Ultrafast chemical exchange and spectral diffusion of solute-solvent complexes probed by two-dimensional IR spectroscopy.

摘要

Various numerical processes used in 2D-IR data processing are explained. The ensemble averaging to add multiple scans can be done in time domain or frequency domain. The advantage and disadvantage of the two methods are discussed. The lost of information by the timing error in interferogram cannot be recovered by the exponential phase factor. It is shown that the dead time error in 2D-IR also cannot be fixed by the phasing process within the dual scan method.;Ultrafast 2D-IR experiment and response function theory are used to examine chemical exchange between solute-solvent complexes and the free solute for the solute phenol and three solvent complex partners, p-xylene, benzene, and bromobenzene, in mixed solvents of the partner and CCl4. The detailed calculations using response function are able to reproduce the experimental results and demonstrate that a method employed previously that used a kinetic model for the volumes of the peaks is adequate to extract the exchange kinetics. The current analysis also yields the spectral diffusion, and shows that the spectral diffusion is significantly different for phenol complexes and free phenol.;The solute-solvent dynamics of the mixed solvent are studied using 2D-IR of the hydroxyl stretch of phenol-OD in three solvents, CCl4, mesitylene, and the mixed solvent of mesitylene and CCl4. Dynamics of the free phenol in CCl4 or the mixed solvent are very similar, and dynamics of the complex in Mesitylene and in the mixed solvent are very similar. However, there are differences in the slowest time scale dynamics between the pure solvents and the mixed solvents. The mixed solvent produces slower dynamics that are attributed to first solvent shell solvent composition variations. The composition variations require a longer time to randomize than is required in the pure solvents, where only density variations occur.;The fast C-H hydrogen bond acceptor substitution reaction is probed using 2D IR chemical exchange spectroscopy by observing the time-dependent growth of off-diagonal peaks in the 2D IR spectra. The measured substitution rate is 1/30 ps for an acetone (AC) molecule to replace a dimethyl sulfoxide (DMSO) molecule in a chloroform (Cf) -DMSO complex and 1/45ps for a DMSO molecule to replace an Ac molecule in a Cf-Ac complex. Free Cf exists in the mixed solvent, and it acts as a reactive intermediate in the substitution reaction, analogous to a SN1 type reaction. From the measured rates and the equilibrium concentrations of Ac and DMSO, the dissociation rates for the Cf-DMSO and Cf-Ac complexes are found to be 1/24 ps and 1/5.5 ps respectively. The difference between the measured rate for the complete substitution reaction and the rate for complex dissociation corresponds to the diffusion limited rate. The estimated diffusion limited rate agrees well with the result from a Smoluchowski treatment of diffusive reactions.;A new observable, the center line slope (CLS) of the 2D spectrum, is presented that greatly simplifies the extraction of the FFCF from experimental data. The CLS otau (CLS om) is the slope of the line that connects the maxima of the peaks of a series of cuts through the 2D spectrum that are parallel to the frequency axis, o tau (om). It is shown analytically to second order in time that the CLS is the Tw dependent part of the FFCF. The procedure to extract the full FFCF from the CLS and IR spectrum is described. It is demonstrated that the CLS o tau is unaffected by Fourier filtering methods (apodization), and by finite pulse durations. CLS om is immune to line shape distortions caused by destructive interference, solvent background absorption, and high optical density of the solute.