1. Near-Infrared Spectroscopy of Small Protonated Water Clusters
International Symposium on Molecular Spectroscopy
J. Philipp Wagner, David C. McDonald II,
Anne B. McCoy and Michael A. Duncan
Jun 20, 2017
Department of Chemistry
University of Georgia
Athens, Georgia 30602

2. Accomodation of the solvated proton
Small protonated water clusters serve as model systems to understand structure of dilute acids in a bottom up approach
Mid-IR spectroscopy and computational chemistry used to unveil structures
Eigen and Zundel as two main structural motifs
Eigen
Zundel C3
2650 cm−1 C2
1000 cm−1
M. Eigen, Angew. Chem. Int. Ed. Engl. 1964, 3, 1-19.
G. Zundel, H. Metzger, Z. Phys. Chem. 1968, 58,
Reviews: H. C. Chang, C. C. Wu, J. L. Kuo, Int. Rev. Phys. Chem. 2005, 24, ; N. Agmon, H. J. Bakker, R. K. Campen, R. H. Henchman, P. Pohl, S. Roke, M. Thämer, A. Hassanali, Chem. Rev. 2016, 116, ; J. A. Fournier, C. T. Wolke, M. A. Johnson, T. T. Odbadrakh, K. D. Jordan, S. M. Kathmann, S. S. Xantheas, J. Phys. Chem. A 2015, 119, 1

3. A AD AAD νs ∼ 3650 cm−1 νas ∼ 3740 cm−1 ∼3715 cm−1 ∼3690 cm−1
Free OH stretches and their characteristic IR absorptions A AD
AAD
νs ∼ cm−1
νas ∼ 3740 cm−1
∼3715 cm−1
∼3690 cm−1
Diminishes with cluster size
Vanishes at H+(H2O)11
Dominant for 8, 9 and 10 water molecules
Appears at H+(H2O)7
Dominant for H+(H2O)11 and bigger clusters
Review: H. C. Chang, C. C. Wu, J. L. Kuo, Int. Rev. Phys. Chem. 2005, 24,
J. M. Headrick, E. G. Diken, R. S. Walters, N. I. Hammer, R. A. Christie, J. Cui, E. M. Myshakin, M. A. Duncan, M. A. Johnson, K. D. Jordan, Science 2005, 308, 2

4. Global minimum energy structures
Both Eigen and Zundel motifs
Smaller clusters open
Ring isomers from 7 onwards
Cage structures at 10 water molecules
Magic number 21 is highly symmetric
Figure: J. A. Fournier, C. T. Wolke, M. A. Johnson, T. T. Odbadrakh, K. D. Jordan, S. M. Kathmann, S. S. Xantheas, J. Phys. Chem. A 2015, 119,
J. M. Headrick, E. G. Diken, R. S. Walters, N. I. Hammer, R. A. Christie, J. Cui, E. M. Myshakin, M. A. Duncan, M. A. Johnson, K. D. Jordan, Science 2005, 308, ; S. Karthikeyan, K. S. Kim, J. Phys. Chem. A 2009, 113, ; S. S. Xantheas, Can. J. Chem. Eng. 2012, 90, 3

5. Magic number cluster H+(H2O)21
Fenn: unusually large 21mer in MS
Analogy to methane clathrate
Single AAD stretch in free OH region
H3O+ unit in surface, H2O in the cage
J. Q. Searcy, J. B. Fenn, J. Chem. Phys. 1974, 61, ; J.-W. Shin, N. I. Hammer, E. G. Diken, M. A. Johnson, R. S. Walters, T. D. Jaeger, M. A. Duncan, R. A. Christie, K. D. Jordan, Science 2004, 304, ; J. A. Fournier, C. J. Johnson, C. T. Wolke, G. H. Weddle, A. B. Wolk, M. A. Johnson, Science 2014, 344, 4

6. Global minimum energy structures
Mid-IR extensively studied in the past
Exploratory Near-IR spectra by Y. T. Lee and H.-C. Chang
NIR absorptions are important for the radiative balance of the Earth (protonated water dimer and trimer are fairly abundant in the atmosphere)
H+(H2O)3
H+(H2O)4
Need for an extensive study of NIR:
Entire accessible spectral range
More cluster sizes
Comparison to VPT2 computations
H+(H2O)4
H+(H2O)5
C. C. Wu, C. Chaudhuri, J. C. Jiang, Y. T. Lee, H. C. Chang, J. Chin. Chem. Soc. 2002, 49, ; K. L. Aplin, R. A. McPheat, J. Atmos. Sol.-Terr. Phys. 2005, 67, 5

7. Ions generated in pulsed electric discharge/supersonic expansion
Experimental setup I
Ions generated in pulsed electric discharge/supersonic expansion
Ar carrier gas seeded with water vapour
Mass-selection of ions of interest
Photodissociation with tunable OPO/OPA laser system
Review: M. A. Duncan, Rev. Sci. Instrum. 2012, 83. 6

8. Produced Cluster Sizes
A broad distribution of protonated water clusters is observed. 7

9. Experimental setup II
OPO
OPA
1 crystal
angle tuned
4 crystals
signal
(not used)
idler
532 nm
1064 nm
KTP
oscillator
KTA
diff. gen.
+ amp of
idler beam
Tunable
cm−1
cm−1
Nd:YAG
10 Hz 8

10. Photodissociation
Photodissociation of mass-selected clusters
IR spectrum is collected as fragmentation yield vs. laser wavelength 9

11. Hydronium
2.03 Å 10

12. Zundel 11

13. Eigen
Controversy: H. Wang, N. Agmon, J. Phys. Chem. A 2017, 121, 12

14. Overview of H+(H2O)n with n=1-413

15. Comparison of larger cluster sizes
Broad band around 5300 cm−1 due to H-bound H3O+
Two sharp bands around 7200 cm−1 assigned to νs and νs+νas
Sharp bands vanish at larger cluster sizes due to decrease of A-type water molecules 14

16. Summary
We have measured NIR spectra of small protonated water clusters up to H+(H2O)8
VPT2 theory is in good agreement with experimental data
Further high-resolution studies are desirable due to the atmospheric relevance of the small clusters 14

17. Thanks for your attention!!!
Acknowledgment
Prof. Dr. Michael A. Duncan
David C. McDonald II
Prof. Dr. Anne B. McCoy
Alexander von Humboldt-Foundation
Thanks for your attention!!!