Ryunosuke Shishido, Asuka Fujii Department of Chemistry, Graduate School of Science, Tohoku University, Japan Jer-Lai Kuo Institute of Atomic and Molecular Sciences, Taiwan Infrared spectroscopy of ((CH 3 ) 3 N) n -H + -H 2 O (n=1-3): Structures and dissociation channels of protonated mixed clusters around a magic number 68 th. International Symposium on Molecular Spectroscopy Ohio State University, Columbus, Ohio, USA June 17, 2013

2 (CH 3 ) 3 N : trimethylamine (TMA) Mass spectrum of (TMA) n -H + -H 2 O Castleman and coworkers, Chem. Phys. Lett. 1991, 178, J. Am. Chem. Soc., 1991, 113, n=2 34 X = TMA closed shell formation at the magic number (n=3) A magic number of ((CH 3 ) 3 N) n -H + -H 2 O protonated mixed clusters proton affinity TMA 225 kcal/mol H 2 O 165 kcal/mol

3 Wei. S. et al., J. Am. Chem. Soc., 1991, 113, Dissociation channels of (TMA) n -H + -H 2 O The major dissociation channel in the meta stable decay of (TMA) n -H + -H 2 O is H 2 O-loss in n  3 while it switches to TMA-loss in n  4 The water molecule at the center preferentially evaporates ?? - H 2 O closed shell structurecharge-dipole structure Large rearrangement of the cluster structure occurs prior to the dissociation?

4 The present study Infrared dissociation spectroscopy of size-selected (TMA) n -H + -H 2 O (n=1-3) in the OH and CH stretch region Cluster structures (at the magic number) Preferential location of the excess proton Measurements of the dissociation channels of the clusters upon the vibrational excitation Dissociation channels of the clusters of which structures are determined by IR spectroscopy

5 (TMA) n -H + -H 2 O H + (TMA) 1 H + (TMA) 2 H + (TMA) 3 H + (TMA) 4 Mass spectrum of (TMA) n -H + -H 2 O n=1n=3n=2n=4 cluster ion source: supersonic jet expansion with discharge The n=3 cluster shows the magic number behavior as previously reported (Castleman and coworkers, 1991).

6 Obs. (H 2 O loss) 1 3 Observed and simulated IR spectra of (TMA) 1 -H + -H 2 O NH freeOH CH H+H+ A single stable structure (the excess proton localizes at the TMA moiety) Strong bands in the 2800 – 3200 cm -1 region The excess proton vibration (N-H + stretch) and Fermi mixing (see MK12&13) ωB97X-D / G(2d,p), S.F.=0.94

7 IR spectra of (TMA) 2 -H + -H 2 O  E 0 = 0.0kJ/mol  E 0 = +0.3 kJ/mol  E 0 = +1.0 kJ/mol (a)Obs. (b)Calc. (c)Calc. (d)Calc.  E 0 = kJ/mol (e)Calc. (H 2 O loss) NH OH NH  H NH CH ： excess proton freeOH Hydrogen-bonded type (2I) Charge-dipole type (2II, 2III, 2IV) H-bonded OH/NH frequencies overestimated under the harmonic approx. Isomer 2I is most probable to interpret the rise of strong absorption at 2700 cm -1

8 Free OH stretch region of the IR spectrum of (TMA) 2 -H + -H 2 O (a)Obs. (b)Calc. (c)Calc. (d)Calc. 2I 2II 2III  E 0 = 0.0kJ/mol  E 0 = +0.3 kJ/mol  E 0 = +1.0 kJ/mol 2I2II 2III 1 3 The two free OH stretch bands in the observed spectrum indicates the contribution of 2II and/or 2II (total ~80% to 2I) Coexistence of the H-bonded type (2I) and charge-dipole type (2II/2III) structures

9 IR spectra of (TMA) 3 -H + -H 2 O  E 0 = 0.0 kJ/mol  E 0 = +2.0 kJ/mol  E 0 = kJ/mol (a)Obs. (b)Calc. (c)Calc. (d)Calc. (H 2 O loss) ： excess proton Closed shell structures (3I and 3II) Charge-dipole structure (3III) No free OH Only closed shell structures contribute to the observed spectrum 3I (H 3 O + ion core) ? 3II (H + TMA ion core)? or

10  E 0 = 0.0 kJ/mol  E 0 = +2.0 kJ/mol (a)Obs. (b)Calc. (c)Calc. Location of the excess proton in (TMA) 3 -H + -H 2 O H 2 O loss ： excess proton the strong absorption at ~2900 cm -1 sharp component CH stretches broad component H-bonded OH stretches 3II (H + TMA ion core) contributes to the observed spectrum (coexistence of 3I is not excluded)

11 Summary of the observed structures of (TMA) n -H + -H 2 O (n = 1 - 3) Vibrational excitation of these structure-determined clusters What dissociation channels are open?

12 2I 3I Dissociation channels upon vibrational excitation mass spectra of fragments The central water preferentially evaporates!

13 Dissociation energies of (TMA) n -H + -H 2 O (n=2&3) Dissociation energy / kJ/mol isomerfragment M06-2X/6-311+G(2d,p) ωB97X-D/6-311+G(2d,p) 2I H2OH2O TMA II H2OH2O TMA I H2OH2O TMA II H2OH2O TMA  38.7  9.8 2I 2II 3I 3II Smaller energy difference between the H 2 O-loss and TMA-loss channels in n=3. Larger dissociation to the TMA channel in n=3

14  We determined the structures of the (TMA) n -H + -H 2 O clusters by size selective infrared spectroscopy and obtained the firm evidence for the closed shell structure at the magic number (n=3).  The preferential evaporation of water, which locates at the center of the H- bond network, was confirmed.  Large rearrangement of the closed shell structure upon the dissociation was suggested. Summary

15 + v=1 h IR (TMA) n-1 -H + -H 2 O TMA (or -H 2 O) (TMA) n -H + -H 2 O (n=1-3) jet expansion + discharge 1 st -Qmass size-selection (TMA) n -H + -H 2 O octopole ion guide mass-selection 2 nd -Qmass (TMA) n -H + -H 2 O (or H + (TMA) n ) IR photodissociation spectroscopy of mass-selected cluster cations

16 density functionals ： ωB97X-D 、 M06-2X 、 B3LYP basis set ： G(2d,p) ZPE and BSSE corrections scaling factor (0.94 at ωB97X-D) Quantum chemical calculations Because of the large molecular size of TMA, the dispersion should be important to evaluate the intermolecular interactions Stable structures, relative energies, and IR spectra of the clusters IR simulations do not show remarkable functional-dependence The energy evaluations by the dispersion-corrected functionals, ωB97X-D (and M06-2X), were mainly referred The ωB97X-D functional is used for IR simulations

17 2I IR spectra calculated by the one - dimensional scan method NH + stretch large low-frequency shift to ~2000 cm -1 OH stretch no much difference from the scaled harmonic frequency

18 ΔE 0 = 0.0 kJ/mol ΔE 0 = +2.0 kJ/mol ΔE 0 = kJ/mol (a)Obs. (c)Calc. (d)Calc. (e)Calc. (b)Obs. IR spectra of (TMA) 3 -H + -H 2 O obtained by monitoring two dissociation channels Two dissociation channels give us the essentially same spectra (TMA) 3 H + -H 2 O has two active dissociation channels (H 2 O-loss and TMA-loss)

19 Comparison of the spectra of (TMA) 3 -H + -H 2 O and H + (H 2 O) 4 The OH stretch band frequency of (TMA) 3 -H + -H 2 O is higher than that of H + (H 2 O) 4 in spite of the larger proton affinity of TMA than water The central water should be neutral (not protonated)

20 All units are kJ/mol Relative energies of the isomers at different calculations levels

21 Proton affinities all units in kcal/mol