1. Stepwise Internal Energy Control for Protonated Methanol Clusters
June 23, 2016
International Symposium on Molecular Spectroscopy
Champaign-Urbana, Illinois, USA
Stepwise Internal Energy Control
for Protonated Methanol Clusters
by Using Inert Gas Tagging
Takuto Shimamori,1 Jer-Lai Kuo,2 Asuka Fujii1
1Department of Chemistry, Graduate School of Science,
Tohoku University,
Sendai, Japan
2Institute of Atomic and Molecular Sciences,
Academia Sinica,
Taiepi, Taiwan

2. Preferred H-bond structures depend on temperature
phase transition of water
ice
liquid water
1 bar
T = 273 K
(Ohmine, MD simulation)
low temp.
high temp.
G = H - TS
(low enthalpy
low entropy)
(high enthalpy
high entropy)
The essentially same phenomenon is expected for gas phase clusters

3. A simple model cluster system:
Relative populations of isomer types in H+(CH3OH)5
Calculated statistical isomer population
under the harmonic approximation
B3LYP/6-31+G(d) L
Population
Temperature / K
(+7.95 kJ/mol)
Cyclic (C)
Liner (L)
flexible, higher vib. state density
microscopic phase transition
“melting” of
the cluster C
(0 kJ/mol)
Isomer population depends on temperature
rigid, lower vib. state density

4. IR spectroscopy of H+(MeOH)5 in the different temp. ranges
Infrared spectroscopy of the size-selected cluster in the OH stretch region
Two extreme cases for the cluster generation;
1. Electronic ionization (~200V) + supersonic jet expansion cooling
H+(MeOH)5 vibrational temperature ~200 K
2. Ar-”tagging” (attachment of an Ar atom)
Thermal vibrational energy of the cluster is restricted to be lower than the binding energy with the “tag” (Ar atom)
H+(MeOH)5-Ar vibrational temperature < ~100 K 4

5. IR dissociation spectroscopy with a tandem Q-mass spectrometer
v=1 IR
H+M4 + M
v=0
H+M5
(H+M5 + Ar)
(H+M5-Ar)
pulsed IR light
(IR OPO/OPA,
LaserVision) 5

6. We have observed only the two extreme cases.
Temperature dependence of the spectra of H+(MeOH)5
Obs. : bare cluster
Simulations
(temp. dependent spectra which reflect isomer population)
Intermediate
temperature ?
(“melting point”)
Obs. : Ar-tagged
cluster
We have observed only the two extreme cases.
Y. C. Li et al., PCCP 2015, 17,

7. Binding energy with the tag
Introduction
Stepwise control of temperature
(internal energy) of protonated clusters
A simple method to control temperature (internal energy)
of protonated clusters
Use of several different tag species
which have different binding energies
H+(MeOH)5-X ; X = C6H6 (Bz), C2H2, CS2, CO, and CO2
Binding energy with the tag
Effective temperature
This study
Experimental confirmation of the principle of the method by IR spectroscopy of these model clusters

8. Introduction Cold ion trap?
Universal technique: temperature controlled ion trap
Even in a cold ion trap experiment, a tag should be introduced to keep the constant fragmentation efficiency in IR dissociation spectroscopy.
However, once a tag is introduced, cluster temperature is in fact restricted by the binding energy with the tag
(at least in relatively high temperature range (100 – 200 K))
Equivalent to the present method

9. Temperature tuning of the n=5 cluster by changing the tag
T = ~200 K
T = 192 K
stronger
tag
(high T)
T = 141 K (L)
164 K (C)
T = 125 K
T = 123 K
weaker
tag
(low T)
T = 120 K
T = 85 K

10. Temperature tuning of the clusterAr
Cyclic
(C)
Linear (L)
Cluster at the “melting point”
ωB97XD / G(3df, 3pd)
Temperature tuning can be performed by changing the tag

11. Anomaly : H+(MeOH)5 with Ne tagging
cyclic + linear ! Ar
cyclic
w/o tagging
linear
Effect by rapid cooling (kinetic trap to local minimum)?
We need to be careful for the case of very low temperature.

12. Summary ▪ IR spectroscopy of H+(MeOH)5 with various tag species
▪ Successful observation of the cluster at the “phase transition”
temperature
▪ Variable control of cluster temperature (internal energy)
by changing the tag species
▪ Anomaly in the tagging should be cared in the case
of very low temperature (weak binding energy)

13. (theoretical analyses) Institute of Atomic and
Collaborators
Fujii Group
(laser spectroscopy)
Tohoku University
(Japan)
Kuo Group
(theoretical analyses)
Institute of Atomic and
Molecular Sciences
(Taiwan)

15. bC Ct L Results and discussion Simulation of n=7 Major population
　3 types of isomers:
　　Linear (L)
　　Cyclic with tail (5,6-member) (Ct)
　　Bicyclic (bC)
　Major population bC Ct L
<100 K
100 ~ 200 K
>200 K

16. Results and discussion IR spectra of n=7 with different tags
H-bonded OH
IR spectra of n=7
with different tags
Bare
Free OH Bz
Bare (Linear)
・Free OH (3678 cm-1)
・H-bonded OH (<3400 cm-1)
Ar-tag (Bicyclic)
・Characteristic H-bonded OH
　　(3400 ~ 3500 cm-1)
Large
C2H2
OH/p CO
Binding energy
CO2
CS2
H-bonded OH
From the spectral similarity Ar
Bz, C2H2-tag Linear
CS2-tag Bicyclic
Small

17. Results and discussion IR spectra of n=7 with different tags
Bare Bz
CO, CO2-tag
・Different from Linear and Bicyclic.
・Observation of Free OH
Large
C2H2 CO
H-bonded OH CO
CO2
Ct(6)
Ct(5)
3000
3200
3400
3600
Wavenumber / cm-1
@ ωB97XD / G(3df, 3pd)
Free OH
Binding energy
CO2
CS2 Ar
Small
Existence of Cyclic-tail (Ct)

18. Results and discussion
Maximum Temperature
@ ωB97XD / G(3df, 3pd)
Table of Tmax (K) Bz
C2H2 CO
CO2
CS2 L
136 98 70 66 68
Ct(6)
160
116 72
107 85
Ct(5)
154
112 67 97 80 bC
118
115 75 88
・Three distinct isomers are observed by tuning the internal energy
　of the cluster.
・The experimental observation is not necessary consistent with
　the calculated prediction.

19. Quantum-harmonic superposition approximation (Q-HSA)
Relative population (Pa) of an isomer is calculated as a function of temperature (T) by the partition function under the harmonic approximation
a cluster is treated as
an ensemble of
harmonic oscillators
sum of partition functions of
all the possible isomers =
( )
the relative population is a function of temperature (b)

20. Vibrational temperature under the Ar “tagging”
Inert gas (Ar) “tagging” (attachment)
Thermal vibrational energy is lower than the binding energy with the “tag”
harmonic vibrational partition function
thermal vibrational energy at T (b)
Tmax = K

21. Carrier gas dependence of the spectra of H+(MeOH)5
・Ne and H2 –tagged clusters
show the coexistence of the
cyclic and linear isomers
H+(MeOH)5-Ne
Ne 9 MPa
H+(MeOH)5-H2
He+H2 9 MPa
(He 90%)
・No carrier gas dependence
but clear tag dependence
H+(MeOH)5-Ar
Ar 9 MPa
・The third body species
in the collisional cooling
process is not important
H+(MeOH)5-Ar
He+Ar 9 MPa
(He 95 %)
・The tag atom (molecule)
plays an essential role in
the trapping at the local
minimum ??
H+(MeOH)5
Ar 9 MPa