infrared spectroscopy of cold, hydrated alkaline-earth salt clusters christopher johnson yale...
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Infrared spectroscopy of cold, hydrated alkaline-earth salt clusters
Christopher JohnsonYale University
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
A B A+ B– A+ B– A+ B–
Complex Ion Pair Separated Ion Pair Solvent-separated Ion Pair
Technique
D2, He
TOF 1
Paul Trap, 10K
Electrospray – Mg(OH)2/Ca(OH)2 in H2O
Ion
Tran
spor
t
OPO/OPA600 – 4500 cm-1
TOF 2
MCPDetector
Magnesium sulfate[Mg2SO4(H2O)n]2+
n=6
n=7
1000 1100 1200 1300 1400 1500 1600 1700 1800Cal
cula
ted
IR S
pect
rum
/ P
redi
ssoc
iatio
n Y
ield
Frequency (cm-1)
SO2
asym
OOPSO2
symH2O
bends
B3LYP/6-311++G(d,p)
MgOH+·(H2O)
1400 1600 2800 3000 3200 3400 3600 3800 4000
Pre
dis
soci
atio
n Y
ield
Frequency (cm-1)
H2Obend
D2
stretch
H2Osym H2O
asym
OHstretch
Frustrated rotation – Duncan & McCoyMg+·D2 (~2917 cm-1) – Bieske Group
MgOH+·(H2O)n
1400 1600 2800 3000 3200 3400 3600 3800 4000
Pred
isso
ciati
on Y
ield
Frequency (cm-1 )
H2Obend D2
stretch
H2Osym
H2Oasym
OHstretch
n=1
n=2
n=3
n=4
n=6
n=5
CaOH+·(H2O)n
1400 1600 2800 3000 3200 3400 3600 3800 4000
Pred
isso
ciati
on Y
ield
Frequency (cm-1 )
H2Obend D2
stretch
H2Osym
H2Oasym
OHstretch n=1
n=2
n=3
n=4
n=5
Band shifts
0 1 2 3 4 5 62800
2900
3500
3600
3700
3800
3900
Ban
d C
ente
r
Number of Water Molecules
D2
stretch
H2Osym
H2Oasym
OHstretch
Filled – MgOpen – Ca
• Convergence of OH stretch and H2O asymmetric stretches
• D2 stretch reports surface charge localization
• What about the broad featues?
0 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
2.0
Cha
rge
(e)
Number of Water Molecules
rM
g-O (Å
)
QMg
QOH
rMg-O
MgOH+ – MP2/6-311++G(d,p)
Ionic H-bond and second shell
OH–·(H2O)n
n=1
n=2
n=3
n=4
n=5
2800 3000 3200 3400 3600 3800 4000
Frequency (cm-1)
n=3
n=4
n=5
MgOH+·(H2O)n
Robertson et al. Science 299, 1367 (2003)
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?
Acknowledgements
Johnson Lab
Chris Leavitt
Anne McCoy
2600 2800 3000 3200 3400 3600 3800 4000
Frequency (cm-1)
CaOH+·(H2O)n
n QMg QOH rMg-O
0 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