Transcript wclust_struct.pptx

67th OSU International Symposium on Molecular Spectroscopy
RH03
Water clusters observed by chirped-pulse
rotational spectroscopy: Structures and
hydrogen bonding
Cristobal Perez, Matt T. Muckle, Daniel P. Zaleski, Nathan Seifert, Brooks H. Pate
Department of Chemistry, University of Virginia, Charlottesville, Virginia, USA
Zbigniew Kisiel
Institute of Physics, Polish Academy of Sciences,
Warszawa, Poland
Berhane Temelso, George C. Shields
Bucknell University, Lewisburg, Pennsylvania, USA
Comparison of chirped pulse and Balle-Flygare type spectra:
 Transitions for the cage water hexamer (frequencies/MHz)
 Chirped pulse spectrum = 8 GHz width recorded overnight
 Balle-Flygare spectrum = 1 MHz width recorded in 10 minutes
VIRGINIA
chirped
IFPAN
BF, cavity
Hexamer structure determination I: Kraitchman equations
 M  M subst parent 
| z | 
( I z  I z )
 M M

 1/ 2
Substitution oxygen atom coordinates (Å) :
Hexamer structure determination II: least squares r0 geometry
 Oxygen framework geometry requires determination of 3N-6=12 internal
structural parameters
 21 experimental moments of inertia are available (parent+six 18O species)
 Fit made with the STRFIT program from PROSPE website
Bobs-Bcalc < 0.31 MHz
Evolution of assumptions concerning the indeterminable structural
parameters of the hydrogens:
Assessed by the value of the total deviation of the least squares fit
(in uÅ2 units)
Comparison of experiment (dark red) and
calculation (semi-transparent):
Experiment = rs
Experiment = r0
Calculation = vibrationally averaged MP2/aug-cc-pVDZ
Try out the rotatable 3D models in Fig.3 of Perez et al., Science 336, 897 (2012)
E = + 0.0 kJ/mol ?!
Water detachment energy (kJ/mol)
expt., r0
9 hydrogen bonds
all 6 H2O’s with 3 H-bonds
calc.,
vibrationally
averaged
MP2/aug-cc-pVDZ
E = + 0.9 kJ/mol ?!
uu{1}
8 hydrogen bonds
2 H2O’s with 2 H-bonds
4 H2O’s with 3 H-bonds
E = + 2.7 kJ/mol
7 hydrogen bonds
4 H2O’s with 2 H-bonds
2 H2O’s with 3 H-bonds
The smallest water clusters and O...O distances:
2.98 Å
2.84 Å
2.80 Å
2.76 Å
Saykally + Wales, Science 336, 814 (2012)
O...O distances in clusters and bulk water:
?
Liu, Brown, Cruzan, Saykally, J.Phys.Chem. A 101, 9011 (1997)
Hexamer O...O distances and the radial distribution function for liquid
water:
Long standing OO distance in liquid water = 2.84 Å
(neutron diffraction)
A.K.Soper, Chem.Phys. 258,121 (2000)
neutron diffraction revised to 2.8 Å
Uwe Bergmann et al.
JCP 127,174504 (2007)
X-ray Raman:
liquid
ice Ih
= 2.81 Å
= 2.76 Å
Cage + Prism + Book,
r0 distances
Book: 2.886 Å
Cage: 2.852 Å
Prism: 2.802 Å
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.
The O...O distance is a known measure of hydrogen bond strength
and in the hexamer isomers these span the range 2.70 to 3.01 Å.
This is a departure from the more uniform picture for (global
minimum) clusters up to the water pentamer in which symmetry
enforces a single O...O distance, systematically decreasing with
cluster size.
The shortest O...O distance in the hexamers is shorter than 2.76 Å in
the pentamer, whereas the longest hexamer O...O distance exceeds
2.98 Å for the water dimer.
The water detachment energies show large variation but
correlate with the O...O distances and with the number of
hydrogen bonds to each water monomer.
The water heptamer, (H2O)7:
 All seven single 18O species assigned
 rs analysis without imaginary coordinates
(unlike in the hexamers)
 Least squares r0 fit possible but rm(1)
geometry is even better: very low
deviation of fit of 0.0069 uA2
 The rm(1) geometry probably accounts for
hydrogen indeterminacies and compares
well with ab initio equilibrium
 Average nearest neighbour OO distances:
rm(1) compared with equilibrium ab initio
r0
= 2.848 Å
rs
= 2.835 Å
rm(1) = 2.823 Å
liquid = 2.81 Å
Similarity between the (H2O)7 and the prism (H2O)6 structures:
Edisp  Ees
All other H-bonds:
Edisp  Ees/2
r0 compared with vibrationally averaged ab initio
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.
C.Perez et al., et al., Science 336, 897 (2012)