Spectroscopic Characterization of N2O5 Halide Clusters 6/22/2017 Joanna K. Denton, Patrick J. Kelleher, Fabian S. Menges, Joseph W. DePalma, Mark A. Johnson

N2O5 Is an Important Atmospheric Species N2O5 is a night time sink for atmospheric NOx species and forms ClNO2. ClNO2 is a pollutant, which is a strong oxidizer of alkanes, kickstarting tropospheric O3 production reactions Understanding N2O5 is key to understanding ClNO2 H2O Cl- Na+ ClNO2 NO3- Tropospheric Cl Produces Ozone O2 NO hν N2O5+Cl- ClNO2+NO3- ClNO2 Cl•+NO2 RH+Cl•+O2 RO2•+HCl RO2•+2NO CO2+HO•+2NO2 NO2+hν NO•+O• O•+O2 O3

XNO2 Formation Mechanism Is Unknown Existing Literature invokes N2O5 dissociation into ions before reacting with aqueous chloride. Is this a water mediated reaction? If so, how many water molecules are required to carry out the reaction? Cl-(aq)+N2O5(aq) Cl-(aq)+NO2+(aq)+NO3-(aq) ClNO2(g)+ NO3-(aq) Let’s look at N2O5 interactions with micro-hydrated halide ions in the gas phase

How Does One Detect N2O5? N2O5 can’t handle the ionization energy Mass Spec. can only detect charged species. N2O5 falls apart if you try to ionize it by electron attachment or impact. It’s not selective for N2O5.†‡ e- + N2O5 NO3- + NO2 NO2- + NO3 NO2- +NO +O2 NO2++NO3+2e- 2.9, 1.31, 1.11 eV exothermic and 10.56 eV endothermic going down See Mark 2004 Mass Spec and Mark 2004 JCP N2O5 can’t handle the ionization energy What is soft ionization scheme? †Mark et al. J. Chem. Phys. 2004. 9891-9897 ‡ Mark et al. J. Mass. Spec. 2004. 147-150

Chemical Ionization Mass Spec (CIMS) I- is a widely used in CIMS to indirectly detect N2O5 by dissociating it to form NO3- . We want to see intact N2O5, so we use a newer CIMS ligand-switching technique to absorb the bond energy by evaporating H2O. If Cl- is so important atmospherically, why resort to I-? I- + N2O5 NO3- + INO2 I-(H2O)n + N2O5 I-N2O5+ nH2O Our Goal: Capture and characterize the X-N2O5 complexes X=I, Br, Cl

Yale Tandem Time of Flight Mass Spec Generate I-(D2O) Clusters Collide with N2O5 Temp.-Contr. RF Paul Trap Accumulate Separate By Mass DC-90° Bender 5x10-7 Torr 3x10-5 Differential Pumping Wiley- McLaren TOF Ion Optics Reflectron MCP Detector Apparatus Cold Ion Vibrational Predissociation Spectroscopy in the IR region 150 200 250 300 m/z I+D2O  I(D2O)n 150 200 250 300 m/z I(D2O)n+N2O5 I-N2O5 N2O5 (3-50 Torr) Low Energy Collision Cell ~10-3 ~760 ESI Humidity Control 2

N2O5 Synthesis Temperature controls shift equilibrium toward N2O5 Ozonator NO+O3 Rxn Imp. 0oC P2O5 Trap Collection Impinger -78oC O3 NO Vent Nylon Wool NO2/ HNO3 N2O5 Temperature controls shift equilibrium toward N2O5 P2O5 prevents H2O from reacting to form HNO3 Nylon wool removes HNO3 It forms in the machine and sticks to the walls anyway. NO2 +NO3 N2O5 NO2+•NO3-

What is the chemical nature of X-N2O5? We made I-N2O5 D2O used to distinguish DNO3 formed from I(D2O)n and those arising from HNO3 sample contamination X-N2O5 Yield Dramatically Reduced I>Br>Cl Why is Cl-N2O5 so much harder to make? ~x10 Cl-(N2O5) Br-(N2O5) ~x5 w I-(N2O5) I- NO3-(DNO3) 191=IDNO3 I-(D2O) What is the chemical nature of X-N2O5? 120 140 160 180 200 220 240 m/z

Yale Tandem Time of Flight Mass Spec Generate I-(D2O) Clusters Collide with N2O5 Cool and Tag Isolate One Mass Excite With Laser Detect Fragmentation and Generate Spectrum Temp.-Contr. RF Paul Trap DC-90° Bender 5x10-7 Torr 3x10-5 Differential Pumping Wiley- McLaren TOF Ion Optics Mass gate Reflectron MCP Detector 600–4500 cm-1 Apparatus Cold Ion Vibrational Predissociation Spectroscopy in the IR region 150 200 250 300 m/z I+D2O  I(D2O)n 150 200 250 300 m/z I(D2O)n+N2O5 I-N2O5 150 200 250 300 m/z 800 1200 1600 2000 I‾(N2O5)•D2 Energy (cm-1) N2O5 (3-50 Torr) Low Energy Collision Cell ~10-3 ~760 ESI Humidity Control 2

What Is the Structure of I-N2O5? This structure shows a perturbed NO3- I‾(N2O5) calc. E=-217 kJ/mol I‾(N2O5) calc. N2O5* νs(NO2) νas(NO2) νas(NON) δs(NO2) E=-116 kJ/mol Cam-B3LYP /6-311++G (3df,3pd) (N,O) Halide: LANL2DZ ECP Final calculated spectrum scaled by 0.93617 And Isolated I- E= 0 kJ/mol I‾(N2O5)•D2 How does this compare to the spectrum of NO3-? Energy (cm-1) 800 1000 1200 1400 1600 1800 2000 *Sodeau et al.; J. Phys. Chem. 1994, 98, 946-951

NO3- νas and νs Split by Dipole Moment NO3- has three modes: a doubly degenerate νas and forbidden νs We expect νs to appear as O-N bonds are unevenly perturbed and we do. I-(N2O5)•D2 concerted νas(NO3) νs(NO2) νs(NO3) I‾(N2O5) calc. νas(NO3) Cam-B3LYP/6-311++G (3df,3pd) (N,O) Halide: LANL2DZ ECP NO3‾•Ar* νas νs(calc) What will we see with other halides? scaled 0.93617 800 1000 1200 1400 1600 1800 2000 Energy (cm-1) I-(N2O5) νas further splits into a concerted νas with νs(NO2) and a lone νas I-(N2O5) νs and νas split farther apart as the INO2 dipole disproportionately perturbs NO3-’s nearest O-N bond, making NO3- more like NO2. *Viggiano et al. J. Chem. Phys. 2008. 129, 064305

We Expect Less Splitting for Lighter Halides 0.2236 Debye 0.9814 Debye 1.9779 Debye Dipole Moment XNO2 Dipole moment decreases for the lighter halides Smaller dipole reduces perturbation of the NO3- ion, thus reducing the displacement of NO3- νs and νas Is This What We See? + - + - + -

NO3- Splitting Follows Decreases in XNO2 Dipole Moment Dipole moments show more charge separation and less disturbance of NO3- Our detection scheme is actually an intermediate state in an XNO2 formation mechanism. I‾(N2O5)•D2 νs(NO3) concerted νas(NO3) νs(NO2) νas(NO3) Br‾(N2O5)•D2 * Cl‾(N2O5)•D2 scaled 0.93617 † Cam-B3LYP/6-311++G(3df,3pd) (N,O) Halide: LANL2DZ ECP Do XNO2 peaks emerge? νs(calc) νas NO3‾•Ar‡ 800 1000 1200 1400 1600 1800 2000 Energy (cm-1) ‡Viggiano et al. J. Chem. Phys. 2008. 129, 064305

ClN2O5 Spectrum Similar to ClNO2 NO3- and NO2 νas splitting increases with decreasing XNO2 dipole moment, coming closer to bands in isolated NO3- Additional peaks provisionally assigned as fermi doublet (2νoop  νs), similar to situation in ClNO2 I‾(N2O5)•D2 O2N-X-O-NO2 νbend (NO2) νs(NO3) concerted νas(NO3) νs(NO2) νas(NO3) νas (NO2) Br‾(N2O5)•D2 O2NX--ONO2 * Cl‾(N2O5)•D2 scaled 0.93617 † XN2O5 anions are intermediates in reaction to form XNO2 Cam-B3LYP/6-311++G(3df,3pd) (N,O) Halide: LANL2DZ ECP νas νNO2 deform νs 2νoop ClNO2‡ 800 1000 1200 1400 1600 1800 2000 Energy (cm-1) ‡ Guirgis et al. Spectrochim. Acta, 1994, 3, 463-472.

The Intermediate is Much Less Stable for Cl-N2O5 than I-N2O5 ON2X-ONO2 goes from the most (I-) to the least (Cl-) stable structure reflecting reduced perturbation by Cl- This is why we don’t see as much INO2 in nature Iodide Reaction pathway 1 2 3 1 2 3 1 2 3 Relative energy [kcal/mol] -50 -10 -20 -40 -30 20 10 Chloride Bromide omegaB97X-D/aug-cc-pVTZ No electric field

Conclusions The I-N2O5 detection scheme using I-(D2O)n isn’t ligand switching, it’s the intermediate to INO2 The I-N2O5 has a much more stable reaction intermediate than Cl-N2O5 ClNO2 forms in the gas phase by direct halide insertion into N2O5. What Next? Independent control of cluster formation and reaction temperatures with two traps. Isolation of other ligand switching intermediates ?

Future Work Preliminary Results Independent control of cluster formation and reaction temperatures Preliminary Results We’ve already made I-N2O5 by adding N2O5 to a single trap

Acknowledgements Johnson Group Prof. Mark Johnson Gerber group Barak Hirshberg Schmuttenmaer group Dr. Gary Weddle Steph Craig Dr. Fabian Menges Chinh Duong Dr. Joe DePalma Nan Yang Olga Gorlova Helen Zeng Patrick Kelleher Evan Perez