Comment pubs.acs.org/JPCA
Comment on “Role of (NO)2 Dimer in Reactions of Fe+ with NO and NO2 Studied by ICP-SIFT Mass Spectrometry” Joshua J. Melko,† Shaun G. Ard,† Joseph A. Fournier,‡ Nicholas S. Shuman,† Jürgen Troe,§ and Albert A. Viggiano*,† †
Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB, New Mexico 87117-5776, United States Sterling Chemistry Laboratory, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States § Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany, and Max-Planck-Institut für Biophysikalische Chemie, D-37077 Göttingen, Germany ‡
J. Phys. Chem. A 2013, 117 (18), 3786−3790. DOI: 10.1021/jp4022272 J. Phys. Chem. A 2013, 117. DOI: 10.1021/jp4069224 S Supporting Information *
O
ur group recently reported that Fe+ does not react appreciably with NO (k ≤ 1.0 × 10−12 cm3 s−1),1 in agreement with a 1995 study by the Bohme group (k < 1.0 × 10−11 cm3 s−1),2 but in contrast with their most recent results (k = 1.8 × 10−11 cm3 s−1).3 To account for the discrepancy, we presented evidence that the more recent measurement by Bohme may have been erroneous due to a small (1−2%) NO2 impurity.1 Further, we argued that a proposed termolecular reaction, needed to explain the otherwise endothermic process, is several orders of magnitude too slow to explain the reported reactivity. The Bohme group offered refutations of some of our arguments in a separate publication (henceforth referred to as Bohme 2013).4 We briefly address their arguments in this comment with more detailed discussion reserved for Supporting Information. The most important point is that no plausible explanation has been provided for our observed lack of NO reactivity when a trap that removes NO2 is included in our inlet lines. Bohme 2013 speculates that their observed reactivity is in fact due to NO dimer and that the dimer is additionally removed by our trap; however, we show here that the lifetime of that species is too short by 10 orders of magnitude to contribute to chemistry in either experiment. Bohme 2013 incorrectly suggests that our initial publication1 misunderstood the proposed termolecular mechanism, the difference being whether Fe(NO)+* reacts directly or is first stabilized (listed as reactions 3 and 4 (stabilized) and 5 and 6 (direct) in their paper4 and our Supporting Information). In fact, we considered both schemes, but detailed only an analysis of the faster of the two (i.e., (3) and (4)). Assuming 100% NO buffer to maximize the rate of (5) and (6), one finds that the upper limits for both sequences are identical at about 10−16 cm3 s−1, 5 orders of magnitude slower than the rate constant reported by the Bohme group (see Supporting Information). The claim that modeling (5) and (6) reproduces the reported data is only possible by assuming an unphysical equilibrium in (5), one that is quite different than calculated by our modeling of reaction (3). An alternative to the termolecular mechanism (5) and (6), whose effective rate constant would be identical to the strong © 2013 American Chemical Society
collision rate constant of (3) and (4), would be the NO dimer mechanism offered in Bohme 2013, which is as follows 2NO ↔ (NO)2
(1a)
(NO)2 + Fe+ → products
(1b)
Because of the small equilibrium concentration of (NO)2, the dimer mechanism can be ruled out, which has also been done previously by the Bohme group.3,5 The suggestion that “small amounts of NO dimer formed in the reservoir go on to react with the metal cation” can certainly be discarded as the bond energy of (NO)2 is on the order of 2 kcal mol−1,6,7 such that the lifetime is smaller than 10−12 s (see Supporting Information). With no plausible mechanism identified, evidence for the reported reactivity must pass a high bar. As reported in Bohme 2013 Table 1,4 two studies (Bohme 1995,2 Viggiano 20121) report no reaction, whereas two studies (Bohme 20038 and 20053) report reaction. We attempted to reconcile these results by pointing out a body of evidence suggesting an NO2 impurity in the Bohme 2003 and 2005 work. As initially conveyed to us by private communication and detailed in Bohme 2013, efforts were made in their experiments to ensure that no NO2 was present. Specifically, care was taken in the design and maintenance of the gas handling system, and monitor reactions were used to check for the presence of NO2. However, NO is known to react with O2, H2O, and air in gas handling lines to form NO2.9−11 Further, the Bohme group does not use any trapping technique, such as our use of Ascarite, which numerous groups consider essential for preventing NO2 contamination when handling NO.12−18 They performed several monitor tests, all of which are problematic, with only subtle indications of impurity possible. Hg+ and GeO+ (not the primary ion Ge+) react with NO2 to produce NO2+. However, NO2+ converts to NO+ rapidly by Received: May 20, 2013 Revised: August 8, 2013 Published: August 28, 2013 9108
dx.doi.org/10.1021/jp404945p | J. Phys. Chem. A 2013, 117, 9108−9110
The Journal of Physical Chemistry A
■
NO2+ + NO → NO+ + NO2 −10
(k = 2.9 × 10
3 −1 19
cm s )
Comment
ASSOCIATED CONTENT
S Supporting Information *
(2)
Further details regarding the second-order reaction mechanism, impurity analyses, experimental setup, NO dimer lifetime, and branching ratio analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
+
Our modeling indicates that under all conditions NO2 is minimal, peaking at a few counts per second at very low NO flows (see graph in the Supporting Information). Bohme 2013 concedes the difficulties in fitting to this level of precision: “The deviation of the FeO+ prof ile f rom linear behavior at low signal intensities (
