Transcript EricDikenOSUtalk2005.ppt

Spectral Observation of Large-Amplitude
Proton Motion and Low-Barrier H-Bonds in
the H3O2– and H2O2– Complexes
International Symposium on Molecular Spectroscopy
60th Meeting – June 20-24, 2005
Eric G. Diken*, Jeffrey M. Headrick, Joseph R.
Roscioli, and Mark A. Johnson
Sterling Chemistry Laboratory
Yale University, P.O. Box 208107
New Haven, CT 06520
Precedence
·
H O complex is thought to mediate the production of OH radicals
· The
and OH ions in the upper atmosphere which are important in combustion.
species are prototypical examples of very strong hydrogen bonds,
· Both
where quantum effects are believed to play a notable role.
The H3O2– complex is important to understand charge transport in water.
(i.e., acid/base chemistry, ion solvation, proton transfer)
2
2
–
–
(analogous to FHF–, BrHBr–, etc..)
complexes are believed to be governed by extremely low-barrier
· Both
hydrogen bonds, where the zero-point level resides above the barrier.
(i.e., enzyme catalysis, structural diffusion)
Low-Barrier Hydrogen Bonds
(large amplitude proton motion)
“symmetric”
OH–· H2O
“asymmetric”
OH–· H2O
O–· H2O
OH–· OH·
Energy
v=1
v=1
v=0
v=0
H
O H O
H
H
O H O
Shared Protons: Quantum Effects
“classically”
asymmetric
(localized)
“quantum mechanically”
symmetric
(delocalized)
Red-shifted Free OH Stretch
H3 O2 –
OH–
H2O2–
INDICATES
CHARGE
DELOCALIZATION!!
Saykally and co-workers, J. Chem. Phys., 83, 5338–5339, 1985.
Very Strong Hydrogen Bonds
ν1 ← 0 = 697 cm-1
H3O2–
ν1 ← 0 = 767 cm-1
H2O2–
Largest OH red-shifts
ever observed to date!
Modern Theory: Vibrational Dynamics
1000 - 1100 cm-1
“shared proton”
650 - 720 cm-1
“out of plane proton”
(x 2)
“free OH stretch”
“O–O stretch”
510 cm-1
Thank you Anne McCoy & Joel Bowman!
3650 cm-1
Spectral Analysis
H3O2–
shared proton
free OH
out-of-plane
x 25
H2O2–
HELP!!
x 25
Badger’s Rule: A Modern Look
Anticipates the base hydrolysis reaction:
X– + H2O → HX + OH–
strong bases
weak
base
weak bases
strong
base
Charge Delocalization: Solvent Induced?
OH–· (H2O)3
?
O–· (H2O)3
CHARGE DELOCALIZES!
Johnson and co-workers, Science, 299, 1367 – 1372, 2003.
Summary & Conclusions
·
The H3O2– and H2O2– complexes have extremely strong hydrogen
bonds that can only be understood in the context of quantum
effects.
infrared spectra indicate that the zero-point level resides
· Our
above the barrier for proton transfer, resulting in large amplitude
proton motion and significant charge delocalization.
of the vibrational dynamics on a full-dimensional
· Inclusion
potential are crucial!
·
We still need theoretical help on the H2O2– complex!!!
Acknowledgments
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Professor Mark A. Johnson
Dr. Nathan I. Hammer
Jeffrey M. Headrick
Joseph R. Roscioli
Joseph C. Bopp
Professor Gary H. Weddle – Fairfield Uni.
Professor Anne B. McCoy – Ohio State
Professor Joel M. Bowman – Emory
Dr. Daniel Chipman – Notre Dame
Early Theoretical Efforts
2.46Ǻ
Assume: O···H···O axis is linear
1-dim Schrodinger Equation via
the Numerov Method
CCSD(T)(fc)/QTZ level of theory
C.C.M. Sampson and W. Klopper, J. Mol. Struct.-Theochem, 586, 201–208, 2002.