^{1}, Dmitriy Yu. Vorobyev

^{1}and Robin M. Hochstrasser

^{1,a)}

### Abstract

The asymmetric stretching vibration of the amphiphilic trifluoroacetate ion and its isotopologue in were investigated with infrared spectroscopy (FTIR), ultrafast infrared pump probe, and two dimensional vibrational photon echo techniques and simulations. Trifluoroacetate ions have a nonexponential depopulation of the first vibrational excited state, which is well described by a kinetic mechanism involving a temperature dependent solvent assisted relaxation to the symmetric stretch mode. The vibrational spectrum of the asymmetric stretch of the isotopologue presents an unusual spectral shape. The frequency-frequency autocorrelation function shows a static term not present in the form, which is caused by an accidental degeneracy with a combinational mode. A newly developed frequency map for carboxylate is used to characterize the processes and dynamics observed in the frequency fluctuations of the carboxylate asymmetric stretch mode in aqueous solution. An assignment of the molecular processes that govern the frequency fluctuations is suggested from an analysis of the solvation shell configurations obtained from molecular dynamics simulations.

This research was supported by grants to RMH from NSF-CHE and NIH-GM12592 with instrumentation supported by NIH-RR001348.

I. INTRODUCTION

II. MATERIALS AND METHODS

A. Sample preparation

B. Linear IR spectroscopy

C. Two dimensional IR spectroscopy

D. Pump-probe spectroscopy

E. Theoretical methods

F. Theoretical frequency-frequency correlation function

III. EXPERIMENTAL RESULTS

A. Linear IR spectra

B. Transient vibrational spectra

C. Transient vibrational spectra temperature dependence

D. Two dimensional IR spectra

IV. DISCUSSION

A. Population relaxation mechanism

B. Frequency-frequency correlation function (FFCF)

C. The spectral difference between TFA-C12 and -C13

D. Solvation dynamics of TFA

V. CONCLUSION

### Key Topics

- Infrared spectra
- 32.0
- Solvents
- 25.0
- Molecular dynamics
- 13.0
- Correlation functions
- 11.0
- Density functional theory
- 10.0

## Figures

Experimental linear IR spectra. TFA-C12 (solid line, lower scale) and TFA-C13 (dashed line, upper scale) in . Inset shows the symmetric stretch band for both samples.

Experimental linear IR spectra. TFA-C12 (solid line, lower scale) and TFA-C13 (dashed line, upper scale) in . Inset shows the symmetric stretch band for both samples.

Experimental linear IR spectra of TFA-C13 (open circles) in . The solid line is the fit with two Lorentzian functions (dashed and dotted lines).

Experimental linear IR spectra of TFA-C13 (open circles) in . The solid line is the fit with two Lorentzian functions (dashed and dotted lines).

Pump-probe dynamics of TFA-C12 (a) and of TFA-C13 (b). Photoinduced transient signal (gray filled circles) and nonexponential fit (black line). The insets show the spectrally resolved signals at 0 fs delay between pump and probe.

Pump-probe dynamics of TFA-C12 (a) and of TFA-C13 (b). Photoinduced transient signal (gray filled circles) and nonexponential fit (black line). The insets show the spectrally resolved signals at 0 fs delay between pump and probe.

Experimental 2D IR vibrational echo spectra for population time , , and . Upper and lower rows correspond to TFA-C12 and TFA-C13, respectively, in .

Experimental 2D IR vibrational echo spectra for population time , , and . Upper and lower rows correspond to TFA-C12 and TFA-C13, respectively, in .

Schematic kinetic diagram for two/three coupled modes with an energy gap. The symmetric and asymmetric carboxylate stretches are denoted as and , respectively, and the combinational mode as . Dotted arrows represent the pathways added to the three level system for the modeling of TFA-C13.

Schematic kinetic diagram for two/three coupled modes with an energy gap. The symmetric and asymmetric carboxylate stretches are denoted as and , respectively, and the combinational mode as . Dotted arrows represent the pathways added to the three level system for the modeling of TFA-C13.

Slope vs population time for TFA-C12 (open circles) and TFA-C13 (filled circles). The smooth lines are the corresponding fits to Eqs. (5) and (6) using the parameters given in Table III.

Slope vs population time for TFA-C12 (open circles) and TFA-C13 (filled circles). The smooth lines are the corresponding fits to Eqs. (5) and (6) using the parameters given in Table III.

Autocorrelation of the frequency fluctuations calculated from the MD/DFT map. Open circles are the FFCF and the solid line is the fit of the correlation function with two exponentials.

Autocorrelation of the frequency fluctuations calculated from the MD/DFT map. Open circles are the FFCF and the solid line is the fit of the correlation function with two exponentials.

Relationship between pure dephasing parameters, and , and the value, , of the normalized FFCF obtained from the slope as defined in the text. (a) shows the possible pairs of and that fit the asymmetric stretch of the carboxylate for TFA-C12 for a given value of as discussed in the text. (b) depicts the normalized FFCF intercept at for the different pairs of and obtained in Fig. 8(a). The upper -axis of (b) displays the corresponding to the displayed in the lower axis. Insets in both Figures are zooms around the values for the parameters that fit the linear IR and the value of from the 2D IR spectrum of TFA-C12.

Relationship between pure dephasing parameters, and , and the value, , of the normalized FFCF obtained from the slope as defined in the text. (a) shows the possible pairs of and that fit the asymmetric stretch of the carboxylate for TFA-C12 for a given value of as discussed in the text. (b) depicts the normalized FFCF intercept at for the different pairs of and obtained in Fig. 8(a). The upper -axis of (b) displays the corresponding to the displayed in the lower axis. Insets in both Figures are zooms around the values for the parameters that fit the linear IR and the value of from the 2D IR spectrum of TFA-C12.

Linear IR spectra of TFA-C12. The open circles are the experimental absorption line shape; the solid line is the fit to experimental parameters as discussed in the text relating to Eq. (7).

Linear IR spectra of TFA-C12. The open circles are the experimental absorption line shape; the solid line is the fit to experimental parameters as discussed in the text relating to Eq. (7).

Solvation shell configurations of TFA with the highest frequency deviations. Idealized solvent conformations for the maximum (a) and minimum (b) frequencies marked with arrows in the inset of Fig. 11. Hydrogen occupancy probability for all the solvent conformations in which the carboxylate asymmetric stretch frequency is calculated to be within 25% of the maximum (c) and 25% of the minimum (d). The transparent and solid isosurfaces of (c) and (d) represent the 66% and 99% probability of finding a hydrogen atom, respectively.

Solvation shell configurations of TFA with the highest frequency deviations. Idealized solvent conformations for the maximum (a) and minimum (b) frequencies marked with arrows in the inset of Fig. 11. Hydrogen occupancy probability for all the solvent conformations in which the carboxylate asymmetric stretch frequency is calculated to be within 25% of the maximum (c) and 25% of the minimum (d). The transparent and solid isosurfaces of (c) and (d) represent the 66% and 99% probability of finding a hydrogen atom, respectively.

Simulated frequency fluctuation of the asymmetric stretch mode. Inset shows a zoom of the marked area. Grey dots and solid black lines represent the frequency fluctuation and its window average, respectively. Dash black line is the average of the frequency fluctuation with respect to the gas phase value at .

Simulated frequency fluctuation of the asymmetric stretch mode. Inset shows a zoom of the marked area. Grey dots and solid black lines represent the frequency fluctuation and its window average, respectively. Dash black line is the average of the frequency fluctuation with respect to the gas phase value at .

## Tables

Parameters of pump-probe dynamics fit for different temperatures and samples.

Parameters of pump-probe dynamics fit for different temperatures and samples.

Parameters obtained from the global fitting of the TFA-C12 pump-probe kinetics at different temperatures. Parameters obtained from individual fits at different temperatures are shown in parenthesis.

Parameters obtained from the global fitting of the TFA-C12 pump-probe kinetics at different temperatures. Parameters obtained from individual fits at different temperatures are shown in parenthesis.

Parameters of the experimental and theoretically predicted FFCFs. The experimental parameters extracted from experimental results and from a simultaneous fit of the linear absorption spectra and of slope of the 2D IR spectrum nodal contour line (marked with ).

Parameters of the experimental and theoretically predicted FFCFs. The experimental parameters extracted from experimental results and from a simultaneous fit of the linear absorption spectra and of slope of the 2D IR spectrum nodal contour line (marked with ).

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