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

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

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

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

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

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, 22042.

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

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

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

Temperature tuning of the cluster Ar Cyclic (C) Linear (L) Cluster at the “melting point” ωB97XD / 6-311++G(3df, 3pd) Temperature tuning can be performed by changing the tag

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.

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)

(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)

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

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

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 / 6-311++G(3df, 3pd) Free OH Binding energy CO2 CS2 Ar Small Existence of Cyclic-tail (Ct)

Results and discussion Maximum Temperature @ ωB97XD / 6-311++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.

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)

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 = 50 - 90 K

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