Microscopic Compatibility between Methanol and Water in Hydrogen Bond Network Development in Protonated Clusters Asuka Fujii, Ken-ichiro Suhara, Kenta Mizuse, Naohiko Mikami Jer-Lai Kuo Department of Chemistry, Graduate School of Science, Tohoku University, Japan School of Physical and Mathematical Sciences, National Nanyang Technological University, Singapore

Hydrogen bond network structures in protonated clusters Mass spectrometry IR dissociation spectroscopy of size-selected cluster cations (Y. T. Lee, 1989) ab initio and MD calculations direct probing of the H-bond network structure size distribution and magic numbers Network structure in large-sized clusters microscopic picture of protic solvents proton motion in liquids nature of hydrogen bond, ….., etc.

3-dimensional (3-D) cage formation in H + (H 2 O) 21 magic number at n=21 dominance of the 3-coor- dinated water at n=21 mass spectrumIR spectrum of free OH ADAAD Fenn Hagen Nishi Castleman Jr. Mikami Duncan,Johnson,Jordan Chang ( ) (2004-5) 3-D cage formation of the hydrogen bond network

Network development in protonated methanol clusters IR spectroscopy of H + (MeOH) n n=4&5 Chang et al. (1999)linear chain to cyclic at n=5 n=4-15 Fujii et al. (2005)bicyclic structure in n>6 IR spectra of n=4-15 bicyclic structure at n=7 terminal of the network development no more complicated cage structure

Hydrogen bond network in protonated water-methanol mixed clusters water : 4-coordination, complicated cage methanol : 3-coordination, simple network hydrogen bond network in the mixed system? What really happens in aqueous alcohol ?

IR spectroscopy of H + (MeOH) m (H 2 O) n m=1-4, n=4-22 (m<<n) This study (the case of m>>n :FD05 by K. Suhara) DFT calculations of the relative stabilities of isomer structures model systems : (MeOH) m (H 2 O) n (m+n=8) and H + (MeOH) 1 (H 2 O) 20 Microscopic compatibility between methanol and water in H-bond network formation

Experiment IR dissociation spectroscopy of size-selected clusters laser-assisted discharge nozzle pick-up type ion source (clusters are formed from bare cations) H+MmWnH+MmWn H + M m W n-1 + W H + M m W n-2 + 2W or IR v=0 v=1

Mass spectrum of H + (MeOH) m (H 2 O) n mixted clusters magic number at m+n=21 (sudden intensity decrease at m+n=22) 3-D cage formation similar to protonated water ? (Castleman et al., 1992) the same behavior as protonated water Cage structure for NH 4 + (H 2 O) 20 (Johnson&Jordan, 2005)

Infrared spectra of H + (MeOH) 2 (H 2 O) n OH stretching vibrational region H-bonded OH stretch :broadened free OH stretch : more informative

Infrared spectra of H + (MeOH) 2 (H 2 O) n in the free OH stretch region AADAD 2-coordinated AD (acceptor-donor) site 3-coordinated AAD (double-acceptor- single-donor) site 3715 cm cm -1 dominance of the AAD (3-coordinated) sites 3-D cage formation at n+m=21

Cluster size dependence of the intensity ratio of the AD/AAD bands Similar size dependence between protonted water and protonated mixed clusters the same 3-D cage formation at the same cluster size Compatible behavior of small number of methanol molecules with water in the H-bond network

Theoretical confirmation of the microscopic compatibility model system : neutral (H 2 O) 8 DFT evaluation of the relative stabilization energies (B3LYP/6-31+G*) (MeOH) 4 (H 2 O) 4 smallest polyhedral cage (cube) 14 orientational isomers substitution of all the AAD sites with methanol

Relative stabilities of the orientational isomers in (H 2 O) 8 and (MeOH) 4 (H 2 O) 4 (numbering of the isomers) Clear correlation between (H 2 O) 8 and corresponding (MeOH) 4 (H 2 O) 4 Compatibility between methanol and AAD water

3-D caged structures and the proton location of H + (MeOH) 1 (H 2 O) 20 DFT energy optimization (B3LYP/6-31+G*) of minima searched by Monte Carlo methods with empirical model potentials the excess proton Proton transfer occurs if the optimization started with the MeOH 2 + core preference to the surface water site proton migration in spite of the larger proton affinity of methanol

Summary IR spectroscopy of H + (MeOH) m (H 2 O) n (m=1-4, n=4-22) in the OH stretch region Spectroscopic evidence for the 3-D cage formation of the mixed clusters at the magic number m+n=21 (m=1-4) Microscopic compatibility between methanol and AAD water in the H-bond network development DFT calculations also support the compatibility in the relative energy The excess proton prefers the surface water site in the (1+20)-mer indicating the proton migration from methanol to water

Microscopic compatibility in (MeOH) m (H 2 O) n (m+n=8) Energy correlation between isomers of (H 2 O) 8 and (MeOH) m (H 2 O) 8-m

Preference of the protonated site Model system : H + (MeOH) 1 (H 2 O) 7 Clear preference of the water site in the cubic isomers (90 isomers) The MeOH 2 + ion core is only stable in the non-cubic isomers.