Vibrational Dynamics of Cyclic Acid Dimers:

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Vibrational Dynamics of Cyclic Acid
Dimers Trifluoroacetic Acid in Gas and Dilute
Solutions
Steven T. Shipman, Pam Douglass, Ellen L.
Mierzejewski, Brian C. Dian, Charlotte E. Hinkle,
Hyun S. Yoo, and Brooks H. Pate
Department of Chemistry University of Virginia
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Motivation

• Acid dimers provide a complex but still
  tractable problem for investigating energy flow
  through hydrogen-bonded networks.
• Can be studied in both the gas and solution
  phase, making them accessible to a wide array of
  experimental techniques.

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The Cast of Characters
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Equilibrium FTIR Spectrum of TFA
Free
Dimer
Dimer OH
Free OH
CO
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Formic Acid Dimer Reaction Surface
Proposed Reaction Surface of Brinkmann et. al. J.
Phys. Chem. A, 2003, 107, 10208-10216.
De2 3360 cm-1
x2
x2
De1 5670 cm-1
Tunneling barrier
DE 980 cm-1
(MP2/TZ2P(f,d)diff)
DE 2270 cm-1
TFA Dimer 2700 cm-1 of Internal Energy at 25oC
DE 2380 cm-1
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Two Color Transient Absorption Spectroscopy


Probe Reference


Variable Delay Stage

InSb

Chopper
Polarizer
Pump Reference


or
Monochromator or Spectrograph
CaF
lens

InSb

2
Transmitted Intensity
Sample

?/2 plate


OPA 1
OPA 2


1 kHz repetition rate Independently tunable
OPAs Tuning range 16 µm, 510 µJ/pulse Gas and
liquid samples


PUMP
PROBE


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Previous Transient Absorption Spectra of TFA (g)
Amplitudes
Amplitudes
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Array Data Monomer and Dimer CO Region
Dimer OH Dimer CO
CO matches O-H bleach (after initial sharp drop)
Monomer CO rise, while noisy, reasonably matches
free O-H rise
Free OH Free CO
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TFA in Dilute Solution
Pump 2925 cm-1
Probe 2925 cm-1
Probe 2925 cm-1 Probe 1730 cm-1
Probe 3500 cm-1
Probe 3400 cm-1
Dimer CO matches O-H in CCl4
No long term structural rearrangement occurs in
solution.
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Proposed Rearrangement Mechanism
0.5 ps
20 ps
1780
(x2)
150 ps
3580
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Pressurized Gas Phase Studies
OH recovery occurs at higher pressures! (200 ps
time constant at 175 psi Ar)
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Whither H-Bond Breaking?
1.2 ps is consistent with previously observed
intra-ring dynamics. (E. Nibbering, T.
Elsaesser, P. Hamm, and others) Longer timescale
for solution was explained as thermalization Is
the solvent acting as a cage?
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Summary

• In the gas phase, O-H stretch excitation
  ultimately causes the dimer to fall apart.
• The gas phase mechanism is as follows
• 1) ring expansion on an ultrafast
  timescale ( 500 fs)
• 2) breaking of one hydrogen bond,
  structural rearrangement ( 18 ps)
• 3) breaking of other hydrogen bond,
  forming monomers ( 100-200 ps)
• The introduction of pressure leads to dimer
  recovery at nearly the gas kinetic rate
  presumably this is catching the open dimer prior
  to complete dissociation (i.e. 2-body vs. 3-body
  collision).
• In solution, no long term structural
  rearrangement occurs. The only kinetics seen are
  a bleach with a biexponential recovery,
  attributed to OH-stretch relaxation and
  thermalization of the excess energy. The solvent
  is acting as a cage, preventing ring opening from
  occurring.

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Acknowledgements
National Science Foundation
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Trifluoroacetic Acid FTIRGas Phase 50C 0.05M
CCl4 25C
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Subtracted FTIR Spectra of TFA
Only contains contributions from species with no
free O-H groups
Only contains contributions from species with
zero or one H-bond
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Array Data O-H Stretching Region Overlay
Pump Dimer band (2953 cm-1) Probe Free OH (3580
cm-1)
Array data matches single-element data fairly
well. The array is inherently noisier, but it has
its advantages
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Array Data No Spectral Diffusion of Transient
0.02
0.01
?OD
0
-0.01
3580
3506
3433
cm-1
3360
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