Atomistic Simulation of Hf-Pyridyl Amido-Catalyzed Chain Transfer Alkene Polymerization Reaction and Its Machine Learning for Extraction of Essential Descriptors: Effect of Microscopic Steric Hindrance on the Monomer Insertion Process.

The microscopic effects of each substituent of the Hf catalyst and the growing polymer on the monomer insertion process were investigated for Hf-pyridyl amido-catalyzed coordinative chain transfer polymerization using the Red Moon method. Since the Hf catalyst has two reaction sites, cis- and trans-sites, we separately applied the appropriate analysis methods to each one, revealing that the naphthalene ring influenced monomer insertion at the cis-one, while the i-Pr group and the hexyl group of the adjacent 1-octene unit did the trans-one. It was interesting to find that the hexyl group of the 1-octene-inserted catalyst (oHfCat) pushes the naphthalene ring toward the cis-site and narrows the space at the cis-site, thus indirectly creating a steric hindrance to cis-insertions. Further, the relative position of the Hf catalyst and the growing polymer was found to be strongly influenced by the patterns of insertion reactions, i.e., cis- or trans-insertions. In particular, it was clarified that, after trans-insertions, the growing polymer on the Hf atom covers the cis-site, making cis-insertion less likely to occur. These studies reveal the microscopic effects of the catalyst substituents and the growing polymer on the catalyst during the polymerization reaction process; these microscopic analyses using the RM method should provide atomistic insights that are not easy to obtain experimentally for advanced catalyst design and polymerization control.

Experimental and Theoretical Study for Core Excitation of Firefly Luciferin in Carbon K-Edge Spectra.

Carbon (C) K-edge X-ray absorption spectra for firefly luciferin were measured and assigned using time-dependent density functional theoretical calculations for luciferin anion and dianion to elucidate the effect of hydroxy-group deprotonation. It was found that the C K-edge spectra for luciferin had four characteristic peaks. The effect of deprotonation of the hydroxy group appears in the energy difference of the first and second peaks of these spectra. This energy difference is 1.0 eV at pH 7 and 2.3 eV at pH 10. The deprotonation of the hydroxy group can be distinguished based on the soft X-ray absorption spectra.

Atomistic Chemical Elucidation of the Higher-Rate Reaction Mechanism in Hf-Pyridyl Amido-Catalyzed Copolymerization of Ethene and 1-Octene: Application of Red Moon Simulation with Polymer Propagation Diagrams.

The Hf-pyridyl amido complex ((pyridylamido)Hf(IV)) is a cationic catalyst activated by ion-pairing with auxiliary catalyst B(C6F5)4 to show high activity for α-olefin polymerization. Previously, it was experimentally observed that the consumption rate of 1-octene in the 1-octene/ethene copolymerization is 3-fold compared to the 1-octene homopolymerization in coordinative chain transfer polymerization using the catalyst HfCat+-B(C6F5)4- ion pair (IP) and the chain transfer agent (CTA) ZnEt2. In the present study, we have performed atomistic chemical simulations of the IP-catalyzed homopolymerization of 1-octene and copolymerization of 1-octene and ethene on the basis of the Red Moon (RM) methodology. Using the analysis by polymer propagation diagrams (PPDs), in the 1-octene homopolymerization and the 1-octene/ethene copolymerization with the 1-octene-inserted catalyst (oHfCat), it is theoretically shown that the propagation reactions intermittently pause due to the steric hindrance of two hexyl groups of the oHfCat and the 1-octene inserted adjacent to the Hf atom. On the other hand, in the polymerizations with the ethene-inserted catalyst (eHfCat), it is reasonably recognized that the propagation reactions occur smoothly at a constant rate, and the polymerization continuously proceeds due to the relatively smaller steric hindrance. In conclusion, it was shown, for the first time, that the RM method can be used to reveal the microscopic effects of monomers and substituents in the polymerization reaction processes. Therefore, our current work using PPDs demonstrates the promising potential of the RM methodology in studying catalytic olefin polymerizations and complex chemical reaction systems in general.

Ultrafast Electronic Relaxation in Aqueous [Fe(bpy)3]2+: A Surface Hopping Study.

Trajectory surface hopping simulations are performed to better understand the electronic relaxation dynamics of [Fe(bpy)3]2+ in aqueous solution. Specifically, the ultrafast relaxation from the photoexcited singlet metal-to-ligand charge-transfer (MLCT) to the metastable quintet metal-centered (MC) states is simulated through the surface hopping method, where the MLCT and MC states of [Fe(bpy)3]2+ in aqueous solution are computed by using a model electronic Hamiltonian developed previously. As a result, most of the trajectories are interpreted to show the sequential relaxation pathways via the triplet MC states, though some are the direct pathway from MLCT to the quintet MC states. Even though the triplet MC states are involved in the relaxation, the population transfer to the singlet MC ground state is very small, and the population of the quintet MC states reaches more than ∼96%, reasonably consistent with the unity quantum efficiency discussed experimentally.

(Pyridylamido)Hf(IV)-Catalyzed 1-Octene Polymerization Reaction Interwoven with the Structural Dynamics of the Ion-Pair-Active Species: Bridging from Microscopic Simulation to Chemical Kinetics with the Red Moon Method.

We performed the atomistic simulation of 1-octene polymerization reaction catalyzed by the ionic pair (IP) consisting of the cationic active species of (pyridylamido)Hf(IV) catalyst, HfCatPn+, and different counteranions (CAs), B(C6F5)4- and MeB(C6F5)3-, at different monomer concentrations. Using a hybrid Monte Carlo/molecular dynamics method, that is, the Red Moon (RM) method, the reaction progress measured by the "RM cycle" was transformed into effective real time using the time transformation theory. Then, the degree of polymerization was found to be consistent with that in the chemical kinetics, a macroscopic theory, and experimental ones. Remarkably, the current simulation has revealed the different dynamical features in the polymerization behavior originating from the CA. Namely, the HfCatPn+-B(C6F5)4- IP mainly forms an outer-sphere IP (OSIP) throughout the polymerization. The HfCatPn+-MeB(C6F5)3- IP, on the other hand, forms an inner-sphere IP (ISIP) in the initial stage of polymerization, and the ratio of ISIP steeply drops after the first monomer insertion because the IP interaction is reduced by the steric hindrance between the inserted monomers and the CA. In conclusion, we have shown that the microscopic IP dynamics interwoven with the polymerization reaction can be computationally observed in the real-time domain by using the RM method. Therefore, our current work demonstrates the promising potential of the RM method in studying catalytic olefin polymerization and complex chemical reaction systems.

Non-π-Allyl Mechanism for the 1,4-cis-Butadiene Polymerization: Theoretical Study of Polymerization via Insertion of Butadiene into Al-C Bond with Cationic Gadolinium Metallocene.

This paper reports a computational study on the specific 1,4-cis polymerization of butadiene catalyzed by the cationic gadolinium metallocene [(C5 Me5 )2 Gd][B(C6 F5 )4 ] combined with excess amount of Al(i Bu)3 . Because this reaction system has no initial Gd-alkyl bond, a mechanism with conventional coordinative chain transfer polymerization (CCTP) is not feasible. Density functional theory (DFT) analyses indicate a novel mechanism in which the cationic Gd plays a crucial role by assisting butadiene insertion into one of the Al-C bond of Al(i Bu)3 . The proposed butadiene polymerization mechanism can account for the specific 1,4-cis selectivity of this catalyst system.

Adsorption Behavior of TEMPO-Based Organic Friction Modifiers during Sliding between Iron Oxide Surfaces: A Molecular Dynamics Study.

Organic friction modifiers (OFMs) added to lubricating oils to reduce friction and wear are crucial for reducing energy loss and CO2 emissions. In our previous studies, we have developed N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide, referred to as C12TEMPO, as a new type of OFM and experimentally demonstrated its superior performance to conventional OFMs of stearic acid and glycerol monooleate. However, the behavior of C12TEMPO adsorbing onto solid surfaces from base oil during sliding, which largely dictates the lubrication performance, is yet to be elucidated. Here, we performed molecular dynamics simulations for confined shear of a C12TEMPO solution in poly-α-olefin between hematite surfaces. Unlike conventional OFMs, which typically have one functional group or multiple functional groups of the same type, C12TEMPO features two functional groups of different types: one amide and one terminal free oxygen radical. The results showed that adsorbed boundary films with a double-layer structure form stably during sliding, owing to double- or single-site surface adsorption and interlayer hydrogen bonding via the two functional groups. Additionally, some molecules in each of the first and second layers also form intralayer hydrogen bonding. Such multitype adsorption is unique and favorable for enhancing the strength of boundary films to withstand heavily loaded and prolonged sliding. The velocity distribution indicates that the first and second layers are solid- and liquid-like, respectively. The second layer could act as a buffer for the first layer, which is the last barrier to prevent solid-solid contact, against shear. We also found that the second layer can act as a reservoir to rapidly repair the once depleted region in the first layer because of the interlayer hydrogen bonding. The combination of the high strength and self-repair ability of the C12TEMPO boundary films can rationally explain the experimentally observed properties of high load-carrying capacity, excellent antiwear effect, and high stability of friction over time.

Polymerization via Insertion of Ethylene into Al-C bond under Mild Conditions: Mechanistic Studies on the Promotion Exerted by a Catalytic Amount of Cationic Gadolinium Metallocene.

The cationic gadolinium metallocene [(C5 Me5 )2 Gd][B(C6 F5 )4 ], when combined with an excess amount of Al(i Bu)3 , efficiently produces polyethylene at 80 °C under 0.8 MPa pressure of ethylene. After quenching, the resulting polyethylene has ethyl group at one end and isobutyl group at the other terminal. Because no Gd-alkyl species appears to be involved, a mechanism with conventional coordinative chain transfer polymerization (CCTP) is not feasible. Density functional theory (DFT) analyses indicate a novel mechanism in which the cationic Gd plays a crucial role by coordinating ethylene and assists the insertion of the coordinated ethylene into Al-C bond.

Atomistic Simulation of the Polymerization Reaction by a (Pyridylamido)hafnium(IV) Catalyst: Counteranion Influence on the Reaction Rate and the Living Character of the Catalytic System.

Atomistic simulation of the 1-octene polymerization reaction by a (pyridylamido)Hf(IV) catalyst was conducted on the basis of Red Moon (RM) methodology, focusing on the effect of the counteranions (CAs), MeB(C6F5)3-, and B(C6F5)4-, on the catalyst activity and chain termination reaction. We show that RM simulation reasonably reproduces the faster reaction rate with B(C6F5)4- than with MeB(C6F5)3-. Notably, the initiation of the polymerization reaction with MeB(C6F5)3- is comparatively slow due to the difficulty of the first insertion. Then, we investigated the free energy map of the ion pair (IP) structures consisting of each CA and the cationic (pyridylamido)Hf(IV) catalyst with the growing polymer chain (HfCatPn+), which determines the polymerization reaction rates, and found that HfCatPn+-MeB(C6F5)3- can keep forming "inner-sphere" IPs even after the polymer chain becomes sufficiently bulky, while HfCatPn+-B(C6F5)4- forms mostly "outer-sphere" IPs. Finally, we further tried to elucidate the origin of the broader molecular weight distribution (MWD) of the polymer experimentally produced with B(C6F5)4- than that with MeB(C6F5)3-. Then, through the trajectory analysis of the RM simulations, it was revealed that the chain termination reaction would be more sensitive to the IP structures than the monomer insertion reaction because the former involves a more constrained structure than the latter, which is likely to be a possible origin of the MWDs dependent on the CAs.

A model electronic Hamiltonian to describe low-lying d-d and metal-to-ligand charge-transfer excited states of [Fe(bpy)3 ]2.

A simple practical method to compute both d-d and metal-to-ligand charge-transfer (MLCT) excited states of iron(II) polypyridyl complexes is proposed for use in simulation studies. Specifically, a model electronic Hamiltonian developed previously for d-d excited states of [Fe(bpy)3 ]2+ is extended to deal with low-lying MLCT excited states simultaneously by including the MLCT electronic configurations into the basis functions of the model Hamiltonian. As a first attempt, parameters in the model Hamiltonian matrix elements are determined by using density functional theory (DFT) and time-dependent (TD-)DFT calculation results as benchmarks. To examine the performance of the model Hamiltonian, the potential energy curves along the interpolation between the lowest singlet and quintet state structures are compared to those from the (TD-)DFT calculations and to those from CASPT2 calculations in literature. The electronic absorption spectrum computed through molecular dynamics simulation is compared to the experimental spectrum. The spin-orbit couplings at the ground state structure are also compared to those from wavefunction-based ab initio electronic structure calculations. The results indicate that the constructed model Hamiltonian provides reasonable information on both the low-lying d-d and MLCT excited states of [Fe(bpy)3 ]2+ .

Theoretical Study of the Wavelength Selection for the Photocleavage of Coumarin-caged D-luciferin.

The equilibrium structures and optical properties of the photolabile caged luciferin, (7-diethylaminocoumarin-4-yl)methyl caged D-luciferin (DEACM-caged D-luciferin), in aqueous solution were investigated via quantum chemical calculations. The probable conformers of DEACM-caged D-luciferin were determined by potential energy curve scans and structural optimizations. We identified 40 possible conformers of DEACM-caged D-luciferin in water by comparing the Gibbs free energy of the optimized structures. Despite the difference in their structures, the conformers were similar in terms of assignments, oscillator strengths and energies of the three low-lying excited states. From the concentrations of the conformers and their oscillator strengths, we obtained a theoretical UV/Vis spectrum of DEACM-caged D-luciferin that has two main bands of shape nearly identical to the experimental UV/Vis spectrum. The absorption bands with maxima ~ 384 and 339 nm were attributed to the electronic excitations of the caged group and the luciferin moiety, respectively, by analysis of the theoretical UV/Vis spectrum. Furthermore, the analysis showed that DEACM-caged D-luciferin is excited in the caged group only by light of wavelength ranging within 400-430 nm, which is in the long-wavelength tail of the 384 nm band. This should be tested to lower damage upon photocleavage.

Conversion of flavonol glycoside to anthocyanin: an interpretation of the oxidation-reduction relationship of biosynthetic flavonoid-intermediates.

An efficient conversion of rutin to the corresponding anthocyanin, cyanidin 3-O-rutinoside, was established. Clemmensen-type reduction of rutin gave a mixture of flav-2-en-3-ol and two flav-3-en-3-ols, which were easily oxidised by air to give the anthocyanin. The interconversion reactions of these flavonoids provide insight into their biosynthetic pathway.

Imine hydrosilylation using an iron complex catalyst: A computational study.

The reaction mechanism for imine hydrosilylation in the presence of an iron methyl complex and hydrosilane was studied using density functional theory at the M06/6-311G(d,p) level of theory. Benzylidenemethylamine (PhCH = NMe) and trimethylhydrosilane (HSiMe3 ) were employed as the model imine and hydrosilane, respectively. Hydrosilylation has been experimentally proposed to occur in two stages. In the first stage, the active catalyst (CpFe(CO)SiMe3 , 1) is formed from the reaction of pre-catalyst, CpFe(CO)2 Me, and hydrosilane through CO migratory insertion into the FeMe bond and the reaction of the resulting acetyl complex intermediate with hydrosilane. In the second stage, 1 catalyzes the reaction of imine with hydrosilane. Calculations for the first stage showed that the most favorable pathway for CO insertion involved a spin state change, that is, two-state reactivity mechanism through a triplet state intermediate, and the acetyl complex reaction with HSiMe3 follows a σ-bond metathesis pathway. The calculations also showed that, in the catalytic cycle, the imine coordinates to 1 to form an FeCN three-membered ring intermediate accompanied by silyl group migration. This intermediate then reacts with HSiMe3 to yield the hydrosilylated product through a σ-bond metathesis and regenerate 1. The rate-determining step in the catalytic cycle was the coordination of HSiMe3 to the three-membered ring intermediate, with an activation energy of 23.1 kcal/mol. Imine hydrosilylation in the absence of an iron complex through a [2 + 2] cycloaddition mechanism requires much higher activation energies. © 2018 Wiley Periodicals, Inc.

Atomistic chemical computation of Olefin polymerization reaction catalyzed by (pyridylamido)hafnium(IV) complex: Application of Red Moon simulation.

We have realized the microscopic simulation of olefin polymerization, that is, the simulation of the catalytic polymerization (CP) reaction system composed of (pyridylamido)hafnium(IV) complex as the catalyst. For this purpose, we adopted Red Moon (RM) method, a novel molecular simulation method to simulate the complex reaction system. First, according to the previous research, with the help of the QM calculation, we proposed a model system and elementary processes and explained the theoretical treatment of the simulation by the RM method (the RM simulation). In addition, we also proposed a macroscopic simulation based on chemical kinetics simulation. Then, we performed two simulations and compared them in terms of the effective time evolution of the three macroscopic physical quantities, the number-average molecular weight Mn , the mass-average molecular weight Mw , and the molar-mass dispersity DM . The comparison showed that the two simulations are in quantitative or partially qualitative agreement with each other. Therefore, it is concluded that the RM simulation could not only simulate the CP reaction process microscopically, but also it is connected essentially to reproduce the time evolution of the macroscopic physical quantities on the basis of its microscopic simulation data. © 2018 Wiley Periodicals, Inc.

Probing the Most Stable Isomer of Zirconium Bis(phenoxy-imine) Cation: A Computational Investigation.

The possibility of coexistence of multiple isomers for zirconium bis(phenoxy-imine) catalyst has been systematically studied by computational approaches. The energetics among the five different isomers of neutral Zr-catalyst have been assessed quantum mechanically. The results suggest that isomer cis-N/trans-O/cis-Me is the most stable among the five isomers in accordance with the general observations of these kinds of phenoxy-imine catalyst. However, for the polymerization reaction, the active species is known to be the cationic form of the Zr-catalyst. The Zr-cation can exist in three different isomers, viz., cis-N/trans-O (A), cis-N/cis-O (B), and trans-N/cis-O (C), and the presence of flexible ligands makes the modeling considerably challenging to determine the most preferable isomers. For the efficient modeling, altogether 80 different structures for each of the three cationic isomers have been generated by using molecular dynamics simulations, and subsequently, the quantum mechanical optimization of these structures has been performed to obtain the most preferable conformation for each isomer. The existing probability derived from the obtained free energy values suggests that isomer C is comparable with isomer A. Even more, isomer A of the cation can be present in two different conformations, where the orientation of side groups is altered at the imine nitrogen atoms. The transition state calculations also confirm that the Zr-cation can exist as a mixture of three structures, "up-down" and "down-down" orientations of the isomers A along with isomer C's "up-up" orientation. However, by varying the substituents at imine nitrogen atoms, one could modulate multimodal to unimodal polymerization behavior of the Zr-catalysts. We believe that this study should provide a starting point for theoretically exploring the mechanistic pathway of the complicated polymerization reactions.

The effect of dynamical fluctuations of hydration structures on the absorption spectra of oxyluciferin anions in an aqueous solution.

In this study, the effect of hydration on the absorption spectra of oxyluciferin anion isomers in an aqueous solution is investigated for elucidating the influence of characteristic hydration structures. Using a canonical ensemble of hydration structures obtained from first-principles molecular dynamics simulations, the instantaneous absorption spectra of keto-, enol-, and enolate-type aqueous oxyluciferin anions at room temperature are computed from a collection of QM/MM calculations using an explicit solvent. It is demonstrated that the calculations reproduce experimental results concerning spectral shifts and broadening, for which traditional methods based on quantum chemistry and the Franck-Condon approximation fail because of the molecular vibrations of oxyluciferin anions and dynamical fluctuations of their hydration structures. Although the first absorption band associated with the lowest energy excitation corresponds to a π-π* transition for all oxyluciferin anion isomers, the changes in this band upon hydration are different among the isomers. In particular, the bands of enol- and enolate-type of oxyluciferin anions are significantly blue-shifted by hydration, whereas those of the keto-type oxylucifeion anion are shifted relatively less. Thus, the order of the first-peak positions in the aqueous solution changed relative to that in vacuo. We ascribe this to the nature of the oxyluciferin anion being more hydrophobic in the keto form as compared with the enol and enolate isomers.

Theoretical insights into the effect of pH values on oxidation processes in the emission of firefly luciferin in aqueous solution.

To elucidate the emission process of firefly d-luciferin oxidation across the pH range of 7-9, we identified the emission process by comparison of the potential and free-energy profiles for the formation of the firefly substrate and emitter, including intermediate molecules such as d-luciferyl adenylate, 4-membered dioxetanone, and their deprotonated chemical species. From these relative free energies, it is observed that the oxidation pathway changes from d-luciferin → deprotonated d-luciferyl adenylate → deprotonated 4-membered dioxetanone → oxyluciferin to deprotonated d-luciferin → deprotonated d-luciferyl adenylate → deprotonated 4-membered dioxetanone → oxyluciferin with increasing pH value. This indicates that deprotonation on 6'OH occurs during the formation of dioxetanone at pH 7-8, whereas luciferin in the reactant has a 6'OH-deprotonated form at pH 9.

Insight into the light-induced spin crossover of [Fe(bpy)3](2+) in aqueous solution from molecular dynamics simulation of d-d excited states.

Molecular dynamics (MD) simulations are performed for d-d excited states of the aqueous [Fe(bpy)3](2+) system using a previously developed model Hamiltonian. Specifically, the characters of d-d excited states and of transitions among these states are explored to gain clues about electronic relaxation during the photo-excited metal-to-ligand charge transfer (MLCT) to the lowest quintet d-d states. By evaluating the spin-orbit couplings in various nuclear configurations through MD simulations, strong mixing among low-lying d-d states with different spin multiplicities is found not to be expected in most of the sampled nuclear configurations except for surface crossing regions. The lifetimes of triplet d-d states are evaluated by Fermi's golden rule using equilibrium MD simulations. The internal conversion from upper-lying triplet to lower-lying triplet states is estimated to occur with a lifetime of order 100 fs accompanied by the distortion of the [Fe(bpy)3](2+) complex structure. This result is consistent with the discussion in another computational study, which evaluated the intersystem crossing rates by Fermi's golden rule using electronic structure calculations. In contrast, the present MD simulations cannot provide a clear picture of intersystem crossings from the lowest triplet d-d state after the above-mentioned internal conversion. Based on this result, possible relaxation mechanisms are discussed.

First-Principles Investigation of Strong Excitonic Effects in Oxygen 1s X-ray Absorption Spectra.

We calculated the oxygen 1s X-ray absorption spectra (XAS) of acetone and acetic acid molecules in vacuum by utilizing the first-principles GW+Bethe-Salpeter method with an all-electron mixed basis. The calculated excitation energies show good agreement with the available experimental data without an artificial shift. The remaining error, which is less than 1% or 2-5 eV, is a significant improvement from those of time-dependent (TD) density functional methods (5% error or 27-29 eV for TD-LDA and 2.4-2.8% error or 13-15 eV for TD-B3LYP). Our method reproduces the first and second isolated peaks and broad peaks at higher photon energies, corresponding to Rydberg excitations. We observed a failure of the one-particle picture (or independent particle approximation) from our assignment of the five lowest exciton peaks and significant excitonic or state-hybridization effects inherent in the core electron excitations.

Vibronic Structures in Absorption and Fluorescence Spectra of Firefly Oxyluciferin in Aqueous Solutions.

To elucidate the factors determining the spectral shapes and widths of the absorption and fluorescence spectra for keto and enol oxyluciferin and their conjugate bases in aqueous solutions, the intensities of vibronic transitions between their ground and first electronic excited states were calculated for the first time via estimation of the vibrational Franck-Condon factors. The major normal modes, overtones and combination tones in absorption and fluorescence spectra are similar for all species. The theoretical full widths at half maximum of absorption spectra are 0.4-0.7 eV and those for the fluorescence spectra are 0.4-0.5 eV, except for phenolate-keto that exhibits exceptionally sharp peak widths due to the dominance of the 0-0' or 0'-0 band. These spectral shapes and widths explain many relevant features of the experimentally observed spectra.