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!