The Solvation-Induced Onsager Reaction Field Rather than the Double-Layer Field Controls CO₂ Reduction on Gold

The selectivity and activity of the carbon dioxide reduction (CO2R) reaction are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in situ-generated CO on Au in the presence of alkali cations, we quantify the total electric field present at catalytic active sites and deconvolute this field into contributions from (1) the electrochemical Stern layer and (2) the Onsager (or solvation-induced) reaction field. Contrary to recent theoretical reports, the CO2R kinetics does not depend on the Stern field but instead is closely correlated with the strength of the Onsager reaction field. These results show that in the presence of adsorbed (bent) CO2, the Onsager field greatly exceeds the Stern field and is primarily responsible for CO2 activation. Additional measurements of the cation-dependent water spectra using vibrational sum frequency generation spectroscopy show that interfacial solvation strongly influences the CO2R activity. These combined results confirm that the cation-dependent interfacial water structure and its associated electric field must be explicitly considered for accurate understanding of CO2R reaction kinetics

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

Orientational Coupling of Molecules at Interfaces Revealed by Two-Dimensional Electronic–Vibrational Sum Frequency Generation (2D-EVSFG)

Photoinduced relaxation processes at interfaces are intimately related to many fields such as solar energy conversion, photocatalysis, and photosynthesis. Vibronic coupling plays a key role in the fundamental steps of the interface-related photoinduced relaxation processes. Vibronic coupling at interfaces is expected to be different from that in bulk due to the unique environment. However, vibronic coupling at interfaces has not been well understood due to the lack of experimental tools. We have recently developed a two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) for vibronic coupling at interfaces. In this work, we present orientational correlations in vibronic couplings of electronic and vibrational transition dipoles as well as the structural evolution of photoinduced excited states of molecules at interfaces with the 2D-EVSFG technique. We used malachite green molecules at the air/water interface as an example, to be compared with those in bulk revealed by 2D-EV. Together with polarized VSFG and ESHG experiments, polarized 2D-EVSFG spectra were used to extract relative orientations of an electronic transition dipole and vibrational transition dipoles at the interface. Combined with molecular dynamics calculations, time-dependent 2D-EVSFG data have demonstrated that structural evolutions of photoinduced excited states at the interface have different behaviors than those in bulk. Our results showed that photoexcitation leads to intramolecular charge transfer but no conical interactions in 25 ps. Restricted environment and orientational orderings of molecules at the interface are responsible for the unique features of vibronic coupling