INFRARED SPECTROSCOPY OF (CH 3 ) 3 N-H + -(H 2 O) n (n = 1-22) Ryunosuke Shishido, Asuka Fujii Department of Chemistry, Graduate School of Science, Tohoku University, Japan Jer-Lai Kuo Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan June 19, 2012 TE06

Introduction Water clusters (H 2 O) n A microscopic model of bulk water 2 Molecular level understanding of H-bond network structures Definitely size selective spectroscopy has never been performed for n > ~10 Size-selective IR spectroscopy of large and neutral water clusters Strategy Excess chargeMass spectrometric techniques Minimum effects of the excess charge to the hydrogen-bond network of water

3 ・ Addition of a metal ion enables us to select cluster sizes even in the large-size region The water moiety would be neutral ・ A high proton affinity "tag“ with an excess proton, which never releases the excess proton M(H 2 O) 36, M = I -, Cs +, Co 2+, Mo 3+ Evan R. Williams and co-workers J. Am. Chem. Soc., 2011, 133 (13), 4810 H-bond network structures are largely different from the neutral water clusters (H 2 O) n ・・・・ H + X protontag

4 TMA can act only as a single proton acceptor in H-bond networks TMA locates at a terminal of H-bond networks TMA has high PA of 225 kcal/mol The excess proton would localize in the TMA moiety Protonated trimethylamine-water clusters The water moiety in TMA-H + -(H 2 O) n is expected to be neutral H+H+ TMA-H + -(H 2 O) 21 Trimethylamine, (CH 3 ) 3 N, TMA

This study ・ Whether the water moiety is neutral in TMA-H + -(H 2 O) n ? 5 ・ IR predissociation spectroscopy of TMA-H + -(H 2 O) n (n = 1-22)

Mass spectrum 6 TMA-H + -(H 2 O) n H + -(H 2 O) n ・ H + -(H 2 O) n ・ TMA-H + -(H 2 O) n No Magic number behavior is seen at n = 21 A magic number appears at n = 21 Closed-cage structure

Free OH stretch bands of TMA-H + -(H 2 O) n 7 n = 1 n = 2 n = 3 n = 4 n = 5 n = 6 n = 7 n = 10 n = 15 n = 20 n = 21 n = 22 ・ The ν 1 and ν 3 band of 1-coord water (terminal of H-bond network) disappears at n ≧ 6 Development into closed(-cage) network at size n = 6 1-coord ν 1 :3650 cm -1 2-coord 3-coord 1-coord ( ν 1 ) 3-coord ( ν 3 ) 2-coord ν 3 :3750 cm -1 ・ The band of 2-coord water shows high-frequency shift ・ The band of 3-coord water (branching point) starts to appear at n = 6

H-Bonded OH stretch bands of TMA-H + -(H 2 O) n 8 ・ At n ≧ 6, the band at 3200 cm -1 becomes prominent Increase of 3-coord sites Development into closed(-cage) network at size n = 6 3-coord n = 4 n = 5 n = 6 n = 7 n = 10 Wavenumber / cm -1 H + -(H 2 O) 6 (H 2 O) 6 This size is much smaller than H + -(H 2 O) n [n~10 in the case of H + (H 2 O) n ]

Comparison of TMA-H + -(H 2 O) n and H + -(H 2 O) n in the free OH stretch region 9 TMA-H + -(H 2 O) n H + -(H 2 O) n Wavenumber / cm -1 n = 20 n = 21 n = 22 ・ H + -(H 2 O) n Intensity of the 2-coord water band suddenly decreases at n = 21 (closed-cage formation) ・ TMA-H + -(H 2 O) n No sudden decrease at n = 21 H + is not transfer to the water moiety 3-coord 2-coord

Summary 10 ・ We observed IR spectra of size selected TMA-H + -(H 2 O) n n = 1~22 ・ No magic number is seen in TMA-H + -(H 2 O) n ・ The observed spectra are different from those of H + -(H 2 O) n n = 1~22 ・ The excess proton localizes in the TMA moiety at any size

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Experiment 12 ~IR predissociation spectroscopy~

13 IR spectra of TMA-H + -(H 2 O) 21 and H + -(H 2 O) 21 TMA-H + -(H 2 O) 21 H + -(H 2 O) 21 Wavenumber / cm -1 The spectrum of TMA-H + -(H 2 O) 21 is different from H + -(H 2 O) 21 The excess proton localizes in the TMA moiety at large size