Time Domain

The above diagram shows the minimum energy structure of formic acid. This structure is a doubly hydrogen bound eight-member ring structure. The hydrogens in the diagram may be replaced with CF3 groups to create trifluoroacetic acid (TFA) with similar characteristics to formic acid. The hydride stretch region in the infrared (IR) is red-shifted, experiences a substantial increase in intensity, and is extremely broad (~ 800-900 cm-1 at room temperature), all characteristic of strongly hydrogen bound system. Ultra-fast time-domain experiments have focused on the study of dimer systems in weakly interacting solvent such as carbon tetrachloride, however, detailed dynamical studies of acid bound dimers in a gas-phase, room temperature environment are lacking, particularly in ultra-fast time domain experiments.
 

Dimer-Monomer Transition

These spectra were recorded in a closed 2.5 mm path length cell while the temperature was varied from room temperature to 160 C. The broad hydride stretch feature from ~ 2400 – 3450 cm-1 is predominant at room temperature and decreases with temperature while the modest amount of free OH present at room temperature, appearing at ~ 3580 cm-1, increases with temperature, suggesting a shift in equilibrium from the dimer to the monomer form of TFA. The inset to the spectra shows an expanded view of the carbonyl stretching region which shows a clear isosbestic point, indicating a transition between species present, namely the dimer and monomer. We endeavor to determine the dissociation reaction mechanism with ultra-fast data collection.

Click image for close up

 

Ultra-Fast Growth and Decay

One of the interesting components of the acid dimer studies is the ability to monitor the growth of the monomer and the decay of the dimer by using ultra-fast time-resolved IR pump-probe techniques. The set up for the experiment involves pumping the dimer at a wavelength of 2953cm-1; the wavelength for the bound OH stretch. The sample is then probed at wavelengths of 2953cm-1, 3580cm-1, and 3450cm-1 to monitor the dimer, free OH (monomer), and potential transient species respectively. The concept of this experiment is to monitor the possible transition from the dimer to the monomer through the transient species.
 

Trifluoroacetic Acid

Here the traces for the dimer band, free OH stretch (monomer), and transient species are shown along with their best fits. Note that the free OH peak has a biexponential rise, and the transient peak has a single exponential rise and a biexponential decay. The data provide supporting information to the monomer-dimer transitions as well as additional information on a transient species. With the sensitivity of our instrumentation we are able to see the transient species grow in from approximately 0-25ps.

 

 

 

  

The transient species mentioned above is a transitional structure from the dimer to the monomer similar to an "intermediate". The exact structure of such a species is unknown, and is hoped to be determined with future testing. There are multiple possibilities for this transient species, and as many as all or none are possible.

Possible Transient Species

       

Work in Progress

We are in the process of replacing our current single-element mid-IR detector with a 32-element array. This new detector will allow us to simultaneously track changes in the absorption spectrum over a broad frequency range (~100 cm-1), greatly aiding in the search for possible transient intermediates. With this new technology, we plan to undertake detailed studies of TFA dynamics in the carbonyl stretching region. The first goal is to look for the presence or absence of an ultrafast isosbestic point (which has implications for the proposed transient intermediates), and the second goal is to see whether and how different pump energies affect the kinetics of the ring-opening process.

The next step for the experiment will be to vary the conditions under which these studies were performed in order to learn more about what physical processes are controlling the kinetic rates. In the gas phase, we will add argon as an inert pressure medium to see if we can ever reach a “liquid-like” limit in which the dimer does not dissociate. As for the solution phase, we plan to study how changes in temperature as well as solvent (particularly solvents capable of hydrogen bonding) alter the results presented here. New data will provide the means to determine a reaction rate and progression.

Please visit our other Time-Domain websites located on the left-hand sidebar or click here to return to the home page.