Vibrational Dynamics of Trifluoroacetic Acid and Formic Acid

Vibrational Dynamics of Trifluoroacetic Acid and Formic Acid in Gas and Dilute Solution: Cracking Open Gas Phase Acid Dimers Pam L. Crum, Brian C. Dian, Hyun S. Yoo, Charlotte E. Hinkle, and Brooks H. Pate Department of Chemistry University of Virginia

Why Acid Dimers? n Lots of work done on acid dimers Understanding hydrogen bonds n Full spectrum (n): initial dynamics n n Dynamics subsequent to dynamics n Look at spectrum at later times (t)

Trifluoroacetic Acid Gas Phase FTIR (50 o. C)

Trifluoroacetic Acid Gas Phase Temperature Dependence

Two Color Transient Absorption Spectroscopy

Trifluoroacetic Acid FTIR Gas Phase: 50 o. C & 0. 05 M CCl 4: 25 o. C

Trifluoroacetic Acid (50 o. C): Dimer Band Bleach Pump 2953 cm-1 Probe 2953 cm-1 D OD v=1 2953 cm-1 v=0 Dimer Band Transition

Trifluoroacetic Acid (50 o. C): “Free OH” Growth v=1 D OD 3580 cm-1 2953 cm-1 Pump 2953 cm-1 Probe 3580 cm-1 v=0 Dimer Band Transition v=0 Free OH Transition

(50 o. C): Trifluoroacetic Acid Pumping 2953 cm-1 Possible Transient Species

Trifluoroacetic Acid (CCl 4) Pumping 2925 cm-1

Possible Interpretation of TFA (g) Time Resolved Data Dissociation Isomerization

Pump Energy Effects on Dynamics: TFA (g) Probing 3580 cm-1 0. 02 D OD 0. 01 Pump Energy 0. 00 2953 cm-1 3131 cm-1 -0. 01 2596 cm-1 -0. 02 0 200 400 600 ps 800 1000

Formic Acid Gas Phase Temperature Dependence

Formic Acid Dimer Reaction Surface Proposed Reaction Surface of Brinkmann et. al. J. Phys. Chem. A, 2003, 107, 10208 -10216. x 2 De 2 = 3360 cm-1 De 1 = 5670 cm-1 x 2 Tunneling barrier DE” = 980 cm-1 Formic Acid Dimer ~ 1100 cm-1 of Internal Energy at 60 o. C (MP 2/TZ 2 P(f, d)+diff) DE = 2270 cm-1 DE” = 2380 cm-1

Formic Acid (60 o. C): Pumping 3107 cm-1

D OD Pump Energy Effects on Dynamics: FA (g) Probing 3580 cm-1 Pump Energy 3107 cm-1 2945 cm-1

Possible Interpretation of Gas Phase Time Resolved Data Isomerization Initial IVR Process

Spectral Diffusion after Excitation * n

Trifluoroacetic Acid (50 o. C) Gas Frequency Scan: Pumping 2953 cm-1 t = 20 ps after excitation t = 175 ps after excitation

Acid Dimers: Other Structures to Consider

Formic Acid Dimers: Unique to Formic Acid

Conclusions n n Gas phase data reveals new dynamics information Trifluoroacetic and Formic Acid dimers show identical dynamics n Formic Acid Energetics not compatible with dissociation n Dynamics seen in gas phase prevented in solution n This is a WORK IN PROGRESS…