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)