1. Haochen Ke, Amy Nicely, James Lisy
MAPPING CONFORMATIONAL ENERGY BARRIERS IN HYDRATED RUBIDIUM ION CLUSTERS
Haochen Ke, Amy Nicely, James Lisy
Department of Chemistry, University of Illinois at Urbana Champaign

2. Experimental Apparatus Spectral Analysis Conclusion and Future Work
Outline
Introduction
Experimental Apparatus
Spectral Analysis
Conclusion and Future Work
I will start with a short introduction of the background, then walk through the apparatus data analysis, and wrap things up and see what is the next step of our study.

3. Introduction
Complex chemistry/biochemistry problems demands molecular level understanding of solvated ion
Non-covalent interaction determines solvated ion structure
ion-water interaction v.s. water-water interaction
Gas-phase cluster ions are ideal model systems to study non-covalent interactions
No counterion and long-range complexities
Easily controlled size and composition
Experimentally obtained structural information
Available high-level ab initio computations
Interest in complex chemistry/biohemistry problems such as ion size-selectivity by ionophores and protein channels has reached a point where an understanding of the nature and behavior of ions in solution at the molecular level is the key to explain and model those systems.
The structure of the solvated ion is predominantly determined by the competition between ion-water Non-covalent interaction and water-water non-covalent interaction.
We study the Non-covalent interaction by probing the Gas-phase cluster ions. Compared with the condensed phase cluster ions, the gas-phase cluster ion is more of an ideal model, because:

4. Introduction
Cluster temperature is determined by binding energy of the most labile ligand
Non argon tagged clusters ~ 350K
Argon tagged clusters ~ 100K
Global minimum-energy conformer is expected for argon tagged clusters
High conformational potential energy conformer trapped in Li+(H2O)3-4Ar clusters [1]
Extend to Rb+(H2O)3-4Ar clusters
Hydrated ion cluster cools down and approach equilibrium state by losing the most labile ligand in the system.
The non argon tagged clusters cools down by losing water which has 40 kJ/mol of binding energy and have a equilibrium temperature of 350K;
Argon tagged clusters cools down by losing argon, which has only 5 kJ/mol of binding energy and therefore results in a much lower equilibrium temperature.
For the For the argon tagger ion clusters, the global minimum-energy conformer is generally expected.
Earlier study has shown that in the Li+(H2O)3-4Ar clusters systems, high conformational potential energy conformer trapped and could coexist with the minimum energy conformer.
We’d like to extend the study to hydrated rubidium clusters where the energy spacing between different conformers are much smaller, therefore requires a more detailed energy analysis.
Now let’s use Lithium system as an example to see how do we study the ion cluster.
[1] Rodriguez, O.; Lisy, J. M. J. Phys. Chem. Lett. 2011, 2 (12), 1444–1448.

5. Energy Analysis IR Photon -[Ar] hν hν L4b L4a Li+(H2O)4Ar Li+(H2O)4
Conformational energy barrier
-[Ar]
Energy hν hν
From ab initio calculations, we know that the L4a configuration with a tetrahedral structure is the minimum energy conformer and the higher energy L4b configuration with a bent hydrogen bond has 9 kJ/mol of conformational potential energy.
The Conformational energy barrier in between denotes the energy required for the L4b conformer to break the hydrogen bond and rearrange into the lowest energy configuration.
The system also has a vibrational energy portion. If the vibrational energy is not enough to overcome the energy barrier, the higher energy conformer will be trapped.
The question here is how do we know which conformer exist in the cluster ensemble? We use the IRPD technique to study the structure of the cluster ion. The IRPD works in this way, If we shoot the infrared laser beam into the cluster ensemble, the conformer could absorb infrared photon and reach an excited state. If the total energy that the excited cluster has is higher than the binding energy of the argon atom in the cluster, the argon atom will be knocked off. We know that different structural conformers have different OH stretching motions and these motions have their own spectral signatures. Therefore, by studying the IRPD spectra in the OH stretching region, we know which conformers are being present.
Conf. P. E. 9 kJ/mol
Vib. E.
Conf. P. E. 0 kJ/mol
[1] Rodriguez, O.; Lisy, J. M. J. Phys. Chem. Lett. 2011, 2 (12), 1444–1448.
L4b
L4a

6. Experimental Apparatus
Triple Quadrupole Mass Spectrometer
Electrostatic Lenses
Conversion Dynode/ Electron Multiplier
Source Chamber
Ion Gun
Detection Chamber
Continuum Surelite II 10 Hz Nd3+:YAG (1064 nm)
Tunable LaserVision OPO/A 1.35~10 µm
Conical Nozzle
DifferentialChamber
Skimmer
Ion Guiding
Quadrupole Ion Guide
Ion Deflector
Ion Guiding
Octapole Ion Guide
Ion Selecting
Quadrupole Mass Filter
Ion Analyzing
Quadrupole Mass Filter
This is the scheme of our apparatus, it consists of two section. Triple Quad spectrometer and light source.
Neutral clusters of water and argon are formed via supersonic expansion through a 30o conical nozzle. Rubidium ions are formed via thermionic emission from a resistively heated tungsten filament coated with an rubidium cloride impregnated paste. The neutral clusters collides with the rubidium ion about 3cm down stream of the nozzle orifice.
The cluster ion then will be guided by electrostatic lens and octapole ion guide through a differential chamber and enter the detection chamber.
The first Quad will select clusters of interest and the mass selected clusters interact with IR photon in the middle of the detection chamber. Fragments will ne guided to the Quad 3 where the fragments are mass selected and data will be recorded on the detector.
Light Source

7. Spectra Analysis Hydrogen-bonded OH stretch Gas phase H2O
O-H symmetric stretching 3657 cm−1 
Gas phase H2O
O-H asymmetric stretching 3756 cm−1 
Hydrogen-bonded OH stretch
[Ar] channel is predominant channel.
Three spectral features, 3550, 3640, 3710;
Gas phase H2O symmetric and asymmetric stretching.
From experience, below 3600 cm-1, the spectral feature has to involve hydrogen-bonded OH stretch, the spectral feature at 3730 and 3770 come from the K rotational subband of C2v symmetry

8. Rb(H2O)3Ar
Conf. P.E. (kJ/mol)
Vib. E. at 100K (kJ/mol)
Ar binding E. (kJ/mol)
H2O binding E. (kJ/mol)
R3a
4.4 5
46.2
R3b 2
3.9
R3c
6.6
R3d
7.9
6.0
R3c
R3a
From gaussian calculation, we found four stable structural isomers of Rb+(H2O)3Ar , Their simulated spectra are plot together with the IRPD spectra to assist spectral assignment.
3540 comes from R3a
3640 comes from R3c, R3d is not very possible.
We have both R3a and R3c features in the Ar loss channel, but the R3c feature dominates in the Ar+H2O channel. No let’s apply a detailed energy analysis

9. Energy Analysis
Rb+(H2O)2
Rb+(H2O)3
Rb+(H2O)3Ar
- [Ar+H2O]
- [Ar]
Energy
Tot. E. 54 kJ/mol
Tot. E kJ/mol hν
Pho. E. 43 kJ/mol hν
Let’s start with the parent Rb+(H2O)3Ar cluster. Upon absorbing a IR photon, it could lose an argon, which requires 5 kJ/mol of energy. It could also lose a water and an argon.
This energy analysis explains the dominance of the R3c features in the argon and water loss channel
Pho. E. 43 kJ/mol
Vib. E. 6.6 kJ/mol
51.2 kJ/mol
Conf. P. E. 4.4 kJ/mol
Vib. E. 4.4 kJ/mol
5 kJ/mol
Conf. P. E. 0 kJ/mol
R3c
R3a

10. Spectra Analysis νfree Free OH Stretch Hydrogen-bonded OH stretch
Three features,
3550 suppressed in the [Ar+H2O]channel
Now let’s move on to the Rb+(H2O)4Ar. Again, the feature at 3640 should be the symmetric OH stretch feature, but the intensity is very low. Based on experience, we know that the symmetric OH stretch and the asymmetric OH stretch should be of similar intensity. Therefore, the feature at 3710 is more likely a free OH stretch feature.
The free Oh stretch is the stretching mode between the oxygen and the free hydrogen atom when the other hydrogen atom is involved in hydrogen bonding. Based on experience, we know that the free OH stretch is usually of higher intensity compared with the asymmetric OH stretch.

11. Vib. Energy at 75K (kJ/mol)
Rb(H2O)4Ar
Conf. P. E. (kJ/mol)
Vib. Energy at 75K (kJ/mol)
Ar binding E. (kJ/mol)
H2O binding E. (kJ/mol)
R4a
4.9
4.5
48.2
R4b
5.6
4.1
R4c
5.7
3.7
R4d
10.8
R4e
13.7
5.1 + =
52.7 kJ/mol
hν = 42.8 kJ/mol
Tot. E. = 52.5 kJ/mol
hν = 44.7 kJ/mol
Tot. E. = 54.4 kJ/mol
R4b
Now we apply the similar spectral and energy analysis.
R4b.
Do a quick energy analysis trying to explain why the H-bonded stretch is suppressed in the [Ar+H2O] channel.

12. Conclusion and Future Work
Hydrated rubidium ion clusters has kinetically trapped conformers.
Rb(H2O)3Ar:
R3c conformer (4.4kJ/mol) is trapped
Rb(H2O)4Ar:
R4b conformer (5.6kJ/mol) is trapped
More accurate energy analysis
Gibbs free energy to replace Conf. P.E.(0K) + Vib. E.(RRKM-EE[2])
We find evidence that:
One thing we will do in the future is applying a more accurate energy analysis. We will calculate the Gibbs free energy directly from the vibrational frequencies, then we will have the energy each conformer has as a function of temperature.
Now the energy is a sum of Conf. P.E.(0K) and Vib. E. calculated at the temperature estimated by the RRKM-EE theory. And is not a function of temperature.
[2] Miller, D. J.; Lisy, J. M. J. Am. Chem. Soc. 2008, 130 (46), 15393–15404.

13. Acknowledgment Prof. James Lisy Dr. Amy Nicely
Dr. Christian van der Linde
Funding
NSF

14. Gibbs Free Energy Analysis