1. Hydrogen Bond Ring Opening and Closing in Protonated Methanol Clusters Probed by Infrared Spectroscopy with and without Ar-Tagging Toru Hamashima, Kenta Mizuse, Yuta Suzuki, Asuka Fujii Tohoku University, Sendai, Japan Jer-Lai Kuo Institute of Atomic and Molecular Sciences, Taipei, Taiwan June 20, 2011 66 th International Symposium on Molecular Spectroscopy

2. Hydrogen bond network structure: temperature dependence a major aim in spectroscopic studies of hydrogen-bonded (H-bonded) clusters structure determination of H-bond networks most stable structures and their cluster size dependence a system at finite temperature (“real world”) not only the most stable structure but also higher energy structures should be important Can we observe large structural changes of clusters with temperature (internal energy) change? +10.3 kJ/mol0 kJ/mol entropy ?

3. Relative population thermal population calculated under the harmonic approximation Temperature (K) Ref : J. Phys. Chem. A 114. 3096 (2010) Temperature dependence of the structures of protonated methanol clusters H + (MeOH) n H + (MeOH) n simple H-bond networks low temperature cyclic structure (and its variations) enthalpy factor (more number of H-bonds) high temperature linear structure entropy factor (flexibility) significant temp. dependence of the dominant isomer structure cycliclinear +7.9 kJ/mol0 kcal/mol H + (MeOH) 5

4. The present work Infrared spectroscopy of size-selected clusters in the OH stretch region H + (MeOH) n and H + (MeOH) n -Ar (n=5-7) Ar-tagging: suppression of the internal (vibrational) energy (lower than the binding energy with Ar: ~500 cm -1 ) pulsed discharge (~400V) + supersonic jet expansion cooling without Ar-tagging (attachment of an Ar atom) vibrational temperature ~200 K with Ar-tagging vibrational temperature < ~100 K we focus on n>4,where linear and cyclic structures compete

5. H+MnH+Mn H + M n-1 + M (H + M n + Ar) IR v=0 v=1 (H + M n -Ar) IR dissociation spectroscopy with the tandem Q-mass spectrometer

6. Observed spectra of H + (MeOH) n and H + (MeOH) n -Ar

7. 220024002600280030003200340036003800 wavenumber (cm -1 ) H + (MeOH) 5 H + (MeOH) 5 -Ar IR spectra of H + (MeOH) 5 free OH H-bonded OH

8. 220024002600280030003200340036003800 wavenumber (cm -1 ) linear structure cyclic structure H + (MeOH) 5 H + (MeOH) 5 -Ar × 1/2 Observed isomer structures of H + (MeOH) 5 +7.9 kJ/mol 0 kJ/mol complete switching with Ar tagging H. –C. Chang et al., JPCA 103, 2941 (1999) low pressure high pressure (B3LYP/6-31+G(d) scaling factor 0.974)

9. 220024002600280030003200340036003800 wavenumber (cm -1 ) H + (MeOH) 6 -Ar H + (MeOH) 6 ×1/2 IR spectra and structures of H + (MeOH) 6 cyclic with tail linear +10 kJ/mol 0 kJ/mol

10. 220024002600280030003200340036003800 wavenumber (cm -1 ) H + (MeOH) 7 H + (MeOH) 7 -Ar IR spectra of H + (MeOH) 7 disappearance of the free OH

11. 24002600280030003200340036003800 wavenumber (cm -1 ) 2200 ×1/2 H + (MeOH) 7 H + (MeOH) 7 -Ar Observed isomer structures of H + (MeOH) 7 bicyclic linear +10.3 kJ/mol 0 kJ/mol

12. n = 6n = 5n = 7 without Ar with Ar Ring closing and opening of H + (MeOH) n with and without Ar tagging H-bond ring closing and opening occur with and without Ar-tagging, respectively

13. Temperature (K) What is the origin of the isomer selectivity? linear cyclic population (B. Dan et al., JPCA 114, 3096 (2010)) temperature dependence of the relative population of the isomers (partition function calc. with harmonic frequencies) Ar tagging well cools down the cluster? w/ Arw/o Ar H + (MeOH) 5

14. Is this temperature effect? – H + (MeOH) 5 with Ne tagging Ne Ar w/o tagging cyclic cyclic + linear ! The observed preference of the low energy cyclic isomers cannot be simply attributed to the lowering of the vibrational temperature. linear

15. Ne-tagging of H + (MeOH) n (n=6 and 7) w/o tagging The essentially same spectra were obtained for the Ar and Ne tagging.

16. Cooling by NeCooling by Ar (Ne) trapped at local minima fall in the global minimum Kinetic trapping at local minima? Ne-taggingstrongly depends on the barrier among the minima? (other explanations would be possible - an open question)

17. Summary ▪ IR spectroscopy of H + (MeOH) n with rare gas tagging ▪ The H-bond ring closing and opening with and without Ar tagging ▪ Effect of the tagging is different between Ar and Ne ▪ Kinetic trapping occurs in the cooling by Ne? – still an open question Ar/Ne

18. Isomer selectivity in H + (H 2 O) 6 by rare gas tagging Eigen isomer Zundel isomer Ne: preference of both the Zundel and Eigen isomers Ar : preference of the Zundel isomer Mizuse & Fujii, PCCP, 13, 7098 (2011)

19. Zundel selection mechanism in the Ar tagging - restriction by the geometrical space? The Zundel isomer has more space around the excess proton and is easier to accept the approach of the Ar atom? Eigen isomer Zundel isomer