spectrum (Figure 2C). Alight source irradiates a Dewar containing the liquid oxygen. It is

Table 1. Lines Observed in the Absorption and Emission Spectra of Liquid Oxygen

Relative Intensity (indicated by the number of x's)

Absorption TransitionTransition

Wavelength/nm SpectrophotometerHandheld Spectroscope

(digital camera)Handheld Spectroscope

(VideoCom)

2 3Σg- (v = 0) f 2 1Δg (v = 0) 634 xxx xxx xxx

2 3Σg- (v = 0) f 2 1Δg (v = 1) 578 xxxxxx xxxxxx xxxx

2 3Σg- (v = 0) f 2 1Δg (v = 2) 534 xx xx x

2 3Σg- (v = 0) f 1Δg þ 1P

gþ (v = 0) 478 xxx xx xx

2 3Σg- (v = 0) f 1Δg þ 1P

gþ (v = 1) 446 x

2 3Σg- (v = 0) f 1P

gþ þ 1P

gþ (v = 0) 380 xx

2 3Σg- (v = 0) f 1P

gþ þ 1P

gþ (v = 1) 362 xx

Emission TransitionTransition

Wavelength/nmMonochromator/

Photomultiplier TubeHandheld Spectroscope

(digital camera)

2 3Σg- (v = 0) r 2 1Δg (v = 0)a 634 xxxxx xxxxx

2 3Σg- (v = 0) r 2 1Δg (v = 1)a 578 xxx

a v: level for both oxygen molecules.

Figure 1. Electronic configurations of the three lowest electronic states ofoxygen. For a more thorough treatment of the electronic structure ofoxygen, see refs 11, 16, 27, and 28.





In the Classroom

then spectrally analyzed in a (mounted) handheld spectroscopeand projected by a focusing lens (f = 150 mm) onto theVideoCom. It is necessary to work in the dark.

Chemiluminescence of Singlet Oxygen

The reaction between hypochlorite anion and hydrogenperoxide in basic conditions will produce excited-state singletoxygen, as first reported by Mallet in 1927 (29). After a briefinduction period of about 10 s, an intense red emission isobserved. This emission was spectrally resolved in two ways.The spectrum recorded with a prism monochromator andphotomultiplier tube (Type 1 P 28A) is shown in Figure 3A.Themonochromator is driven by a step motor. The analog signalwas input to a Cassy interface (Leybold didactic) and thenprocessed using the Cassy-Lab software. The image photo-graphed with a digital camera aimed through the ocular of ahandheld spectroscope pointed at the reaction vessel is shown inFigure 3B. To calibrate the wavelength axis, a mercury vaporspectrum (from the laboratory ceiling lighting) is shown forcomparison (Figure 3C).

Discussion: Experimental and Theoretical Evidence for anOxygen Dimer

Other discussions of the history of the search for O4 can befound in refs 19, 30 and 31. The earliest work on the magnetismof liquid oxygen and its absorption spectrum suggested theexistence of O4 molecules (9, 12, 16). Since then, oxygen dimershave been made (the references cited represent only some of theliterature on this subject) under “soft” conditions, as in a gas cellat cold temperatures (15, 23, 32-35), in O2 þ O2 molecularbeam scattering (10), and inmolecular beam expansions (36, 37),and metastable excited states of (O2)2* have been produced(19, 38). Evidence for oxygen dimers has been observed in theatmosphere (13). That some O4 species exists that is more thanjust a molecule pair is implied, for example, by the observation ofthe reaction 2O2fO3þO formation at energies lower than thebond dissociation energy of O2 (36). In fact, rovibrationalspectra have been obtained for the [O2(

3Σg-)v=0]2 f

[O2(1Δg)v=0]2 transition in van der Waals dimers, an indication

that these states of the (O2)2/O4 species exist for longer than justcollisional interactions (34).

Figure 2. (A) Liquid oxygen absorption spectrum as seen through ahandheld spectroscope. (B) UV-vis spectrum of liquid oxygen recordedwith a double-beam spectrophotometer (Perkin-Elmer 555) showingfeatures at 630, 578, 548, 446, 380, and 362 nm. Measurementsbeyond 350 nm are prevented by the Duran glass. (C) The absorptionspectrum seen through the handheld spectroscope as recorded by theLeybold didactic VideoCom.

Figure 3. (A) Spectrum of the Mallet reaction recorded with the prismmonochromator, photomultipler tube, and Cassy interface. (B) Photo-graph of the Mallet reaction chemiluminescence spectrum taken with adigital camera through the ocular of a handheld spectroscope. (C)Mercury emission of the laboratory ceiling lighting taken as a calibrationspectrum.

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_Journal of Chemical Education 43

In the Classroom

Only van der Waals complexes with some spin couplingbetween the O2 monomers can explain the experimental ob-servations made thus far. For the dimer formed under softconditions (molecular-beam scattering), the relative contribu-tion to the O2-O2 binding energy from spin-spin interactionsbetween two ground-state monomers has been found to be about15% of that from the van der Waals interaction (10).

Reviewing and summarizing the data of all these sources, thebinding energies of the dimer formed from ground-state O2 areon the order of 10-20 meV and the O2-O2 separation is about3-3.6 Å: those that correlate to excited-state O2 molecules havewell depths of about 6-8 meV and separations of 3.2 - 4.0 Å.(For comparison, standard reference tables give the bond energyof a single O-O bond as about 1.5 eV and the bond length asabout 1.5 Å.) A metastable state is seen at 4.1 eV above theseparated ground-state O2 and found to be made from ground-state O2 and a highly excited 1Σu

- state of O2. This state musthave a lifetime on the order of microseconds to survive todetection (38). High-energy repulsive states of (O2)2 have beeninferred in photoelectron detachment studies of O4

- thatcorrelate to 23Σg

- and to 1Δg þ 3Σg- (39).

A dimeric species must be responsible for the chemilumines-cence in the Mallet reaction owing to the relaxation of two 1Δg

O2 to two ground-state O2. As in the absorption, this transition isallowed only when two molecules are coupled together. Weaklybound states of (O2)2 with both O2 in

1Δg have been identifiedspectroscopically and predicted theoretically, as mentioned above.That such a dimer is being produced is reasonable in light of Khanand Kasha's (18) determination that only excited-state oxygenmolecules can be produced in the Mallet reaction.

The possibility of reactive processes concomitant withelectronic transitions, that is, that the oxygens trade partners,has been evaluated. An interesting result obtained in anion-molecule collision, neutralization, dissociative reionizationreaction indicates that, for metastable excited states of (O2)2, noexchange reaction takes place. The decomposition products ofthe dissociation of 16O2

18O2 are 16O2 and 18O2; no mixedspecies, 16O18O, was observed (38). For reactions involving theground states of the oxygen monomers, one notices that only theπ* electrons (using, for simplicity, the notation for the separatedoxygen monomers) can delocalize in this simple picture, the σpand π remain in the original molecules (top part of Figure 4). Infact, delocalization of theπ* could even lead to a strengthening ofthe original O-O double bonds in the dimer, since it withdrawssome antibonding electron density from the original bonds.Thus, a reactive process can be ruled as being unlikely. In thetop part of Figure 4, we show a simple “frontier molecularorbital” picture of the interaction of the antibonding π* orbitalsof two oxygen molecules in an “H” geometry, in which theoxygenmolecules are parallel to each other,D2h symmetry group.Other limiting geometries of approach are shown in the bottomleft of Figure 4. Of these, ab initio calculations have yieldedstable structures only for theH andX approaches. Singlet, triplet,and quintet states are possible from 3Σg

- þ 3Σg-. For these, an

H geometry is the lowest energy species for the singlet and triplet,while the quintet is most stable in an X geometry,D2d, where thebond axes of the separated monomers are at an angle to eachother (40). The singlet states of the dimer that dissociates tothe 1Δg þ 1Δg excited states of O2 are equally stable in H orX geometries (34). This reconciles the early postulation of a“floppy” but “somewhat rigid” complex (32).

Although theoretical calculations indicate its existenceas a possibility (20, 42), a completely covalently bound, cyclic,O4 molecule (bottom right of Figure 4) has not beenobserved. It is expected to be a puckered ring of D2dsymmetry, with bond lengths similar to that in O2. Therecan be at least two reasons for this: First, formation of cyclictetraoxygen in the gas phase is endothermic by about 4.27 eV(but it is bound by a low barrier to redissociation to 2O2

molecules, on the order of 0.5 eV) (30). Second, statisticalmechanical calculations of the phase space available to free-pair, metastable-pair, and true dimeric (covalently bound)states of oxygen showed relative fractions of about 51%, 47%,and 2%, respectively, near -183 �C, the boiling point ofoxygen, and with the covalently bound-state fraction drop-ping off to zero above this temperature (43). Thus, cyclic O4

is not a likely product in the gas or liquid phases. Alternativelimiting structures for O4 have been considered and elimi-nated, for example, a branchedD3h shown in the bottom rightof Figure 4.

An extensive molecular dynamics study of liquid oxygen (8)showed that the electronic structure of O2 in the liquid is notdifferent from the gas, the energy levels are mostly broadened buthardly shifted. Thus, it is reasonable to extrapolate the results forO4 in the liquid to those in the gas phase. For the liquid, it wasalso found that the most stable geometry inO4 is theH structure.Magnetic saturation was observed at O2-O2 separations lessthan 3.1 Å and a persistence of magnetic alignment was observedto separations as large as 4.4 Å.

Figure 4. (Top panel) Possible bonding combinations of the two π*orbitals of two oxygen molecules to form a 1B1u state of O4 in D2h

symmetry (H geometry). The z-axis is perpendicular to the plane of thepaper. To make the B3g orbital, the π* lobes are mixed above andbelow the plane of the paper; for the B2u, they are in-plane. (Bottomleft panel) The limiting geometries of approach of two oxygenmolecules: for H and X, the approach axis is shown out of the planeof the paper as indicated by the wide skewed arrow, and for T and Lapproach, the collision geometry is in the plane. (Bottom right panel)The calculated lowest energy geometry for cyclic, puckered, O4 anda further considered, but eliminated, covalently bound O4 species(10, 34, 40, 41).





In the Classroom

Conclusion

Considering all the experimental and calculational datacollected thus far, we recommend no longer referring to a “one-photon-two-molecule” mechanism when discussing the absorp-tion and chemiluminescence spectra of oxygen. Rather, they aredue to spin-coupled and van derWaals-coupledmetastable boundstates. The oxygen molecules interact strongly enough with oneanother so that the spin and symmetry forbidden transitionsbetween 3Σg

- and 1Δg of the monomers become allowed in thecoupled dimer between 2(3Σg

-) and 2(1Δg).

Acknowledgment

We would like to thank Steffen Rieger for his photographicservices and Regina Hornstein and Hans Strecker for theirtechnical assistance.

Notes1. It has been estimated that oxygen dimers formed in the atmo-

sphere contribute about 1-2% of the total absorption of solarradiation (15). They alone cannot be responsible for the bluecolor of the sky. The most important reason for the sky beingblue is Rayleigh scattering.

2. Standard cuvettes are too small and not thermally insulated; theliquid oxygen evaporates too quickly.

Literature Cited

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