Structures of the lowest energy nonamer and decamer water clusters from chirped-pulse

rotational spectroscopy

Cristobal Perez, 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

68th OSU International Symposium on Molecular Spectroscopy TH0868th OSU International Symposium on Molecular Spectroscopy TH08

The genealogy of water nonamer and decamer clusters:

Assigned + 18O analysis AssignedDE /kcal mol-1

Complete sets of singly 18O substituted species were assigned for three out of the five assigned nonamer species

In the case of the nonamer3 spectroscopic species, which is assigned to cluster 9-D1, there were initially two missing species but effective degeneracy of two pairs of 18O species was eventually resolved out by careful consideration of relative intensities

Structural analysis encountered several difficulties:

rs analysis was hindered by multiple imaginary coordinates

rm(1) analysis only reached satisfactory numerical stability after

an ab initio based simplifying assumption of equal length pillars connecting the upper and lower rings

The water nonamer clusters, (H2O)9 :

Structural analysis:

The isotopic sets consisting of the parent and all single 18O isotopic species can be treated in several ways:

Experiment: Calculation:

rs geometry unclear

r0 geometry vibrationally averaged

geometry

rm or reSE geometry equilibrium

geometry Programs KRA and EVAL were used for the rs and STRFIT for

the rm(1) evaluations, all from the PROSPE website

Imaginary coordinates in substitution analysis of water nonamers:

9-D1

5 imaginary coordinates for cluster 9-D13 for cluster 9-S13 for cluster 9-S2

All are c-coordinates

rs /Å

The underlying reason for nonamer structural difficulties:

Oxygen atoms O1, O4, O7, O8, O9 are all very close to the ab inertial plane so that their c coordinates are very small.

9-D1

The two degenerate 18O substituted pairs are:

O2, O3O5, O6

Comparison of calculation and experiment for the nonamers:

The complete geometry is for RI-MP2/aug-cc-pVDZ calculation and the blue water units are at the transition point between two alternative minima

The smaller spheres are experimental rs coordinates of the oxygen atoms

The perplexing misalignment between experimental and calculated inertial coordinates:

Relative magnitudes of apparent ground state dipole moment components:

a ++

b +++c -

9-S2 9-S2TS 9-S2A

Misalignment is caused by orientation of just one hydrogen atom:

The diagrams compare ab initio principal coordinates (complete water molecules) with experimental substitution coordinates (small circles)

The responsible hydrogen is in the nonbonded OH belonging to the only water unit bound by two (not three) hydrogen bonds

The performance of least-squares geometry fits for the nonamer clusters:

The fits are to: 30 rotational constants3N-6 = 21 internal coordinates define the O frameworkEqual pillars assumption reduces these to 18 for r0 (21 for rm

(1))

The water nonamers (H2O)9: the pattern of short/long OO distances identifies the species

Complete sets of singly 18O substituted species were assigned for two out of the four assigned decamer species

The structural analysis turned out to be easier than for the nonamers due to lack of imaginary coordinates (although the two blue oxygen atoms are close to the ac plane)

The water decamer clusters, (H2O)10 :

rs /Å

The water decamers (H2O)10 = stacked pentameric rings

Identical foreground rings, but co- or contra-rotating

background rings

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 2.80 Åby improved deconvolution of gOO,gOH,gHH

Water cluster O...O distances and the radial distribution function for liquid water:

Long standing OO distance in liquid water = 2.84 Å(neutron diffraction)

Uwe Bergmann et al.JCP 127,174504 (2007) X-ray Raman:

liquid = 2.81 Åice Ih = 2.76 Å

Nearest neighbour averages (94 values):

<r0 > = 2.824 Å<rm

(1) > = 2.802 Å

Cluster OO distances (9 clusters, 255 values)

18O substitution resulted in determination of oxygen framework geometries for 3 water nonamer clusters, and 2 water decamer clusters

Consideration of the patterns of short-long OO distances allowed unambiguous assignment of spectroscopic species to ab initio calculated carriers

The oxygen framework geometries all turn out to be for the most stable clusters of a given size:

the nonamers are D1, S1 and S2 (E = 0, 1.0, 1.4 kJ/mol resp.)

the decamers are PPD1, PPS1 (E = 0, 0.2 kJ/mol resp.)

Even at the level of oxygen framework geometries as determined by 18O substitution there are visible effects of ground state averaging of hydrogen atom positions

The structures of 9 water clusters determined so far by 18O substitution lead to an average ground state OO distance that is only 0.02Å longer than that in liquid water

CONCLUSIONS: