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

2. 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•+O RO2•+HCl
RO2•+2NO CO2+HO•+2NO2
NO2+hν NO•+O•
O•+O2 O3

3. 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

4. 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- + N2O NO3- + NO2
NO2- + NO3
NO2- +NO +O2
NO2++NO3+2e-
2.9, 1.31, 1.11 eV exothermic and 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
‡ Mark et al. J. Mass. Spec

5. 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- + N2O NO3- + INO2
I-(H2O)n + N2O I-N2O5+ nH2O
Our Goal: Capture and characterize the X-N2O5 complexes X=I, Br, Cl

6. 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

7. 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 +NO N2O NO2+•NO3-

8. 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

9. 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

10. 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
/ G
(3df,3pd) (N,O)
Halide: LANL2DZ ECP
Final calculated spectrum scaled by
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,

11. 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/ G
(3df,3pd) (N,O)
Halide: LANL2DZ ECP
NO3‾•Ar*
νas
νs(calc)
What will we see with other halides?
scaled
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 ,

12. We Expect Less Splitting for Lighter Halides
Debye
Debye
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? + - + - + -

13. 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 †
Cam-B3LYP/ 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 ,

14. 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 †
XN2O5 anions are intermediates in reaction to form XNO2
Cam-B3LYP/ 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,

15. 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
Relative energy [kcal/mol]
-50
-10
-20
-40
-30 20 10
Chloride
Bromide
omegaB97X-D/aug-cc-pVTZ No electric field

16. 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 ?

17. 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

18. 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