Observation of Isomer Trapping in Li + (H 2 O) 4 Ar Cluster Ions Dorothy J. Miller and James M. Lisy Department of Chemistry University of Illinois at Urbana Champaign

Triple Quadrapole Mass Spectrometer  Neutral water clusters are formed via a supersonic expansion.  Fully expanded neutral clusters collide with lithium cations produced via thermionic emission.  Clusters are cooled through the evaporative loss of the most labile species, there is no collisional cooling.  Tandem MS-MS method: Select parent ion cluster, dissociate with IR radiation, detect fragment cluster ion. Indirect action/depletion Source lenses Detector lenses Cross beam ion gun Ion selecting quadrupole Ion analyzing quadrupole Ion guiding quadrupole CD/CEM Nozzle Nd 3+ :YAG Laser (1064 nm) 20 ns pulse 20 Hz Tunable LiNbO 3 OPO

n-m n- 2 n- 1 n E final = E n-(m-1) - KE n-(m-1) -BE n- (m-1) E n-2 =E n-1 -KE n-1 - BE n-1 E n-1 =E n -KE n - BE n Evaporative Cooling  Cooling efficiency determined by the evaporating ligands’ binding energy  Assume statistical energy distribution  effective cluster ion temperature E final [Li + (H 2 O) 4 ] Energy Δ E ≈ BE H2O E n [Li + (H 2 O) n ] 0- Δ E ≈ BE Ar E final [Li + (H 2 O) 4 Ar] E n [Li + (H 2 O) 4 Ar n ] 0- Energy Terminal temperature ~100 K H 2 O evap. = larger energy loss Terminal temperature ~400 K Ar evap. = smaller energy loss

Why is knowledge of the effective cluster temperature important?  Gas-phase ion clusters are used to model biological, biochemical, and chemical systems Biological temperature range ~250 – 300 K Room temperature 298 K  Cluster ions can retain significant internal energy when binding energies are high B.E. Li + (H 2 O) 3 ···H 2 O 66.8 kJ/mol  Structures/configurations may vary with internal energy  Higher temps may lead to entropically favored isomers and low temperatures favor the global minimum isomer

Internal Energy Simulation  Internal energy-dependant evaporation rates are calculated using RRKM theory  RRKM rates are used to simulate total internal energy population distributions and relative fragmentation in the evaporative ensemble  Internal energy distributions and vibrational frequencies are used to estimate the cluster ion temperature  The molecular partition function and changes in free energy are calculated from ab initio energy and frequency calculations

Free Energy Ordering of Li + (H 2 O) 4 Δ G (kJ/mol) Temperature (K) Li + (H 2 O) 4 ΔE eq ΔE ZPE (3+1) linear (3+1) bent (4+0)00 MP2/aug-cc-pVDZ

Li + (H 2 O) 4 Evaporation of Water Experimental spectrum best agrees with the calculated spectrum for the tetrahedral nonhydrogen-bonded isomer RRKM-EE temperature ~400K MP2/aug-cc-pVDZ

Δ G (kJ/mol) Temperature (K) Li + (H 2 O) 4 Ar ΔE eq ΔE ZPE (3+1) linear (3+1) bent (4+0)00 MP2/aug-cc-pVDZ Free Energy Ordering of Li + (H 2 O) 4 Ar

Experimental spectrum best agrees with that calculated for the bent hydrogen- bonded isomer RRKM-EE temperature ~100K Li + (H 2 O) 4 Ar Evaporation of Argon MP2/aug-cc-pVDZ

Li + (H 2 O) 4 – The effect of argon Frequency (cm -1 ) IRPD Cross Section (cm 2 ) Li + (H 2 O) 4 Li + (H 2 O) 4 Ar

Calculations ≠ Experiment  Preliminary ion approach calculations indicate that there is a low (or no) barrier going from Li + ···(H 2 O) 4 to the bent hydrogen-bonded isomer  Cluster ions are formed by impacting an ion into a fully formed neutral cluster  cyclic water four MP2/aug-cc-pVDZ

 No collisional cooling, only evaporative cooling  tetrahedral isomer not formed after ion impact  To form the tetrahedral 4+0 isomer, two hydrogen bonds must be broken Cluster Rearrangement Barrier ~30 kJ/mol B.E. Li + (H 2 O) 4 ···Ar ~ 3.8 kJ/mol  No collisional cooling, only evaporative cooling  tetrahedral isomer not formed after ion impact  Mass select Li + (H 2 O) 4 Ar clusters  Once in the bent conformation, insufficient internal energy to overcome the rearrangement barriers MP2/aug-cc-pVDZ

Conclusions  Neither ab initio or thermodynamic calculations predict that the Li + (H 2 O) 4 Ar (3+1 bent) isomer should be observed 4+0 tetrahedral isomer is predicted at all temperatures  Dynamic “trapping” may play a role  Low or no barrier to form the 3+1 bent isomer from Li + ···(H 2 O) 4  Large (~30 kJ/mol) barrier to cluster rearrangement  Rapid argon cooling is trapping the higher energy conformer in the molecular beam

Acknowledgements The Lisy Group: Jason Rodriguez, Matthew Ackerman, Amy Willmarth, Jordan Beck and James Lisy Timothy Vaden Hanneli Hudock NCSA support staff $$ UIUC Block Grant National Science Foundation ACS-PRF

Connection between hydrated ions and condensed phase (solutions)  Mass-selectivity allows probing of size- and composition-dependences  With no heat bath, knowledge of internal energy distribution (‘temperature’) is key  Structures/configurations may vary with internal energy  Balance of competing intermolecular forces can reflect energetic and entropic factors

Rearrangement Barriers Barrier ~30 kJ/mol  Mass select Li + (H 2 O) 4 Ar cluster ions  Once in the bent conformation, insufficient internal energy to overcome the rearrangement barriers MP2/aug-cc-pVDZ B.E. Li+(H2O)4···Ar ~ 3.8 kJ/mol