Article pubs.acs.org/JPCA
Role of (NO)2 Dimer in Reactions of Fe+ with NO and NO2 Studied by ICP-SIFT Mass Spectrometry Voislav Blagojevic, Michael J. Y. Jarvis, Gregory K. Koyanagi, and Diethard K. Bohme* Department of Chemistry, Centre for Research in Mass Spectrometry and Centre for Research in Earth and Space Science, York University, Toronto, Ontario M3J 1P3, Canada ABSTRACT: In a recent publication by J. J. Melko et al. (J. Phys. Chem. A 2012, 116, 11500−11508) on the reactions of Fe+ cations with NO and NO2, these authors made a number of assertions regarding the work previously published in our laboratory. Melko et al. assert that our previously reported data was erroneously analyzed, resulting in our misreporting of the Fe+ + NO2 reaction branching ratio for NO+. Also, they proposed that this alleged misreporting made it likely for the second-order chemistry observed in our Fe+ + NO experiments to be a product of an impurity of NO2 in our NO reagent and, furthermore, that our reported rate coefficient for the effective second-order chemistry was unreasonably high on the basis of their model calculations. Despite extensive private communications in which we presented detailed data supporting our original data analysis to Melko et al., these authors proceeded to publish their critique without any reference to this data. Here, we present the data communicated by us to Melko et al. and show that our result reported earlier for the Fe+ + NO2 reaction branching ratio to form NO+ is accurate and, furthermore, that there is no evidence for a sufficient NO2 impurity in any of our NO experiments. We suggest that the discrepancy in the results observed by us and Melko et al. may be attributed to a reaction with the dimer (NO)2. This possibility was dismissed in our earlier work as the dimer concentration under the flow tube conditions was calculated to be below 10−5% of the monomer, but the new results of J. J. Melko et al. raise the dimer reaction as a real possibility. Finally, J. J. Melko et al. appear to have misunderstood the mechanism of the second-order NO chemistry that we had proposed.
1. INTRODUCTION In their recent publication, J. J. Melko et al. (henceforth referred to as the Viggiano group) presented experimental results for reactions of the Fe+ cation with NO and NO2.1 These results differed from the results previously published by our group in 1995,2 2003,3 and very recently4 for NO2 and in 20033 and 20055 for NO. The Viggiano group further undertook to scan and reanalyze our published data for the Fe+ + NO2 reaction and reached the conclusion that the NO+ branching ratio that we reported was erroneous, that it should have been 25% rather than the 0% that we reported. This result, they argued, could then explain our observed formation of FeO+ and NO+ in the reaction of Fe+ and NO in terms of NO2 impurity in our NO reagent gas of approximately 2. In particular, they asserted that the reaction of Fe+ with NO that we attributed to second-order chemistry, as well as the reactions of other atomic metal cations with NO that we reported, could be attributed to an NO2 impurity (or metal cation excited states). Furthermore, the Viggiano group presented a model calculation that demonstrated, according to them, that a second-order reaction in NO should be too slow to observe under the experimental conditions of the Selected Ion Flow Tube (SIFT). We will show here that all of the assertions made the Viggiano group in their recent publication1 are false: that our © 2013 American Chemical Society
data was properly analyzed and clearly shows that the reaction of Fe+ with NO2 produces only FeO+ as a primary product, that there is no NO2 impurity in our NO reagent above the previously reported upper limit of 0.5%, and that second-order reactions are not too slow to observe under the experimental conditions of the Selected Ion Flow Tube (SIFT).
2. EXPERIMENTAL METHODS The details of the Inductively Coupled Plasma−Selected Ion Flow Tube (ICP−SIFT) setup and its application to systematic studies of metal cation reactivity have been discussed previously.2−5 Two issues are relevant for this discussion: the excited state population of the Fe+ cation created in the ICP source and contamination of the gas handling and reservoir system. In the ICP source, the two most abundant spin states of Fe+ are the 6D (61.6% population, energy 0.0−0.12 eV) and 4F (29.4% population, energy 0.23−0.39 eV), accounting for 90% of the Fe+ population. The populations are calculated using the Maxwell−Boltzmann distribution at the temperature of the Received: March 4, 2013 Revised: April 6, 2013 Published: April 12, 2013 3786
dx.doi.org/10.1021/jp4022272 | J. Phys. Chem. A 2013, 117, 3786−3790
The Journal of Physical Chemistry A
Article
source plasma (appox. 5500 K) and do not take into account any cooling or quenching that might occur between the ICP source and the reaction region of the flow tube. The Fe+ state distribution in the ICP source is different from that produced in the Electron Impact (EI) source used by the Viggiano group. Furthermore, unlike the EI source, which can vary the excited state population by increasing or decreasing electron energy, the ICP source provides no means for controlling the state distribution. The reservoir system used to mix and handle reagent gases, including NO, is regularly (nightly) baked by a heat lamp under vacuum to prevent contamination. Baking temperature is approximately 60 °C. This procedure is especially important for nitrogen containing species, such as NO and NO2, which are strongly adsorbed to the stainless steel surface of the reservoir system. The NO and NO2 studies were performed separately, NO in the summer of 2002 and NO2 in the winter of 2003, so there is no possibility of cross-contamination of NO2 into the NO study. Each study was conducted with all 59 cations available from the ICP source, typically by performing 4−6 experiments per day with 1−2 physical mixtures of the reagent gas. Any contamination would have exhibited itself in multiple experiments and would not be restricted to a single experiment. Unlike Viggiano’s group, which uses MKS thermal dispersion mass flow controllers to measure and control the flow of all gases, we measure our reagent gas flow using a combination of a flow restriction capillary and two differential pressure transducers. Flow is calculated from the pressure difference between the two ends of the capillary and the reservoir pressure. The flow is controlled by a fine needle valve. Since there are no temperature-sensitive components, the entire system, including the pressure transducers, can be baked to minimize impurities. Finally, we record that our NO was purchased from Linde with a listed purity of 99.5% and the following impurities: N2O + N2 (0.4%), CO2 (1000 ppm), and H2O (50 ppm). The NO used by the Viggiano group was obtained from Specialty Gases of America and had the same listed purity of 99.5% (impurities not listed).
Table 1. Summary of Rate Coefficients Measured for the Reactions of Fe+ with NO and NO2 at Room Temperaturea reaction Fe+ + NO
experiment
−11b
