Isotope Specific Kinetics of Hydroxyl Radical (OH) with Water (H₂O):  Testing Models of Reactivity and Atmospheric Fractionation

Abstract

Gas-phase hydrogen (H) abstractions from molecules by free radicals have been studied extensively. They form the simplest class of elementary reactions and also play a key role in atmospheric chemistry and so are the centerpiece of models of reactivity. Despite intense scrutiny, two fundamental mechanistic issues remain unresolved: (1) Do H abstractions proceed directly or indirectly? (2) Do thermodynamic or electronic interactions determine their reaction barrier? The thermoneutral identity reaction, OH + H2O → H2O + OH, provides an excellent opportunity to answer these questions. Several theoretically predicted H2O−HO complexes raise the possibility of an indirect mechanism, while no thermodynamic forcing influences the reaction barrier. To examine the various reactivity models, the isotopic scrambling reactions 18OH + H2 16O → H2 18O + 16OH and 16OD + H2 16O → H16OD + 16OH are studied in a high-pressure flow reactor. The measured rate constants are (2.3 ± 1.0) × 10-13 exp[−(2100 ± 250)/T] cm3 molecule-1 s-1 over the range 300−420 K ((2.2 ± 1.0) × 10-16 at 300 K) and (3 ± 1.0) × 10-16 cm3 molecule-1 s-1 at 300 K, respectively. The similarity between the room temperature rates indicates a small secondary isotope effect. While the strong temperature dependence reveals that the predicted complexes do not stabilize the isotope exchange transition state sufficiently to bring its energy below the reactants, the small preexponential factor indicates that the complexes pose entropic constraints. Therefore, the reaction mechanism appears to be indirect. This is clarified by tracing the evolution of reagent electronic interactions and geometrical transformations along the reaction path. Activation energies of isotope exchange reactions are used to constrain the thermoneutral intercept for themodynamically based reactivity models. These thermochemical models are shown to be unreliable. However, a correlation between theoretical (ab initio) and experimental reaction barriers does capture gross reactivity trends. These measurements also exclude kinetic fractionation by OH as an important contributor to the isotopic fractionation of water in the earth's atmosphere.

Signature of Arctic surface ozone depletion events in the isotope anomaly (Δ<sup>17</sup>O) of atmospheric nitrate

“…In this regard, the situation encountered here is similar to the OH isotopic budget of the stratosphere : indeed, the dryness and the low temperature prevailing in the stratosphere have led modelers to predict significant isotope anomalies in the stratosphere up to 40 ‰ according to Lyons (2001), above 30 ‰ for Zahn et al (2006). Due to the increase of temperature and the concomitant increase of the water vapor content and to the high temperature dependency of the isotopic equilibrium (Dubey et al, 1997), the x factor exhibits a marked decrease over the course of the measurement campaign, from values around 70 % at the beginning of the measurement campaign to values lower than 20 % at the end of the campaign. Therefore, if OH was produced with a significant isotope anomaly, this anomaly would be present at the beginning of the measurement campaign but would have largely disappeared toward the end of the campaign.…”

“…Therefore we cannot estimate the 17 O value of the produced OH. Dubey et al (1997) (Michalski et al, 2003). Given the fact that H 2 O represents an enormous non-anomalous oxygen reservoir in the atmosphere with respect to OH, such a reaction tends to erase the isotope anomaly of OH.…”

“…Assuming that all chemical sinks do not intrinsically induce mass-independent fractionation for OH, the isotope anomaly of OH at steady state is : To assess the degree of suppression of the isotope anomaly acquired during the formation of OH, we compute the value of the x factor during our measurements campaign, using temperature, humidity, the mixing ratio of CO and CH 4 measured at Alert (D. Worthy, personal communication, 2006) and temperature dependent kinetic rates (Atkinson et al, June 2006 version;Dubey et al, 1997). The absolute water vapor concentration was deduced from relative humidity and temperature measurements using Bolton (1980) , with P water in hPa and T in K).…”

Isotopic constraints on heterogeneous sulfate production in Beijing haze

“…SO 2 has 17 O = 0 ‰ due to the rapid isotopic exchange with abundant vapor water whose 17 O is near 0 ‰ (Holt et al, 1981). S(IV) oxidation by H 2 O 2 and O 3 leads to 17 O(SO 2− 4 ) = 0.7 and 6.5 ‰, respectively, on the basis of 17 O(H 2 O 2 ) = 1.4 ‰ (Savarino and Thiemens, 1999) and assuming 17 O(O 3 ) = 26 ‰ (Vicars and Savarino, 2014;Ishino et al, 2017 (Dubey et al, 1997;Luz and Barkan, 2005;Lee et al, 2002;Bao et al, 2000). Sulfate produced by NO 2 oxidation is suggested to occur either via a radical chain mechanism (Shen and Rochelle, 1998), via oxygen-atom transfer from OH − (Clifton et al, 1988), or from O 2 based on experimental results of He et al (2014) (Fu, 2014).…”

Ab initio Study on the Complex Forming Reaction of OH and H2O in the Gas Phase

“…Experimental studies with isotope labeling of H atoms have shown that the above H atom transfer is extremely (Dubey et al, 1997). The theoretical calculated rate constant for the same reaction has been reported as 1.67 × 10 -16 cm 3 molecule -1 s -1 at 300 K (Uchimaru et al, 2003).…”