1. Infrared spectroscopy of cold, hydrated alkaline-earth salt clusters
Christopher Johnson
Yale University

2. Why study salts? Dynamics of complimentary charges in water
Dissolution in finite systems
Fundamental chemical process
Charging at aqueous surfaces
Aerosol structure, composition, and reactivity
Complex
Ion Pair
Separated Ion Pair
Solvent-separated Ion Pair A B A+ B– A+ B– A+ B–

3. Technique Paul Trap, 10K TOF 1 TOF 2 D2, He Ion Transport MCP Detector
OPO/OPA
600 – 4500 cm-1
Electrospray – Mg(OH)2/Ca(OH)2 in H2O

4. Magnesium sulfate [Mg2SO4(H2O)n]2+ n=6 n=7 OOP SO2 sym H2O SO2 bends
asym
H2O
bends
n=6
n=7
1000
1100
1200
1300
1400
1500
1600
1700
1800
Calculated IR Spectrum
/ Predissociation Yield
Frequency (cm-1)
B3LYP/ G(d,p)

5. MgOH+·(H2O) Frustrated rotation – Duncan & McCoy
sym
H2O
asym
1400
1600
2800
3000
3200
3400
3600
3800
4000
Predissociation Yield
Frequency (cm-1) OH
stretch
H2O
bend D2
stretch
Frustrated rotation – Duncan & McCoy
Mg+·D2 (~2917 cm-1) – Bieske Group

6. MgOH+·(H2O)n Predissociation Yield Frequency (cm ) sym asym OH H2O
1400
1600
2800
3000
3200
3400
3600
3800
4000
Predissociation Yield
Frequency (cm -1 )
H2O
bend D2
stretch
sym
asym OH
n=1
n=2
n=3
n=4
n=6
n=5

7. CaOH+·(H2O)n Predissociation Yield Frequency (cm ) asym sym OH H2O
1400
1600
2800
3000
3200
3400
3600
3800
4000
Predissociation Yield
Frequency (cm -1 )
H2O
bend D2
stretch
sym
asym OH
n=1
n=2
n=3
n=4
n=5

8. Band shifts Convergence of OH stretch and H2O asymmetric stretches
D2 stretch reports surface charge localization
What about the broad featues? 1 2 3 4 5 6
2800
2900
3500
3600
3700
3800
3900
Band Center
Number of Water Molecules D2
stretch
H2O
sym
asym OH
Filled – Mg
Open – Ca
MgOH+ – MP2/ G(d,p) 1 2 3 4 5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
1.7
1.8
1.9
Charge (e)
Number of Water Molecules
rMg-O (Å)
QMg
QOH
rMg-O

9. Ionic H-bond and second shell
OH–·(H2O)n
n=1
n=2
MgOH+·(H2O)n
n=3
n=3
n=4
n=4
n=5
n=5
Robertson et al. Science 299, 1367 (2003)

10. Conclusions
Sulfate SO2 asymmetric stretches very sensitive to solvation of counter-ions
Hydroxide salts in early stages of dissolution
Possible early onset of second shell formation
Quantitative description of broad feature?

11. Acknowledgements
Johnson Lab
Anne McCoy
Chris Leavitt

12. CaOH+·(H2O)n

13. n
QMg
QOH
rMg-O
1.40
-0.40
1.74 1
1.34
-0.49
1.75 2
1.28
-0.52
1.77
3 (avg)
1.22
-0.58
1.82
4 (avg)
1.21
-0.63
1.86
5 (avg)
1.20
-0.67
1.92