Transcript 67th_OHIO_shishido_ver5.pptx

June 19, 2012
TE06
INFRARED
SPECTROSCOPY OF
(CH3)3N-H+-(H2O)n (n = 1-22)
Ryunosuke Shishido, Asuka Fujii
Department of Chemistry, Graduate School of Science,
Tohoku University, Japan
Jer-Lai Kuo
Institute of Atomic and Molecular Sciences,
Academia Sinica, Taipei, Taiwan
Introduction
Water clusters (H2O)n
A microscopic model of bulk water
Molecular level understanding of H-bond network structures
Definitely size selective spectroscopy has never been performed
for n > ~10
Size-selective IR spectroscopy of large and neutral water clusters
Strategy
Excess charge
Mass spectrometric techniques
Minimum effects of the excess charge to the hydrogen-bond network of water
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・Addition of a metal ion enables us to select cluster sizes
even in the large-size region
M(H2O)36, M = I-, Cs+, Co2+, Mo3+
Evan R. Williams and co-workers
J. Am. Chem. Soc., 2011, 133 (13), 4810
H-bond network structures are largely different from the neutral water clusters
・A high proton affinity "tag“ with an excess proton, which
never releases the excess proton
The water moiety would be neutral
(H2O)n・・・・H+X
proton
3
tag
Trimethylamine, (CH3)3N, TMA
Protonated trimethylamine-water clusters
TMA can act only as a single proton acceptor in H-bond networks
TMA locates at a terminal of H-bond networks
TMA has high PA of 225 kcal/mol
The excess proton would localize
in the TMA moiety
H+
TMA-H+-(H2O)21
The water moiety in TMA-H+-(H2O)n is expected to be neutral
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This study
・Whether the water moiety is neutral in TMA-H+-(H2O)n ?
・ IR predissociation spectroscopy of TMA-H+-(H2O)n
(n = 1-22)
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Mass spectrum
21
TMA-H+-(H2O)n
H+-(H2O)n
21
22
22
・H+-(H2O)n
A magic number appears at
n = 21
+
Closed-cage structure
・TMA-H+-(H2O)n
No Magic number behavior
is seen at n = 21
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Free OH stretch bands of TMA-H+-(H2O)n
1-coord (ν1)
(ν3)
2-coord
n=1
3-coord
n=2
1-coord
ν1:3650 cm-1
ν3:3750 cm-1
n=3
n=4
n=5
n=6
n=7
n = 10
n = 15
n = 20
3-coord
2-coord
・The ν1 and ν3 band of 1-coord water (terminal of
H-bond network) disappears at n ≧ 6
・The band of 2-coord water shows high-frequency
shift
・The band of 3-coord water (branching point)
starts to appear at n = 6
n = 21
n = 22
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Development into closed(-cage) network
at size n = 6
H-Bonded OH stretch bands of TMA-H+-(H2O)n
・At n ≧ 6, the band at 3200 cm-1
becomes prominent
n=4
Increase of 3-coord sites
n=5
n=6
Development into closed(-cage)
network at size n = 6
n=7
This size is much smaller than H+-(H2O)n
[n~10 in the case of H+(H2O)n]
n = 10
3-coord
Wavenumber / cm-1
H+-(H2O)6
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(H2O)6
Comparison of TMA-H+-(H2O)n and H+-(H2O)n
in the free OH stretch region
H+-(H2O)n
TMA-H+-(H2O)n
3-coord
2-coord
・H+-(H2O)n
n = 20
Intensity of the 2-coord water
band suddenly decreases
at n = 21 (closed-cage formation)
n = 21
・TMA-H+-(H2O)n
No sudden decrease at n = 21
n = 22
Wavenumber / cm-1
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H+ is not transfer to
the water moiety
Wavenumber / cm-1
Summary
・We observed IR spectra of size selected TMA-H+-(H2O)n n = 1~22
・No magic number is seen in TMA-H+-(H2O)n
・The observed spectra are different from those of H+-(H2O)n n = 1~22
・The excess proton localizes in the TMA moiety at any size
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Experiment ~IR predissociation spectroscopy~
v=1
[ TMA-H+-(H2O)n ]
n = 1-22
discharge
mass
selection
1st-Qmass
[ TMA-H+-(H2O)n-1]
+ H2O
hIR
dissociation
[ TMA-H+-(H2O)n]
octopole ion guide
mass
selection
2nd-Qmass
[ TMA-H+-(H2O)n-1 ]
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IR spectra of TMA-H+-(H2O)21 and H+-(H2O)21
TMA-H+-(H2O)21
The spectrum of TMA-H+-(H2O)21
is different from H+-(H2O)21
The excess proton localizes
in the TMA moiety at large size
H+-(H2O)21
Wavenumber / cm-1
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