Deuterated Water Dimer Jacob T Stewart and Benjamin J McCall Department of chemistry University of Illinois Why water clusters Water is ubiquitous on Earth and essential to life Complicated molecular structure due to hydrogen bonding ID: 654731
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Slide1
Rotationally-Resolved Spectroscopy of the Bending Modes of Deuterated Water Dimer
Jacob T. Stewart
and Benjamin J. McCall
Department of chemistry, University of IllinoisSlide2
Why water clusters?
Water is ubiquitous on Earth and essential to life
Complicated molecular structure due to hydrogen bonding
Studying small water clusters aids in understanding interactions between water moleculesSlide3
What do we know about water dimer?
(H
2
O)
2
and (D
2O)2 extensively studied in microwave and far-IR (rotations and intermolecular modes)Data used to develop potential energy surfacesIntramolecular stretches have been measured at high resolutionNo rotationally-resolved spectra of bending modes
far-IR probes
inter
molecular vibrations
mid-IR
probes
intra
molecular
vibrationsSlide4
Previous work on bending modes of water dimer
Gas phase spectra of (H
2
O)
2
observed by cavity
ringdown spectroscopyNo rotational resolution, difficult to determine band centers
Could not observe tunneling patterns
Paul et al.,
J. Phys. Chem. A, 103, 2972 (1999).Slide5
Previous work on bending modes of water dimer
Spectra taken in the
Saykally
group of a He/D
2
O expansion
Possible hints of (D2O)2 features
Laser stopped working (damaged mirrors)
Huneycutt
, PhD thesis, University of California, Berkeley, 2003.Slide6
Tunneling in water dimer
Keutsch
, F. N., &
Saykally
, R. J.
PNAS
, 98 (2001) 10533.
Three large amplitude motions lead to tunneling between 8 equivalent minima
Splittings
caused by tunneling can be observed experimentallySlide7
rigid
dimer
acceptor
switching
interchange
bifurcation
Experimentally determined
splittings
are a measure of barriers on the potential energy surface
Keutsch
, F. N., &
Saykally
, R. J.
PNAS
,
98
(2001) 10533.
Bottom half are “
1’s
”
Top half are “
2’s
”
Tunneling in water dimerSlide8
Expected band structure
Either perpendicular (
Δ
K
a
= ±1) or parallel bands (
ΔKa = 0)Selection rules only allow 1s ↔ 1s or 2s ↔ 2s
Two sets of bands separated by acceptor switching tunneling
Each set composed of three bandsSlide9
Producing and measuring clusters
Clusters were generated in a continuous supersonic slit expansion (150 µm × 1.6 cm)
Gas was bubbled through D
2
O at room temperature
Ar
at
~250 torr
He at
~900
torrUsed cavity
ringdown spectroscopy to obtain spectrumSlide10
Overview of the spectrum
Ar
expansion
Most features also present in He
Studies with D
2O/H
2O mixtures confirm (D2O)
2Slide11
Identifying (D2O)2 bands
K
a
= 1 ← 0 band of donor bend
R(0) lines confirm assignment
Actually three overlapping bandsSlide12
Identifying (D2O)2 bands
K
a
= 2 ← 1 band of donor bend
Lack of R(0) lines confirm assignment
Actually three overlapping bandsSlide13
Other component of acceptor switching splitting
2.4 cm
-1
1’s
2’s
K
a
= 1
←
0Slide14
Other component of acceptor switching splitting
0.9 cm
-1
1’s
2’s
K
a
= 2
← 1Slide15
Acceptor switching splitting in the excited state
Using previous estimates of Paul et al. for the ground state, we can calculate excited state splitting
For
K
a
= 1 in excited state, acceptor switching splitting is 19 GHz (17 GHz in ground state)For Ka
= 2 in excited state, acceptor switching splitting is 44 GHz (42 GHz in ground state)Exciting donor bend has little to no effect on acceptor switchingSlide16
Trying to assign interchange tunneling levels
Exciting donor bend perturbs interchange tunnelingSlide17
Band center
Band center can be calculated from assignment
After taking tunneling into account, band center is 1182.2 cm
-1
About 10 cm
-1
lower than matrix studiesClose agreement with calculations on ab initio surfaceSlide18
Conclusions
Observed
first rotationally resolved spectrum of donor bend of water dimer
Found excitation of donor bend has basically no effect on acceptor switching tunneling
Excitation of donor bend appears to perturb the interchange tunneling, making detailed fit
difficult
Additional bands should be accessible with more widely tunable laser
TJ12, 2015 McPherson, 4:40Slide19
Acknowledgments
McCall Group
Claire
Gmachl
Richard
Saykally
Springborn
Endowment
http://bjm.scs.illinois.eduSlide20Slide21
Determining cluster size
Add H
2
O to sample and observe how lines decrease
Assume statistical ratio of D
2
O, H2O, and HODCluster size can be determined by a linear relationship21
Cruzan et al.,
Science
,
271
(1996), 59.
Slide22
Determining cluster size
Our data from cluster of lines near 1195.5 cm
-1
Measured each concentration 10 times
Slope = 3.9 ± 0.2
Consistent with dimer