1. AN INFRARED SPECTROSCOPIC STUDY ON THE FORMATION OF THE HYDROGEN BONDED INCLUSION-STRUCTURES IN THE PROTONATED METNANOL WATER CLUSTERS
Marusu katada1, Po-Jen Hsu2, Asuka Fujii1, Jer-Lai Kuo2
(1Tohoku University, Japan,
2IAMS, Academia Sinica, Taiwan)
I’m Marusu Katada
This is title of my talk.

2. Introduction
Protonated methanol(M)-water(W) mixed clusters (H+(M)n-W1)
Size dependence
H-bond network structures
Mass spectrometry (H+MnW1)
H+M9W1
magic
number
cluster ion intensity
A large ring of methanol
water ion core locates on the center
cluster size (n)
For the last several decades, in order to understand microscopic picture of solvation, structures of hydrogen-bonded mixed cluster have been extensively studied.
Size dependence of hydrogen bond networks of this system has attracted much interest.
This is the previously reported mass spectrometry data of protonated methanol water mixed clusters.
Sizes 8 and 9 are magic number.
To explain structure of these magic number clusters, it has been proposed that these size clusters form such a water inclusion-structure.
A large ring of methanol is formed, hydronium ion, locates on the center of this methanol ring.
J. F. Garvey, et. al., J. Am. Chem. Soc., 114, 3684, (1992)
Magic number clusters have been proposed that these size clusters form a water-inclusion structures.

3. Introduction
Protonated methanol(M)-water(W) mixed clusters (H+(M)n-W1)
Size dependence
H-bond network structures
Mass spectrometry (H+MnW1)
H+M9W1
magic
number
cluster ion intensity
Viewpoint of topology
Multi-ring (MR)　structures
No free OH
cluster size (n)
From the view point of hydrogen-bond topology,
This structure is also called multi-ring structures.
These particular structures maximize the number of hydrogen-bonds in cluster with full solvation of the water ion core.
As a results, free OH band is not seen.
J. F. Garvey, et. al., J. Am. Chem. Soc., 114, 3684, (1992)
Magic number clusters have been proposed that these size clusters form a water-inclusion structures.

4. Introduction
Protonated methanol(M)-water(W) mixed clusters (H+(M)n-W1)
Size dependence
H-bond network structures
Mass spectrometry (H+MnW1)
H+M9W1
magic
number
cluster ion intensity
Viewpoint of topology
Multi-ring (MR)　structures
No free OH
cluster size (n)
However, no firm experimental evidence for such multi ring structures.
J. F. Garvey, et. al., J. Am. Chem. Soc., 114, 3684, (1992)
Magic number clusters have been proposed that these size clusters form a water-inclusion structures.
No firm experimental evidence for such multi-ring (MR) structures.

5. This study
H-bond networks of protonated methanol-water H+MnW1 clusters (n = 6-10) by IR spectroscopy and quantum chemical calculations.
Hydrogen-bond (H-bond) network structures depending on size and temperature
Confirmation of multi ring (MR) structure formation
Then, in this study, we explore the hydrogen-bond networks of the protonated methanol-water mixed clusters in this size range,
by IR spectroscopic study and quantum chemical calculations.
We examine size and temperature dependence of hydrogen-bond networks.
We try to confirm multi ring structure formation.
In order to clarify, we measure IR spectra of clusters at two different temperatures.
One is bare clusters, that is relatively warm, and the other is Ar-tagged clusters, that is typically at around 50 K.
The theoretical IR spectra were obtained by statistical weight calculations of possible isomers. MR
Experiments: Warm (bare) and cold (Ar-tagged) clusters
Theory: Statistical weight calculations of possible isomers

6. Experimental H+(M)6-10(W)1 H+(M)6-10(W)1-Ar
He-Ar (95-5%) 60 atm
Infrared photo dissociation spectroscopy
Tandem-type mass spectrometer
object
H+(M)6-10(W)1
H+(M)6-10(W)1-Ar
Vibrational
predissociation
Sample:
methanol-d3 (M)
IVR
v = 1
super sonic jet
electron
ionization
Here I explain the experimental method.
IR spectra were recorded by infrared predissociation spectroscopy using a tandem-type mass spectrometer.
A mass spectrometer was equipped with linearly aligned tandem quadrupole mass filters connected by an octopole ion guide.
This is scheme of the experiment.
The sample of methanol-d3 vapor was seeded in the He-Ar carrier gas, stagnation pressure of 60 atm, and it was expanded to a vacuum chamber.
Cluster ions were generated in the jet by electron ionization using an electron gun.
The cluster ions were size selected by the first quadrupole mass filter.
Then they were introduced into the octopole ion guide.
Within the octopole ion guide, the mass-selected cluster ions were irradiated by IR laser and sent to the second quadrupole mass filter.
The second quadrupole mass filter was tuned to pass only the fragment mass.
IR spectra of size-selected cluster were recorded by monitoring the fragment ion intensity while scanning the IR laser frequency.
IR irradiation
(scan)
Mass selection
(2nd Q-pole)
clusters were generated
v = 0
fragments ion monitor
while scanning IR laser
H+MmW1
Mass selection
(1st Q-pole)

7. Theoretical Calculation
Harmonic Superposition Approximation (HSA)
Systematically search the local minimum structures of H+(MeOH)n(H2O)1 by the multi-scale modeling approach
Relative energetics of structural isomers and all calculated IR spectra are based on B3LYP/6-31+G(d)
HSA to sum up the contribution of all thermally populated isomers
The width of H-bonded OH stretching
𝐼total(𝜔, 𝑇)=Σa𝐼a ω Pa(𝑇)
𝐼a ω = spectral intensity
Pa(𝑇) = population weight of the a-th isomer
scaling factor
Each isomer is regarded as an ensemble of harmonic oscillators.
Its population weight is estimated by statistical weight of the oscillators.
In theoretical calculations, we use Harmonic Superposition Approximation method.
First systematically search the local minimum structures of the　clusters was performed by the multi-scale modeling approach.
All structurally distinct isomers were optimized by using B3LYP/6-31+G(d) and their IR spectra were calculated.
Each isomer is regarded as an ensemble of harmonic oscillators.
Its population weight is estimated by statistical weight of the oscillators, and this factor shows temperature dependence.
Calculated HSA IR spectra were obtained by summation of the population weighted contribution of each isomer spectrum.
The bands width is given by this formula.
This formula is designed to provide larger width with increasing red-shift of the OH band by the hydrogen bond.
x =
β = 1.9
𝜔freeOH = 3678 cm-1
Γ=𝑥(𝜔freeOH−𝜔)𝛽

8. * The Hydrogen bond networks of H+(M)6W1 – (Ar) Ar-tagged bare
anharmonicity ?
Ar-tagged K
bare K
The simulations at 50 K and 100K qualitatively agree with the observed spectra.
Now, I discuss the hydrogen bonding networks in the size 6.
This black line is the observed IR spectrum.
And the red line is the calculated spectrum.
This is the spectrum of Ar-tagged cluster, and the calculated spectrum at 50K is the best fit.
On the other hand, this is spectrum of bare cluster, and this is well simulated at 100K.
The simulations qualitatively agree with the observed spectra.
It is interesting to compare Ar-tagged spectrum with bare spectrum.
The band structure is clearly different.
This indicates isomer distribution changes depending on temperature.
I think difference of here around might be due to anharmonicity.
The band structure is clearly different between Ar-tagged and bare.
(depending on temperature)
Isomer distribution changes.

9. The Hydrogen bond networks of H+(M)6W1 – (Ar)
isomer population
Ar-tagged
Double rings
Single ring K
bare
Relative population K
This graph shows how major isomer changes with elevation of temperature.
These spectral simulations are based on this isomer population, and the qualitative agreement with the observed spectra demonstrates the reliability of this calculated population.
Blue line means isomer population of double ring structures, and green line means isomer population of single ring structures 50
100
The calculated spectra are based on isomer distribution.
Temperature (K)
The qualitative agreement demonstrates the reliability of calculated population.

10. The Hydrogen bond networks of H+(M)6W1 – (Ar)
Relative potential energy
&
Entropy
The relative stability
DR has more number of hydrogen bonds than SR.
DR is more stable than SR.
DR is dominant in the cold condition.
isomer population
Relative population 50
100
Temperature (K)
Double ring structures (DR)
Double rings
By the way,
The relative stability of the clusters is determined by the competition between relative potential energy and entropy.
Double ring structure has more number of hydrogen bonds than single ring structure.
Therefore Double ring structures is more stable than single ring structures.
On the other hand, single ring structures are more flexible than double ring structures
This means single ring structures have more number of low frequency modes in view point of entropy
Therefore, double ring structure is dominant in the cold condition.
and single ring structure is predominant as temperature becomes higher.
because entropy factor.
Single ring structures (SR)

11. The Hydrogen bond networks of H+(M)6W1 – (Ar)
Relative potential energy
&
Entropy
The relative stability
isomer population
Relative population 50
100
Temperature (K)
Single ring
Double ring structures (DR)
By the way,
The relative stability of the clusters is determined by the competition between relative potential energy and entropy.
Double ring structure has more number of hydrogen bonds than single ring structure.
Therefore Double ring structures is more stable than single ring structures.
On the other hand, single ring structures are more flexible than double ring structures
This means single ring structures have more number of low frequency modes in view point of entropy
Therefore, double ring structure is dominant in the cold condition.
and single ring structure is predominant as temperature becomes higher.
because entropy factor.
SR are more flexible than DR.
SR have more number of low frequency modes.
SR is predominant as temperature becomes higher.
Single ring structures (SR)
(entropy factor)

12. The Hydrogen bond networks of H+(M)6W1 – (Ar)
Ar-tagged
50 K
isomer population
Double rings Ar
bare
Single ring
bare
100 K
Relative population
50 K
100 K
Temperature (K)
Ar-tagged
bare
These structures are one of the possible conformers.
From this date and information of temperature by comparing the observed spectra with calculated spectra,
we conclude that the structure of Ar-tagged cluster is such double rings type, and bare cluster is single ring type.
Of course, we should note that this structure is one of the possible conformers.
Many conformers may coexist and contribute to the observed spectra.
Many conformers may coexist and contribute to the observed spectra.
Double ring structures (DR)
Single ring structures (SR)

13. The Hydrogen bond network of H+(M)7W1 – (Ar)
The simulations reproduce well the observed spectra.
Ar-tagged K
The free OH band is missing in the cold condition.
absence of free OH
all OHs form H-bonds
only multi-ring structure
bare K
multi-rings
weak free OH
isomer population
Single ring Ar
bare
Next I show the results of size 7.
This shows IR spectrum of Ar-tagged and simulation at 50K, and spectrum of bare cluster, and this is simulated well at 150K.
The simulations reproduce well the observed spectra.
Here we note the free OH band is missing in the cold condition.
This means all OH groups in the cluster form hydrogen bonds.
This is a unique spectral sign of multi ring structure.
Among all the possible isomers, multi ring structure satisfy this requirement.
Then only multi ring structures are formed in the cold condition.
This is the plot of the calculated isomer population, and this also supports the multi-ring structure at low temperature.
In the warmer bare cluster, a weak free OH band is seen.
This indicates the different isomers also formed.
From this plot, at 150 K, single ring structures are predominant.
We conclude that the major isomer would be single ring structure.
Relative population
50 K
150 K
Temperature (K)
Multi-ring structure (MR)
Single ring structure (SR)

14. The Hydrogen bond network of H+(M)8-10W1 – (Ar)
n = 8 Ar-tagged
n = 8 bare K K
absence of free OH
absence of free OH
n = 9 Ar-tagged
n = 9 bare
n = 10 Ar-tagged
n = 10 bare
These are the IR spectra of size 8, 9 and 10.
We do not perform a quantum chemical calculation at size9 and size 10,
Because the calculation cost is too much.
We see the free OH band disappears in all the spectra.
This indicates multi ring structure is dominant in wide range of temperature in these sizes.
Because more compact structures, like multi ring structure, tend to gain greater binding energy as the size increase.
H+M9W1
Free OH bands are not seen in the observed temperature range.
Only MR structures are formed in these size range. MR

15. H-bond networks of H+MnW1
Warm (100 ~ 150 K)
Cold (~50 K)
H+M6W1-Ar
e.g. H+M6W1
n = 6
Double ring structures
size (H+MnW1)
n = 7
e.g.H+M9W1
Single ring structures
In summary,
In the size 6 and 7, single ring structures are formed in the warm condition.
And double ring (n = 6) and Multi ring (n = 7) structures are formed in the cold condition.
In the size 8, 9 and 10, only multi ring (MR) structures are formed.
n =
Multi-ring structure
Multi-ring structures

16. Mass spectrometry and This study
Cold (~50 K)
Warm (100 ~ 150 K)
Mass spectrometry (H+MnW1)
n = 7 MR SR
magic
number
cluster ion intensity
n =
only MR
cluster size (n)
J. F. Garvey, et. al., J. Am. Chem. Soc., 114, 3684, (1992)
This is mass spectrometry date I showed in the begging of my talk.
Sizes 8 and 9 are magic number.
In my study, I found that multi ring structures actually begins at the size 7.
Why is not size 7 cluster magic number?
this was measured at the warm condition, higher than 200 K.
Then, the observed magic number behavior is consistent with the multi-ring isomer formation in the warm condition
Mass spectrometry
Temperature > ~200 K (warm condition)
The observed magic number behavior is consistent with the multi-ring isomer formation in the warm condition.

17. H-bond networks of H+MnW1
We performed infrared spectroscopy and quantum chemical calculations of the H+M6-10W1 clusters.
The multi ring (MR) structures formation was proved in size n > 6.
Cold (~50 K)
Warm (100 ~ 150 K)
H+M6W1-Ar
e.g. H+M6W1
n = 6
Double ring structure
size (H+MnW1)
n = 7
e.g.H+M9W1
Single ring structure
This is summary of my talk.
We performed infrared spectroscopy and quantum chemical calculations of the protonated methanol water mixed clusters.
We confirmed the formation of multi ring structures in n greater than 6.
Thank you for your attention.
n =
Multi-ring structure
Multi-ring structure

19. Introduction(protonated site)
Protonated methanol(M)-water(W) mixed clusters (H+(M)n-W1)
H-bond network structures
Protonated site: W or M ? or ?
H3O+ ion core
(protonated site)
MeOH2+ ion core
(protonated site)
identification of its protonated site has attracted much interest.
Let me simply consider the preferred protonated site in this mixed system.
The proton affinities of water and methanol are these values.
These values suggest that methanol would be the preferred protonated site.
Proton affinity (kcal/mol)
Methanol :180
Water: 165
Methanol is preferred protonated site?
size n < 7
Linear(L), Tree(T), Single ring(SR), Double ring(DR) type
size n > 8
Tree(T), Multi ring(MR) type
・the protonated site(ion core)
3 < n < 7
coexistence (MeOH2+, H3O+)
n > 8
only H3O+
J. F. Garvey, et. al., J. Phys. Chem. A, 104, 5197, (2000)
N, mikami et. al., J. Phys. Chem. A, 113, 2323, (2009)
and many others

20. This study IR spectroscopy & quantum chemical calculations
of protonated methanol-water H+MnW1 clusters (n = 6-10)
Size and temperature dependence of hydrogen-bond (H- bond) network structures
Confirmation of multi ring (MR) structure formation
Size and temperature dependence of protonated site change (proton switching)
Our second purpose is to study size and temperature dependence of the proton switching.
We determine the critical size for the protonated site change from methanol to water.
Critical size for the protonated site change from methanol to water.
The protonated site can be determined by the free OH stretch region.

21. Analysis of the protonated site by the Free OH stretch region (H+M6W1)
Ar-tagged
water K
If the protonated site is water (H3O+),
a free OH band of a water should be missing.
methanol
If the protonated site is methanol (MeOH2+),a free OH band of methanol and water appear.
bare
sim.
@100 K
Free OH site SR DR
These are expanded spectra in the free OH stretch vibration region.
The spectrum of Ar-tagged cluster shows only free OH band of water.
But the spectrum of bare cluster shows two free OH bands.
They are bands of methanol and water.
If the protonated site is water, its free OH band should be missing.
On the other hand, if a protonated site is methanol, free OH bands of methanol and water appear.
This is because the charged site is preferentially solvated, and only water molecule is included in the cluster.
These spectra indicate, in the cold condition, methanol is the protonated, but in warm condition, both of methanol and water can be protonated.
methanol
water
Charge site is preferentially solvated.
Low temp. Ar-tagged cluster : Free OH of water
MeOH2+
High temp. bare cluster: Free OH both of
methanol and water
H3O+ and MeOH2+

22. Analysis of the protonated site by the Free OH stretch region (H+M6W1)
relative population of protonated site
water
methanol
Ar-tagged K
bare
sim.
@100 K
50 K
100 K
H3O+
Relative population
MeOH2+
From calculation result
in the cold condition (50 K)
This is the calculated relative population of the protonated site.
The vertical axis is relative population of the protonated site, and the horizontal axis is cluster temperature.
This red line is population of methanol ion core and the green line is population of water ion core.
The methanol ion core is preferred in the cold condition.
But the water ion core is preferred in the high temperature.
This simulation supports well the observed switching of the protonated site with temperature.
MeOH2+
Temperature (K)
in the warm condition (100 K)
H3O+ and MeOH2+
The simulation (relative population of the protonated site) also supports the observed protonated site switching with temperature.

23. Analysis of the protonated site by the Free OH stretch region (H+M7W1)
Ar-tagged K
Low temp. Ar-tagged cluster : No Free OH
methanol
formed Multi ring structures
The protonated site should be water
(H3O+).
bare
sim.
@150 K
High temp. bare cluster : only Free OH of methanol
If the protonated site is water,
the free OH band of water is missing.
relative population of ion core site
The protonated site is water (H3O+).
From the free OH region, the protonated site of the size 7 cluster also can be determined.
As we just found, the free OH band disappears in the cold condition, and this means the protonated site is water, because in the multi ring structures, water should be protonated site.
On the other hand, only free OH band of methanol molecules shows in the warm condition.
This indicates also in warm condition, the protonated site is water.
This is simulation of the relative population of the protonated site.
The simulation also predicts the dominance of the water ion core in all the temperature range, and this agrees well with the observation.
H3O+
simulation
low temperature : H3O+
high temperature : H3O+
Relative population
MeOH2+
The simulation agrees well with the observation.
Temperature (K)

24. The Hydrogen bond network structures and ion core site of H+(M)8-10W1 – (Ar)K K
absence of free OH
absence of free OH MR
These are the IR spectra of size 9 and of size10.
Multi ring structure is dominant in wide range of temperature in these sizes.
This indicates multi ring structure means that the protonated site is water.
Only MR structures is formed.
H3O+ is preferred.
The protonated site is water in this size region.

25. SUMMARY (protonated site)
The proton (ion core) switching was also confirmed.
size n = 6
In the cold condition
Protonated site is methanol.
temperature dependence
In the warm condition
Protonated site is water.
H+M6W1 – Ar
(cold condition)
H+M6W1
(warm condition)
size n = 7-10
Protonated site is water regardless of temperature.

26. H-bond structures depending on Dissociation channel
isomer distribution
loss W K
Double rings Ar
bare
Single rings
Tree
Relative population
loss M K
50 K
100 K
Temperature (K)
Loss W
Single ring structures
to dissociate selectively
Loss M
Tree structures

27. Morphology MeOH2+ ion core H3O+ ion core many isomers
Two type multi ring structures, water-inclusion and methanol-inclusion
in order to distinguish,,,
require to measure in the mid-IR region (~1400-~2000 cm-1)

28. Water do not form single acceptor configuration.
typically
~3680 cm-1 Free OH of methanol
~3700 cm-1 Free OH of dangling water
(~ 3650 cm-1 & ~ 3750 cm-1 Free OH of n1, n3)
Free OH of methanol
Free OH of water
single acceptor
configuration
double acceptor configuration
single acceptor – single donor configuration
single acceptor
configuration
n1, n3 are not showed in the observed spectra.
n1, n3
Water do not form single acceptor configuration.
水分子が水素結合ネットワークの末端にあるときだけ対称伸縮と反対称伸縮が現れる
Water does not exist on the end of the hydrogen bonding network.

29. major dissociation channel spectra
H+(M)nW1

30. Discussion
The reason why multi ring structures became main structure with size increase.
blue line = the isomer distribution of multi rig structures
and in the size n > 8, the clusters form only multi ring structures.
Because more compact structures, like multi ring structure, tend to gain greater binding energy as the size increase.
A distortion decreases with size increase.

31. The charged site is preferentially solvated.
If the protonated site is water, the free OH band of water is missing.
charge – dipole interaction > dipole – dipole interaction
The charged site is preferentially solvated.

32. Relative stability of multi-ring structures
MR構造の説明？
Relative stability of multi-ring structures
As the cluster size increases, compact structure (MR & DR) tend to gain greater binding energy.
entropy DG
distortion of H-bond networks
The distortion of H-bond networks of MR structures decrease as a size increase.
size
The calculated Gibbs free energy in Figure 6c favors the open structures
(L, T, C, and Ct) at small sizes (m < 7), because of rich low frequency
modes. On the other hand, the tC and T structures
have much lower Gibbs free energies than the others at m > 8.
The differences among the Gibbs free energies of the different
structures are ∼ 8 kcal/mol at 190 K.

33. The Hydrogen bond network structures of H+(M)8W1 – (Ar)
Ar-tagged K
The intensity of the free OH band is remarkably weak even in bare cluster.
absence of free OH
Main structure is MR.
isomer distribution
bare K Ar
ホットなクラスターでもFreeOHの強度は著しく弱い
bare
The simulation also predicts the dominance of MR in the wide temperature range
(< 150 K).
Multi ring structure (MR)
MR + Single ring structure (SR)

34. Analysis of the protonated site by the Free OH stretch region
Ar-tagged K
Low temp. Ar-tagged cluster : No Free OH
High temp. bare cluster : Free OH of methanol
(very weak)
methanol
bare
sim.
@100 K
low temperature : H3O+ ion core
high temperature : H3O+ ion core
simulation
low temperature : H3O+ ion core
high temperature : H3O+ ion core
The simulation agrees well with the observation.

35. SUMMARY
We performed infrared spectroscopy and quantum chemical calculations of the H+M6-10W1 clusters.
The multi ring (MR) structure formation was proved in n > 6, and the proton (ion core) switching was also confirmed.
multi ring structure
e.g.H+M9W1
H+M6W1 - Ar
H+M6W1