Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra.

Understanding proton transfer (PT) dynamics in condensed phases is crucial in chemistry. We computed a 2D map of N 1s X-ray photoelectron/absorption spectroscopy (XPS/XAS) for an organic donor-acceptor salt crystal against two varying N-H distances to track proton motions. Our results provide a continuous spectroscopic mapping of O-H···N↔O-··· H+-N processes via hydrogen bonds at both nitrogens, demonstrating the sensitivity of N 1s transient XPS/XAS to hydrogen positions and PT. By reducing the O-H length at N1 by only 0.2 Å, we achieved excellent theory-experiment agreement in both XPS and XAS. Our study highlights the challenge in refining proton positions in experimental crystal structures by periodic geometry optimizations and proposes an alternative scaled snapshot protocol as a more effective approach. This work provides valuable insights into X-ray spectra for correlated PT dynamics in complex crystals, benefiting future experimental studies.

First-principles simulation of X-ray spectra of graphdiyne and graphdiyne oxides at the carbon K-edge.

The experimental C 1s near-edge X-ray absorption fine-structure (NEXAFS) spectra of graphdiyne (GDY) show an evident change at different exposure periods, which is explained by oxidation. Herein, to better understand the structure-spectra relationship and the influence of oxidization, we performed a first-principles simulation of the NEXAFS spectra and X-ray photoelectron spectra (XPS) of both pure GDY and its four typical graphdiyne oxides (GDO) at the carbon K-edge. Pure GDY contains one sp2-hybridized (C1) and two sp-hybridized (C2, C3) carbons, while oxidation introduces more nonequivalent carbons. The experimental NEXAFS spectrum exhibits the lowest peak at ca. 285.8 eV. It was found that the C 1s → π* excitation from the sp2-hybridized carbon atoms (C1) in pure GDY and the sp-hybridized atoms (C2, C3) in GDOs contributes to this peak. The two weak resonances at around 289.0 and 290.6 eV in the experiment are contributed by the carbon atoms bonded to the oxygen atoms. Meanwhile, we found that oxidization leads to an increase in the C 1s ionization potentials (IPs) by 0.3-2.7 eV, which is consistent with the XPS experiments. Our calculations provide a clear explanation of the structure-spectra relationships of GDY and GDOs, and the signatures are useful for estimating the degree of oxidation.

Interpreting the Cu-O2 Antibonding Nature in Two Cu-O2 Complexes from Cu L-Edge X-ray Absorption Spectra.

Cu-O2 structures play important roles in bioinorganic chemistry and enzyme catalysis, where the bonding between the Cu and O2 parts serves as a fundamental research concern. Here, we performed a multiconfigurational study on the copper L2,3-edge X-ray absorption spectra (XAS) of two copper enzyme model complexes to gain a better understanding of the antibonding nature from the clearly interpreted structure-spectroscopy relation. We obtained spectra in good agreement with the experiments by using the restricted active space second-order perturbation theory (RASPT2) method, which facilitated reliable chemical analysis. Spectral feature interpretations were supported by computing the spin-orbit natural transition orbitals. All major features were assigned to be mainly from Cu 2p to antibonding orbitals between Cu 3d and O2 π*, Cu 3d-πO-O* (type A), and a few also to mixed antibonding/bonding orbitals between Cu 3d and O2 π, Cu 3d ± πO-O (type M). Our calculations provided a clear illustration of the interactions between Cu 3d and O2 π*/π orbitals that are carried in the metal L-edge XAS.

Efficient degradation of the refractory organic pollutant by underwater bubbling pulsed discharge plasma: performance, degradation pathway, and toxicity prediction.

It is essential to develop an efficient technology for the elimination of refractory contaminants due to their high toxicity. In this study, a novel underwater bubbling pulsed discharge plasma (UBPDP) system was proposed for the degradation of Orange II (OII). The degradation performance experiments showed that by enhancing the peak voltage and pulse frequency, the degradation efficiency of OII increased gradually. The removal efficiencies under different air flow rates were close. Reducing OII concentration and solution conductivity could promote the elimination of OII. Compared with neutral and alkaline conditions, acidic condition was more beneficial to OII degradation. The active species including ·OH, ·O2-, 1O2, and hydrated electrons were all involved in OII degradation. The concentrations of O3 and H2O2 in OII solution were lower than those in deionized water. During discharge, the solution pH increased while conductivity decreased. The variation of UV-vis spectra with treatment time indicated the effective decomposition of OII. Possible degradation pathways were speculated based on LC-MS. The toxicity of intermediate products was predicted by the Toxicity Estimation Software Tool. Coexisting constituents including Cl-, SO42-, HCO3-, and humic acid had a negative effect on OII removal. Finally, the comparison with other technology depicted the advantage of the UBPDP system.

Manipulating spatial alignment of donor and acceptor in host-guest MOF for TADF.

Thermally activated delayed fluorescence (TADF) was achieved when electron-rich triphenylene (Tpl) donors were confined to a cage-based porous metal-organic framework (MOF) host (NKU-111) composed of electron-deficient 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine (Tpt) acceptor as the ligand. The spatially separated donor and acceptor molecules in a face-to-face stacking pattern generated strong through-space charge transfer (CT) interactions with a small energy splitting between the singlet and triplet excited states (∼0.1 eV), which enabled TADF. The resulting Tpl@NKU-111 exhibited an uncommon enhanced emission intensity as the temperature increased. Extensive steady-state and time-resolved spectroscopic measurements and first-principles simulations revealed the chemical and electronic structure of this compound in both the ground and low-lying excited states. A double-channel (T1, T2) intersystem crossing mechanism with S1 was found and explained as single-directional CT from the degenerate HOMO-1/HOMO of the guest donor to the LUMO+1 of one of the nearest acceptors. The rigid skeleton of the compound and effective through-space CT enhanced the photoluminescence quantum yield (PLQY). A maximum PLQY of 57.36% was achieved by optimizing the Tpl loading ratio in the host framework. These results indicate the potential of the MOFs for the targeted construction and optimization of TADF materials.

On the choice of shape and size for truncated cluster-based x-ray spectral simulations of 2D materials.

Truncated cluster models represent an effective way for simulating x-ray spectra of 2D materials. Here, we systematically assessed the influence of two key parameters, the cluster shape (honeycomb, rectangle, or parallelogram) and size, in x-ray photoelectron (XPS) and absorption (XAS) spectra simulations of three 2D materials at five K-edges (graphene, C 1s; C3N, C/N 1s; h-BN, B/N 1s) to pursue the accuracy limit of binding energy (BE) and spectral profile predictions. Several recent XPS experiments reported BEs with differences spanning 0.3, 1.5, 0.7, 0.3, and 0.3 eV, respectively. Our calculations favor the honeycomb model for stable accuracy and fast size convergence, and a honeycomb with ∼10 nm side length (120 atoms) is enough to predict accurate 1s BEs for all 2D sheets. Compared to all these experiments, predicted BEs show absolute deviations as follows: 0.4-0.7, 0.0-1.0, 0.4-1.1, 0.6-0.9, and 0.1-0.4 eV. A mean absolute deviation of 0.3 eV was achieved if we compare only to the closest experiment. We found that the sensitivity of computed BEs to different model shapes depends on systems: graphene, sensitive; C3N, weak; and h-BN, very weak. This can be attributed to their more or less delocalized π electrons in this series. For this reason, a larger cluster size is required for graphene than the other two to reproduce fine structures in XAS. The general profile of XAS shows weak dependence on model shape. Our calculations provide optimal parameters and accuracy estimations that are useful for x-ray spectral simulations of general graphene-like 2D materials.

Vibrationally-Resolved X-ray Photoelectron Spectra of Six Polycyclic Aromatic Hydrocarbons from First-Principles Simulations.

Vibrationally resolved C 1s X-ray photoelectron spectra (XPS) of a series of six polycyclic aromatic hydrocarbons (PAHs; phenanthrene, coronene, naphthalene, anthracene, tetracene, and pentacene) were computed by combining the full core hole density functional theory and the Franck-Condon simulations with the inclusion of the Duschinsky rotation effect. Simulated spectra of phenanthrene, coronene, and naphthalene agree well with experiments both in core binding energies (BEs) and profiles, which validate the accuracy of our predictions for the rest molecules with no high-resolution experiments. We found that three types of carbons i (inner C), p (peripheral C bonded to three C atoms), and h (peripheral C bonded to an H atom) show decreasing BEs. In linear PAHs (the latter four), h-type carbons further split into h1 or h2 (on inner or edge benzene ring) subtypes with chemical shifts of ca. 0.2-0.4 eV. All major Franck-Condon-active modes are characterized to be in-plane vibrations: low-frequency (<800 cm-1) C-C ring deformation modes play an essential role in determining the peak asymmetries; and for each h-type carbon a high-frequency (ca. 3600 cm-1) C*-H stretching mode is responsible for the high-energy tail. We found that core ionization leads to reduction of all C*-C and C*-H bond lengths and ring deformation with a definite direction. Based on theoretical spectra of four linear PAHs, we found asymptotic relations and anticipated possible spectral features for even larger linear PAHs. Our calculations provide accurate reference spectra for XPS characterizations of PAHs, which are useful in understanding the vibronic coupling effects in this family.

A theoretical library of N1s core binding energies of polynitrogen molecules and ions in the gas phase.

Polynitrogen molecules and ions are important building blocks of high energy density compounds (HEDCs). High energy bonds formed at the N sites can be effectively probed by X-ray photoelectron spectroscopy (XPS) at the N K-edge. In this work, with the density functional theory and the ΔKohn-Sham scheme, we simulated the N1s ionic potentials (IPs) of 72 common polynitrogen molecules [tetrazoles, pentazole (N5H), diazines, triazines, tetrazines, furazans, oxazoles and oxadiazoles], ions [pentazolate anion (cyclo-N5-), pentazenium cation (N5+), etc.], and molecular (NH3⋯N5H, H2O⋯N5H) and ionic (NH4+⋯N5-, H3O+⋯N5-) pairs, as well as mononitrogen relatives. These constitute a small theoretical database for absolute N1s IPs with an average accuracy of ca. 0.3 eV. To understand the structure-IP relationship within this family, effects of side substituent and bridging groups, local bonding types (amine or imine N), charge and protonation states, and vibronic coupling were analyzed based on selected systems. This study in the gas phase collects inherent chemical shifts of nitrogen in high-energy NN and NC bonds, which provides an essential reference into XPS interpretations of more complex HEDCs in the solid state. We especially highlight the evident N1s chemical shifts induced by protonation for nitrogen in the five-membered ring (N5H versus cyclo-N5-, ca. 7 eV; NH3⋯N5H versus NH4+⋯N5-, ca. 3 eV; H2O⋯N5H versus H3O+⋯N5-, ca. 2 eV), and suggest XPS as a sensitive tool in determining the hydrogen positions in pentanitrogen-based HEDCs.

Breaking inversion symmetry by protonation: experimental and theoretical NEXAFS study of the diazynium ion, N2H.

As an example of symmetry breaking in NEXAFS spectra of protonated species we present a high resolution NEXAFS spectrum of protonated dinitrogen, the diazynium ion N2H+. By ab initio calculations we show that the spectrum consists of a superposition of two nitrogen 1s absorption spectra, each including a π* band, and a nitrogen 1s to H+ charge transfer band followed by a weak irregular progression of high energy excitations. Calculations also show that, as an effect of symmetry breaking by protonation, the π* transitions are separated by 0.23 eV, only slightly exceeding the difference in the corresponding dark (symmetry forbidden) and bright (symmetry allowed) core excitations of neutral N2. By DFT and calculations and vibrational analysis, the complex π* excitation band of N2H+ is understood as due to the superposition of the significantly different vibrational progressions of excitations from terminal and central nitrogen atoms, both leading to bent final state geometries. We also show computationally that the electronic structure of the charge transfer excitation smoothly depends on the nitrogen-proton distance and that there is a clear extension of the spectra going from infinity to close nitrogen-proton distance where fine structures show some, although not fully detailed, similarities. An interesting feature of partial localization of the nitrogen core orbitals, with a strong, non-monotonous, variation with nitrogen-proton distance could be highlighted. Specific effects could be unraveled when comparing molecular cation NEXAFS spectra, as represented by recently recorded spectra of N2+ and CO+, and spectra of protonated molecules as represented here by the N2H+ ion. Both types containing rich physical effects not represented in NEXAFS of neutral molecules because of the positive charge, whereas protonation also breaks the symmetry. The effect of the protonation on dinitrogen can be separated in charge, which extends the high-energy part of the spectrum, and symmetry-breaking, which is most clearly seen in the low-energy π* transition.

Theoretical assessment of vibrationally resolved C1s X-ray photoelectron spectra of simple cyclic molecules.

An interface between our in-house DynaVib package and quantum chemistry software Gamess-US is implemented for computing vibrationally-resolved K-edge X-ray photoelectron spectra (XPS) of molecules at the density functional theory level with both the full (FCH) and equivalent (ECH, or Z+1) core-hole approximations. To assess the influence of theoretical parameters (core-hole methods, vibronic coupling models, and basis sets), vibrationally-resolved C1s XPS of six simple cyclic molecules [furan, pyrrole, thiophene; benzene (C6H6 and C6D6); pyridine] were evaluated in the gas phase by both core-hole methods in combination with two time-independent vibronic coupling models, the Duschinsky rotation (DR) method and the linear coupling model (LCM). We achieved excellent/acceptable performance for FCH/Z+1 simulations in comparison with experiments. The most accurate method FCH-DR correctly reproduced all experimental features and gave an accuracy of ca. 0.2 eV in absolute binding energies (BEs). The choice of the vibronic model is less sensitive to that of electronic structure method. Results indicate that Z+1 overestimates the core-hole effect on the geometry of the ionization state.

Theoretical Spectroscopic Studies on Chemical and Electronic Structures of Selenocysteine and Pyrrolysine.

The chemical and electronic structures of the 21st and 22nd proteinogenic amino acid selenocysteine (Sec), pyrrolysine (Pyl), and their derivatives (deprotonated and protonated ions) were extensively characterized for the first time. Through the fragment based step-by-step research on their potential energy surface (PES), electronic energies of the most stable conformers of Sec, Pyl and the related ions were finally determined at the advanced CBS-QB3 and DSD-PBEP86-D3(BJ)/aug-cc-pVTZ levels, respectively, with the identification of many new low-energy conformers. The infrared spectra (IR) at 298 K of the most abundant conformers in different forms were scaled by comparison with the anharmonic frequency calculations and analyzed comparing with the experimental spectra of similar molecules. The characteristic soft X-ray spectra (including X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine-structure spectra (NEXAFS)) of the most stable conformers at 498 K were also simulated. In particular, the two possible protonated configurations of Pyl can be clearly distinguished by their different spectral features. Furthermore, a small binding energy intersection appeared around 293 eV at the C 1s edge between the canonical and protonated Pyl conformers, which is different from all the previous studies. This work thus filled the gap in our knowledge by providing detailed information on the chemical and electronic structures of Sec and Pyl and will be a useful guidance for future experimental research.

Transient X-ray Absorption Spectral Fingerprints of the S1 Dark State in Uracil.

Low-lying dark nπ* states play an important role in many photophysical and photochemical processes of organic chromophores. Transient X-ray absorption spectroscopy (TXAS) provides a powerful technique for probing the dynamics of valence states by exciting the electrons into high-lying core excited states. We employ multiconfigurational self-consistent field calculations to investigate the TXAS of uracil along its nonradiative photodecay pathways. An open issue is whether dark nπ* state S1 (n is the lone pair localized on an oxygen atom) is accessible when bright ππ* state S2 is selectively excited. Vertical core excitations were calculated along the potential energy surfaces of the three lowest states, S0-S2, interpolated between two minima and two minimum-energy conical intersections. Computed TXAS data from the C, N, and O K edges show distinct spectral fingerprints of the dark state in all spectral regimes. At the O 1s edge, the nπ* state has a very strong absorption at 526-527 eV, while at the C (N) 1s edge, by contrast, there is almost zero (very weak) absorption at 279-282 eV (397-398 eV). All K-edge spectra can be used to sensitively detect the dark states. Our proposed O 1s feature has already been observed in a recent TXAS experiment with thymine. Natural transition orbital analysis is used to interpret all dominant features of the three lowest-valence states along the reaction coordinate and reveal some important valence fine-structure information from the core excitation.

The role of transition dipole phase in atomic attosecond transient absorption from the multi-level model.

Based on a multilevel model considering enough bound electronic states of atoms, we theoretically study the role of the transition dipole phase (TDP) in the attosecond transient absorption (ATA) spectrum of helium in intense laser fields. By solving the stationary Schrödinger equation with B-spline basis sets, we first calculate the transition dipole moments with well-defined phases between the bound states. Using the modified multilevel model, we reveal that the TDP plays an important role in determining the spectral structures if two or more paths populate the excited states from the ground state. Our multilevel model with the accurate TDP is convenient to address the origin of atomic ATA spectral structures by freely removing or adding specific electronic states and has been justified by comparing with the ATA spectra via directly solving the time-dependent Schrödinger equation. Hopefully, further incorporating macroscopic propagation into the model will provide indepth physical insights into experimental ATA spectra.

Accurate K-edge X-ray photoelectron and absorption spectra of g-C3N4 nanosheets by first-principles simulations and reinterpretations.

We performed a density functional theory (DFT) study on X-ray photoelectron (XPS) and absorption (XAS) spectra of graphitic carbon nitride (g-C3N4) nanosheets at the N and C K-edges. A combined cluster-periodic approach was employed to calculate XPS spectra, in which the core ionic potential (IP) of the solid 2D material was computed by subtracting the work function (obtained with periodic conditions) from the gas phase IP (obtained with large cluster models). With amino-terminated supermolecules of different sizes, we obtained convergent spectra and provide new assignments for 5 nitrogen [1 sp2; 4 sp3 (bridging, tertiary, and primary/secondary amino nitrogens)] and 4 carbon (all bonded with three nitrogens) local structures. A good agreement with experiments was obtained, with the N1s (C1s) main peak position differing by 0.1-0.2 eV (0.5-0.8 eV). Our simulations show that N1s XPS of pure g-C3N4 contains only two major features at 398.6 and 401.2 eV, contributed from sp2-N and sp3-N, respectively. The chemical shifts of all sp3-N are so close (deviating by 0.3-0.6 eV) that terminal amino groups -NHx (x = 1, 2) will only be distinguished in high-resolution measurements. In C1s XPS, all carbons show similar (deviation < 0.2 eV) IPs, as determined by the same nearest neighbors. We further excluded the effect of shake-up satellites that may change our XPS interpretations by equivalent core hole time-dependent DFT (ECH-TDDFT) simulations. The effect of vibronic coupling is small (redistribution is only 0.1-0.3 eV to the higher-energy region) in the N1s edge as estimated from the asymmetric main peak shape, and negligible in the C1s edge. Quicker size convergence was found in XAS than XPS. In N1s XAS, we identified a weak π* spectral feature at 400-401 eV for both -NHx and tertiary nitrogens. Our study provides a clear theoretical reference for X-ray spectral fingerprints of different local structures, which is useful for analysis of g-C3N4 based materials with various designed or unavoidable structural modifications. We also highlight our combined cluster-periodic approach in calculating the K-edge XPS spectra of general 2D materials which predicts accurate absolute values.

Study on the light medium separation of waste plastics with hydrocyclones.

The separation of waste plastics is an important part of solid waste recycling. Based on the density difference between high density polyethylene (HDPE) and polypropylene (PP), this paper used the experimental research and CFD numerical simulation to study the separation performance by using the light medium separation technology with hydrocyclones. Results showed that with the increase of feed flow rate, the pressure drop increased as a power function, the Newton efficiency peaked at the feed flow rate of 3.6 m3/h and peaked at the volume ratio of PP to HDPE particles of 2.0 respectively.

Thermally Activated Delayed Fluorescence in an Organic Cocrystal: Narrowing the Singlet-Triplet Energy Gap via Charge Transfer.

Harvesting non-emissive spin-triplet charge-transfer (CT) excitons of organic semiconductors is fundamentally important for increasing the operation efficiency of future devices. Here we observe thermally activated delayed fluorescence (TADF) in a 1:2 CT cocrystal of trans-1,2-diphenylethylene (TSB) and 1,2,4,5-tetracyanobenzene (TCNB). This cocrystal system is characterized by absorption spectroscopy, variable-temperature steady-state and time-resolved photoluminescence spectroscopy, single-crystal X-ray diffraction, and first-principles calculations. These data reveal that intermolecular CT in cocrystal narrows the singlet-triplet energy gap and therefore facilitates reverse intersystem crossing (RISC) for TADF. These findings open up a new way for the future design and development of novel TADF materials.

On the spectral profile change in the Q band absorption spectra of metalloporphyrins (Mg, Zn, and Pd): A first-principles study.

We performed a systematic study of the vibrationally resolved absorption spectra in the Q band of three metalloporphyrins (Mg, Zn, and Pd) to understand the spectral changes in this series, including both the Franck-Condon (FC) and Herzberg-Teller (HT) contributions. The ground (S0) and the lowest singlet excited (S1) states were, respectively, simulated by the static and time-dependent density functional theory, with which the Duschinsky rotation effect was considered. Different functionals and basis sets were tested and compared with experiment. Results show that the long-range corrected functional CAM-B3LYP can nicely describe the spectral fingerprints of these metalloporphyrins, while the B3LYP functional significantly underestimates the FC contributions. We found that the absorption fine structures of these molecules are mainly caused by the HT vibronic couplings. The experimentally observed enhancements to the on-site 0-0 absorption peak in the series of Mg, Zn, and Pd are nicely reproduced. Enhanced absorption intensity is caused by larger FC contributions of molecules with heavier metal ions. The structure-spectroscopy relationship was analyzed, and it was found that the smaller cavity size of the porphyrin ring can significantly enhance the oscillator strength of the S0 → S1 transition.

Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy.

Attosecond X-ray pulses are short enough to capture snapshots of molecules undergoing nonadiabatic electron and nuclear dynamics at conical intersections (CoIns). We show that a stimulated Raman probe induced by a combination of an attosecond and a femtosecond pulse has a unique temporal and spectral resolution for probing the nonadiabatic dynamics and detecting the ultrafast (∼4.5 fs) passage through a CoIn. This is demonstrated by a multiconfigurational self-consistent-field study of the dynamics and spectroscopy of the furan ring-opening reaction. Trajectories generated by surface hopping simulations were used to predict Attosecond Stimulated X-ray Raman Spectroscopy signals at reactant and product structures as well as representative snapshots along the conical intersection seam. The signals are highly sensitive to the changes in nonadiabatically coupled electronic structure and geometry.

Nonlinear Spectroscopy of Core and Valence Excitations Using Short X-Ray Pulses: Simulation Challenges.

Measuring the nonlinear response of electrons and nuclei to attosecond broadband X-ray radiation has become possible by newly developed free electron lasers and high harmonic generation light sources. The design and interpretation of these novel experiments poses considerable computational challenges. In this chapter we survey the basic description of nonlinear X-ray spectroscopy signals and the electronic structure protocols which may be used for their simulation.

Study of double core hole excitations in molecules by X-ray double-quantum-coherence signals: a multi-configuration simulation.

The multi-configurational self-consistent field method is employed to simulate the two-dimensional all-X-ray double-quantum-coherence (XDQC) spectroscopy, a four-wave mixing signal that provides direct signatures of double core hole (DCH) states. The valence electronic structure is probed by capturing the correlation between the single (SCH) and double core hole states. The state-averaged restricted-active-space self-consistent field (SA-RASSCF) approach is used which can treat the valence, SCH, and DCH states at the same theoretical level, and applies to all types of DCHs (located on one or two atoms, K-edge or L-edge), with both accuracy and efficiency. Orbital relaxation introduced by the core hole(s) and the static electron correlation is properly accounted for. The XDQC process can take place via different intermediate DCH state channels by tuning the pulse frequencies. We simulate the XDQC signals for the three isomers of aminophenol at 8 pulse frequency configurations, covering all DCH pathways involving the N1s and O1s core hole (N1sN1s, O1sO1s and N1sO1s), which reveal different patterns of valence excitations.