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

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, 225-245. Reviews: H. C. Chang, C. C. Wu, J. L. Kuo, Int. Rev. Phys. Chem. 2005, 24, 553-578; N. Agmon, H. J. Bakker, R. K. Campen, R. H. Henchman, P. Pohl, S. Roke, M. Thämer, A. Hassanali, Chem. Rev. 2016, 116, 7642-7672; 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, 9425-9440. 1

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 ∼ 3650 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, 553-578. 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, 1765-1769. 2

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, 9425-9440. 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, 1765-1769; S. Karthikeyan, K. S. Kim, J. Phys. Chem. A 2009, 113, 9237-9242; S. S. Xantheas, Can. J. Chem. Eng. 2012, 90, 843-851. 3

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, 5282-5288; 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, 1137-1140; J. A. Fournier, C. J. Johnson, C. T. Wolke, G. H. Weddle, A. B. Wolk, M. A. Johnson, Science 2014, 344, 1009-1012. 4

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, 769-775; K. L. Aplin, R. A. McPheat, J. Atmos. Sol.-Terr. Phys. 2005, 67, 775-783. 5

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

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

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 4850-7350 cm−1 2050-4550 cm−1 Nd:YAG 10 Hz 8

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

Hydronium 2.03 Å 10

Zundel 11

Eigen Controversy: H. Wang, N. Agmon, J. Phys. Chem. A 2017, 121, 3056-3070. 12

Overview of H+(H2O)n with n=1-4 13

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

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

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!!!