Niranjan Govind

Strong coupling to the vacuum electromagnetic field of optical cavities has been proposed as a route to control ground-state chemical reactivity, enabling “photonic substituent effects.”

Excited-state intramolecular proton transfer is a prototypical ultrafast process that forms the foundation for many chemical and biological phenomena and is highly sensitive to changes in the underlying potential energy landscape. Here, we develop neural network potentials for 10-hydroxybenzo[h]quinoline that describe ground- and excited-state dynamics with near ab initio accuracy. These neural network potentials enable efficient sampling of the proton transfer coordinate and incorporation of nuclear quantum effects through path-integral molecular dynamics. The free energy profiles show that excited-state intramolecular proton transfer proceeds on a largely barrierless excited-state surface, with nuclear quantum effects reducing and smoothing the residual barrier. Non-equilibrium simulations yield an average proton transfer time of ∼11.5 fs for quantum hydrogen, consistent with experiment, and reveal a clear isotope effect, with slower dynamics for deuterium and classical nuclei. Mechanistic analysis identifies transient proton donor–acceptor contraction as the key enabling coordinate, while the proton crosses the midpoint prior to reaching the minimum of this coordinate, indicating a dynamically driven transfer. Subtle changes in the conjugated backbone precede proton transfer, reflecting early electronic reorganization and coherent coupling between proton motion and backbone vibrations. Bond-length alternation and vibrational coherence analysis reveal isotope-dependent coherence loss, with deuteration producing stronger damping and more localized proton/backbone dynamics relative to hydrogen. These results establish neural network potential-based approaches as an effective framework for simulating excited-state proton transfer with explicit nuclear quantum effects.

Quantum dynamical (QD) descriptions of the coupled electronic and nuclear dynamics can deliver full information on photochemical reactions providing a firm basis for the simulation of their time-resolved spectroscopic signatures. Yet, treating all vibrational degrees of freedom explicitly rapidly becomes computationally intractable. Here, we introduce a hybrid reactive-kernel/bath (RK/B) partitioning of the vibronic Hamiltonian into an anharmonic reactive kernel and a parametrically coupled harmonic bath. Using the cis–trans photoisomerization of a minimal retinal chromophore model PSB3, we perform nonadiabatic QD simulations on a hierarchy of models of increasing dimensionality and simulate two representative N K-edge X-ray Raman signals, resonant inelastic X-ray scattering (RIXS) and TRUECARS, to assess how the vibrational bath shapes them. We observe that RIXS, which is sensitive to the electronic populations, is governed almost entirely by the reactive coordinates and is already reproduced by the bare reactive kernel. Conversely, TRUECARS, which is sensitive to the electronic coherence, is shaped by the coupling modes, whose explicit inclusion is indispensable and is accurately captured by a quadratic bath treatment. These results suggest that RIXS and TRUECARS provide complementary, mode-selective views of photochemical dynamics, and establish the RK/B strategy as a computationally affordable route to investigate multimode effects on the quantum dynamics and nonlinear X-ray signals of complex photoactive systems.

We present a comprehensive experimental and computational spectroscopic study of the trinuclear cyanide-bridged mixed-valence complex trans-[RuIIL4{(μ-NC)FeIII(CN)5}2]4– (FeRuFe), where L = pyridine or 4-methoxypyridine dissolved in methanol and water. Four complementary spectroscopies spanning 250 meV to 7.1 keV are combined to probe the electronic structure at all three metal sites. These include infrared, visible/near IR, Ru L-edge, and Fe K-edge X-ray absorption spectroscopy probing the cyanide stretches, the metal-to-metal charge transfer transition, and electronic structure around the Ru and Fe atoms, respectively. The experimental spectra show that the two terminal Fe sites, which are crystallographically equivalent in the solid state, become inequivalent in solution. The IR spectra reveal that the solvent-exposed terminal cyanide broadens markedly in water relative to methanol. The solvation environment removes the degeneracy of the near IR metal-to-metal charge transfer transition and we measure two components separated by 190–340 meV. A weak pre-edge feature at the Ru L3-edge, present in three of the four systems, reports on Fe–Ru coupling through the cyanide bridge. To interpret the experimental spectra, we performed TDDFT calculations using complementary descriptions of the solvated complex. In one approach, the transition metal complex is treated quantum mechanically (QM) and the solvent is treated at the molecular mechanics (MM) level within a QM/MM framework. We also performed targeted calculations in which molecules in the first solvation shell are included explicitly in the QM region along with the solute, while the remaining solvent is represented as background point charges taken from the QM/MM force field. We find that core transitions localized at Fe or Ru sites are well reproduced within the QM/MM framework, while spectroscopic observables sensitive to intersite coupling or specific solute–solvent interactions benefit from explicit quantum mechanical treatment of the first solvation shell. This work establishes a systematic framework for simulating spectroscopic observables from the IR through the X-rays to characterize electronic coupling in multimetallic mixed-valence complexes mediated by the solvent.

Excited-state intramolecular proton transfer is a prototypical ultrafast process that forms the foundation for many chemical and biological phenomena and is highly sensitive to changes in the underlying potential energy landscape. Here, we develop neural network potentials for 10-hydroxybenzo[h]quinoline that describe ground- and excited-state dynamics with near ab initio accuracy. These neural network potentials enable efficient sampling of the proton transfer coordinate and incorporation of nuclear quantum effects through path-integral molecular dynamics. The free energy profiles show that excited-state intramolecular proton transfer proceeds on a largely barrierless excited-state surface, with nuclear quantum effects reducing and smoothing the residual barrier. Non-equilibrium simulations yield an average proton transfer time of ~11.5 fs for quantum hydrogen, consistent with experiment, and reveal a clear isotope effect, with slower dynamics for deuterium and classical nuclei. Mechanistic analysis identifies transient proton donor–acceptor contraction as the key enabling coordinate, while the proton crosses the midpoint prior to reaching the minimum of this coordinate, indicating a dynamically driven transfer. Subtle changes in the conjugated backbone precede proton transfer, reflecting early electronic reorganization and coherent coupling between proton motion and backbone vibrations. Bond-length alternation and vibrational coherence analysis reveal isotope-dependent coherence loss, with deuteration producing stronger damping and more localized proton/backbone dynamics relative to hydrogen. These results establish neural network potential-based approaches as an effective framework for simulating excited-state proton transfer with explicit nuclear quantum effects.

The complicated physicochemical properties of metal complexes that exhibit thermal spin crossover make it difficult for routine electronic structure calculations to yield an accurate transition temperature prediction, T1/2. The difficulty lies in the intricate connection between the spin-crossover energy, which is a molecular spectroscopic property, and T1/2, a condensed-phase property. Here we show how to obtain spin-crossover energies systematically by reverse engineering of experimental T1/2 data. The protocol is based upon fitting the range separation parameter, ω, in the hybrid LC-ωPBE density functional to reproduce the experimental T1/2 values for a series of metal complexes. We provide insights into the sources of variations of at least ±15 kJ mol–1 found from common exchange and correlation functionals by comparing their performance against our reference data. By analysis of the sensitivity of transition temperatures to ±1% shifts in the range separation parameter, we determined a typical uncertainty of ±50 K for them, and a ±2 kJ mol–1 uncertainty in the extracted spin-crossover energies due to ±1% variations of T1/2. Lastly, we present results from the high-level, all-electron coupled cluster method for eight of the smaller molecules in the reference data set, and discuss the influence of the truncation of the excitation series upon the spin state energies.

Gas phase methods were used to obtain novel thermodynamic data and compared to theoretical calculations to help elucidate the role of actinide 5f electrons in bonding.

Proton-coupled electron transfer (PCET) is foundational to catalysis, bioenergetics, and energy conversion, yet directly observing the interplay between electronic redistribution, protonation, and solvent reorganization remains challenging. We combine femtosecond optical spectroscopy, ultrafast N K-edge X-ray absorption spectroscopy, and time-resolved X-ray solution scattering to capture the steps of a sequential PCET reaction in water with atomic-site specificity. Using a ruthenium polypyridyl model complex, we resolve the electron redistribution upon photoinduced metal-to-ligand charge transfer and subsequent ( ~ 460 ps) protonation at a ligand nitrogen, as well as the concomitant rearrangement of the first-solvation-shell. Combined with advanced electronic structure and molecular dynamics simulations, our measurements reveal a marked localization of the excited-state electron density at the protonated N site, together with a switch from N···HO to NH···O hydrogen-bonds. These results establish a multimodal X-ray framework for mechanistic insight into PCET and its control in catalysis, artificial photosynthesis, and biological energy flow.

Iron-cyanide complexes with unsaturated ligand coordination and unusual metal oxidation states are transient and highly reactive in the condensed phase, posing significant experimental and theoretical challenges for investigating their intrinsic properties. In this work, a series of exotic gaseous iron-cyanide complex dianions, [Fe(CN)n]2– (n = 4–6), are generated by electrospraying ferricyanide solution and studied using photoelectron spectroscopy. The GW-based many-body perturbation theory for the first time is applied to interpret the electronic structure of isolated 3d transition-metal complex ions. The experimental adiabatic/vertical detachment energies (ADEs/VDEs) for [Fe(CN)4]2–, [Fe(CN)5]2–, and [Fe(CN)6]2– are 0.35/0.65 eV, 1.54/1.74 eV, and 2.61/2.73 eV (±50 meV), respectively. The evGW calculation, which outperformed a hierarchy of GW methods in predicting VDE and fitting spectral envelope, assigns predominant isomers as quintet tetrahedral [Fe(CN)4]2–, quartet trigonal bipyramidal [Fe(CN)5]2–, and triplet octahedral [Fe(CN)6]2–. The covalency of the ligand field and Fe-CN orbital interactions are further assessed by supplementary analysis.