Successive Adsorption of Cations and Anions of Water–1-Butyl-3-methylimidazolium Methylsulfate Binary Mixtures at the Air–Liquid Interface Studied by Sum Frequency Generation Vibrational Spectroscopy and Surface Tension Measurements

We have investigated the surface behavior of 1-butyl-3-methyl­imidazolium methylsulfate ([bmim]­[MS]) aqueous solutions by sum frequency generation vibrational spectroscopy (SFG-VS) and surface tension measurements, including the adsorption of ions and its relationship with surface tension. At very low [bmim]­[MS] concentrations, SFG-VS data indicate that with increasing mole fraction of [bmim]­[MS], adsorption of cations at the interface rapidly increases, whereas the surface tension rapidly decreases. When cation adsorption to the surface is close to saturation, the change of the surface tension tends to be gradual. When the mole fraction of [bmim]­[MS] reaches 0.1, anions begin to adsorb to the interface, leading to the changes of the orientation angle of cations and the aggregation behavior of cations and anions at the interface. The previously reported unusual minimum point in the surface tension curve of [bmim]­[BF₄] aqueous solution suggested to be caused by successive adsorption of cations and anions was not observed for [bmim]­[MS] aqueous solution. SFG-VS spectra and the surface tension curve of [bmim]­[MS] aqueous solution indicate that anion adsorption does not significantly affect the surface tension. These results provide important information about the surface behavior of ionic liquid aqueous solutions and the effect of adsorption of ions on the surface tension

Development of Two-Dimensional Electronic-Vibrational Sum Frequency Generation (2D-EVSFG) for Vibronic and Solvent Couplings of Molecules at Interfaces and Surfaces

Many photoinduced excited states’ relaxation processes and chemical reactions occur at interfaces and surfaces, including charge transfer, energy transfer, proton transfer, proton-coupled electron transfer, configurational dynamics, conical intersections, etc. Of them, interactions of electronic and vibrational motions, namely, vibronic couplings, are the main determining factors for the relaxation processes or reaction pathways. However, time-resolved electronic-vibrational spectroscopy for interfaces and surfaces is lacking. Here we develop interface/surface-specific two-dimensional electronic-vibrational sum frequency generation spectroscopy (2D-EVSFG) for time-dependent vibronic coupling of excited states at interfaces and surfaces. We further demonstrate the fourth-order technique by investigating vibronic coupling, solvent correlation, and time evolution of the coupling for photoexcited interface-active molecules, crystal violet (CV), at the air/water interface as an example. The two vibronic absorption peaks for CV molecules at the interface from the 2D-EVSFG experiments were found to be more prominent than their counterparts in bulk from 2D-EV. Quantitative analysis of the vibronic peaks in 2D-EVSFG suggested that a non-Condon process participates in the photoexcitation of CV at the interface. We further reveal vibrational solvent coupling for the zeroth level on the electronic state with respect to that on the ground state, which is directly related to the magnitude of its change in solvent reorganization energy. The change in the solvent reorganization energy at the interface is much smaller than that in bulk methanol. Time-dependent center line slopes (CLSs) of 2D-EVSFG also showed that kinetic behaviors of CV at the air/water interface are significantly different from those in bulk methanol. Our ultrafast 2D-EVSFG experiments not only offer vibrational information on both excited states and the ground state as compared with the traditional doubly resonant sum frequency generation and electronic-vibrational coupling but also provide vibronic coupling, dynamical solvent effects, and time evolution of vibronic coupling at interfaces

Negligible Isotopic Effect on Dissociation of Hydrogen Bonds

Isotopic effects on the formation and dissociation kinetics of hydrogen bonds are studied in real time with ultrafast chemical exchange spectroscopy. The dissociation time of hydrogen bond between phenol-OH and <i>p</i>-xylene (or mesitylene) is found to be identical to that between phenol-OD and <i>p</i>-xylene (or mesitylene) in the same solvents. The experimental results demonstrate that the isotope substitution (D for H) has negligible effects on the hydrogen bond kinetics. DFT calculations show that the isotope substitution does not significantly change the frequencies of vibrational modes that may be along the hydrogen bond formation and dissociation coordinate. The zero point energy differences of these modes between hydrogen bonds with OH and OD are too small to affect the activation energy of the hydrogen bond dissociation in a detectible way at room temperature