Transcript MK02.ppt

68th. International Symposium on Molecular Spectroscopy
Ohio State University, Columbus, Ohio, USA
June 17, 2013
Infrared spectroscopy of ((CH3)3N)n-H+-H2O
(n=1-3): Structures and dissociation
channels of protonated mixed clusters
around a magic number
Ryunosuke Shishido, Asuka Fujii
Department of Chemistry, Graduate School of Science,
Tohoku University, Japan
Jer-Lai Kuo
Institute of Atomic and Molecular Sciences, Taiwan
A magic number of ((CH3)3N)n-H+-H2O
protonated mixed clusters
n=2
3
4
(CH3)3N : trimethylamine
(TMA)
X = TMA
closed shell formation
at the magic number (n=3)
Mass spectrum of (TMA)n-H+-H2O
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Castleman and coworkers,
Chem. Phys. Lett. 1991, 178 ,411-418
J. Am. Chem. Soc., 1991, 113 ,1960-1969
proton affinity TMA 225 kcal/mol
H2O 165 kcal/mol
Dissociation channels of (TMA)n-H+-H2O
The major dissociation channel in the meta stable decay of
(TMA)n-H+-H2O is H2O-loss in n3 while it switches to TMA-loss in n4
The water molecule at the center preferentially evaporates ??
- H2O
Wei. S. et al., J. Am. Chem. Soc.,
1991, 113 ,1960-1969
closed shell structure
charge-dipole structure
Large rearrangement of the cluster structure occurs prior to the dissociation?
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The present study
Infrared dissociation spectroscopy of size-selected (TMA)n-H+-H2O (n=1-3)
in the OH and CH stretch region
Cluster structures (at the magic number)
Preferential location of the excess proton
Measurements of the dissociation channels of the clusters upon the vibrational excitation
Dissociation channels of the clusters of which
structures are determined by IR spectroscopy
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Mass spectrum of (TMA)n-H+-H2O
H+(TMA)2
(TMA)n-H+-H2O
n=1
n=2
H+(TMA)3
n=3
n=4
The n=3 cluster shows
the magic number behavior
as previously reported
H+(TMA)1
H+(TMA)4
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cluster ion source:
supersonic jet expansion
with discharge
(Castleman and coworkers, 1991).
Observed and simulated IR spectra of (TMA)1-H+-H2O
Obs. (H2O loss)
nNH
freeOH
n1
n3
freeOH
H+
nCH
Strong bands in the 2800
– 3200 cm-1 region
nNH
ωB97X-D / 6-311+G(2d,p), S.F.=0.94
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A single stable structure
(the excess proton localizes
at the TMA moiety)
The excess proton
vibration (N-H+ stretch)
and Fermi mixing
(see MK12&13)
IR spectra of
(TMA)2-H+-H2O
Hydrogen-bonded type (2I)
(b)Calc.
(a)Obs.
nCH
(H2O loss)
nNH
freeOH
nOH
freeOH
nCH
DE0 = 0.0kJ/mol
Charge-dipole type (2II, 2III, 2IV)
H-bonded OH/NH frequencies
(c)Calc.
nNH
DE0 = +0.3 kJ/mol
overestimated under
the harmonic approx.
(d)Calc.
nCH
nNH
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freeOH
nCH
DE0 = +1.0 kJ/mol
Isomer 2I is most probable to
interpret the rise of strong
absorption at 2700 cm-1
freeOH
nCH
(e)Calc.
DE0 = +24.4 kJ/mol
： excess proton
nNH
freeOH
Free OH stretch region of the IR spectrum of (TMA)2-H+-H2O
n1
n3
2I
2II
(a)Obs.
(b)Calc.
2I
2III
DE0 = 0.0kJ/mol
(c)Calc.
2II
DE0 = +0.3 kJ/mol
(d)Calc.
DE0 = +1.0 kJ/mol
2III
The two free OH stretch bands in the observed spectrum
indicates the contribution of 2II and/or 2II (total ~80% to 2I)
Coexistence of the H-bonded type (2I) and
charge-dipole type (2II/2III) structures
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IR spectra of (TMA)3-H+-H2O
(a)Obs.
(H2O loss)
Closed shell structures
(3I and 3II)
(b)Calc.
Charge-dipole structure (3III)
DE0 = 0.0
kJ/mol
No free OH
(c)Calc.
DE0 = +2.0
kJ/mol
structures
Only closed shell
contribute to the observed
spectrum
3I (H3O+ ion core) ?
or
3II (H+TMA ion core)?
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(d)Calc.
DE0 = +21.0
kJ/mol
： excess proton
Location of the excess proton in (TMA)3-H+-H2O
(a)Obs.
the strong absorption at ~2900 cm-1
H2O loss
sharp component
CH stretches
(b)Calc.
DE0 = 0.0 kJ/mol
broad component
H-bonded
OH stretches
(c)Calc.
DE0 = +2.0 kJ/mol
： excess proton
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3II (H+TMA ion core) contributes
to the observed spectrum
(coexistence of 3I is not excluded)
Summary of the observed structures
of (TMA)n-H+-H2O (n = 1 - 3)
Vibrational excitation of these
structure-determined clusters
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What dissociation channels
are open?
Dissociation channels upon
vibrational excitation
2I
3I
The central water preferentially evaporates!
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mass spectra of fragments
Dissociation energies of (TMA)n-H+-H2O (n=2&3)
isomer
fragment
H 2O
Dissociation energy / kJ/mol
M06-2X/6-311+G(2d,p)
ωB97X-D/6-311+G(2d,p)
34.2
29.7
2II
D38.7
2I
TMA
H 2O
72.4
28.9
68.4
29.7
D38.7
2II
TMA
H 2O
67.1
55.3
68.4
47.9
TMA
H 2O
55.6
50.1
57.7
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3I
3I
D9.8
D9.8
3II
TMA
50.4
Smaller energy difference between the H2O-loss
and TMA-loss channels in n=3.
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2I
3II
51.8
Larger dissociation to
the TMA channel in n=3
Summary
 We determined the structures of the (TMA)n-H+-H2O clusters by size selective
infrared spectroscopy and obtained the firm evidence for the closed shell
structure at the magic number (n=3) .
 The preferential evaporation of water, which locates at the center of the Hbond network, was confirmed.
 Large rearrangement of the closed shell structure upon the dissociation was
suggested.
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IR photodissociation spectroscopy of
mass-selected cluster cations
v=1
(TMA)n-H+-H2O
(n=1-3)
jet expansion
+ discharge
1st-Qmass
hn IR
size-selection
(TMA)n-H+-H2O
octopole ion guide
(TMA)n-1-H+-H2O
+ TMA (or -H2O)
2nd-Qmass
mass-selection
(TMA)n-H+-H2O
(or H+(TMA)n)
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Quantum chemical calculations
Stable structures, relative energies,
and IR spectra of the clusters
density functionals：ωB97X-D、M06-2X、B3LYP
basis set ：6-311+G(2d,p)
ZPE and BSSE corrections
scaling factor (0.94 at ωB97X-D)
IR simulations do not show remarkable functional-dependence
Because of the large molecular size of TMA, the dispersion
should be important to evaluate the intermolecular interactions
The energy evaluations by the dispersion-corrected functionals,
ωB97X-D (and M06-2X), were mainly referred
The ωB97X-D functional is used for IR simulations
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IR spectra calculated by the one - dimensional scan method
2
NH+ stretch
large low-frequency shift
to ~2000 cm-1
OH stretch
no much difference from
the scaled harmonic
frequency
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IR spectra of (TMA)3-H+-H2O obtained by
monitoring two dissociation channels
(a)Obs.
(TMA)3H+-H2O has two active dissociation
channels (H2O-loss and TMA-loss)
(b)Obs.
(c)Calc.
ΔE0 = 0.0 kJ/mol
Two dissociation channels give us
the essentially same spectra
(d)Calc.
ΔE0 = +2.0 kJ/mol
(e)Calc.
ΔE0 = +21.0 kJ/mol
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Comparison of the spectra of
(TMA)3-H+-H2O and H+(H2O)4
The OH stretch band frequency of
(TMA)3-H+-H2O is higher than that of
H+(H2O)4 in spite of the larger proton
affinity of TMA than water
The central water should be neutral
(not protonated)
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Relative energies of the isomers at different calculations levels
All units are kJ/mol
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Proton affinities
Water
Benzene
Methanol
Ethanol
Naphthalene
Acetone
Phenol
Pyrene
Anthracene
Azulene
Pyridine
Trimethylamine
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165.2
179.3
181.9
185.6
191.9
194.1
195.3
207.7
209.7
221.1
222.3
225.1
all units in kcal/mol