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+(H2O)21

magic number at n=21 dominance of the 3-coor-dinated water at n=21

mass spectrum IR spectrum of free OH

ADAAD

FennHagenNishiCastleman Jr.

MikamiDuncan,Johnson,JordanChang

(1974-91)(2004-5)

3-D cage formationof 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(H2O)n (@Sendai)

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 (@Singapore)

model systems : (MeOH)m(H2O)n (m+n=8) and H+(MeOH)1(H2O)20

Microscopic compatibility between methanol and water in H-bond network formation

MeOHH2O

Ne

PulsedValve

UV laser(355 nm)

-300V

Experiment

IR dissociation spectroscopy of size-selected clusters

laser-assisted discharge nozzle

pick-up type ion source(clusters are formed frombare cations)

H+MmWn

H+MmWn-1 + W

H+MmWn-2 + 2Wor

IRv=0

v=1

Mass spectrum of H+(MeOH)m(H2O)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 structurefor NH4

+(H2O)20

(Johnson&Jordan,2005)

Infrared spectra of H+(MeOH)2(H2O)n

OH stretching vibrational region

H-bonded OH stretch :broadened

free OH stretch : more informative

Infrared spectra of H+(MeOH)2(H2O)n in the free OH stretch region

AAD AD2-coordinated

AD (acceptor-donor) site

3-coordinated

AAD (double-acceptor-single-donor) site

3715 cm-1

3695 cm-1

dominance of theAAD (3-coordinated) sites

3-D cage formationat n+m=21

Cluster size dependence of the intensity ratio of the AD/AAD bands

Similar size dependencebetween protonted water and protonated mixed clusters

the same 3-D cageformation at the same cluster size

Compatible behavior of small number of methanolmolecules with water in the H-bond network

Theoretical confirmation of the microscopic compatibility

model system : neutral (H2O)8

DFT evaluation of the relativestabilization energies (B3LYP/6-31+G*)

(MeOH)4(H2O)4

smallest polyhedral cage (cube)

14 orientational isomers

substitution of all the AAD sites with methanol

Relative stabilities of the orientational isomers in (H2O)8 and (MeOH)4(H2O)4

(numbering of the isomers)

Clear correlation between (H2O)8 and corresponding (MeOH)4(H2O)4

Compatibility between methanol and AAD water

3-D caged structures and the proton location of H+(MeOH)1(H2O)20

DFT energy optimization (B3LYP/6-31+G*) of minimasearched by Monte Carlo methodswith empirical model potentials

the excess proton

Proton transfer occurs if the optimizationstarted with the MeOH2

+ core

preference to the surface water site

proton migration in spite of the largerproton affinity of methanol

Summary

IR spectroscopy of H+(MeOH)m(H2O)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(H2O)n (m+n=8)

Energy correlation between isomersof (H2O)8 and (MeOH)m(H2O)8-m

Preference of the protonated site

Model system : H+(MeOH)1(H2O)7

Clear preference of the water sitein the cubic isomers (90 isomers)

The MeOH2+ ion core is only stable

in the non-cubic isomers.