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IDENTIFICATION OF THE CAGE, PRISM, AND BOOK ISOMERS OF WATER HEXAMER AND THE PREDICTED LOWEST ENERGY...
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Transcript of IDENTIFICATION OF THE CAGE, PRISM, AND BOOK ISOMERS OF WATER HEXAMER AND THE PREDICTED LOWEST ENERGY...
IDENTIFICATION OF THE CAGE, PRISM, AND BOOK ISOMERS OF WATER HEXAMER AND THE PREDICTED LOWEST ENERGY HEPTAMER AND
NONAMER CLUSTERS BY BROADBAND ROTATIONAL SPECTROSCOPY
CRISTÓBAL PÉREZ, MATT T. MUCKLE, DANIEL P.ZALESKI, NATHAN SEIFERT, BROOKS H. PATE. Department of Chemistry. University of Virginia.
ZBIGNIEW KISIEL. Institute of Physics, Polish Academy of Sciences. Poland.
BERHANE TEMELSO, GEORGE C. SHIELDS, Dean’s Office, College of Arts and Sciences, and Department of Chemistry, Bucknell University.
Previous Studies
Dimer Trimer Tetramer Pentamer
Liu, K.; Brown, et al. Nature 1996, 381, 501. Liu, K.; Brown, M.G.; Cruzan, J.D.; Saykally, R.J. Science 1996, 271, 62. Pugliano, N.; Saykally, R.J. Science 1996, 257,1 937. Cruzan, J.D. et al Science 1996, 271, 59.
Hexamer “Cage”
Saykally’s group
Extensive work in water clusters by THz VRT Spectroscopy observing trimer, tetramer, pentamer and “cage” form of the water hexamer.
Figures from R. J. Saykally and D. J. Wales, Science 336, 814 (2012)
Water Hexamer
Extensively studied ab-initio as a benchmark system. Structural transition point wherein three-dimensional structures become more
stable than the planar ring structures for the smaller clusters. Several low-energy isomers predicted: Prism, Cage and Book within an energy
separation of 1 kJ/mol or less.
Figures from: Bates, D.M.; Tschumper, G.S. J. Phys. Chem. A 2009, 113, 3555
Water Hexamer
Cyclic, planar, S6 hexamer has been found using HENDI Spectroscopy K. Nauta, R. E. Miller, Science, 287, 293 (2000).
Hexamer Cage has been found using terahertz VRT Spectroscopy Liu, K. et al. Nature, 381, 501 (1996).
Hexamer book found using IR spectroscopy coupled with size-selection techniques in OH stretch region. Steinbach, C. et al. PCCP, 6, 3320 (2004). E. G. Diken, et al., J. Phys. Chem. A 108, 64 (2004).
No direct Structural Information
CP-FTMW Experiment
• CP-FTMW Spectroscopy in the 7-18 GHz frequency range.
• Optimal results found using a gas mixture of neon blown over a reservoir of DI water at a total pressure of 6 atm @ RT.
• 18O isotopomer measurements with a reservoir mixture of 1:6 H2
18O/H216O using Ne as carrier
gas at a total pressure of 6 atm @ RT.
Brown, G. G.; Dian, B.C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. Rev. Sci. Instru. 2008, 79, 053103.
Results
• The water dimer shows the most intense spectrum..• Hexamer Cage shows a 200:1 signal-to noise ratio.• Hexamer prism is the only one to show tunneling.• Heptamer “prism” and Nonamer also assigned on basis of rotational constants.• Trimer, tetramer and pentamer are nonpolar in their ground state.
650,000 averages spectrum and simulations for the assigned species
Science 336, 897-901 (2012).
Results
Experimental Theory
EquilibriumVib.
Averaged
Hexamer Cage
A /MHzB /MHzC /MHz
2161.8762(8)1129.3153(5)1066.9627(5)
2249.21149.81097.9
2172.4 1129.2 1070.3
Hexamer Prism
A /MHzB /MHzC /MHz
1658.224(4)1362.000(2)1313.124(2)
1697.21416.61358.3
1644.5 1373.6 1312.5
Hexamer Book
A /MHzB /MHzC /MHz
1879.4748(8)1063.9814(6)775.0619(3)
1898.31106.3809.5
1856.1
1080.5 788.7
Vibrationally averaged structures provided by Shields group.
Science 336, 897-901 (2012).
Results
Experimental
Theory
Equilibriu
mVib. Avg.
Heptamer PR1
A /MHzB /MHzC /MHz
1304.4355(3) 937.8844(6) 919.5236(6)
1333.7 974.1 954.2
1297.5 950.7 926.8
Nonamer D2dDD
A /MHzB /MHzC /MHz
774.7442(7)633.5403(6)570.6460(5)
810.9 630.6 585.9
791.2 619.8 574.1
Science 336, 897-901 (2012).
In Neon estimated population ratio for cage:prism:book is 4:4:1. In Argon only the cage is observed, giving evidence that the cage global-
minimum energy structure for the water hexamer. Heptamer and Nonamer are observed in argon global-minimum
configurations.
Population and Isotopic Substitution
Assignment of 6 separate rotational spectra, each onewith a single 18O water incorporated in the structureof the cluster.
Science 336, 897-901 (2012).
† - prism transitions‡ - book transitions^ - heptamer transitions
Ruoff, R.S et al. J. Chem. Phys. 1990, 93, 3142.
Population and Isotopic Substitution
Science 336, 897-901 (2012).
None of them showing fine structure Tunneling quenched upon isotopic substitution
Population and Isotopic Substitution
Science 336, 897-901 (2012).
Population and Isotopic Substitution
(A,B,C) = (IA,IB,IC)
I = Σ mi∙ri2
(Aˊ,Bˊ,Cˊ) = (IAˊ,IBˊ,ICˊ)
|ra|,|rb|,|rc|
H218O
Hexamer Substitution Structures
Equilibrium StructuresB. Temelso, et al.
B. Temelso, K. A. Archer, G. C. Shields, J. Phys. Chem. A, 115, 12034 (2011).
Vibrationally averagedstructures provided by Shields group
NEXT TALKR0 Structure.
Heptamer Substitution Structure
To get all 18O transitions resolvable at signal-to-noise ratio > 3:1.
Reduced bandwidth Spectra (250MHz-1GHz)
Much deeper average was required.
New oscilloscope made this possible, recording 2 million FIDs overnight
(NS Spectrum 650.000 FIDs in two days with the old Scope).
Repetition rate is limited by pumping speed.
100Gs/s 33GHz 12x Faster.
Heptamer Substitution Structure
Seven separate broadband spectra upon isotopic substitution!
414 303
250MHz Bandwidth (7500MHz-7750MHz) and 1.4million averages.
Heptamer Substitution Structure
MP2/6311G++(d,p) vs rs structure
B. Temelso, K. A. Archer, G. C. Shields, J. Phys. Chem. A, 115, 12034 (2011).
NEXT TALK!
Structural analysis to obtain two experimental structures.
Ro structure Rm1 structure
RH03
Residual Spectrum
Acknowledgements
National Science Foundation (NSF) grants CHE-0960074 and CHE-0848827.
The Institute of Physics, Polish Academy of Sciences (Poland).
National Science Foundation (NSF) grants CHE-0116435, CHE-0521063, and CHE-0849677 as part of the MERCURY high-performance computer consortium (www.mercuryconsortium.org).
Thanks for your attention!