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Water Hexamer Anion
Water Hexamer Anion
Electron attachment and detachment
Electron attachment and detachment
?
Water Hexamer Water Hexamer • Water hexamer smallest cluster whose local minima
have 3D structure• Many experimental and theoretical studies, eg. D.C.
Clary and J. K. Gregory J Phys. Chem. A 101 6813 1997• Find cage type isomer global minimum• For formation of the anion (H2O)-
n n=6 is a ‘magic number’. It gives a strong peak in the mass spectrum. Others are: n=2,7,11.
• Also investigated by photoelectron spectroscopy, N. I. Hammer et al. J. Phys. Chem. A 109 7896 2005
• Found evidence for surface bound state with AA motif• Calculations suggest in smaller water clusters, eg.
tetramer AA motif is robust, eg. M. A. Johnson et al. Science, 306, 675, 2004; J.M. Herbert PCCP 8 68, 2006
• Water hexamer smallest cluster whose local minima have 3D structure
• Many experimental and theoretical studies, eg. D.C. Clary and J. K. Gregory J Phys. Chem. A 101 6813 1997
• Find cage type isomer global minimum• For formation of the anion (H2O)-
n n=6 is a ‘magic number’. It gives a strong peak in the mass spectrum. Others are: n=2,7,11.
• Also investigated by photoelectron spectroscopy, N. I. Hammer et al. J. Phys. Chem. A 109 7896 2005
• Found evidence for surface bound state with AA motif• Calculations suggest in smaller water clusters, eg.
tetramer AA motif is robust, eg. M. A. Johnson et al. Science, 306, 675, 2004; J.M. Herbert PCCP 8 68, 2006
Neutral IsomersNeutral Isomers
Book Cage
Boat
Prism
Chair
Vertical Binding EnergiesVertical Binding Energies
Isomer E/kcalmol-1 VEBE/eV
Chair 8.01 0.12
Boat 1.87 -0.035
Prism 0.95 -0.031
Book 0.65 -0.075
Cage 0.0 -0.075
• Neutral Energies on AMEOBA force-field parameterized to reproduce results from MP2/CBS calculations, VEBE = E(neut) -E(an)•Anion energies calculated using MP2/6-311(1+,3+)G**•Negative VEBE would be unbound in complete BS limit
AA Water Hexamer Anion
AA Water Hexamer Anion
E(neut)/kcal mol-1 12.7
VEBE/eV 0.389
• Weakly bound water cluster anion• Calculations by Hammer suggest that the
AA type isomer observed experimentally has the following geometry
• Weakly bound water cluster anion• Calculations by Hammer suggest that the
AA type isomer observed experimentally has the following geometry
Other Anion FormOther Anion Form
E(neut)/kcal mol-1 2.33
VEBE/eV 0.049
Anion SOMO
•Does not show any isomerization in 20 AIMD trajectories on the anion PES - each ~1500fs at 50K
• e- not associated with 1 water molecule - bound by collective action
Electron DetachmentElectron Detachment•Assume e- detached with no change in configuration of nuclei
•Find Minimum Energy Pathway to the nearest local minimum on the NEUTRAL surface
•Gradient following method - TINKER
12.6 kcal mol-1
0.95 kcal mol-1
0.0 kcal mol-1
AA
Open Prism
Cage
-e-
MEP to Open PrismMEP to Open Prism
•MD simulation at 1K to show change in configuration along MEP of neutral surface
•Changes within 700fs - fast even at low temperature
•Little change in configuration
QuickTime™ and aMotion JPEG OpenDML decompressor
are needed to see this picture.
Open PrismOpen Prism
E(neut)/kcal mol-
1
0.95
VBE/eV -0.016
Anion
SOMO
t/fsVBE/eV
/D
340 0.0351 5.430
342 0.0358 5.443
1140
0.0500 5.510
1146
0.0562 5.602
1150
0.0527 5.550
1151
0.0511 5.460
1286
0.0400 5.417
1287
0.0412 5.425
1288
0.0410 5.403
Fluctutions in Dipole Moment
Fluctutions in Dipole Moment
•Classical MD simulation on neutral surface at 100K using TINKER and AMOEBA force-field shows large fluctuations in dipole moment
•Standard deviation of dipole moment,Dipole = 0.71D
Snapshots from trajectorySnapshots from trajectory
340fs Anion SOMO
1146fs AnionSOMO
Ionize with no change in configuration
Sudden ApproximationSudden Approximation
? eV
???
Anion surface
Neutral surface
+e-
Run AIMD simulation on anion surface to find unknown minimum
Prism
Open Prism
Conformational ChangeConformational Change•From ionized snapshot at 340fs - run AIMD on the anion surface
•MP2/6-311(+1,+3)G**
•Would expect AA isomer formation since calculations on tetramer have shown AA isomer robust and evidence from photoelectron spectroscopy
QuickTime™ and aMotion JPEG OpenDML decompressor
are needed to see this picture.
‘Product’ on Anion Surface‘Product’ on Anion Surface
VBE = 0.29eV
SOMO of ???•Not a local minimum
•Other trajectories show this behavior
•None show formation of AA type isomer
•Is this typical? Have we sampled sufficiently?
• How can we reconcile this with the experimental evidence for the AA configuration?
at 230fs
Possible ExplanationsPossible Explanations1) Formation of the AA isomer a comparatively rare
event, we need more sampling.
Since a 200fs trajectory takes > 1 week at this level of theory this would be difficult. Perhaps a MODEL HAMILTONIAN approach such as the Drude model of Jordan et al. could reduce the cost.
2) Formation of the AA state goes through the cavity bound state as an intermediate
3) AA formation takes place through an isomer which has fewer initial hydrogen bonds, eg. linear or chair
SummarySummary
• Low energy structures in neutral due to much hydrogen bonding
• Electron capture can occur in these due to dipole fluctuations
• So far AIMD calculations show the formation of cavity bound states
• No AA type formation observed. Is this as a result of insufficient sampling?
• Model Hamiltonian type calculations needed to increase efficiency of AIMD calculations
• Low energy structures in neutral due to much hydrogen bonding
• Electron capture can occur in these due to dipole fluctuations
• So far AIMD calculations show the formation of cavity bound states
• No AA type formation observed. Is this as a result of insufficient sampling?
• Model Hamiltonian type calculations needed to increase efficiency of AIMD calculations
AcknowlegementsAcknowlegements
I, for one, welcome our new
Overlords
John Herbert
Leif Jacobson
Adrian Lange
OSC
Ohio State University