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