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Water clusters observed by chirped-pulse rotational spectroscopy: Structures and
hydrogen bonding
Cristobal Perez, Matt T. Muckle, Daniel P. Zaleski, Nathan Seifert, Brooks H. PateDepartment of Chemistry, University of Virginia, Charlottesville, Virginia, USA
Zbigniew KisielInstitute of Physics, Polish Academy of Sciences,
Warszawa, Poland
Berhane Temelso, George C. ShieldsBucknell University, Lewisburg, Pennsylvania, USA
67th OSU International Symposium on Molecular Spectroscopy RH0367th OSU International Symposium on Molecular Spectroscopy RH03
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
IFPANBF, cavity
VIRGINIAchirped
Hexamer structure determination I: Kraitchman equations
2/1)(|| parentsubst
zz IIMM
MMz
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 = r0Experiment = rs
Calculation = vibrationally averaged MP2/aug-cc-pVDZ
Try out the rotatable 3D models in Fig.3 of Perez et al., Science 336, 897 (2012)
expt., r0
calc., vibrationallyaveraged MP2/aug-cc-pVDZ
9 hydrogen bonds
all 6 H2O’s with 3 H-bonds
E = + 0.0 kJ/mol ?!
Water detachment energy (kJ/mol)
8 hydrogen bonds
2 H2O’s with 2 H-bonds4 H2O’s with 3 H-bonds
uu{1}
E = + 0.9 kJ/mol ?!
7 hydrogen bonds
4 H2O’s with 2 H-bonds2 H2O’s with 3 H-bonds
E = + 2.7 kJ/mol
The smallest water clusters and O...O distances:
2.98 Å 2.84 Å2.80 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)
?
A.K.Soper, Chem.Phys. 258,121 (2000)
neutron diffraction revised to 2.8 Å
Hexamer O...O distances and the radial distribution function for liquid water:
Book: 2.886 Å
Prism: 2.802 ÅCage: 2.852 Å
Cage + Prism + Book,r0 distances
Uwe Bergmann et al.JCP 127,174504 (2007) X-ray Raman:
liquid = 2.81 Åice Ih = 2.76 Å
Long standing OO distance in liquid water = 2.84 Å(neutron diffraction)
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:
r0 = 2.848 Å liquid = 2.81 Å
rs = 2.835 Å
rm(1) = 2.823 Å
rm(1) compared with equilibrium ab initio
Similarity between the (H2O)7 and the prism (H2O)6 structures:
r0 compared with vibrationally averaged ab initio
Edisp Ees
All other H-bonds:Edisp Ees/2
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)
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