Title: Vibrational Dynamics of Cyclic Acid Dimers:
1Vibrational 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
2Motivation
- 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.
3The Cast of Characters
4Equilibrium FTIR Spectrum of TFA
Free
Dimer
Dimer OH
Free OH
CO
5Formic 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
6 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
7Previous Transient Absorption Spectra of TFA (g)
Amplitudes
Amplitudes
8Array 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
9TFA 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.
10Proposed Rearrangement Mechanism
0.5 ps
20 ps
1780
(x2)
150 ps
3580
11Pressurized Gas Phase Studies
OH recovery occurs at higher pressures! (200 ps
time constant at 175 psi Ar)
12Whither 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?
13Summary
- 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.
14Acknowledgements
National Science Foundation
15Trifluoroacetic Acid FTIRGas Phase 50C 0.05M
CCl4 25C
16Subtracted 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
17Array 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
18Array Data No Spectral Diffusion of Transient
0.02
0.01
?OD
0
-0.01
3580
3506
3433
cm-1
3360
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