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

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

Why worry about internal energy (effective temperature)? Cluster ions have high binding energies. Stable species can retain significant internal energy. B.E.: Li+(H2O)4 66.8 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.

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).

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

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.

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

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, 4628-4631 (2002)

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

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 95 - 140 147 - 212 250 - T.D. Vaden, C.J. Weinheimer and JML, JCP 121, 3102 (2004)

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

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?

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.

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.

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.

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) 2949-58.

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

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

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. FA09

Does Temperature Have an Effect? Water Loss →Warm Clusters T ~ 250-350 K Broader bands. H-Bonding onset at n=5 (Cs+ 4). Larger size than cold (Ar) species. Structural isomers

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

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

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.