Kyung Hwan Kim

Radical-arene contact complexes have long been proposed as key intermediates for selectivity control in radical reactions, but their solution-phase structures and dynamics underlying this role have remained experimentally inaccessible. Here, we use time-resolved X-ray solution scattering to directly observe transient I•–arene contact complexes in benzene and mesitylene. Our results show that I•–benzene adopts an η1/η2-like off-center geometry rather than an η6 π-complex. In contrast, I•–mesitylene exhibits a localized η1 geometry directed toward a CH site. DFT calculations support the relative energetic preferences of these contact motifs, and AIMD simulations reveal rapid dynamic averaging among closely related off-center configurations in benzene but a much narrower orientational range in mesitylene. These findings resolve the detailed structures, interaction motifs, and dynamics of I•–arene contact complexes and show that aromatic substitution substantially reshapes each of these features. Taken together, these results provide a structural and dynamic framework for understanding selectivity in radical reactions.

Nickel–iridium metallophotoredox catalysis has emerged as an effective strategy for promoting C–C and C–heteroatom bond-forming reactions, overcoming the often sluggish reductive elimination from Ni(II) intermediates, thereby expanding the synthetic utility of nickel catalysis. Despite extensive study, however, its precise mechanism has remained largely unresolved because of the experimental difficulty of disentangling the two catalytic centers and the complex electronic structure of nickel. Here, we directly probe the ultrafast electronic dynamics of both metal centers using element-specific femtosecond time-resolved X-ray absorption spectroscopy at an X-ray free-electron laser (XFEL). We identify a transient higher-energy triplet metal-to-ligand charge-transfer (3MLCT) state of nickel that appears exclusively in the presence of the iridium photocatalyst, providing evidence for the energy transfer within a preassociated Ni–Ir complex. Comparison with quantum calculations further shows that this state promotes access to a repulsive Ni–C σ–σ* state, thereby suggesting a possible excited-state pathway relevant to reductive elimination. These findings reveal how the iridium photocatalyst activates the nickel catalytic cycle and highlight selective population of optically dark yet reactive excited states as a general strategy for enabling otherwise inaccessible reaction pathways.

The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering. By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down. We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water.

Photochemistry of aromatic compounds has attracted significant interest for their distinctive excited-state behavior and potential applications as energy materials. Isoelectronic atomic substitution, which involves replacing ring atoms while maintaining the same number of electrons in aromatic compounds (e.g., from C–C to B–N), offers a promising strategy for modulating these properties by tuning electronic structures. However, a fundamental understanding of its effect on the excited-state dynamics and reactivity remains elusive. In this study, we investigate the photochemical properties of oxaborine, using benzene and azaborine as references, to elucidate the effects of isoelectronic atomic substitution. Potential energy surface (PES) calculations reveal that oxaborine undergoes barrierless ring distortion toward S1/S0 conical intersection (CI), followed by barrierless Dewar isomerization directly from the S1/S0 CI. Using nonadiabatic molecular dynamics simulations, we observe a rapid S1 decay time (125 fs) and a notably high Dewar isomerization yield (23.3%), consistent with the results of PES calculations, highlighting the unique photochemical properties of oxaborine. Orbital-level analysis reveals that atomic substitution breaks the π orbital degeneracy and destabilizes the S1 state, explaining the much faster S1 decay for azaborine and oxaborine. The p orbital energy modulations from O–B or N–B substitution weaken the meta-bridge bond while enhancing para-bridge C–C interactions, favoring Dewar isomerization. These findings offer a comprehensive understanding of the effect of isoelectronic atomic substitution in aromatic compounds and demonstrate its potential as a design strategy for controlling photochemical properties.

Recent experiments and theoretical studies strongly indicate that water exhibits a liquid-liquid phase transition (LLPT) in the supercooled domain. An open question is how the LLPT of water can affect the properties of aqueous solutions. Here, we study the structural and thermodynamic properties of supercooled glycerol-water microdroplets at dilute conditions (χ_g = 3.2% glycerol mole fraction). The combination of rapid evaporative cooling with femtosecond X-ray scattering allows us to outrun crystallization and gain access to the deeply supercooled regime down to T = 229.3 K. We find that the density fluctuations of the glycerol-water solution or, equivalently, its isothermal compressibility, κ_T, increases upon cooling. This is confirmed by molecular dynamics simulations, which indicate that the presence of glycerol shifts the temperature of maximum κ_T from T = 230 K in pure water down to T = 223 K in the solution. Our findings elucidate the interplay between the complex behavior of water, including its LLPT, and the properties of aqueous solutions at low temperatures, which can have practical consequences in cryogenic biological applications and cryopreservation techniques.

Here, we investigate the hypothesis that despite the existence of at least two high-density amorphous ices, only one high-density liquid state exists in water. We prepared a very-high-density amorphous ice (VHDA) sample and rapidly increased its temperature to around 205 ± 10 K using laser-induced isochoric heating. This temperature falls within the so-called “no-man’s land” well above the glass-liquid transition, wherein the IR laser pulse creates a metastable liquid state. Subsequently, this high-density liquid (HDL) state of water decompresses over time, and we examined the time-dependent structural changes using short x-ray pulses from a free electron laser. We observed a liquid–liquid transition to low-density liquid water (LDL) over time scales ranging from 20 ns to 3 μs, consistent with previous experimental results using expanded high-density amorphous ice (eHDA) as the initial state. In addition, the resulting LDL derived both from VHDA and eHDA displays similar density and degree of inhomogeneity. Our observation supports the idea that regardless of the initial annealing states of the high-density amorphous ices, the same HDL and final LDL states are reached at temperatures around 205 K.

The phase transition between water and ice is ubiquitous and one of the most important phenomena in nature. Here, we performed time-resolved x-ray scattering experiments capturing the melting and recrystallization dynamics of ice. The ultrafast heating of ice I is induced by an IR laser pulse and probed with an intense x-ray pulse which provided us with direct structural information on different length scales. From the wide-angle x-ray scattering (WAXS) patterns, the molten fraction, as well as the corresponding temperature at each delay, were determined. The small-angle x-ray scattering (SAXS) patterns, together with the information extracted from the WAXS analysis, provided the time-dependent change of the size and the number of liquid domains. The results show partial melting (~13%) and superheating of ice occurring at around 20 ns. After 100 ns, the average size of the liquid domains grows from about 2.5 nm to 4.5 nm by the coalescence of approximately six adjacent domains. Subsequently, we capture the recrystallization of the liquid domains, which occurs on microsecond timescales due to the cooling by heat dissipation and results to a decrease of the average liquid domain size.