Deuterated water hexamer observed by chirped-pulse rotational

spectroscopy

International Symposium on Molecular Spectroscopy, 69th MeetingChampaign-Urbana, June 16-20, 2014

L. Evangelisti, C. Perez, S. Lobsiger, N.A. Seifert, D.P. Zaleski, B.H. PateDepartment of Chemistry, University of Virginia

Z. Kisiel Institute of Physics, Polish Academy of Sciences. Poland.

B. Temelso, G.C. Shields Dean’s Office, College of Arts and Sciences, and Department of Chemistry, Bucknell University

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: Temelso B. et al. Phys. Chem. A 2011, 115, 12034

RI-MP2/CBS low energy isomers of (H2O)6 sorted in order of increasing electronic energy

Previous studies

• 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.

CAGE

BOOK

PRISM

Previous studies

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 is the global-minimum energy structure for the water hexamer.

∆E* = +0.22

∆E = +0.07

δZPE δZPE

* kcal/mol: CCSD(T)~MP2+∆MP2_CCSDt Anharmonic VPT2/MP2/CBS+ +∆MP2_CCSDt

Computational studies

Electronic energy

ZPE – (H2O)6

ZPE – (D2O)6

Replica Exchange Path Integral MD

CP-FTMW

• CP-FTMW Spectroscopy in the 2-18 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.

• 18O isotopomer measurements with a reservoir mixture of 1:6 D2

18O/D216O using Ne as carrier

gas at a total pressure of 6 atm.

cage prism book (H2O)6 (D2O)6 (H2O)6 (D2O)6 (H2O)6 (D2O)6

A/MHz 2161.88745(72) 1932.0564(24) 1658.22093(75) 1493.9052(12) 1879.47557(24) 1661.1986(16)B/MHz 1129.31549(40) 1012.7078(12) 1361.99984(80) 1218.3566(12) 1063.98110(17) 940.6792(28)C/MHz 1066.96280(39) 940.8584(11) 1313.12449(74) 1185.6460(11) 775.06295(13) 690.23483(77)DJ/kHz 0.6851(31) 0.4978(66) 0.5274(92) 0.4094(94) 0.9222(12) 0.452(14)DJK/kHz -0.169(17) [ 0.] 2.642(40) 1.662(58) 1.0201(50) 0.851(47)DK/kHz 2.704(69) 1.58(13) -1.861(41) -0.961(63) 1.2682(62) [ 0.]dJ/kHz 0.0229(17) 0.0245(81) 0.0617(58) 0.0511(63) 0.12496(59) -0.0121(91)dK/kHz 0.44(16) [ 0.] -3.65(15) -2.92(27) 2.1097(58) [ 0.]N linesb 103 35 56 51 137 25σ/kHzc 5.82 7.76 6.81 8.23 5.65 9.33

Results

∆E = +0.22

∆E = +0.07

δZPEδZPE

∆E = +0.07

Results

There is remarkable agreement between experiment and computation concerning the variation in O...O bond distances

... allowing more confident, detailed insight into the hydrogen bonding network for each isomer.  

r0

all Hr0

all Dr0

all Hr0

all Dr0

all Hr0

all D

Avg.O···O 2.885 2.876 2.851 2.846 2.803 2.796∆O···O(D-H) -0.009 -0.005 -0.007

PRISM CAGE BOOK

Results

Acknowledgements

Pate Group

NSF grants CHE-0960074

MC-IOF 328405

Thank you for your attention

Questions / Comments?

This (work) provides a consistent picture of the diversity of hydrogen bonding appearing at the hexamer cluster level ...

... which is a small scale prelude to the known diversity in the structure of liquid water.

Computational studiesMP2 CCSD(T)~MP2+∆MP2_CCSDt

Harmonic Anharmonic Harmonic Anharmonic

PR-D2O 0.18 0.22 0 0

CA-D2O 0.08 0.11 0.08 0.07

BK-D2O 0 0 0.21 0.17

PR-H2O 0.33 0.33 0 0.07

CA-H2O 0.17 0.08 0.02 0

BK-H2O 0 0 0.06 0.13

Relative CCSD(T)/CBS harmonic Gibbs free energy (∆∆G0) of the prism, cage and book hexamers as a function of temperature for (H2O)6 and (D2O)6.

T / KT / K

Computational studiesMP2 CCSD(T)~MP2+∆MP2_CCSDt

Harmonic Anharmonic Harmonic Anharmonic

PR-D2O 0.18 0.22 0 0

CA-D2O 0.08 0.11 0.08 0.07

BK-D2O 0 0 0.21 0.17

PR-H2O 0.33 0.33 0 0.07

CA-H2O 0.17 0.08 0.02 0

BK-H2O 0 0 0.06 0.12

Relative CCSD(T)/CBS anharmonic Gibbs free energy (∆∆G0) of the prism, cage and book hexamers as a function of temperature for (H2O)6 and (D2O)6.

Computational studiesMP2

Harmonic Anharmonic

PR-D2O 0.18 0.22

CA-D2O 0.08 0.11

BK-D2O 0 0

PR-H2O 0.33 0.33

CA-H2O 0.17 0.08

BK-H2O 0 0CCSD(T)~MP2+∆MP2_CCSDt

Harmonic Anharmonic

0 0

0.08 0.07

0.21 0.18

0 0.07

0.02 0

0.06 0.14

PR-D2O

CA-D2O

BK-D2O

PR-H2O

CA-H2O

BK-H2O

Electronic energy

ZPE – (H2O)6

ZPE – (D2O)6