First-principles theory of the nitrogen interstitial in hBN: a plausible model for the blue emitter.

Color centers in hexagonal boron nitride (hBN) have attracted considerable attention due to their remarkable optical properties enabling robust room temperature photonics and quantum optics applications in the visible spectral range. On the other hand, identification of the microscopic origin of color centers in hBN has turned out to be a great challenge that hinders the in-depth theoretical characterization, on-demand fabrication, and development of integrated photonic devices. This is also true for the blue emitter, which is a result of irradiation damage in hBN, emitting at 436 nm wavelength with desirable properties. Here, we propose the negatively charged nitrogen split interstitial defect in hBN as a plausible microscopic model for the blue emitter. To this end, we carried out a comprehensive first-principles theoretical study of the nitrogen interstitial. We carefully analyzed the accuracy of first-principles methods and showed that the commonly used HSE hybrid exchange-correlation functional fails to describe the electronic structure of this defect. Using the generalized Koopman's theorem, we fine-tuned the functional and obtained a zero-phonon photoluminescence (ZPL) energy in the blue spectral range. We showed that the defect exhibits a high emission rate in the ZPL line and features a characteristic phonon side band that resembles the blue emitter's spectrum. Furthermore, we studied the electric field dependence of the ZPL and numerically showed that the defect exhibits a quadratic Stark shift that is perpendicular to plane electric fields, making the emitter insensitive to electric field fluctuations in the first order. Our work emphasizes the need for assessing the accuracy of common first-principles methods in hBN and exemplifies a workaround methodology. Furthermore, our work is a step towards understanding the structure of the blue emitter and utilizing it in photonics applications.

Toward Large-Scale Restricted Active Space Calculations Inspired by the Schmidt Decomposition.

We present an alternative, memory-efficient, Schmidt decomposition-based description of the inherently bipartite restricted active space (RAS) scheme, which can be implemented effortlessly within the density matrix renormalization group (DMRG) method via the dynamically extended active space procedure. Benchmark calculations are compared against state-of-the-art results of C2 and Cr2, which are notorious for their multireference character. Our results for ground and excited states together with spectroscopic constants demonstrate that the proposed novel approach, dubbed as DMRG-RAS, which is variational and free of uncontrolled method errors, has the potential to outperfom conventional methods for strongly correlated molecules.

Sensitivity of coupled cluster electronic properties on the reference determinant: Can Kohn-Sham orbitals be more beneficial than Hartree-Fock orbitals?

Coupled cluster calculations are traditionally performed over Hartree-Fock reference orbitals (HF-CC methodology). However, in the literature it has been repeatedly raised whether the use of a Kohn-Sham reference (KS-CC methodology) might result in improved performance relative to HF-CC. In the present study, we re-examine the relation of HF-CC and KS-CC methods by comparing the results of widely applied truncated CC calculations (CCSD, CCSD(T), CCSDT) to the limit of full configuration interaction (FCI), which serves as an undebatable reference point of accuracy. Based on a series of CC calculations on diatoms and transition metal complexes, we demonstrate that no systematic improvement of coupled cluster electronic energies, densities and chemical reaction energies is expected when changing from HF to a KS reference. Nevertheless, fortuitous error cancellations might occasionally result in illusory improvement compared to HF-CC. Altogether, the application of KS-CC is not advantageous over HF-CC, but it is also not unreasonable as the choice of reference has negligible influence on the results at sufficiently high CC levels. In addition, KS-CC can be a particularly useful alternative if difficulties are encountered in HF or HF-CC convergence. It is also notable that KS-CC results are found to be practically independent of the chosen density functional, which implies that almost any KS-CC method can be used in place of HF-CC.

Exploring Hydrogen Evolution Accompanying Nitrogen Reduction on Biomimetic Nitrogenase Analogs: Can Fe-N xH yIntermediates Be Active Under Turnover Conditions?

Nitrogen reduction reaction (N2RR) carried out on biomimetic catalytic systems is considered to be a promising alternative for the traditional Haber-Bosch ammonia synthesis. Unfortunately, the selectivity of the currently known biomimetic catalysts is poor, as they also catalyze the unproductive hydrogen evolution reaction (HER). In the present computational study, we examine the HER activity of early N2RR intermediates in EP3 (E = B, Si) ligated single-site biomimetic iron complexes by calculating and comparing the activation Gibbs free energies of HER and N2RR elementary steps. We find that, in contrast to previous suggestions, early N2RR intermediates are not likely sources of HER under turnover conditions, as the barriers of the competing N2RR steps are significantly lower. Consequently, future research should focus on preventing other potential HER mechanisms, e.g., hydride formation, rather than accelerating the consumption of early N2RR intermediates as proposed earlier to design more efficient biomimetic catalysts.

Identifying the Rate-Limiting Elementary Steps of Nitrogen Fixation with Single-Site Fe Model Complexes.

Biomimetic nitrogen fixation provides an attractive alternative for the century-old Haber-Bosch process; however, the performance of the currently available molecular biomimetic catalysts is very limited. In this work, we are aiming to understand the catalytic cycle of one of the most promising biomimetic complex families that can be the cornerstone of future computer-aided rational design of biomimetic complexes. We calculate the Gibbs free energy of all elementary reaction steps of homogeneous dinitrogen reduction to NH3 on single-site iron complexes with EPPP tetradentate ligands (E = B, Si). We examine all possible mechanisms and identify the dominant pathways and the critical elementary steps that can be rate-determining in the catalytic cycle of nitrogen fixation. We find that the catalytic mechanism depends on the applied ligand and that the distal pathway observed with E = B is the most favorable route regarding the catalytic performance. Our calculations also reveal the lack of thermodynamic driving force in the last steps of the catalytic cycle that can be responsible for the low catalytic activity of the studied biomimetic catalysts. Our results can serve as a starting point for the rational design of biomimetic complexes, which should focus on establishing a steadily decreasing Gibbs free energy profile, as suggested by the Sabatier principle.

Theoretical Evidence for the Utilization of Low-Valent Main-Group Complexes as Rare-Synthon Equivalents.

We examine by the means of computational chemistry the ability of two phosphasilenes to transfer the phosphinidene moiety to four double bonded organic functional groups (>C=C<, -N=N-, >C=O, and >C=S) in the presence of different bulky ligands. We show that large bulky groups in the reactants can sterically prohibit the otherwise favored association of reactants and phosphasilenes and instead a new phosphinidene transfer reaction can occur. We find that the transfer reaction mechanism is generally present independent from the functional group and by introducing large enough trimethylsilyl or tert-butyl-dimethylsilyl ligands it can be used to transfer phosphinidene to organic functional groups such as thioformaldehydes or diazenes, respectively. We propose that by exploiting the complex bonding nature of low-valent main group complexes they can act as synthetic equivalents of hitherto unknown very reactive synthons. We encourage experimentalists to explore the reactivity of their main-group complexes by varying the size of the bulky substituents on the reactants that can result in new unexpected chemistry.

Fluorogenic probes reveal a role of GLUT4 N-glycosylation in intracellular trafficking.

Glucose transporter 4 (GLUT4) is an N-glycosylated protein that maintains glucose homeostasis by regulating the protein translocation. To date, it has been unclear whether the N-glycan of GLUT4 contributes to its intracellular trafficking. Here, to clarify the role of the N-glycan, we developed fluorogenic probes that label cytoplasmic and plasma-membrane proteins for multicolor imaging of GLUT4 translocation. One of the probes, which is cell impermeant, selectively detected exocytosed GLUT4. Using this probe, we verified the 'log' of the trafficking, in which N-glycan-deficient GLUT4 was transiently translocated to the cell membrane upon insulin stimulation and was rapidly internalized without retention on the cell membrane. The results strongly suggest that the N-glycan functions in the retention of GLUT4 on the cell membrane. This study showed the utility of the fluorogenic probes and indicated that this imaging tool will be applicable for research on various membrane proteins that show dynamic changes in localization.

Molecular tailoring: a possible synthetic route to hexasilabenzene.

The possible synthesis of hexasilabenzene was studied as the consecutive reaction of three disilyne units in order to find a suitable substituent. Although there is a reaction pathway which leads to hexasilabenzene in the case of hydrogen (A) and phenyl (B) groups, and it is thermodynamically and kinetically favourable, the reaction can easily proceed toward octasila species which makes it impossible to keep the synthesis under control and prepare hexasilabenzene. In contrast to this, using a methylated terphenyl (D) substituent, the addition of the third disilyne unit to the four-membered silicon ring (D5) is highly unfavourable because of the steric hindrance of the substituents. The terphenyl group (C), however, seems to be a perfect substituent because the reaction pathway leading to substituted hexasilabenzene consists of thermodynamically favourable steps and small activation barriers, and further reaction is hindered by the bulky substituents. We suggest synthesizing hexasilabenzene from terphenyl-halosilanes, performing reductive dehalogenation.