1. Temperature Effects in Hydrated Alkali Metal Ions
James M. Lisy and Dorothy J. Miller
Department of Chemistry
University of Illinois at Urbana-Champaign
OSU International Symposium on Molecular Spectroscopy
June 23, 2006
FA09

2. Acknowledgements Funding provided by UIUC University of Georgia
National Science Foundation – Chemistry
American Chemical Society - PRF
UIUC
Dr. Tim Vaden
Mr. Jason Rodriguez
Mr. Matt Ackerman
University of Georgia
Prof. Michael Duncan
Mr. Prosser Carnegie
FA03
FA02

3. Why worry about internal energy (effective temperature)?
Cluster ions have high binding energies.
Stable species can retain significant internal energy.
B.E.: Li+(H2O) kJ/mol.
At low temperatures:
Global minimum energy structure favored.
Maximize non-covalent interactions (hydrogen bonds).
At higher temperatures:
Internal energy can disrupt non-covalent interactions.
Hydrated ions stable; structures with higher entropy.
If cluster ions are to serve as realistic models:
Comparisons to chemical systems at relevant temperatures.
Biochemical systems – physiological temperatures.

4. Temperature effects in NH4+(H2O)5
Vary expansion conditions
Reduce temperature
Change isomer distribution
By adjusting the temperature!
H. C. Chang, Y. S. Wang, Y. T. Lee, and H. C. Chang, IJMS 179/180, 91 (1998).

5. Argon-Tagging I-(H2O)1-6 Br-(H2O)3Ar3 Br-(H2O)3Ar Br-(H2O)3
M.A. Johnson and co-workers
JPCA 102(18) 3076, 1998
Br-(H2O)3Ar3
Br-(H2O)3Ar
Br-(H2O)3
M.A. Johnson and co-workers
JCP 110(15) 7129, 1999

6. Triple Quadrupole Mass Spectrometer
Nd3+:YAG Laser
(1064 nm)
20 ns pulse width
20 Hz
Tunable LiNbO3
OPO
Source lenses
Detector lenses
Cross beam
ion gun
Ion selecting quadrupole
Ion analyzing quadrupole
Ion guiding quadrupole
CD/CEM
Differential Pumping
Nozzle
Nd3+:YAG Laser
(1064 nm)
10 ns pulse width
10 Hz
LaserVision OPO/OPA 1.4 – 10 mm
Cluster ions form by ion impact into fully formed neutral cluster.
Excess energy (collision + solvation) ~5-10 eV dissipated by evaporation.
100 msec flight time to first quad.
Tandem MS-MS method: Select ion cluster, dissociate with IR laser, detect fragment ion. Action spectra.

7. Evaporation Pathways - Temperature
ΔE ≈ BE Ar
Efinal [Cs+(H2O)Ar]
En [Cs+(H2O)Arn]
Energy 0-
Efinal [Cs+(H2O)]
Energy
ΔE ≈ BE H2O
En [Cs+(H2O)n] 0-
Each water loss corresponds to a large energetic drop.
But the large binding energies, leave significant internal energy in the cluster. Warm clusters
Each argon loss corresponds to a small energetic drop.
But the low binding energy results in a lower final internal energy. Cool clusters

8. Temperature Control: Cs+(H2O)1
Evaporation of water
High internal energy
Low IRPD σ
Broad bands, low S/N
Cs+(H2O)Ar
Evaporation of argon
Lower internal energy
Higher S/N
Rotational structure
T. D. Vaden, B. Forinash, JML, JCP 117, (2002)

9. Evaporative and collisional cooling are not the same!
T. D. Vaden, P. D. Carnegie, E. D. Pillai, M. A. Duncan and JML, PCCP, accepted.

10. M+(H2O)Ar Temperatures: Does the Ion Play a Role? Experiment and Theory
The use of the Evaporative Ensemble to estimate the temperature (often unknown) of the ion clusters can be tested against experiment.
Texp (K)
125(5)
175(5)
130(5)
207(25)
TRRKM-EE
250 -
T.D. Vaden, C.J. Weinheimer and JML, JCP 121, (2004)

11. Information from simple systems
Method of preparation is important.
Expansion/collisional cooling, the standard molecular beam technique.
Depends on conditions, more collisions  more cooling.
Not easy to model, must measure internal energy.
Evaporative cooling depends on constituents.
Can model, binding energy determines temperature.
Must consider ion-neutral and neutral-neutral interactions

12. Cluster Size – Steps to Solvation
Ion-water interaction depends on size of alkali cation.
Strength also depends on hydration number, decreasing with increasing #’s.
Water-water interactions are also present.
How and when do these two interactions become competitive?
Are there differences between ions?

13. M+(H2O)2Ar Spectra nasym nsym
Bands at 3640 and 3710 cm-1 are due to symmetric and asymmetric O-H stretch.
No evidence of hydrogen bonds.

14. M+(H2O)3Ar Spectra
nsym
No H-bonds for Li+ and Na+, slight H-bonding for K+ and extensive H-bonds for Cs+. Smallest cluster size with H-bonding.

15. M+(H2O)4Ar Spectra
Cs+ ?
All clusters exhibit extensive H-bonding, with bands below 3550 cm-1.
Expect similar structures for Li+ , Na+ and K+. Cs+ does not follow the trend.

16. Consider Cs+(H2O)2-4Ar Cs+ has weakest electrostatic interaction
Favors hydrogen bonds.
Formation has Cs+ approach “cold” water complex in Ar cluster.
Electrostatic interaction can not disrupt cyclic water configuration.
Will this impact the other hydrated ions?
Cs+(H2O)n structures from Kolaski, Tarakeshwar, Kim and co-workers (unpublished), and Kim and co-workers JPCA 2004; 108(15)

17. Li+(H2O)4Ar Calculation Comparison
Experimental spectrum
MP2 aug-cc-pVDZ

18. Free Energy Ordering of Li+(H2O)4Ar Isomers
RRKM-EE Temp X
Calculations and
Observations not
consistent, Why?

19. Dynamics in Argon Evaporation
For (H2O)n 3≤n ≤6, cyclic structures in Ar.
Ion can approach along min. PES.
For Cs+, argon evap. + weak electrostatic interaction  cyclic cluster ions (n=3,4).
For other alkali ions, strong electrostatic interaction disrupts cyclic structure.
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20. Does Temperature Have an Effect? Water Loss →Warm Clusters
T ~ K
Broader bands.
H-Bonding onset at n=5 (Cs+ 4).
Larger size than cold (Ar) species.
Structural isomers

21. K+(H2O)5 Temperature Analysis
RRKM-EE Temp
5 isomers within 5 kJ/mol, 3 within 2 kJ/mol !

22. Temperature Effects for K+(H2O)2-5; Cooling with Argon
Larger clusters: cooling sharpens and/or generates
new H-bonded
features.
Warm
Small clusters: sharper bands
but no new
features
Cool

23. Summary – Hydrated Ions
Temperature (internal energy) has a major impact.
Low temps favor minimum-energy structures with more H-bonds.
High temps favor entropy-driven isomers with fewer H-bonds.
Competition between intermolecular interactions results in size- and composition-dependence.
As ion size increases ion-water interaction decreases.
Number of waters affects the strength of the ion-water interaction.
Water-water interactions eventually compete with ion-water interaction.
Evaporative cooling by argon “traps” stable configurations that are not the global energy minimum on the PES.
Responsible for Cs+ cyclic water structures.
Hydrogen-bonded Li+(H2O)4 species FA09 – Dotti Miller.