Trapping and Structural Characterization of the XNO2Â·NO3â (X = Cl

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Trapping and Structural Characterization of the XNO·NO¯ (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated X¯ + NO Reactions with Cryogenic Vibrational Spectroscopy 2

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Patrick J. Kelleher, Fabian S Menges, Joseph W. DePalma, Joanna K. Denton, Mark A. Johnson, Gary H. Weddle, Barak Hirshberg, and Robert Benny Gerber J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02120 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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The Journal of Physical Chemistry Letters

Trapping and Structural Characterization of the XNO2·NO3ˉ (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated Xˉ + N2O5 Reactions with Cryogenic Vibrational Spectroscopy Patrick J. Kelleher,a Fabian S. Menges,a Joseph W. DePalma,a Joanna K. Denton,a and Mark A. Johnsona* a

Department of Chemistry, Yale University, New Haven, Connecticut 06525, United States

Gary H. Weddleb b

Department of Chemistry, Fairfield University, Fairfield, Connecticut 06824, United States

Barak Hirshberg,c and R. Benny Gerberc,d c

Institute of Chemistry and the Fritz-Haber Center for Molecular Dynamics, The Hebrew University, Jerusalem 9190401, Israel d

Department of Chemistry, University of California, Irvine, California 92697, United States

*M. A. Johnson. Tel: +1 203 432 5226, Email: [email protected]

Abstract The heterogeneous reaction of N2O5 with sea spray aerosols yields the ClNO2 molecule, which is postulated to occur through water-mediated charge separation into NO3ˉ and NO2+ ions followed by association with Clˉ. Here we address an alternative mechanism where the attack by a halide ion can yield XNO2 by direct insertion in the presence of water. This was accomplished by reacting Xˉ(D2O)n (X=Cl, Br, I) cluster ions with N2O5 to produce ions with stoichiometry [XN2O5]ˉ. These species were cooled in a 20 K ion trap and structurally characterized by vibrational spectroscopy obtained using the D2 messenger tagging technique. Analysis of the resulting band patterns with DFT calculations indicates that they all correspond to exit channel ion-molecule complexes based on the association of NO3ˉ with XNO2, with the NO3ˉ constituent perturbed in the order I>Br>Cl. These results establish that XNO2 can be generated even when more exoergic reaction pathways involving hydrolysis are available, and demonstrate the role of the intermediate [XN2O5]ˉ in the formation of XNO2.

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I. Introduction Dinitrogen pentoxide (N2O5) is an important species in the atmospheric chemistry of nitrogen oxides. Reactions of N2O5 with water to produce HNO3 serve as sinks for NOx (NOx = NO, NO2),7-8 and heterogeneous reactions of N2O5 with chloride-containing aerosol particles lead to the production of nitryl chloride, ClNO2,9-12 which can be subsequently photolyzed to produce reactive Cl radicals.13-14 Although laboratory studies15 have demonstrated that N2O5 can also react with bromide containing water droplets to produce BrNO2 and Br2, field measurements have been unable to detect BrNO2,16 despite the fact that Brˉ is also present in aerosols, albeit in lower concentrations than Clˉ. In either case, the factors that govern the yield of XNO2 from heterogeneous reactions of N2O5 are not well understood.17 In fact, ClNO2 formation is often assumed to occur after hydrolysis of N2O5 into NO3ˉ and NO2+ ions, followed by reaction of NO2+ cations with aqueous chloride9, 18 or gaseous HCl.19 In the gas phase, on the other hand, N2O5 reacts roughly at the collision rate with each of the halide ions to form NO3ˉ and XNO2.20 In this study, we address the chemical rearrangements in play when N2O5 collides with Xˉ(D2O)n ions in the gas phase, which introduces a variety of reaction pathways as outlined in Fig. 1. 1-6

In these reactions, one role for water appears to be that of a spectator (a1 in Fig. 1), leading to formation of Xˉ(N2O5) upon N2O5 collision with Xˉ(D2O).10 This process has, in the X = I case, become the method of choice for ambient detection of N2O5 using chemical ionization mass spectrometry (CIMS).18, 21-23 The analogous Xˉ(N2O5) ions have not previously been reported for X = F, Cl and Br,16 however, and calculations (vide infra) suggest that the hydrolysis pathways b) and c) in Fig. 1 should become increasingly important for the lighter halides. This raises the possibility that the relative pKa values of HX Figure 1. Possible reaction paths (A, B, and C) may contribute to this propensity. Here we report the for the formation of mass spectrometrically formation of the Clˉ(N2O5) and Brˉ(N2O5) ions by observed ionic species (red) after collision of collisions of N2O5 with Clˉ(D2O)n and Brˉ(D2O)n clusters X(D2O)‾ (X = Cl, Br, I) with N2O5 in the second in the gas phase, and characterize the structures of the differential pumping region of the mass three Xˉ(N2O5) species (X = Cl, Br, I) using “messenger spectrometer (Fig. S1). tagging” vibrational spectroscopy.24 We are particularly interested in understanding whether these species exist as electrostatically bound halide ion-molecule complexes arising from “ligand switching” of N2O5 for the solvent water molecule, or correspond to trapped intermediates deep into the reaction path leading to XNO2 formation. These structures are determined through analysis of the resulting vibrational band patterns with frequency calculations for minimum energy geometries. We also report the relative energies of stationary points associated with three different water-mediated reaction pathways, obtained with electronic structure theory.



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II. Results and Discussion A description of the experimental and synthetic procedures is included in the Supplementary Information, with a schematic diagram of the custom built photofragmentation mass spectrometer used for these experiments in Fig. S1. Xˉ(D2O)n (X = I, Br, Cl; n = 0-8) clusters were generated by electrospray ionization (ESI) of ~1 mM solutions of [EMIM]+Xˉ ([EMIM]+ = 1-ethyl-3-methylimidazolium) at 1 atm in a humidity-controlled (D2O vapor) chamber. D2O was used in these experiments so that nitric acid produced via reactions of N2O5 with the Xˉ(D2O)n clusters (yielding DNO3) could be differentiated from species containing HNO3 that is inevitably produced in the N2O5 synthesis or through reaction of N2O5 with H2O on the vacuum chamber walls or in the delivery lines. The hydrated halide clusters underwent reaction with N2O5 at ambient temperature at a partial pressure of ~0.13 Pa and a total pressure of ~0.67 Pa. The reaction products arising from N2O5 collisions with Xˉ(D2O)n are indicated in the mass spectrum displayed in Fig. 2. As expected, the Iˉ(N2O5) product is efficiently generated (Fig. 2c) provided at least one water molecule is attached to the iodide reactant. Specifically, only NO3ˉ was observed when bare Iˉ was used as the chemical ionization reagent (Fig. S2), in agreement with the early kinetics studies by Ferguson and co-workers20 and CIMS experiments by Kercher and co-workers.10 We also observe anionic products containing nitric acid that are uniquely traced to the halide hydrates via transfer of the isotopic label (e.g., Iˉ(DNO3) and NO3ˉ(DNO3)), thus establishing that the solvent water molecules can play a reactive role in the N2O5 uptake (paths b and c in Fig. 1). For the Cl case, the Clˉ(N2O5) yield is significantly smaller relative to that of NO3ˉ(DNO3) species in comparison with the behavior of the other halides.

Figure 2. Mass spectra of ions following reaction of N2O5 with Xˉ(D2O)n ions in a linear octopole ion guide. The intensities are normalized to the NO3ˉ(DNO3) peak at m/z = 126 (denoted by dashed line). Other major observed peaks include NO3ˉ (m/z = 62), NO3ˉ(HNO3) (m/z = 125), and Xˉ(N2O5) (red) (m/z = 143/145, 195/197, and 235 for X = Cl, Br, and I, respectively). All peaks and their assignments are summarized in Table S1. Note that the HNO3 peaks are due to N2O5 decomposition in the inlet system.

To better understand the observed trends in the reactivity, we carried out electronic structure calculations to survey the halide dependence of the potential energy landscapes underlying the reaction pathways highlighted in Fig. 1. Stationary points along three reactive pathways (A, B and C) are displayed in Fig. 3 along with their energies relative to the Xˉ(H2O) + N2O5 asymptotes. The molecular identities, relative energies, and the energies of neutral ligand evaporation are collected in Table 1. In Path A (red), the water molecule plays the role of a spectator in the direct insertion reaction Xˉ + N2O5 → NO3ˉ + XNO2 observed previously in flow tube experiments.20 Paths B (blue) and C (green), on the other hand, correspond to two different hydrolysis reactions. Species labeled X1A,1B,1C in Fig. 3 are the 4 ACS Paragon Plus Environment

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lowest energy structures along each pathway and are not observed here, but are included because they lead to the observed masses corresponding to X2A,2B,2C. Calculated structures of the Cl1A,1B,1C and Cl2A,2B,2C are displayed at the right of Fig. 3. These binding motifs are representative of those calculated for all three halides, and the specific structures for all stationary points plotted are included in Figs. S3-S5. Observed products Cl2A and Cl2B are essentially NO3ˉ ions complexed with neutral molecules, and are formed by evaporation of water (Path A) to yield NO3ˉ(ClNO2) [Cl2A], or HCl (Path B) to generate NO3ˉ(HNO3) [Cl2B]. The Cl2C ion along Path C is best described as solvation of the Clˉ ion by neutral HNO3, which arises by loss of HNO3. In all three halides, the X2A structures feature XNO2 binding to the NO3- ion with the halide atom pointing toward one of the oxygen atoms of the nitrate ion at a skew angle relative to the N-O bond axis of the ion. This low symmetry arrangement is reminiscent of the bent structure of the OH group in nitric acid. We note that each halide is also calculated to support a local minimum corresponding to a simple Xˉ(N2O5)·H2O ion-molecule complex (with intact N2O5 and H2O molecules, see Fig. S3), which corresponds to the intermediate in a trivial ligand switching reaction (i.e., displacement of H2O by an intact N2O5 molecule). This configuration is quite unstable, however, lying far (19, 25, and 37 kcal/mol for Cl, Br, and I, respectively) above those arising from chemical rearrangement.

Figure 3. Relative energy pathways for the reaction of Clˉ(H2O), Brˉ(H2O), and Iˉ(H2O) (left to right) with N2O5 calculated at the ߱B97X-D/aug-cc-pVDZ level of theory (including ZPE corrections). In all three cases, the zero of energy is set to the separated hydrated ion and N2O5 molecule. Example structures are given for the first two steps in the reaction of Clˉ(H2O) with N2O5 for the three pathways considered (Cl1A, Cl1B, Cl1C), where the boxes indicate ions observed in the mass spectrometer. The chemical formulas and relative energies for all species are included in Table 1, and the corresponding minimum energy structures can be found in Figures S4-6.

The most important qualitative information obtained by inspection of the halide dependence of the three paths is that the direct insertion path (A) lies far above those arising from hydrolysis for Clˉ, while the opposite is the case for Iˉ. This suggests an explanation for the observed difficulty in preparing Clˉ(N2O5) through collisions with Clˉ(D2O), and why the Iˉ(D2O) system has been adopted as the 5 ACS Paragon Plus Environment


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empirical method of choice for CIMS detection of N2O5.10 Another interesting aspect of the energetic profiles is that the binding energy (D0) of the NO3ˉ(XNO2) complexes (X2A) dramatically increases in going from Clˉ to Iˉ, again favoring retention of the NO3ˉ(INO2) ion once it is formed by the direct insertion pathway. Table 1. Chemical formulas and relative energies (in kcal/mol) at the ߱B97X-D/aug-cc-pVDZ level of theory including ZPE corrections for species plotted in Fig. 3. ∆Evap is included for the evaporation of neutral H2O, HX or HNO3 between steps 1 and 2.

Halide

0

1

2

∆E

Path A Xˉ(D2O) + N2O5 NO3ˉ(XNO2)(H2O)

H 2O vap

3

NO3ˉ(XNO2) + H2O NO3ˉ + XNO2 + H2O

Cl

0

-28.89

10.96

-17.93

-4.82

Br

0

-34.27

9.88

-24.39

-3.48

I

0

-44.13

8.78

-35.35

-4.43

NO3ˉ(HNO3)(HX)

HX ∆E vap

NO3ˉ(HNO3) + HX

NO3ˉ + HNO3 + HX

Path B Xˉ(D2O) + N2O5 Cl

0

-49.12

12.85

-36.26

-5.92

Br

0

-42.02

12.15

-29.87

0.48

I

0

-37.07

13.90

-23.17

7.18

Xˉ(HNO3)2

HNO 3 ∆E vap

Xˉ(HNO3) + HNO3

NO3ˉ + HNO3 + HX

Path C Xˉ(D2O) + N2O5 Cl

0

-51.04

21.41

-29.63

-5.92

Br

0

-45.73

19.65

-26.08

0.48

I

0

-39.86

16.50

-23.36

7.18

At a qualitative level, the large increase in D0 for the NO3ˉ(XNO2) complex is consistent with what one might expect for ion-molecule complexes involving a common anion, where the interaction is dominated by the dipole moment of the neutral partner. The D0 values for the NO3ˉ(XNO2) ions and the dipole moments of the neutral XNO2 molecules are listed in Table 2 (vectors displayed in Fig. S7), and indeed the calculated dipole moment of INO2 (1.76 D) is almost a factor of four higher than that of ClNO2 (0.49 D). Moreover, if one considers the FNO2 case, the direction of the dipole changes and the equilibrium structure of NO3ˉ(FNO2) occurs with the –NO2 moiety pointing toward an oxygen atom of the nitrate rather than the halide, which is the binding motif for Cl, Br and I (Fig. S8). Table 2. Dipole moments of neutral XNO2 species and binding energies, D0, of the NO3ˉ(XNO2) complexes. (+) direction of dipole is defined as pointing from N towards X, as indicated on the calculated minimum energy structures in Fig. S7.

F

XNO2 Dipole (Debye) -0.31

NO3ˉ(XNO2) D0 (kcal/mol) -

Cl

0.49

13

X


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Br

0.89

21

I

1.76

31

To establish that the predicted X2A structural motif displayed in Fig. 3 is, in fact, prepared in the cluster reaction, we next turn to vibrational spectroscopy to compare the observed band patterns with those predicted for the various X2A structures. Vibrational spectra of the three ions were obtained using the so-called messenger tagging method,24 as described in the supplementary materials. Briefly, the ions are stored and cooled to around 20 K in a cryogenic RF (Paul) ion trap,25 during which time weakly bound D2 molecules are condensed onto the target ions. The D2 molecules are readily photodissociated upon resonant excitation of vibrational transitions in the ion, thus revealing the linear absorption spectrum when the photofragmentation yield is plotted as a function of the photon energy. The efficient photodissociation of the D2 tag molecules with excitation down to 800 cm-1 indicates that it does not interact strongly with the ion. The D2 molecule is predicted to attach to an oxygen atom of the NO3ˉ moiety within the Xˉ(N2O5) complexes, where it is calculated to induce minimal (

Trapping and Structural Characterization of the XNO2Â·NO3â (X = Cl

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