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

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‘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