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. 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 TH0568th 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 16O 6 Single 18O15 Double 18O20 Triple 18O (or triple 16O)15 Double 16O 6 Single 16O 1 All 18O-----------------------------------------------------------------64 = 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 414 303 transition in the 1:3 18O:16O spectrum:10M averages

Single 18ODouble 18OTriple 18O All 16O

** **

*

*

Improvement in S/N in chirped-pulse water cluster spectra:

Science, 20120.55M averages, cage 505←404 CAGE

Current10M averages, 414←303 BOOK

Visibility of singly 18O substituted species

Current

The BOOK hexamer 414 303 transition in the 3:1 18O:16O spectrum:

Single 16O Double 16O Triple 16O

9.6M averages

All 18O

Visibility of triply substituted isotopic species:

*** * * ** * **

Analysis of the spectra:

All 16O All 18O

CAGE 103 50 a + c

PRISM 56 38 a + b

BOOK 137 51 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, ΔJ, ΔJK 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:

r0 geometry vibrationally averaged

geometry

rm or reSE geometry equilibrium

geometry

Program STRFIT from the PROSPE site used for the analysis (allows r0, rm

(1), rm(1L) , rm

(2) , reSE fits)

The CAGE water hexamer:

The PRISM water hexamer:

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

UD{1} UU{1}

The CAGEwater 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 16O/18O isotopic combinations

The total number of measured water hexamer species is thus 3x64=192

In the r0 fits progress from 7 to 64 isotopic species increases deviation of fit by a factor of two

For 7 isotopic species the change from r0 to rm(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 rm

(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)1/23

The rm(1) model seems to be the optimum for determining the oxygen framework

geometry for this cluster size (rm(2) does not fare well, while attempts to move

outside the oxygen framework by deuteration are in progress)

CONCLUSIONS: