Transcript whex.pptx

68th OSU International Symposium on Molecular Spectroscopy
TH05
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
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)
Isotopic species possible on O substitution in hexamer water clusters:
1
All 16O
6
Single 18O
15
Double 18O
20
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.
The BOOK hexamer 414 303 transition in the 1:3 18O:16O spectrum:
10M averages
*
*
Triple 18O
Double 18O
* **
Single 18O
*
All
16O
Improvement in S/N in chirped-pulse water cluster spectra:
Visibility of singly 18O substituted species
Current
10M averages, 414←303 BOOK
Current
Science, 2012
0.55M averages, cage 505←404 CAGE
The BOOK hexamer 414 303 transition in the 3:1 18O:16O spectrum:
9.6M averages
All
18O
Single 16O
Double 16O
Triple 16O
Visibility of triply substituted isotopic species:
*
*
*
*
*
**
*
*
*
Analysis of the spectra:
 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:
All
16O
All
18O
CAGE
103
50
a + c
PRISM
56
38
a + b
BOOK
137
51
b
 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

rm or reSE geometry

vibrationally averaged
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 CAGE
water hexamer:
CONCLUSIONS:
 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)1/23
 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)