Reactions of O2 (1Δg) and O2 (1Σ+g)

11 Pages·1964·0.718 MB·English

by  L. W. BADER, E. A. OGRYZLO

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M. Reactions of 0 2 ( I&) and O2 (lCg+)
A
0
5
57: BY L. W. BADERA ND E. A. OGRYZLO
2:
22 Dept. of Chemistry, University of British Columbia, Vancouver 8, Canada
0
2
25/ Received 14th January, 1964
1/
1
on A method of obtaining measurable concentrations of 02(1Ag) and Oz(lXC,+) for kinetic studies
er has been developed, and a number of reactions of these excited molecules are described. Evidence
est is presented for simultaneous electronic transitions in a weakly bound complex between two 02(1Ag)
h
oc molecules resulting in emission bands at 6340 and 7030A. The phenomenon is discussed in terms
R of the association theory of gases.
of " "
y
ersit The (0,l) 1- 58 u, band of the (1 A, - 3Z;) system is one of the more intense features
niv of the atmospheric " day glow " (the (0,O) 1.27 p band is re-absorbed by the lower
U
y atmosphere).l A number of excitation mechanisms have been suggested by Vallance
d b Jones ;2 however, all of them depend on assumptions about the deactivation cross-
e
d sections for O@A,) which have not yet been determined. A prominent feature
a
wnlo of the " night air glow " is the (0,l) 8645 A band of the (1Z$-3E;) system (the
o (0,O) 7619 band is also re-absorbed). Here again deactivation cross-sections
D
4. have not been determined and Young and Sharpless 3 have indicated that no satis-
96 factory excitation mechanism has yet been proposed.
1
y Both of these species are known to be present in electrically discharged oxygen
ar
u together with oxygen atoms, where the ratio of O@Z;) : 0 : 02(1Ag) : 02(1Zi) is
n
a
1 J usually about 1 : 0.1 :0 .05 : 0.0015.4 When these products are pumped out of the
n 0 discharge into a fast flow system the atom concentration is essentially unaltered
d o since both the gas phase and wall recombination are sl0w.5 The 02(1Zl) concen-
e
h tration is still large; however, since it is continually being formed by the recom-
s
bli bination of atoms, its reactions cannot be studied conveniently in this system. There
u
P is, in the literature, only indirect evidence for the presence of 02(1Ag) in the dis-
charge products. Foner and Hudson6 showed the presence of a species with an
appearance potential about 1 eV below that of ground-state oxygen. They sug-
gested that it might be 02(1Ag), in which case about 10 % of the oxygen could be in
this state. Elias, Bgryzlo and Schiff 5 presented calorimetric evidence that was
consistent with about 10 % 02(1Ag) in the discharge products. They also described
the following three methods that could be used to remove atoms preferentially from
the gas stream. (i) NO2 can be added to the stream. Atoms are rapidly removed
and some calorimetrically-detectable excited molecules remain. However, the
presence of NO and NO2 then complicates any kinetic studies. (ii) A coil of silver
wire placed in the gas stream rapidly recombines the atoms and more slowly de-
activates the excited molecules. (iii) The distillation of mercury through the dis-
charge creates a mercuric oxide deposit after the discharge which removes atoms
but not excited molecules. Using these sources of excited molecules we have begun
an investigation of some of their reactions.
EXPERIMENTAL
A typical Pyrex fast-flow system5 was equipped with Edwards needle valves and a
581./min vacuum pump. The discharge was maintained in air-cooled 13 mrn Pyrex tubing
by a Raytheon 2450 rncfsec, 100 W generator. Two light-traps separated it from the
46
View Article Online
L. W. BADER AND E. A. OGRYZLO 47
reaction-tube. The 20mm (int. diam.) reaction-tube was jacketed and insulated so that
the temperature could be varied between 173 and 373°K. Two different spectrographs
could be placed against an optical window at the end of the reaction tube. A low dis-
persion (5 4.6) Hilger-Watts glass prism monochromator was equipped with a 27 c/sec
M. mechanical chopper, an RCA-7265 (or 7 102) liquid-nitrogen-cooled photomultiplier and
0 A a conventional tuned a.c. amplifier. With this instrument very weak emissions could be
5 detected between 3000 and 13,OOOA. A Jarell-Ash (5 6.3) grating spectrograph blazed
7:
2:5 at 7500 A with a dispersion of 20 A/mm in first order was used when greater resolution was
2 required.
2
20 An isothermal calorimetric detector 5 could be moved along the reaction tube. Its cobalt
5/ surface removes all detectable excited molecules from the gas stream. By operating the
2
1/ coil at a constant temperature the total energy content of the gas stream could be
1 " "
n determined.
o
er For most work, ordinary tank oxygen was used without further purification. When
est necessary, it was dried by passing it over P205 and through a liquid-nitrogen trap. Nitrogen-
h
oc free oxygen was prepared by thermally decomposing KMnO4. Other gases were obtained
R
of from the Matheson Company. Ozone was prepared with a commercial generator and
y stored on silica gel at dry-ice temperature.
sit Preliminary studies indicated that under conditions where atoms were completely
er
v removed from the gas stream by a silver wire, the excited molecule concentration was also
ni
U greatly reduced. This was not found to be the case when mercury was used to destroy
by the atoms, and hence the later method was used in all studies reported in this paper.
d
e
d
oa RESULTS
nl
w
o Curve (a), fig. 1, shows a typical example of the energy liberated to a cobalt
D
4. detector by the products of electrically discharged oxygen from which all the atoms
96 have been removed. The presence of the slightest trace of atoms is easily detectable
1
y
ar
u
n
a
J
1
0
n
o
d
e
h
s
bli
u
P
20 30 40
distance from discharge (cm)
FIG. 1 .-Curve (a),h eat measured by cobalt detector in excited molecule stream ; P = 3 mm Hg ;
T = 300°K. Curve (b), heat measured when a partial pressure of 0.03 mm Hg of water is added
after the discharge.
by the yellow-green NO2 emission to which the eye is very sensitive. When all
the atoms have been removed, a weak red emission becomes visible. Fig. 2 shows
the emission detectable with the Hilger-Watts spectrograph and a cooled 7102
photomultiplier. The most prominent peak is at 7619A. This is undoubtedly
the (0,O)b and of the (lX;-3Z;) intercombination. The band at 8645A is then
the (0,l) transition in the same system. Photographs of these bands with the higher
View Article Online
48 REACTIONS OF 02(1Ag) AND 02(2S;)
dispersion grating spectrograph show the characteristic rotational fine structure of
the (lZl-3S;) transition, confirming the assignment. We have been unable to
detect either the (1,l) band at 7708 A or the (1,O) band at 6882 A, though the
transition probabilities are comparable to those for the (0,O) and (0,l) transitions
M. respectively. This is taken as evidence for the absence of appreciable concentrations
A
0 of vibrationally excited oxygen.
5
7:
5
2:
2
2
0
2
5/
2
1/
1
n
o
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c
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of
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er
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ni 0 0
U 0
by I I
d / /
e
d /
a 0
o
nl
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o
D
4.
6
9
1
y
ar
u
n
a
J
1
0
n
o
d
e
h
blis 6340 7030 7619 8645 12700
Pu A <A>
FIG. 2.-Emission spectrum of electrically discharged oxygen with all atoms removed obtained
with Hilger-Watts glass-prism spectrometer, slit 500 microns, and liquid nitrogen cooled RCA-
7102 photomultiplier. Spectral sensitivity of RCA-7102 given by dotted curve.
The peak at 12,700A is undoubtedly the (0,O) band of the (1Ag-3Z;) inter-
combination, confirming previous mass spectrometric 6 and calorimetric 5 evidence.
Since the sensitivity of the photomultiplier at 12,700 A is less than 10-2 of its maximum
value at 8000A, the emission is fairly intense.
The peaks at 6340 and 7030A cannot be attributed to any known transition
in 02. They have been observed in a number of chemiluminescent reactions in
solution ; 79 8 however, Khan and Kasha's 9 recent assignment of these peaks to
solvent shifted (0,O) and (0,l) (1Zl-3Zi) transitions must be in error. Fig. 3
shows the 6340 A band photographed with a grating spectrograph (20 &mm dis-
persion). Neon calibration lines are superimposed on it. Only a structureless
diffuse band could be obtained. To eliminate the possibility that it is an impurity
"
emission ", nitrogen-free oxygen was prepared and thoroughly dried with P205.
The intensities of the peaks were essentially unaltered. Furthermore the addition
of nitrogen and water before the discharge, and NO?, NO, H20, N20, Ar, He,
View Article Online
L. W. BADER AND E. A. OGRYZLO 49
CO, C02, NH3 and H2 after the discharge had no significant effect on either the
6340 or the 7030A peak. The 7619A peak was, however, affected by a number
of gases. Water had the most marked effect (heavy water was about half as effec-
M. tive). A small amount of water added after the discharge completely removed the
7619 and 8645A bands and left the 6340, 7030 and 12,700A bands unaltered.
A
50 This observation shows that the two new bands do not involve 02(1X$).
7: The lack of reactivity of the species responsible for this emission, the presence
5
2 2: of only two bands, and the large spacing between the bands allows us to eliminate
02 other eexxcciitteedd ssttaatteess ooff 00 22 ss uucchh aass CC33AA,,,, AA33EE;; aanndd ccllZZ;;..
2
5/ r
1/2 r --
1
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er
st
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c
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of
y
sit .r.
er
niv ."
U
y
b
d
e
d
a
o
nl
w
o
D
4. 6217 6266 6334 6302 6402
96 1 (A)
1
ary FIG. 3.-The 6340A band obtained with a Jarell-Ash f 6-3 grating spectrograph, 20&m dis-
u persion, 30 micron slit, 72 h exposure on Kodak 103 a F spectroscopic plate. Small peaks are
n
Ja superimposed neon calibration lines.
1
0
on A study of the relationship between the 02(1Ag) concentration and the new bands
ed provided more positive evidence for their source. A direct comparison of the
h
blis 6340 peak and the 12,700 A peak was not possible because of the extremely variable
Pu sensitivity of the cooled photomultiplier in the 12,700 A region. However, the
detector could be used to measure the 02(1Ag) calorimetrically. Curve (b), fig. 1,
illustrates the effect of water (added after the discharge) on the energy content of
the gas stream. Spectroscopically the removal of 02(lXg) can be seen by the reduc-
tion of the 7619A emission intensity to less than 1 % of its original value. The
02(1Ag) concentration remains unaltered. In all of our experiments we have found
that the 6340 A emission intensity is proportional to the square of the heat measured
by the detector. A typical set of results is given in table 1, where it is assumed
that the heat measured by the detector is due entirely to 02(1Ag).
TABLE1
P = 4-4 ~llfnH g ; 0 2f low = 160 pmoles/sec ;
T = 25°C ; H20 flow = 0.5 ,u moles/sec.
experiment 1
microwave power output (watts) 100 20
02(1 AJ flow (pmoles/sec) 2.5 1.7
C = 02(1Ag) concentration (moles/cm3) 3 . 71~0- 9 2 . 61~0- 9
I = 6340 A emission intensity (arbitrary units) 55.0 27-5
11/12 = 2-00; Ci/C2 = 142 ; (Ci/C2)2 = 2.03
View Article Online
50 REACTIONS OF 02(lAg) AND 02(1E:)
In view of these observations, and the fact that twice the electronic energy of
02(1Ag) is equal to 15765 cm-1 or 6343 A, we propose the following processes to
account for the two emission bands.
AM. 2O2('A,M)~ O~+(O4),=+, hv(6340 A),
0
5 M
57: 20,(1Ag)+O:-(04),=1 +hv(7030 A).
2:
22 To determine whether the emitting species (0:)i s simply a colliding pair of
0
5/2 molecules or a stabilized dimer, the temperature of the reaction tube was varied.
1/2 Fig. 4 shows the emission spectrum recorded with an RCA-7265 photomultiplier.
1
n
o
er
st
e
h
c
o
R
of
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sit
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v
ni
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y
b
d
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a
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D
4.
6
9
1
y
ar
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a
J
1
0
n
o
d
e
h
s
bli
u
P
1
6340 7030 76 19
1 (A)
FIG. 4.-Effect of temperature on the excited molecule emission. RCA-7265 photomultiplier,
500 micron slit. Curve (a) obtained at 25°C ; curve (b) at -29°C.
Because of its different spectral response it yields 7619 and 6340A peaks of com-
parable height. Curve (a) was obtained at 25°C and curve (b) at -29°C. In the
flow system the pressure remains constant when the temperature is changed in a
small region of the system. Assuming ideal behaviour (P = cRT), the concentra-
tion of all independent species will be inversely proportional to the temperature.
Hence the concentration of 02(1CB+) would be expected to increase by a factor of
2981244 = 1.22 when the temperature is changed between 25 and -29°C. The ratio
of the 761 9 A emission intensities at these temperatures (obtained by integrating under
the curves in fig. 4) is 1-21 which is the expected result within experimental error.
The ratio of the emission intensities for the 6340A band is, however, 1.97. Since the
View Article Online
L. W. BADER AND E. A. OGRYZLO 51
emission is proportional to the square of the 02(1Ag) concentration, a factor of
(1 *22)2= 1-49i s due to the concentration change alone. The additional decrease we
attribute to the dissociation energy of 0:. If we assume that the process consists of
M. the following steps :
A K
0 02('Ag)+ 02('Ag)+ M+O,('Ag, 'A,)+M, (3)
5
2:57: 04('Ag, '~i~k )-+o~("~XXi);+ hv(6340 A), (4)
2
2
0 where M is any third body, k is the emission probability defined by I = d(hv)/dt =
2
5/ k[04(1Ag, lAg)], I is the 6340 A band emission intensity, K is the equilibrium con-
2
11/ stant for the formation of dimolecular complexes (abbreviated to dimols in the rest
on of this paper), i.e.
er
est K = C04(lAg, 1Ag)llCo2(1Ag)129
h
oc and the dimol concentration is governed by K (i,e., that reaction (4) is the rate-
R
of controlling step) ; then since
y
sit 04(1Ag,lAg) = I/k and 02(1Ag) = P/RT
er
niv (where P = partial pressure of 02(lAg)),
U .'.
y K = RZIT2/kP2.
b
d
e Hence, on substitution into the thermodynamic relationship,
d
a
o
nl In (K1IK2) = AU(T~-TI)/RTIT~,
w
Do In (IlT:/I2T,2) = AU(T~-TI)/RT~T~,
4.
6 where AU is the change in internal energy and equals the bond dissociation energy
9
1
y D at some average temperature T. Substituting into this equation the values for
uar Z and T given above, one obtains - AU = D(02(lAff)- 02(1Aff)) = 600 cal.
n
a
J
1
0 REACTIONS WITH OZONE
n
o
d The effect of the addition of 0 3t o the excited molecule stream is shown in fig. 5.
e
ublish yVeilsluoawll-yg rteheen eNffOec;t! iesm dirsasmioant icw hsiinchce rtehseu lwtse afrko mre dt heem irsesaicotnio ins :m a0sk +e Nd Oby- ,NthOe 2s t+roh nvg.
P
It can be seen that the 02(1Zgf) concentration is lowered by more than a factor of
10, suggesting that the dissociation is due to this species, i.e.
02(12,')+ 03+202(3~,)+0 . (5)
On the other hand, the 6340 and 7030 A bands increase when 0 3i s added. If this
means that the 02(1Ag) concentration increases, it may be produced in the reaction,
o(~P)0+3( 1A)+02(1Ag)+ 0,(~2;). (6)
This explanation is not entirely satisfactory in view of the effect of the addition of
water shown in fig. 6, where 99 % of the Oz(lX,+) has been removed by water.
Ozone is, however, still strongly dissociated. No change in the 02(1Zi) concen-
tration occurs and surprisingly the 02(1 Ag) concentration (taken from the 7030
peak height) seems. to have risen. Clearly, eqn. (5) and (6) are not sufficient. It
is possible, however, that these results can be explained by some energy chain that
is only initiated by excited molecules.
NO2 - GLO W
When NO2 is added to the gas stream containing 02(1E:) and 02(1Ag), the
concentrations of the excited molecules are not greatly altered even when large
View Article Online
52 REACTIONS OF 02(1Ag) AND 02(1Ei)
M.
A
0
5
7:
5
2:
2
2
0
2
5/
2
1/
1
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b
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a
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nl
w
o
D
4.
6
y 19 63I 40 70I 30 71I1 9 86I 45 12I7 0
uar A (A)
n
a
1 J FIG. 5.-Effect of ozone on emission. Curve (+a) , excited molecules only; curve (b), excited
0 molecules ozone.
n
o
d
e
h
s
bli
u
P
I
63 40 7030 7619 8545 12700
(4
A
FIG. 6.-Effect of water on ozone reaction. Curve (a), excited molecules+water; curve (b),
excited molecules, water and ozone.
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L. W. BADER AND E. A. OGRYZLO 53
amounts are added. However, as the NO2 concentration is increased the normal
red emission is masked by a higher energy emission that has a spectral distribution
very similar to that due to the 0 plus NO reaction shown in fig. 5. However, there
can be no atoms present in this system because of the large NO2 concentration.
M.
When H20 is added to the stream the NO2 glow decreases in intensity. It disappears
A
0 when sufficient water is added to remove completely 02(1Z3.
5
7:
5
2 2: DISCUSSION
2
0
2 We believe that the 6340 and 7030A bands described in this paper can be ex-
5/
1/2 plained only by the presence of dimolecular complexes (dimols) of 02(1Ag), with
n 1 a bond dissociation energy of about 600cal. The two bands then arise from
er o simultaneous electronic transitions in the loosely associated pair. The 6340 A
est emission arises when only ground vibrational states of 0 2 are involved, while the
h
oc 7030A band arises when one of the 0 2 m olecules ends up in its first excited vibra-
of R tional level. Though such simultaneous electronic transitions do not seem to have
y been reported previously in emission, they have been proposed to account for a
ersit number of absorption bands in high pressure systems.
niv Rank et aZ.10 reported the appearance of some new absorption bands in pure
U HCl and HC1 mixtures with Ar and Xe that appear at high pressures. The intensity
y
d b of the bands was found to be proportional to the square of the pressure. From
de the temperature variation of the absorption they calculated the following bond
a
nlo dissociation energies :
w
Do D(HC1-HC1) = 2.14 kcal, D(HC1-Ar) = 1.1 kcal, D(HC1-Xe) = 1.6 kcal.
964. Hoijtink et aZ.11 reported a study of the absorption spectrum of mixtures of
y 1 naphthalene and 0 2 in chloroform. They observed a new band at 29,000cm-1
uar with an intensity proportional to the concentration of naphthalene and 02. They
n
a attribute the band to a simultaneous transition in a naphthalene-02 complex.
J
01 The effect of a change in temperature was not reported so that it is not possible to
on say whether dimols are responsible.
d
e More relevant to our work is the absorption spectrum of liquid and gaseous
h
s
bli oxygen. Ellis and Kneser 12 were the first to attribute peaks observed in liquid
Pu oxygen at 6290 and 5770 to the process :
where the 6290A peak arises when all the species are in their ground vibrational
state, and the 5770A peak arises when one of the 02(1Ag) molecules ends up in
its first excited vibrational level. Salow and Steiner 13 studied the pressure depend-
ence of these bands in highly compressed gases, and found that they depended
on the square of the oxygen pressure and were independent of the pressure of added
gases, confirming Ellis and Kneser's assignment, More recently Dianov-Klokov 14
studied the temperature dependence of the band intensities. He found that they
changed little with temperature, possibly increasing slightly between 77 and 298°K.
Dianov-Klokov therefore concluded that the absorption was due to an 0 4 c ollision
complex.
This result might appear inconsistent with our emission studies. However,
the different temperature dependence may be due to the fact that the absorption
intensities were measured at a constant density of 1.17 g/cm3, which is the density
of the liquid. Under these conditions the average intermolecular distances are
fixed at the values that would be assumed in our proposed dimol. Hence, changing
the temperature could well have no appreciable effect on the absorption intensity.
View Article Online
54 REACTIONS OF 02(1Aq) AND 02(lC;)
For any given gaseous system there are two distinct sources for both simultaneous
and induced transitions in molecules : (i) collision complexes, or (ii) dimols (stabil-
ized dimolecular complexes). With the information available at present it is not
possible to state when the shorter intermolecular distance in the collision complex
M. is more important to the transition probability than the longer lifetime of the dimol.
A
0 However, it is certain that a number of simultaneous and induced transitions can
5
7: be explained only by dimols, and there appears to be no evidence in the literature
5
2: inconsistent with the idea that dimols are universally responsible for such transitions
2
02 at low temperatures.
2
5/ In a system containing 02(3X;), 02(1 A& and 02(1XB+) we might expect the follow-
2
1/ ing dimols to be present :
1
n
o 3q);
er (1) 04(3x;, (2) 04(?4g, lAg);
hest (3) 04(1q, lq); (4) 0d3q¶‘$1 ;
c
o
of R (5) 04(3z;, lg); (6) 04(1q, lAg).
y At any given temperature the dimol equilibrium concentration will depend on its
sit
er bond dissociation energy and the concentration of the appropriate molecules. The
v
ni dissociation energy of dimole (1) can be estimated from PVT data. With some
U
y assumptions about the form of the interaction potential, the interaction energy E
b
d can be calculated from the second virial coefficient. Assuming a Leonard Jones
e
ad (6 : 12) potential one obtains 15 ~(02-02)= 230 cal. Unfortunately, AU and
o
nl thus the dissociation energy at some temperature T, is not easily calculated from
w
Do E. However, Stogryn and Hershfelder 15 have calculated Kp (the equilibrium
4. constant for dimol formation) from the second virial coefficient. Curve (a) in fig. 7
6
9 shows a plot of loglo Kp against 1/T from their data. From the slope of the curve
1
ary at 273”K, - AH = 840 cal, and therefore - AU(273”K) = D(O2-02) = 300 cal.
nu These results, however, still depend on the assumed form for the interaction potential.
a
1 J A more direct estimate of AH can be obtained from “association theory”.16
0
n Assuming that the deviations from ideal gas behaviour at moderate pressures is
o
d due to (a) the size of the molecules, and (b) the formation of dimols, trimols, etc.,
e
sh we can write the equation of state (for one mole of gas) :
bli .
Pu P(V-b)= RT(1-pKD-p2KT-. .),
where b = excluded volume = 4 (molecular volume) ; KD and KT are the equilbrium
constants for the formation of dimols and trimols respectively.
If we consider conditions where only the dimol term is significant we obtain upon
rearranging :
PV/RT = 1 - P(K, - b/RT).
A plot of PV/RT against P for oxygen at several temperatures is shown in fig. 8.
The slope is equal to -(Kp-b/RT). Assuming a value of b = 0.0318 the values
of Kp obtained are plotted in fig. 7, curve (b). From the slope of the curve at
273”K, -AH = 940, and - AU = D(O2-02) = 400 cal. It is not possible to
decide between this value and that obtained from Stogryn and Hershfelder’s data.
Dimol (2) with a dissociation energy somewhat greater than dimol (1) gives rise
to the 6340 and 7030A bands. Dimols (3) and (6) could give rise to bands at 3810
and 4760A respectively. We have not been able to detect any emission at these
wavelengths. This may be due to the lower concentration of O#C:), or to a smaller
dissociation energy for the dimols. It is possible, however, that one of them is
responsible for the N02-glow. From the change in the 7619A emission band
intensity with temperature it appears that emission from dimol (5) does not add
View Article Online
L. W. BADER AND E. A. OGRYZLO 55
to the emission in this band. This is consistent with the fact that the molar ex-
tinction coefficient for the highly compressed gas does not differ significantly from
that of the low pressure gas.139 14 Because the 12,700A band intensity could not
M.
A
0
5
7:
5
2:
2
2
0
2
5/
2
1/
1
n
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y
b
d
e 0
d
oa 1/Tx lo3 (OK-1)
nl
w FIG. 7.-A plot of the loglo of the equilibrium constant Kpf or dimol formation against l/Tfor oxygen.
o
D Curve (a), from the data of Stogryn and Hershfelder; curve (b), from associate theory.
4.
6
9
1
y
ar
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1
0
n
o
d
e
h
s
bli
u
P
P (atm)
FIG. 8.-Plot of PV/RT against P for oxygen from the data of Birdsall, Jenkens, Dipaslo, Beattie
and Apt, J. Chern. Physics, 1955, 23,441.
be accurately measured, it was not possible to determine whether dimol (6) con-
tributed to this band. From the magnitude of the pressure-induced absorption 14
one might expect it to be significant at about 1 atm pressure. However, the rota-
tional fine structure observed under these conditions by Herzberg and Henberg16
suggests that it is a simple molecular absorption.