3 Available Reference Data for Iodine Oxides
3.2 C HEMICAL K INETICS R EFERENCE D ATA
In the same way the published chemical kinetic rate coefficients for the main reactions involved in the formation and consumption of IO and OIO are far from being clearly understood. Determination of rate coefficients and cross sections are often critically linked, as most experiments yield ratios of absolute cross section divided by the rate coefficients of the relevant reactions. In addition to the aforementioned studies, which either focussed on spectroscopic aspects or covered spectroscopic aspects and chemical kinetics simultaneously, also a large number of studies on chemical kinetics of formation and consumption of iodine oxides exist [Turnipseed et al. 1995, Gilles et al. 1997, Turnipseed et al. 1997, Atkinson et al.
1999, Larin et al. 1999, Canosa-Mas et al. 1999,Vipond et al. 2002]. By analogy to the well
established case of chlorine for the self reaction of IO in principle five channels are possible, if different isomeric structures for I2O2 are disregarded:
IO + IO+ M → I2O2 + M (R3.1a)
IO + IO → I2 + O2 (R3.1b)
IO + IO → 2 I + O2 (R3.1c)
IO + IO → IOO + I (R3.1d)
IO + IO → OIO + I (R3.1e)
Tab. 3.1 lists rate coefficients published for the IO self reaction. Most publications report a rate coefficient for the overall reaction and a combination of either I-atom producing channels or non-I-atom producing channels. Results for branching ratios are only a few and the agreement is not satisfying. Channel (R3.1a) is frequently considered as a significant source for the I2O2 dimer [Sander 1986, Harwood et al. 1997, Bloss et al. 2001] even though up to now no unequivocally spectroscopic evidence for its existence has been published. For channel (R3.1b) Sander [1986] and Laszlo et al. [1995] placed an upper limit of 5% and Harwood et al. [1997] one of 30% of the overall (R3.1) rate. Unpublished work from our lab yielded an upper limit of 7%. Energy considerations showed that this channel is inhibited by an energy barrier [Plane 2003, private communication].
Channel (R3.1d) postulates the existence of IOO, which was never observed except in matrix isolation experiments [Maier and Bothur 1997]. Ab initio calculations and energy considerations showed IOO to be only very weakly bound if stable at all. The only channel of which existence up to now seems to be considerably well established is therefore channel (R3.1e) as a source to the observed OIO. Ab initio calculations by Misra and Marshall [1998]
predict OIO to be the major product with ∆fH°(OIO)=76.7kJ⋅mol-1. In time resolved measurements the formation of OIO always follows a pattern which is highly correlated in extent and time to the observed IO concentrations. But still simulations and modelling of observed optical density against time have not yet been able to reproduce the observations.
Especially the consumption of OIO, which occurs to be faster than that of IO is still an open question of concern.
Reference Reaction rate coefficient
[cm3⋅⋅⋅⋅mole-1⋅⋅⋅⋅s-1]
Cox and Coker 1983
IO + IO →M I2O2
IO + IO → I + IOO → 2I + O2
near gas kinetic collision rate Jenkin and Cox
1985 IO + IO → products (2.8±0.4)⋅10-11 p-depend., 200mbar (8.0±2.7)⋅10-12 p-independent
Sander 1986
IO + IO → IOOI* →M I2O2 (a)
→ I2 + O2 (b)
→ 2I + O2 (c) IO + IO → IOIO* → I + OIO (e)
kprod: 5.3+4.7-2.6⋅10-11
(kc+0.5ke)/kprod=0.45 at 28mbar kb < 5%
Stickel et al.
1988 IO + IO → products kprod: (6.6±2)⋅10-11 Jenkin et al.
1991 IO + IO →M I2O2 Laszlo et al.
1995 Huie et al.
1995
IO + IO →M OIO + I (e)
→ I2 + O2 (b)
→ 2I + O2 (c)
kprod: (8.0±1.8)⋅10-11
Harwood et al.
1997
IO + IO →M I2O2 (a)
→ I2 + O2 (b)
→ 2I + O2 (c)
→ OIO + I (e)
kprod: (9.9±1.5)⋅10-11
(ka+0.5ke)/kprod=0.78 at 1000mbar kb<30%
Atkinson et al.
1999 IO + IO → prod. kprod.: (10±3)⋅10-11 Ingham et al.
2000 IO + IO → prod. kprod: (9.3±1)⋅10-11 Vipond et al.
2002
IO + IO →M I2O2 (a)
→ I2 + O2 (b)
→ 2I + O2 (c)
→ OIO + I (e)
kprod : (9.3±1.9)⋅10-11 at 2 Torr ka≈0 at given pressure
kb≈0
kc>0.56±0.20 ke<0.44±0.20 Bloss et al.
2001
IO + IO →M I2O2 (a)
→ I2 + O2 (b)
→ 2I + O2 (c)
→ OIO + I (e)
kprod : (8.2±1)⋅10-11 0.42 < ka/kprod < 0.55
kb/kprod ≤ 0.05 0.07 < kc/kprod < 0.15 0.30 < ke/kprod < 0.46 Dillon et al.a
2004
IO + IO →M I2O2 (a)
→ I2 + O2 (b)
→ 2I + O2 (c)
→ OIO + I (e)
kprod: (9.8±1.2)⋅10-11
(kc+0.5ke)/kprod=0.4 at 26.7mbar
Table 3.1 For the self reaction of IO mostly overall rate coefficients have been published, where the more recent determinations differ significantly from the earlier ones. Results on detailed branching ratios are scarce. Different intermediates IOIO and IOOI have been considered. But even though a large branching ratio of approximately 50% is postulated for that channel, spectroscopic evidence for its existence is still missing. )a: private communication 2004.
In I+O3 photolysis experiments undertaken in the context of the present work a significant formation of vibrationally excited IO at ν"=1 and 2 was observed at optical densities of up to 40% of that of ground state IO(4←0) absorptions [Spietz and Gómez-Martín, unpublished work]. Harwood [1997] also observed absorptions by vibrationally excited IO in O+CF3I photolysis, but they reached only less than 10% of the (4←0) transition. The role of these excited species up to then remained unclear.
Further doubts about our present understanding of IO chemistry arise from the contradictory observations of pressure dependence in the temporal behaviour of IO concentration. While in systems without ozone no pressure dependence for the consumption of IO is observed [e.g.
Sander 1986, Laszlo 1995], other observations in systems with O3 give evidence for the existence of pressure dependent reactions of IO consumption [e.g. Cox and Coker 1983 compared to Clyne and Cruse 1970, Jenkin and Cox 1985] as well as possibly its formation [Spietz and Gómez-Martín, unpublished work]. The possible formation of the I2O2 dimer is a critical issue in this context. If this channel existed, it would by necessity be pressure dependent. Similarly, reaction of IO and OIO forming I2O3 could be a plausible sink for IO and OIO, especially because strong sinks are needed to explain the fast OIO consumption:
IO + OIO+ M → I2O3 + M (R3.2)
Both analogous reactions for bromine [Rowley et al.1996] and chlorine [NIST 1998] are established. But again such a reaction – if significant - would introduce a likewise significant pressure dependence into the consumption of IO. Which is not what has been observed in experiments without O3.
With respect to the formation of I2 an IO-catalysed mechanism was proposed by Harwood et al. [1997]:
I + IO+ M → I2O + M (R3.3)
I2O + I → I2 + IO Cl: [9.6⋅10-11cm3/(molec.⋅sec)] (R3.4)
net.: 2 I → I2 (M3.1)
Observations by Bloss et al. [2001] support this prediction. If (R3.4) would be similarly fast
reduced below a certain threshold depending on mixing ratios. Again this would produce a pressure dependence, which is not observed in mixtures without O3.
In summary our understanding of the mechanism of IO and OIO formation and consumption is far from satisfactory. Even though the role of vibrationally excited IO in its formation appears to be significant, the mechanism remains to be clarified and its impact quantified.
While the overall rate coefficient of its self reaction at least among the recent publications seems to converge near 9 to 10⋅10-11cm3⋅molec-1⋅s-1 the individual channels can not be called well established. This is severely limiting the possibilities and reliability of determining an absorption cross section for OIO. This even more as the mechanism of OIO consumption remains completely unclear. Whether reaction with itself or with IO, most studies indicate in any case large rate coefficients of the order 10-10cm3⋅molec-1⋅s-1. Under such circumstances a determination of cross sections, which has to rely on a knowledge of the chemical mechanism and rate coefficients would be highly dependent on the used kinetic reference data with all its uncertainties. A more independent determination is desirable.