current research Singlet Oxygen in the Environmental Sciences Singlet Molecular Oxygen and Photochemical Air Pollution J. N. Pitts, Jr., Ahsan U. Khan, E. Brian Smith, and Richard P. Wayne Department of Chemistry, University of California, Riverside, Calif. 92502
Singlet molecular oxygen (loz)may play a significant role as a n oxidant in photochemical air polution. Reaction of electronically excited oxygen with olefinic substances produces thermally unstable hydroperoxides which may be involved in the rapid conversion of NO into NOz, a process not well understood in photochemical air pollution. Several mechanisms for the formation of ' 0 2 are examined critically in relation to their possible importance in the chemistry of urban atmospheres. In each, the excitation energy is derived ultimately from the sun's radiation, but the energy may be utilized by direct absorption of radiation by ground state 3 0 2 , by photolysis of an atmospheric contaminant to form excited loz in the primary step, by spin-conserved energy transfer mechanism in which a n atmospheric contaminant absorbs solar radiation and transfers its excitation to ground state 30z,or by exothermic chemical reactions involving atmospheric contaminants which themselves originated in a photochemical process.
A
lthough considerable understanding of photochemical air pollution of the type originally encountered and characterized in the Los Angeles basin has been attained (Altshuller and Bufalini, 1965; Leighton, 1961; Stern, 1968), several crucial aspects of the problem are not yet explained. For example, a key process in the formation of photochemical smog is the photoconversion to nitrogen dioxide of nitric oxide from automobile exhaust gases, with the concurrent disappearance of the hydrocarbons and the buildup of oxidants, principally ozone and peroxyacyl nitrate (PAN), and other oxidized organic material such as aldehydes and alkyl nitrates. The reaction system is extremely complex under normal field conditions, and it is also complex when studies are conducted in large photochemical reaction chambers which are used as atmospheric models. While a great deal of useful information has been derived from studies in test chambers, current mechanisms advanced to explain such phenomena as the photochemical conversion of nitric oxide into nitrogen dioxide in the presence of hydrocarbons can account for only approximately 50 of the hydrocarbons consumed in the process. We recently came t o the conclusion that a new oxidant, electronically excited singlet molecular oxygen, 02('A,), 02('S,), or both, may be involved in these systems; that is, in test chambers as well as in polluted urbai, atmospheres,
and that hitherto unexplained chemical and possibly biological effects might be attributed to the excited species (Khan, Pitts, et a/., 1967; Pitts, 1967). It was pointed out that a n energy transfer mechanism could be responsible for production of singlet oxygen in the atmosphere. In this paper we present more detailed arguments for such a mechanism and also suggest other possible processes, some of which are not photochemical, that could also result in the formation of singlet molecular oxygen in polluted atmospheres. The term singlet molecular oxygen is used in this discussion states. It is probable that the to refer to either the 'A, or I&+ 'A, state is the more important in atmospheric processes, but unless otherwise indicated we do not distinguish between the reactivities of the two states.
Inorganic System In the strictly inorganic system NO-N02-02, it has been clearly demonstrated that over a wide pressure range of NOz, the sole primary photochemical process at wavelengths shorter than about 4200 A. is the photodissociation of NO2 into nitric oxide and atomic oxygen (Equation 1) (Blacet, Hall, et a/., 1962; Ford and Endow, 1957; Ford and Jaffee, 1963; Pitts, Sharp, et al., 1963, 1964; Sato and Cvetanovic, 1958, 1959; Schuck and Stephens, 1968).
NO2
+ hv-
X
< 4200 A .
NO
+0
(1)
In the lower atmosphere, oxygen atoms, formed in this primary photochemical act, react with oxygen in a three-body process to form ozone: 0 -t 0 2 - k M + 0 3
+
(2)
Subsequent reactions in the system are complex but if one makes reasonable steady-state approximations for atomic oxygen and ozone, the following important equilibrium can be derived (Altshuller and Bufalini, 1965; Schuck and Stephens, 1968):
NOz
+ Oz
hv(X 4200 A,)
+NO+03
(3)
System NO- NO2-Oz-Hydrocarbon
The thermal and photochemical reactions in the purely inorganic system, although complicated, appear to raise n o fundamental chemical problems. However, when a reactive hydrocarbon-e.g., an olefin or an alkylbenzene, both of which are emitted in automobile exhaust-is added to the system, a highly complex set of reactions ensues. These reactions lead to production of a host of compounds characteristically found in photochemical smog. In addition to Volume 3, Number 3, March 1969 241
ozone, these products include, for example, formaldehyde, acetaldehyde, acrolein, PAN, and alkyl nitrites, as has been amply demonstrated by the work of Altshuller, Cvetanovic, Haagen-Smit, Schuck, Stephens, Tuesday, and Wayne and their coworkers [detailed references are given by Altshuller and Bufalini (1965); Schuck and Stephens (1968); Stern (1968)l. A perplexing aspect of the photochemical production of smog in these systems is the remarkably rapid, and highly efficient, conversion of N O to NOz which has been observed by all investigators. The thermal reaction 2 N 0 O 2 .-+ 2N02 is too slow a t the relatively low concentrations of N O in the atmosphere to account for this rapid conversion. Furthermore, the ozonosphere restricts the radiation reaching the surface of the earth to wavelengths greater than about 2900 A. Hence, direct photochemical decomposition of nitric oxide or simple olefins is not possible. The sole absorber of light in chamber studies is N O z and the photoconversion of N O to NOz is therefore particularly remarkable because it is a rare example of a system which has, as Leighton pointed out, a negative quantum yield for the absorber--i.e., the molecule which absorbs light and photodissociates is ultimately produced in larger amounts than are initially present. Clearly, a unique and efficient overall photochemical process or set of processes must exist, and, as Altshuller and Bufalini (1965) state, “A satisfactory mechanism has not been developed to fully explain the process by which the oxidation of nitric oxide occurs.” However, several investigators have suggested a general mechanism, including reactions of N O with radicals such as RC03, ROs, and R C 0 2 ,to generate NO*. Heicklen and Cohen (1968) have recently proposed an alternative mechanism in which the unsymmetrical radical intermediate formed by the thermal reaction of NO with Oz, ONOO, reacts photochemically to give N O z and ozone as overall products.
+
ON00
+hv
02
NO2
+
0 3
(4)
While the species O N 0 0 has been characterized by Guillory and Johnston (1963, 1965) and the mechanism may be important in the strictly inorganic system, its applicability to photochemical smog generation is not clear.
known for some time to contribute to the day and night “airglows.” Wayne (1967) notes that the + 3Z0- transition of molecular oxygen gives rise to the “atmospheric band” which was first produced in emission in the laboratory by Kaplan (1947) and then observed in the airglow by Meinel (1950). Valiance-Jones and Harrison (1958) made the first observation in the airglow of the “infrared atmospheric” band which results from the ‘A,-,38,- radiative transition in oxygen. The possibility that singlet molecular oxygen plays a role in the formation of photochemical smog in the lower atmosphere was first considered by Leighton in 1961. It was concluded that excitation by direct absorption of solar radiation could not lead to significant stationary concentrations of the I.&,+ state in the lower atmosphere. Furthermore, there was not evidence for the reaction with hydrocarbons of either state of singlet oxygen a t room temperature. Subsequent studies of the spectroscopy (Arnold, Ogryzlo, et a/., 1964; Khan and Kasha, 1963) and of the chemical reactivity of singlet oxygen in the gas phase (Arnold and Ogryzlo, 1967; Izod and Wayne, 1968b; McNeal and Cook, 1967) necessitate a re-examination of the problem. It was established in 1964 by Corey and Taylor and by Foote and Wexler that singlet 0 2 ,produced in a microwave discharge or by the HZO2 NaOCl reaction (Khan and Kasha, 1963), respectively, reacts rapidly with olefinic hydrocarbons in solution to produce peroxides. The products are identical to the products of sensitized photooxidation which had been well established, and both groups concluded that the reactant in such sensitized photooxidation is singlet 0 2 . The original literature on various aspects of singlet molecular oxygen and photosensitized oxidations has been well summarized in recent reviews by Foote (1968), Gollnick (1$68), Gollnick and Schenck (1967), and Wayne (1967, 1969) and in the proceedings of the “International Oxidation Symposium” (1967); n o attempt is made, therefore, to furnish a detailed literature survey in this article. T) pica1 peroxide products formed in solution by either chemical oxidation by 02(’A0) or by dyesensitized photooxidation are shown in Table I (taken directly from Foote’s review) for several substrates.
+
Singlet Molecular Oxygen in the Atmosphere A simplified potential energy diagram for the oxygen molecule is given in Figure 1. “Forbidden” optical transitions from the two low-lying singlet states to the ground state are well established:
+ hv (7619 A,) OS(~Z,-) + hv (12,690 A.) Oz(3Z,-)
-+
Oz(’Z,+) +
Oz(’A,)
(6)
The numbers cited are for the wavelength of maximum intensity of the 0, 0 bands, and correspond approximately t o energies of 22.4 and 37.4 kcal. per mole for the IA0 and I&,+ states, respectively. Calvert and Pitts (1966), Gill and Laidler (1958), Griffith (1964), and Wayne (1967, 1969) discuss electronically excited molecular oxygen in greater detail than is possible here. The emission of radiation from electronically excited singlet oxygen molecules present in the upper atmosphere has been 242 Environmental Science & Technology
I
I
04
08
I I 2
ohq-
X’X;
INFRARED ATMOSPHERIC SYSTEM
~‘1;-
x’z;
ATMOSPHERIC S Y S T E M
B3Xi-
X’X;
SCHUMANN-RUNGE
I 16
I 2 4
I
2 0
SYSTEM
I
1
I
2 8
3 2
36
INTERNUCLEAR DISTANCE ( A i Fig.
1
Figlire 1. Potential energy curves for several states of the oxygen molecule (Calvert and Pitts, 1966)
nitrogen in the presence of methanol, the peroxidic product was identical with that obtained from the reaction of dimethylfuran with singlet oxygen in methanolic solution. Large yields of relatively pure hydroperoxides were obtained Compound Product from the gas-phase reactions, suggesting that they could well be used on a preparative scale. Bayes (1964) showed that if the absorption coefficients for the direct absorption processes 5 and 6 were corrected to allow for collisional line broadening (assuming a Lorentzian line shape), the rate of absorption of solar radiation by oxygen in the lower atmosphere became significant. As a result of these calculations and experiments, Bayes concluded, in contrast to Leighton’s views, that although the direct transitions shown in Equations 5 and 6 are strongly forbidden, OOH OCH, the absorptions “may be strong enough to contribute significantly to the reactions of photochemical smog.” Furthermore, ‘6+11 H5 0 c 6 w e [ 6 H 5 lie felt that collisional deactivation of O&A0) by 0 2 or N2 was highly inefficient and concluded that the long lifetime / \ C6H5 \ ,/ C6H5 would “permit the average excited oxygen in polluted air to 0 0 encounter an olefin molecule” (and hence presumably to form 0 a peroxide). In the atmosphere of the Los Angeles basin, the rate of this reaction might compete with the rate of reaction of olefins with oxygen atoms produced by NOz photolysis. Although Bayes’s proposal was advanced in 1964, it has OOH received relatively little attention from those interested in photochemical air pollution, in part, perhaps because direct irradiation of olefin-air mixtures in chambers did not produce smog symptoms. Our hypothesis that singlet oxygen may be important in Taken from Foote (1968). a. Foote and Wexler (1964) ; b. unstable product; c. unreactive acceptor, very low yield; d . methanol adduct of photochemical smog accepts the view of Bayes that OZ(’A,) unstable endoperoxide; r . presumably formed by loss of CO from has a long lifetime even at atmospheric pressure [recent intermediate peroxide; f . Foote, Wexler, er a/. (1968). determinations by Clark and Wayne (1968) of the quenching rate of Oz(’Ag)by atmospheric gases suggest that the half-life may be about 5 X lop2 second], and that it exhibits a high degree of reactivity with olefins in the gas phase (or in heterProduction of Singlet Oxygen in Atmosphere by ogeneous systems). However, we further propose that, in Direct Absorption o,f Solar Radiation addition to its being formed by direct absorption of radiation, In 1964, Bayes demonstrated that O2(’A0)generated by the there are other possible modes of generation of O2(lA0)in the H 2 0 z NaOCl reaction, reacted with tetramethylethylene, atmosphere a t rates greater than those available from direct TME, rapidly and efficiently in the gas phase to give the same absorption of sunlight by Oz. These are (1) by energy transfer, peroxide product (I) (identified and analyzed by gas chroma(2) by direct photolysis of ozone, and (3) by certain exothermic tography) as in solution (Bayes, 1964; Winer and Bayes, 1966). chemical reactions, some of which are known to occur in smog and in which molecular oxygen is a product. In the subsequent sections of this paper, we shall expand on the (7) energy transfer mechanism proposed in our earlier communication (Khan, Pitts, et al., 1967), discuss ( 3 ) in some detail, and refer briefly to (2). This work has recently been extended in our laboratory Ozone photolysis in the lower atmosphere as a source of (Broadbent, Gleason, et al., 1968). Singlet O2 was generated OZ(’A)is discussed in detail by Kummler, Bortner, et al. (1969). in a microwave discharge and the discharged gas, treated with mercury vapor to remove 0 atoms, was mixed with tetraProduction of Singlet Molecular Oxygen by methylethylene vapor; the only product was tetramethylethylEnergy Trunsjer Mechanism ene hydroperoxide, identified by infrared and N M R spectrosThe mechanism proposed bypasses the problem of very copy and gas chromatography. Yields were measured using weak direct absorption of radiation by suggesting that sigG L C and up to 100 conversions to tetramethylethylene nificant concentrations of singlet oxygen are produced hydroperoxide were obtained. The gas-phase infrared spectra indirectly by triplet electronic energy transfer from elecof the products were obtained using a 10-meter path multiple tronically excited organic molecules. A high yield of singlet reflection cell and characteristic bands of the hydroperoxide oxygen can be obtained if solar radiation is absorbed first were observed. by an organic molecule and then, on collision, electronic Similar studies have been made with 2,5-dimethylfuran. energy is transferred from this donor (in its triplet state) t o New bands were observed in the infrared spectrum of the oxygen to produce electronically excited oxygen in one of its gas stream, and, when the mixture was condensed in liquid
Table I. Chemical Oxygenations with Singlet Molecular Oxygen of Typical Substrates“
-Q-
-63-
00‘
+
Volume 3, Number 3, March 1969 243
singlet states. The overall mechanism may be represented as
W o )
+ hv
-
Wi)
(8)
+ D(T1)
(9)
intersystem
D(s~) D(TJ
+ Od3.Z,-)
-
crossing
DGo)
+ O#A,
or
(10)
where D represents the donor organic molecule which absorbs the sunlight, and the symbols S o , S1, and T1 represent its ground state and first excited singlet. and triplet states, respectively (Figure 2 ) . Kummler (1968) has demonstrated the presence of 02('A,) in an irradiated benzaldehyde-molecular oxygen system. This experiment appears to offer dramatic confirmation of the validity of the energy transfer mechanism in gas-phase processes. Furthermore, benzaldehyde is an atmospheric contaminant (resulting from the oxidation of aromatic hydrocarbons present in gasoline), with a further substantial extinction coefficient in the near ultraviolet. The mechanism for the production of singlet oxygen by transfer of electronic energy from triplet states of organic molecules, Reactions 8 to 10, was flrst proposed by Kautsky (1939), but went unnoticed until recent photochemical and spectroscopic results led to the present general acceptance of the Kautsky scheme as one of the two prime mechanisms for photosensitized oxidations (Foote, 1968; Gollnick, 1 968). It is particularly significant that a wide range of different chemical species of donor molecules may be effective in producing singlet 02.and that energy transfer has been shown for heterogeneous gas-solid systems as well as gas-liquid and homogeneous gaseous systems. Thus, in his classic paper, Kautsky (1939) adsorbed biologically important dyestuffs such as chlorophyll and porphyrins on solid supports of silica gel or aluminum oxide and suggested that singlet molecular oxygen was formed on illumination of the surface in the presence of normal oxygen, and that "free" singlet 0 2 migrated to the acceptor which was adsorbed on another solid support separated from the donor. Rawls and Van Santen (1968) recently demonstrated the role of singlet oxygen in the autoxidation of fatty acids by passing singlet O2 from a microwave discharge over a thin film and showing that a conjugated hydroperoxide was formed. They noted that the singlet On reacted a t least 1000 times as fast as the ground state triplet O2 molecule. The same oxidation products were formed when biologically significant dyestuffs such as chlorophyll-a were introduced into a fatty acid film illuminated with visible light in the presence of air. Triplet energy transfer at gas-solid interfaces has been demonstrated for the system naphthalene-cis-penta-l,3-diene (and in several other systems including those with 9,lOanthraquinone as donor) by Daubendiek, Magid, et al. (1968). On irradiation of the solid aromatic hydrocarbon in the presence of the diolefin, isomerization of the olefin to the trans form is observed. If gaseous olefins quench triplet aromatics in the solid phase, molecular oxygen might also act as a quencher to yield singlet excited oxygen. Since this paper was submitted, Wasserman and coworkers (1969) and Kearns and coworkers (1969) using e.s.r. techniques, and our group, using a germanium diode detector to monitor the emission from 02('A,), have demonstrated that this does, in fact, occur.) The transfer of electronic energy is thus seen to be of considerable generality. Furthermore, the transfer of energy a t 244
Environmental Science & Technolog)
"Tso
-
x31,
DONOR
02
Figure 2. Schematic representation of triplet energy transfer from a donor organic molecule to molecular oxygen
gas-solid interfaces may be of considerable significance in view of the presence of organic particulate matter in polluted urban air. Absorption of Sunlight by Organic Donor Molecules Let us now consider in more detail the energy transfer mechanism for singlet O2production in polluted atmospheres. The sequence of processes 8, 9, and 10 replaces direct absorption processes 5 and 6 suggested by Bayes. Actually the two mechanisms are not exclusive. We feel that singlet O 2 is produced by both routes, but that for several reasons, particularly the low efficiency of the direct absorption of 0 2 ,the energy transfer mechanism is the more important in urban air. The rate of formation of singlet O2 in our mechanism will depend on the rate of absorption by the donor molecules. These donors must absorb radiation a t wavelengths longer than 2900 A . because of the cut-off in solar radiation in the lower atmosphere. With this stipulation. the donors may be aromatic hydrocarbons directly emitted to the atmosphere, carbonyl compounds such as aldehydes, which may be either primary contaminants or secondary contaminants produced by photochemical reactions of olefins in smoggy atmospheres, or a variety of other absorbing species. Energy transfer from the donor 3( K , T * ) states of polynuclear aromatic hydrocarbons to the acceptor, ground state oxygen, 3?;v-, to produce singlet oxygen is known to be efficient (Foote, 1968: Kawaoka, Khan, ef al.. 1967) and similar considerations seem to apply to transfer of " n , K*) energy from carbonyl compounds. Recent experimental work by Gollnick (1 967), Trozzolo and Fahrenholtz (1968), and Kummler (1968) supports this view. The absorbing donor molecule may be in the solid, liquid, or gaseous state, or adsorbed on a solid. Since it is virtually impossible to estimate accurately the rate of absorption (vs. scattering) of sunlight by particulate matter in smog, we shall not consider further the contributions to the production of singlet oxygen by energy transfer from solids (or aerosol droplets). It is even difficult to estimate the rate of absorption of solar radiation by gaseous pollutants (Nader, 1967), and the following approximate calculations are intended merely to show that the rate of singlet oxygen production may be significant. In considering absorption by potential donor molecules, we note that the absorption spectrum of a smoggy atmosphere (taken after smog formation had reached a steady state) has two distinct absorption bands (Leighton, 1961; Stair, 1955):
a n absorption band extending from the solar cut-off up t o 3200 A. and a band extending from 3400 to 4000 A. For the present purpose of estimating the concentration of singlet 0 2 in the smoggy atmosphere, we confine our attention to the absorption band around 3200 A. This band is a true molecular absorption band and not a n artifact solely due to molecular and particulate scattering (Leighton, 1961; Stair, 1955): The absorption originates essentially from ozone (Hartley and Huggins bands) and from organic molecules. U p to 80% of the solar radiation, about 3200 A., may be attenuated on a particularly smoggy day (Stair, 1955). For the present purpose, the maximum concentration of singlet 0 2 in a polluted atmsophere may be estimated by assuming unit efficiency of the energy transfer process. The total concentration of hydrocarbons in a polluted atmosphere is of the order of 0.5 p.p.m. (Schuck, Pitts, e t al., 1966). Of these hydrocarbons, 0.5 are polynuclear aromatics (Begeman, 1964). If these polynuclear aromatic hydrocarbons are assumed t o be homogeneously distributed throughout a column u p to the inversion height of 1 km. (Altshuller and Bufalini, 1965), the rate of excitation may be calculated. For the hydrocarbons under consideration, the molar decadic extinction coefficient is of the order of lo4 in the spectral region considered, so that roughly one quarter of the photon flux is absorbed. A similar result may also be obtained from consideration of the extinction coefficients and concentrations of carbonyl compounds in polluted atmospheres. For a photon flux of 101j photons an-? sec.-', the production rate of excited organic molecules in the 1-km. column is approximole liter-'sec.-' This rate thus corresponds mately 4 X t o a maximum rate of singlet oxygen formation by the energy transfer process. In reality the production rate may be smaller, since alternative de-excitation paths may exist for the organic species. (This is especially true, of course, in the case of the carbonyl compounds which may decompose.) A comparable calculation of the absorption by NOz of solar ultraviolet radiation in the wavelength region 2900 t o 3850 A . (Leighton, 1961) suggests that the absorption rate by NOz and by organic compounds will be identical when the nitrogen dioxide concentration is about 10 p.p.h.m. The calculations therefore give the important and rather unexpected result that the absorption by polynuclear aromatic hydrocarbon and carbonyl compounds in typical polluted atmospheres may be significant relative to absorption by NOz in the photochemical reactive region. Ozone and Singlet Oxygen
Ultraviolet photolysis of ozone is thought to give rise to a singlet molecular oxygen product (DeMore and Raper, 1966; Norrish and Wayne, 1965). Wayne (1967) has reviewed the implications of several production processes for singlet O z in the upper atmosphere and concludes that a mechanism involving O3 photolysis is the most probable. Kummler, Bortner, et al. (1968) have suggested that 0 3 photolysis could also be a source of singlet molecular oxygen in polluted atmospheres, a n d that, in particular, 0z('Ag) produced in the photolysis of O3 can play a significant part in atmospheric oxidation processes. The exact mechanism of singlet oxygen formation on O3 photolysis appears not t o be so simple as initially believed. Izod and Wayne (1968a) have demonstrated that 02(lA,) is produced o n ultraviolet photolysis of ozone but that molecular oxygen must be present for the appearance of the singlet
species. This condition is, of course, met in the atmosphere and it is suggested that the most probable course of events is 0 3
+ hv
-
O('D)
+ OZ(state not determined)
(11)
The analogous excitation reactioli for Oz('2,+) O(Q)
-
+ oz
+
oz(12,+) o(3p)
(1 3)
has been demonstrated by Young and Black (1967), who suggest that the process is highly efficient. Izod and Wayne (1968b), on the other hand, believe that only one Oz('Z,+) molecule is formed for about 500 O('D) atoms deactivated, and that Reaction 12, forming O,('A,) could be considerably more efficient. Some recent experiments (Wayne, 1968), have shown that 02('A,) is formed in Reaction 12when vacuum ultraviolet photolysis of 0 2 is the source of O('D). [The nature of the state formed is of some importance in this context, since state in laboratory studies it is the la,, rather than the I&+, which reacts with olefins (Arnold, Kubo, et al., 1968; Khan and Kearns, 19671.1 The occurrence of the energy transfer process 12 o r 13 leads to two further considerations. First, the excitation of singlet oxygen results in the formation of two oxidizing species, singlet molecular oxygen and ground state atomic oxygen : the excess photochemical energy carried by O('D) has been made available for photooxidation. Secondly, this doubling numbers of oxidants makes it important to look for other atmospheric sources of O(lD). [We have to exclude the photolysis of NO2 itself, since the formation of O(lD) in the reaction
NO2
+ h~
.-c
NO
+ O('D)
(14)
requires more energy than is available in the solar radiation reaching polluted atmospheres.] Singlet Oxygen as Product of Exothermic Chemical Reactions
We have to examine now the possibility that singlet molecular oxygen can be the product of a n exothermic chemical reaction in which photochemical excitation is not the direct source of energy (although it may well be the indirect source). So far as polluted atmospheres are concerned, the most obvious reaction (first pointed out to these authors by E. A. Schuck) is that between nitric oxide and ozone
NO+03+NOz+02
(1 5 )
The reaction is highly exothermic and leads to electronic excitation of NO2 (Clyne, Thrush, et al., 1964). There has been some feeling that where the exothermicity of a chemical reaction is not equilibrated about the various degrees of freedom, the excess energy will appear initially in the newly formed bond. However, it may well be that Oz(lAp) is a product of the reaction as well as electronically excited NOz since this would lead to conservation of spins [or, alternatively, that in the presence of molecular oxygen an energy transfer process NO2t 0 2 +. NO? ' 0 2 (16)
+
+
(dagger above a formula here represents electronic excitation) can occur]. Ozone might also react in a similar manner with a variety of other atmospheric contaminants (such as sulfides) to yield Volume 3, Number 3, March 1969
245
singlet molecular oxygen as a product. Processes of this type are known to lead to singlet O 2 formation in condensed phases (Murray and Kaplan, 1968). Whether or not these excitation mechanisms make a significant contribution in polluted atmospheres will depend on both the rate constants for the reactions and on the concentrations of the several contaminants. Data are not yet available on these matters, but such possible excitation mechanisms should not be overlooked. Murray and Kaplan have now demonstrated that this type of process does occur in the gas phase and have also suggested its possible role in air pollution (Murray and Kaplan, 1968). Ground state atomic oxygen may participate in several plausible mechanisms for the formation of singlet oxygen
+ - io2+ o2
+ o(3~) +M
o(3~)
o(3p)
+
0 3
+ NO2
O(")
-+
lo2
M
+ NO
' 0 2
(17) (1 8)
(19)
Of these, direct evidence has been found for the production of 02('Z,') in Reaction 17 and circumstantial evidence for its formation in Reaction 18 (Young and Black, 1965, 1966), although these processes probably proceed too slowly, and the concentrations of oxygen atom: are too low, to lead to significant concentrations of 02(lZ,+) in polluted atmospheres. Clyne, Thrush, e t ul. (1965) found no evidence for the formation of 02('Z,+) in Reaction 18 or 19, although Arnold, Browne, e t ul. (1965) suggest that some 02('1,)may be formed. Perhaps the most telling piece of evidence against singlet oxygen as a major product of the ozone Reaction 18 is that the quantum yield for O 3photolysis by red light (where 3P atoms alone can be formed in the primary step) does not differ significantly from two, which it would if 0 2 were produced in Reaction 18(Castellano and Schumacher, 1962). Purticipution of Singlet Oxygen in Photoconversion of NO to NO:!
A number of possible sources of singlet molecular oxygen in polluted atmospheres may operate singly or together to yield relatively high atmospheric concentrations of singlet oxygen. This singlet oxygen can lead to rapid oxidation of nitric oxide to nitrogen dioxide. The reaction
NO
+
' 0 2
+
NO2
+0
(20)
must be rejected, since it requires more energy than can be provided even by O2('Z8+) in its ground vibrational state [vibrationally "hot" species, even if formed, could not survive sufficient nonreactive collisions to make Reaction 20 of importance with the prevailing (NO);(O2) (N?) ratio]. However, the oxidation may proceed in the presence of olefinic substances via a thermally unstable hydroperoxide intermediate:
Reactions 23 and 24 are steps in the currently accepted mechanism for the oxidation of NO to NO2 (Altshuller and Bufalini, 1965; Schuck and Stephens, 1968). Reactions such as 21 have been shown to occur with several olefins for 02('A,) and may possibly occur with 02(:2,+). The essential idea in this scheme is that singlet oxygen formed by the energy transfer from the donor organic molecules, D ,will react with olefinic hydrocarbons to give hydroperoxides. These hydroperoxides are thermally unstable and sensitive to surface catalysis. They decompose to give free radicals of the same type as those formed in the direct attack of atomic oxygen on olefins. The most important advantages of this scheme are that energy transfer from the triplet state of hydrocarbons can lead to appreciable concentrations of singlet molecular oxygen, and it is no longer necessary to invoke the formation of vibrationally excited oxygen to account for the required production rate; and the electronically excited oxygen molecules may produce hydroperoxides which in turn generate the radicals needed in the oxidation of NO to NO2. The presence of Reaction 15 in this scheme as the major loss process for ozone suggests that, while nitric oxide is still present, only a small stationary concentration of ozone can exist. The exact value of this concentration depends on the rate of generation of atomic oxygen and the production of ozone in Reaction 2 compared with the rate of Reaction 15. Once most of the nitric oxide has been consumed, the ozone concentration can increase, and this prediction is borne out by experimental evidence, such as that summarized in Figure 3.
Conclusions Singlet molecular oxygen may not only be an important species in the smog-forming reactions discussed, but may also play a significant role in the pathology of man, animals, and plants; in this connection, Foote (1968) has mentioned the implications of the production of skin cancer by photosensitizing dyes and polycyclic hydrocarbons. Furthermore, singlet molecular oxygen may well be involved in the degradation in polluted atmospheres of susceptible natural and synthetic substances with consequent economic loss. It is, therefore, apparent that a considerable extension of the preliminary experiments and speculations about singlet oxygen in the context of atmospheric pollution would be amply justified, in terms both of the financial losses and of the health hazards that might be ameliorated in urban communities.
+
' 0 2
+ &C+-
--+-c=c1 H
thermal
-C
