Structures of the cage, prism and book hexamer water clusters from multiple isotopic substitution Simon Lobsiger, Cristobal Perez, 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 68th OSU International Symposium on Molecular Spectroscopy TH05

C.Perez et al., et al., Science 336, 897 (2012)

Chirped-pulse spectrometer improvements:  Improvements in stability of averaging in the multi-FID per one gas pulse mode  Averaging of over 10M FIDs now possible at a rate of 270K/h  Stability is sufficient for coaddition of separate runs  New broadband horn antennas  Up to 5 supersonic expansion nozzles Described in: C.Perez et al., CPL 571, 1-15 (2013)

1 All 16 O 6 Single 18 O 15 Double 18 O 20 Triple 18 O (or triple 16 O) 15 Double 16 O 6 Single 16 O 1 All 18 O = the total number of isotopic species possible for a given hexamer water cluster Since there are three observable hexamer water cluster isomers there are a total of 192 possible species. Isotopic species possible on O substitution in hexamer water clusters:

The BOOK hexamer 4 14  3 03 transition in the 1:3 18 O: 16 O spectrum: 10M averages Single 18 ODouble 18 OTriple 18 O All 16 O * * * * * *

Improvement in S/N in chirped-pulse water cluster spectra: Science, M averages, cage 5 05 ←4 04 CAGE Current 10M averages, 4 14 ←3 03 BOOK Visibility of singly 18 O substituted species Current

The BOOK hexamer 4 14  3 03 transition in the 3:1 18 O: 16 O spectrum: Single 16 O Double 16 OTriple 16 O 9.6M averages All 18 O

Visibility of triply substituted isotopic species: * **** * * * **

Analysis of the spectra: All 16 OAll 18 O CAGE10350  a +  c PRISM5638  a +  b BOOK13751 bb  Data sets for the limiting species of the three conformers were first refined in order to determine confident values of all quartic centrifugal distortion constants. Numbers of lines:  Isotopic data sets ranged from 11 to over 50 lines, depending on substitution multiplicity. The lines were fitted by floating only A,B,C, Δ J, Δ JK and deviations of fit were all around 10 kHz.  Watson’s asymmetric rotor Hamiltonian + programs AUTOFIT, JB95, AABS, SPFIT/SPCAT were used

Structural analysis:  The many different isotopic substitutions can only be accounted for with least-squares structure fitting methods and the main choices are: Experiment:Calculation: r 0 geometry  vibrationally averaged geometry r m or r e SE geometry  equilibrium geometry  Program STRFIT from the PROSPE site used for the analysis (allows r 0, r m (1), r m (1L), r m (2), r e SE fits)

The CAGE water hexamer:

The PRISM water hexamer:

Is the water CAGE hexamer UU{1} or UD{1} ? UD{1} UU{1}

The CAGE water hexamer:

 Improvements in the quality of chirped-pulse spectra of water clusters allowed observaton of all 64 isotopic species for each hexamer cluster that result from 16 O/ 18 O isotopic combinations  The total number of measured water hexamer species is thus 3x64=192  In the r 0 fits progress from 7 to 64 isotopic species increases deviation of fit by a factor of two  For 7 isotopic species the change from r 0 to r m (1) fits improves deviation of fit by a factor of two, and change to 64 isotopic species does not have an appreciable further effect on the deviation of the r m (1) fit  The improvement in precision in OO distances with 64 isotopic species is close to a factor of five, which is greater than from the square root of the ratio of the used isotopic species (64/7) 1/2  3  The r m (1) model seems to be the optimum for determining the oxygen framework geometry for this cluster size (r m (2) does not fare well, while attempts to move outside the oxygen framework by deuteration are in progress) CONCLUSIONS: