microscopic compatibility between methanol and water in hydrogen bond network development in...
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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.