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

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.

The Cast of Characters TFA monomer 8-membered ring dimer Open dimer / Linear dimer 6-membered ring dimer

Equilibrium FTIR Spectrum of TFA Dimer OH Free OH Free Dimer C=O

x2 Formic Acid Dimer Reaction Surface x2 Tunneling barrier TFA Dimer ~ 2700 cm -1 of Internal Energy at 25 o C  E = 2270 cm -1 Proposed Reaction Surface of Brinkmann et. al. J. Phys. Chem. A, 2003, 107, De 2 = 3360 cm -1 De 1 = 5670 cm -1  E” = 980 cm -1  E” = 2380 cm -1 (MP2/TZ2P(f,d)+diff)

Two Color Transient Absorption Spectroscopy OPA 2 PUMP OPA 1 PROBE Variable Delay Stage CaF 2 lens Sample λ/2 plate Chopper Probe Reference Pump Reference InSb Monochromator or Spectrograph 32-element MCT Array 1 kHz repetition rate Independently tunable OPAs Tuning range ~1–6 µm, ~5–10 µJ/pulse Gas and liquid samples Transmitted Intensity Polarizer or

Previous Transient Absorption Spectra of TFA (g) Amplitudes / a1 = -7 mOD / a2 = -11 mOD / a1 = OD / a2 = 0.12 OD / a3 = 6 mOD

Array Data – Monomer and Dimer C=O Region C=O matches O-H bleach (after initial sharp drop) Monomer C=O rise, while noisy, reasonably matches free O-H rise Free OH Free CO Dimer OH Dimer CO

TFA in Dilute Solution Dimer C=O matches O-H in CCl 4 Probe 2925 cm -1 Pump 2925 cm-1 Probe 3500 cm -1 Probe 3400 cm -1 Probe 2925 cm -1 Probe 1730 cm -1 No long term structural rearrangement occurs in solution.

Proposed Rearrangement Mechanism (x2) ps ps 20 ps

Pressurized Gas Phase Studies OH recovery occurs at higher pressures! (200 ps time constant at 175 psi Ar)

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?

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 (~ 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.

Acknowledgements Time Domain Pam Douglass Ellen Mierzejewski Frequency Domain Gordon Brown Dr. Brian Dian (Purdue) Kevin Douglass Jason Pajski National Science Foundation

Trifluoroacetic Acid FTIR Gas Phase: 50°C & 0.05M CCl4: 25°C

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

Array Data – O-H Stretching Region Overlay Array data matches single-element data fairly well. The array is inherently noisier, but it has its advantages… Pump: Dimer band (2953 cm -1 ) Probe: Free OH (3580 cm -1 )

Array Data – No Spectral Diffusion of Transient Time (ps) ΔOD cm Time (ps) cm ΔmOD

Evo M HPR Spitfire FS HP Spitfire PS Evo Tsunami T2 PST3 PS T1 FS OPA 800 FS Synched in time Experiment Setup Laser System