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Structures of the lowest energy nonamer and decamer water clusters from chirped-pulse rotational spectroscopy Cristobal Perez, 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 68th OSU International Symposium on Molecular Spectroscopy TH08

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The genealogy of water nonamer and decamer clusters: Assigned + 18 O analysis Assigned E /kcal mol -1

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Complete sets of singly 18 O 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 18 O species was eventually resolved out by careful consideration of relative intensities Structural analysis encountered several difficulties: r s analysis was hindered by multiple imaginary coordinates r m (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 :

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Structural analysis: The isotopic sets consisting of the parent and all single 18 O isotopic species can be treated in several ways: Experiment:Calculation: r s geometry unclear r 0 geometry vibrationally averaged geometry r m or r e SE geometry equilibrium geometry Programs KRA and EVAL were used for the r s and STRFIT for the r m (1) evaluations, all from the PROSPE website

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Imaginary coordinates in substitution analysis of water nonamers: 9-D1 5 imaginary coordinates for cluster 9-D1 3 for cluster 9-S1 3 for cluster 9-S2 All are c-coordinates r s /Å

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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 18 O substituted pairs are: O2, O3 O5, O6

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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 r s coordinates of the oxygen atoms

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The perplexing misalignment between experimental and calculated inertial coordinates: Relative magnitudes of apparent ground state dipole moment components: a ++ b +++ c

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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

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The performance of least-squares geometry fits for the nonamer clusters: The fits are to: 30 rotational constants 3N-6 = 21 internal coordinates define the O framework Equal pillars assumption reduces these to 18 for r 0 ( 21 for r m (1) )

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

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Complete sets of singly 18 O 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 : r s /Å

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The water decamers (H2O) 10 = stacked pentameric rings Identical foreground rings, but co- or contra-rotating background rings

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O...O distances in clusters and bulk water: Liu, Brown, Cruzan, Saykally, J.Phys.Chem. A 101, 9011 (1997) ?

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A.K.Soper, Chem.Phys. 258,121 (2000) neutron diffraction revised to 2.80 Å by improved deconvolution of g OO,g OH,g HH 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, (2007) X-ray Raman: liquid = 2.81 Å ice Ih = 2.76 Å Nearest neighbour averages ( 94 values) : = Å = Å Cluster OO distances ( 9 clusters, 255 values)

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18 O 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 18 O substitution there are visible effects of ground state averaging of hydrogen atom positions The structures of 9 water clusters determined so far by 18 O substitution lead to an average ground state OO distance that is only 0.02Å longer than that in liquid water CONCLUSIONS:

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