Open-shell Singlet States Research Articles

Nickel-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination

The fluoroalkyl-containing organic molecules are widely used in drug discovery and material science. Herein, we report ligand regulated nickel(0)-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination, providing an atom- and step-economical synthesis route of two types of fluoroalkyl substituted silanes with exclusive regioselectivity. The anti-Markovnikov addition products (β-fluoroalkyl substituted silanes) are formed with monodentate phosphine ligand. Noteworthy, the bidentate phosphine ligand promote the generation of the more challenging Markovnikov products (α-fluoroalkyl substituted silanes) with tetrasubstituted saturated carbon centers. This protocol features with easy available starting materials and commercially available nickel catalysis, a wide range of substrates and excellent regioselectivity. The structure divergent products undergo a variety of transformations. Comprehensive mechanistic studies including the inverse kinetic isotope effects demonstrate the regioselectivity controlled by ligand structure through α-CF3 nickel intermediate. DFT calculations reveal a distinctive mechanism involving an open-shell singlet state, which is crucial for generating intricate tetra-substituted Markovnikov products.

Spin-orbit charge transfer from guanine and 9-methylguanine radical cations to nitric oxide radicals and the induced triplet-to-singlet intersystem crossing.

Nitric oxide (●NO) participates in many biological activities, including enhancing DNA radiosensitivity in ionizing radiation-based radiotherapy. To help understand the radiosensitization of ●NO, we report reaction dynamics between ●NO and the radical cations of guanine (a 9HG●+ conformer) and 9-methylguanine (9MG●+). On the basis of the formation of 9HG●+ and 9MG●+ in the gas phase and the collisions of the radical cations with ●NO in a guided-ion beam mass spectrometer, the charge transfer reactions of 9HG●+ and 9MG●+ with ●NO were examined. For both reactions, the kinetic energy-dependent product ion cross sections revealed a threshold energy that is 0.24 (or 0.37) eV above the 0K product 9HG (or 9MG) + NO+ asymptote. To interrogate this abnormal threshold behavior, the reaction potential energy surface for [9MG + NO]+ was mapped out at closed-shell singlet, open-shell singlet, and triplet states using density functional and coupled cluster theories. The results showed that the charge transfer reaction requires the interaction of a triplet-state surface originating from a reactant-like precursor complex 3[9MG●+(↑)⋅(↑)●NO] with a closed-shell singlet-state surface evolving from a charge-transferred complex 1[9MG⋅NO+]. During the reaction, an electron is transferred from π∗(NO) to perpendicular π∗(9MG), which introduces a change in orbital angular momentum. The latter offsets the change in electron spin angular momentum and facilitates intersystem crossing. The reaction threshold in excess of the 0K thermochemistry and the low charge-transfer efficiency are rationalized by the vibrational excitation in the product ion NO+ and the kinetic shift arising from a long-lived triplet intermediate.

Introducing Novel Materials with Diffuse Electrons for Applications in Redox Catalysis and Quantum Computing via Theoretical Calculations

Diamond-like structures, where carbon atoms have been replaced with Li+ and C–C bonds with diamines, have currently been introduced as new materials, which can host diffuse electrons in the periphery of each lithium tetra-amine center. These materials display a diverse range of properties behaving as metals or semiconductors depending on the diamine chain length. Multi-reference wavefunction and density functional theory calculations were employed to study the electronic structure of these materials. Initially, gas phase calculations are performed on isolated (NH3)3LiNH2(CH2)1–10H2NLi(NH3)3 molecular strings. One diffuse electron surrounds the periphery of each −NH2Li(NH3)3 terminus. The two terminal electrons couple into a triplet and open-shell singlet states, which are nearly degenerate for long chains and as closed shell singlets for short. At intermediate lengths, the wavefunction of the ground-state singlet state mixes both open- and closed-shell configurations raising doubts about which configuration should be considered for density functional theory calculations. Observations from gas phase calculations accurately predict properties from the condense phase density functional theory calculations carried out for proposed crystalline Li-diamine materials, offering an avenue for further development and insight. Spin-polarized and unpolarized calculations are performed for the whole range of hydrocarbon sizes reporting geometrical and electronic band structures, spin density contours, and density of states. Diffuse electrons can be used for redox reactions or can serve as qubits for quantum computing. Future work will focus on decorating the hydrocarbon backbone with functional groups and/or bulky units, in order to facilitate or block the association between neighboring electrons for more controlled quantum computing applications and propose materials for selective redox catalysis.

Does the Spin State and Oriented External Electric Field Boost the Efficiency of Fe(II) Pincer Catalyst toward CO2Hydrogenation Reaction?

In this study, we have explored the catalytic reactivities of four PNP-pincer supported Fe(II) complexes, namely, [(iPrPNMeP)FeH2(CO)] (1), [(iPrPNMeP)FeH(CO)(BH4)] (2), [(iPrPNHP)FeH2(CO)] (3), and [(iPrPNMeP)FeH(BH4)] (4) (iPrPNMeP = MeN{CH2CH2(PiPr2)}2 and iPrPNHP = HN{CH2CH2(PiPr2)}2) toward reductive CO2 hydrogenation for formate production. Our density functional theory and ab initio complete active space self-consistent field study have identified three fundamental steps in this catalytic transformation: (i) anchoring of the CO2 molecule in the vicinity of the metal using noncovalent interactions, (ii) catalyst regeneration via H2 cleavage, and (iii) formate rebound step leading to catalytic poisoning. The variations in the catalytic efficiency observed among these catalysts were attributed to either easing of steps (i) and (ii) or the hampering step (iii). This can be achieved in various chemical/non-chemical ways, for instance, (a) incorporation of strong-field ligands such as CO facilitating single-state reactivity and eliminating two-state reactivity that generally enhances the rate and (b) inclusion of Lewis acids such as LiOTf and strong bases found to either avoid catalytic poisoning or ease the H-H cleavages, to enhance the rate of reaction (c) evading mixing of excited open-shell singlet states to the ground closed-shell singlet state that hampers the catalytic regeneration. We have probed the role of oriented external electric fields (OEEFs) in the entire mechanistic profile for the best and worst catalyst, and our study suggests that imposing OEEFs opposite to the reaction axis (z-axis) fastens the catalytic regeneration step and, at the same time, hampers catalytic poisoning. The application of OEEFs is found to regulate the energetics of various spin states and can hamper two-state reactivity, therefore increasing the efficiency. Thus, this study provides insights into the CO2 hydrogenation mechanism where the role of bases/Lewis acid, ligand design, spin states, and electric field in a particular direction has been established and is, therefore, likely to pave the way forward for a new generation of catalysts.

High-throughput UV-photofragmentation studies of thymine and guanine.

High-throughput photofragmentation studies of thymine and guanine were performed at 257 and 343 nm and for a wide range of ionisation laser intensities. Combining a continuous laser-based thermal desorption source with femtosecond multiphoton ionisation using a 50 kHz repetition rate laser allowed us to produce detailed 2D maps of fragmentation as a function of incident laser intensity. The fragmentation was distinctly soft, the parent ions being at least an order of magnitude more abundant than fragment ions. For thymine there was a single dominant fragmentation channel, which involves consecutive HNCO and CO losses. In contrast, for guanine there were several competing ones, the most probable channel corresponding to CH2N2 loss through opening of the pyrimidine ring. The dependence of parent ion abundance on the ionisation laser intensity showed that at 257 nm the ionisation of thymine is a 1 + 1 resonance enhanced process through its open-shell singlet state.

Issues on DFT+U calculations of organic diradicals.

The diradical state is an important electronic state for understanding molecular functions and should be elucidated for the in silico design of functional molecules and their application to molecular devices. The density functional theory calculation with plane-wave basis and correction of the on-site Coulomb parameter U (DFT+U/plane-wave calculation) is a good candidate of high-throughput calculations of diradical-band interactions. However, it has not been investigated in detail to what extent the DFT+U/plane-wave calculation can be used to calculate organic diradicals with a high degree of accuracy. In the present study, using typical organic diradical molecules (bisphenalenyl molecules) as model systems, the discrepancy in the optimum U values between the two electronic states (open-shell singlet and triplet) that compose the diradical state is detected. The calculated results show that the reason for this U value discrepancy is the difference in electronic delocalisation due to π-conjugation between the open-shell singlet and triplet states, and that the effect of U discrepancy becomes large as diradical character decreases. This indicates that it is necessary to investigate the U value discrepancy with reference to the calculated results by more accurate methods or to experimental values when calculating organic diradicals with low diradical character. For this investigation, the local magnetic moments, unpaired beta electron numbers, and effective magnetic exchange integral values can be used as reference values. For the effective magnetic exchange integral values, the effects of U discrepancy are partially cancelled out. However, because the effects may not be completely offset, care should be taken when using the effective magnetic exchange integral value as a reference. Furthermore, a comparison of DFT+U and hybrid-DFT calculations shows that the DFT+U underestimates the HOMO-LUMO gap of bisphenalenyls, although a qualitative discussion of the gap is possible.

Singlet Fission and Aromaticity.

Qualitative aromaticity-based arguments are often used to explain singlet fission (SF) properties in polycyclic conjugated systems. It has been shown by Fowler and collaborators that magnetically induced ring currents are associated with transitions between occupied and unoccupied molecular orbitals (MOs). Since SF has to do with relative energies of electronic states, it was hypothesized that induced currents may indicate SF properties. The quantitative aromaticity of linear oligoacene and several doubly boron-doped anthracenes and phenanthrenes, in their closed-shell singlet, open-shell singlet (where applicable), and triplet electronic states, has been studied using nucleus-independent chemical shift (NICS)-XY scan methods. It is shown that quantitative magnetic aromaticity can be used to identify SF compounds, at least for initial screening. Thus, the induced current features that are indicative of singlet fission (SF) ability have been found to be global current and local current at each ring of the systems. This conclusion was verified for very different systems─a tetracyclic, nitrogen-containing quinone (6), diphenyl benzofurane (7), and cibalackrot (8), all reported to be SF systems. Finally, it is predicted that some isomers of the doubly boron-doped linear[4]phenylene should show SF properties.

Computational Exploration of the Intersystem Crossing from the X̃3A2 to the ã1A1 State in Boryl Nitrenes upon Photoexcitation.

The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state X̃3A2 with light of wavelength λ > 550 nm. A computational study of a model boryl nitrene using complete active space self-consistent field (CASSCF) theory provides evidence for the population of the highly reactive electronic state ã1A1 upon irradiation. Potential energy scans connecting different critical points (minima, minimum energy crossing points, and conical intersections) reveal two possible pathways that could relax photoexcited boryl nitrene from the Franck-Condon region of Ã3B1 to the ã1A1 state minimum. Considering the energy barriers to relaxation from one electronic state to another and the magnitude of spin-orbit couplings, the energetically most favorable pathway involves photoexcitation to Ã3B2, followed by intersystem crossing to the open-shell singlet state (b̃1A2) and internal conversion to ã1A1. The relevant minimum energy crossing point is about 7-8 kcal mol-1 higher in energy than the Franck-Condon region.

Computational Study of Electronic Properties in Oligoacenes

Calculations were performed for closed shell singlet states, open shell singlet states and open-shell triplet states using DFT and B3LYP functional in oligoacenes. 6-31+G(d,p) basis set was used to calculate energies for polyacenes in Hartrees at various levels of spin-state configuration. For systems beyond pentacene, open shell sing let state is more stablilized for hexacene, heptacene, octacene and nonacene and singlet triplet gap decreases as size of polyacene increases. This work is used to describe unrestricted DFT calculations, spin contamination and quantum mechanical applications..

Diradicals or Zwitterions: The Chemical States of m -Benzoquinone and Structural Variation after Storage of Li Ions

Diradicals or Zwitterions: The Chemical States of <i>m</i> -Benzoquinone and Structural Variation after Storage of Li Ions

Catalytic Activity of Aliphatic PNP Ligated CoIII/I Amine and Amido Complexes in Hydrogenation Reaction—Structure, Stability, and Substrate Dependence

Structures, energies, and stability of aliphatic PNP ligated CoIII/CoI amine and amido complexes in different spin (singlet, triplet, and open-shell singlet) states and coordination spheres have been computed. Spin exchange is found for the interconversion between the singlet CoIII amine and the triplet CoI amine complexes as well as the singlet CoIII amido and the triplet CoI amido complexes. For the hydrogenation of CH2O, PhCHO, and PhCOCH3 having lower Gibbs free energy barriers than that of the interconversion from the singlet CoIII amine and the triplet CoI amine complexes, both singlet CoIII amine and triplet CoI amine complexes are active catalysts and both catalytic cycles are independent, and the triplet CoI amine complex is more active than the singlet CoIII amine complex. For the hydrogenation of PhCOOCH3 having higher barrier than that of catalyst interconversion, the singlet CoIII amine complex is the sole active catalyst and more active than the triplet CoI amine complex. For the hydrogenation of CH3COCH3, both catalytic cycles can be competitive depending on reaction temperature. These reveal the substrate dependent mechanisms. The correlation between the hydride affinity of the substrate and the H– transfer barrier for aldehydes and ketones as well as between the deprotonation energy of alcohol and the reverse barrier of H+ transfer indicates that the energy of the transition state of the H–/H+ transfer via the corresponding most favorable mechanism can be estimated by the hydride affinity of substrate and the deprotonation energy of the product.

Conversions of localized excess electrons and spin states under external electric field: Inter‐cage electron‐transfer isomer (C20F20)3&amp;K2

AbstractBy doping two potassium atoms among three C20F20 cages, peanut‐shaped single molecular solvated dielectron (C20F20)3&amp;K2 was presented theoretically. The triplet structure with two separated excess electrons respectively inside left and middle cages (isomers I or II) are thermodynamically more stable than both open‐shell (OS) and close‐shell (CS) singlet ones with lone pair of excess electrons inside middle cage. Applying an oriented external electric field (×10−4 au) of −20 or a larger one and then releasing it, left‐to‐right transfer occurs for both triplet excess electrons and a new kind of inter‐cage electron‐transfer isomers (III or IV) forms. Triplet structures (I–IV) with three redox sites may be new members of mixed‐valent compounds, namely, Robin‐Day Class II. For electrified I of (C20F20)3&amp;K2, the OS singlet state in small‐ranges of field strengths (−30 to −5 and 5–30), triplet one in mid‐ranges of field strengths (−120 to −30 and 30–111), and CS singlet one in large‐ranges of field strengths (≥111 and ≤−120) are, respectively, ground states.

Energy decomposition analysis based on broken symmetry unrestricted density functional theory.

In this paper, the generalized Kohn-Sham energy decomposition analysis (GKS-EDA) scheme is extended to molecular interactions in open shell singlet states, which is a challenge for many popular EDA methods due to the multireference character. Based on broken symmetry (BS) unrestricted density functional theory with a spin projection approximation, the extension scheme, named GKS-EDA(BS) in this paper, divides the total interaction energy into electrostatic, exchange-repulsion, polarization, correlation, and dispersion terms. Test examples include the pancake bond in the phenalenyl dimer, the ligand interactions in the Fe(ii)-porphyrin complexes, and the radical interactions in dehydrogenated guanine-cytosine base pairs and show that GKS-EDA(BS) is a practical EDA tool for open shell singlet systems.

Corrigendum: Open‐Shell Characters, Aromaticities and Third‐Order Nonlinear Optical Properties of Carbon Nanobelts Composed of Five‐ and Six‐Membered Rings

In the original article,1 we reported that electronic structures and optical and magnetic response properties of an open-shell carbon nanobelt (IF-CNB) are evaluated by the spin-unrestricted density functional theory (U-DFT) method using the long-range corrected LC-BLYP exchange correlation functional, namely, at the LC-UBLYP/6-311+G** level. However, only in the calculations of magnetic response properties, we found that the results are obtained by the spin-restricted DFT (R-DFT) solution with <S2>=0. Therefore, the magnetically-induced current (MIC) density maps given in Figures 6 and S6 (in the Supporting Information) as well as the nucleus-independent chemical shift (NICS) values given in Figure S5 for the open-shell IF-CNB are found to be those evaluated at the LC-RBLYP/6-311+G** level. We sincerely apologize for this mistake. In a previous study,2 on the other hand, we compared the MIC density results from the R- and U-DFT solutions for oligoacenes and dicyclopenta-fused oligoacenes by using the gauge-including MIC (GIMIC) method. We found that the diatropic (aromatic) contributions of aromatic (4n+2)π systems with open-shell singlet electronic structures tend to be exaggerated when the R-DFT solution was employed primarily because of its very small energy gaps for the orbital transitions contributing to the diatropic current. Therefore, we have recalculated the MIC density map for IF-CNB at the LC-UBLYP/6-311+G** level of approximation and compared the result with that at the LC-RBLYP/6-311+G** level. The results are shown in Figure 1. Even though strong contributions of diatropic (aromatic) ring current were predicted everywhere around the belt at the LC-RBLYP level (Figure 1a), such strong aromatic contributions of ring current are found to be suppressed at the LC-UBLYP level (Figure 1b). In particular, paratropic (anti-aromatic) contributions are found to be predominant inside the belt whereas diatropic contributions are found to appear outside the belt at the LC-UBLYP level. Such paratropic/diatropic contributions inside/outside the hoop are similar to those of [6]annulene (benzene). However, the amplitudes of integrated MIC values of IF-CNB (1-3 nA/T) are found to be much smaller than that of the closed-shell [6]annulene (11.9 nA/T) evaluated at the LC-RBLYP/6-311+G** level [2]. Figure 2 shows the side-views of the MIC density map and the integrated values of MIC density for each bond calculated at the LC-RBLYP and LC-UBLYP levels. Even though the amplitudes of integrated MIC values are decreased, the net induced ring currents along the belt are predicted to flow clockwise at the LC-UBLYP level. Magnetically induced current (MIC) density vector plots of IF-CNB evaluated at the LC-RBLYP/6-311+G** (a) and LC-UBLYP/6-311+G** (b) levels on the plane perpendicular to the z-axis and 1 Å above the center of mass. Diatropic currents rotate clockwise, while the paratropic ones anticlockwise. The values in the color bar are given in a.u. The current vectors are scaled with a factor of 30 Å a.u.-1, where 1 a.u.=100.63 nA T−1 Å−2. Integrated MIC values [nA/T] of each bond with along with MIC maps for IF-CNB at the LC-RBLYP/6-311+G** (a) and LC-UBLYP/6-311+G** (b) levels. Positive and negative values represent clockwise (diatropic) and counter-clockwise (paratropic) contributions of ring current, respectively. Spatial integration of MIC densities was performed on the bisector plane of the bond with a width of 2.0 Å. The length of the plane is 8.0 Å from the center of the mass of the belt. Here, the current vectors are scaled with a factor of 20 Å a.u.−1. Figure 3 shows the results of NICS(0) values on the five- and six-membered rings, which characterize aromatic nature on each ring. In both R- and U-DFT cases, positive (negative) NICS values at the center of five- (six-) membered ring are obtained, whereas, the U-DFT results are found to exhibit more antiaromatic nature than the R-DFT results. This feature may be associated with the emergence of the odd electron density in the U-DFT solution. NICS(0) values [ppm] at the centers of five- and six-membered rings calculated at the LC-RBLYP/6-311+G** (a) and LC-UBLYP/6-311+G** levels. NICS(0) values of symmetry-independent positions are only shown. In summary, the MIC density map of IF-CNB in Figure 6 in the original article should be replaced by Figure 1b, the correct integrated MIC values corresponding to Figure S6 in the Supporting Information are shown in Figure 2b, and the NICS(0) values in Figure S5 (Supporting Information) are presented in Figure 3b. Judging from the results at the LC-UBLYP level, IF-CNB is expected to show not so strong, but fairly weak aromatic nature over the belt in the open-shell singlet state.

Exploring the mechanism of alkene hydrogenation catalyzed by defined iron complex from DFT computation.

UB3LYP computation including dispersion and toluene solvation has been carried to elucidate the mechanisms of alkene hydrogenation catalyzed by bis(imino)pyridine iron dinitrogen complex (iPrPDI)Fe(N2)2, which has low stability towards N2 dissociation. The coordinatively unsaturated complexes, (iPrPDI)Fe(N2) and (iPrPDI)Fe(1-C4H8), favor open-shell singlet ground states. On the basis of our computations, we propose a new mechanism of 1-butene coordination and hydrogenation after N2 dissociation. The hydrogenation of 1-butene undergoes a concerted open-shell singlet transition state involving H2 dissociation, C-H bond formation and C=C bond elongation, as well as the subsequent C-H reductive elimination. In the whole alkene hydrogenation, the H-H bond cleavage is the rate-determining step. Graphical abstract The alkene hydrogenation catalyzed by redox-active pyridine(diimine)-chelate iron complex follows the open-shell singlet state path.

Anharmonic Vibrational Analysis of Biomolecules and Solvated Molecules Using Hybrid QM/MM Computations.

Quantum mechanics/molecular mechanics (QM/MM) calculations are applied for anharmonic vibrational analyses of biomolecules and solvated molecules. The QM/MM method is implemented into a molecular dynamics (MD) program, GENESIS, by interfacing with external electronic structure programs. Following the geometry optimization and the harmonic normal-mode analysis based on a partial Hessian, the anharmonic potential energy surface (PES) is generated from QM/MM energies and gradients calculated at grid points. The PES is used for vibrational self-consistent field (VSCF) and post-VSCF calculations to compute the vibrational spectrum. The method is first applied to a phosphate ion in solution. With both the ion and neighboring water molecules taken as a QM region, IR spectra of representative hydration structures are calculated by the second-order vibrational quasi-degenerate perturbation theory (VQDPT2) at the level of B3LYP/cc-pVTZ and TIP3P force field. A weight-average of IR spectra over the structures reproduces the experimental spectrum with a mean absolute deviation of 16 cm–1. Then, the method is applied to an enzyme, P450 nitric oxide reductase (P450nor), with the NO molecule bound to a ferric (FeIII) heme. Starting from snapshot structures obtained from MD simulations of P450nor in solution, QM/MM calculations have been carried out at the level of B3LYP-D3/def2-SVP(D). The spin state of FeIII(NO) is likely a closed-shell singlet state based on a ratio of N–O and Fe–NO stretching frequencies (νN–O and νFe–NO) calculated for closed- and open-shell singlet states. The calculated νN–O and νFe–NO overestimate the experimental ones by 120 and 75 cm–1, respectively. The electronic structure and solvation of FeIII(NO) affect the structure around the heme of P450nor leading to an increase in νN–O and νFe–NO.

Is the Inversion of Phosphorus Trihalides (PF3, PCl3, PBr3, and PI3) a Diradical Process?

This work explores possible reaction paths for the inversion of a series of trigonal pyramidal phosphorus trihalides, PF3, PCl3, PBr3, and PI3, and it especially addresses the question of whether and when the bonding of the lowest-energy species along the inversion paths should be described as a hyper-open-shell diradical. The various paths for inversion are calculated using a single-reference method within the framework of Kohn-Sham density functional theory and also with multireference wave function methods. Our calculated results using both kinds of methods show that, for all the halogens studied (F, Cl, Br, and I), the lowest-energy singlet path for the inversion occurs by the formation of a C2 v transition structure rather than a D3 h transition structure. This geometrical preference agrees with what has been inferred previously based on closed-shell singlet calculations. But in the present study, we examined not only closed-shell singlet transition states but also open-shell singlet states and triplet states for calculating stationary points and inversion paths, and for some of the phosphorus trihalides, we found that paths involving open-shell configurations are lower in energy than those restricted to closed-shell configurations. We analyzed the changes along the paths in terms of hybridization and orientation of the frontier orbitals and in terms of locally avoided crossings, and the extent of the diradical character was quantified by calculating the effective number of unpaired electrons. Even for the singlet inversion path that goes via a D3 h structure, the barrier for PF3, PCl3, and PBr3 is higher for a closed-shell singlet spin state than for the open-shell singlet configuration. Furthermore, the energy of the triplet D3 h structure is below even that of the open-shell D3 h singlet for PCl3, PBr3, and PI3. This necessitates rethinking the role of open-shell states in nominally closed-shell processes.

Switching the Spin State of Pentafluorophenylnitrene: Isolation of a Singlet Arylnitrene Complex.

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements, which diminish the use of arylnitrenes as intermediates in organic synthesis. While the closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed-shell singlet states of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis acid BF3 under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two molecules of BF3 is formed. Although the closed-shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25 kcal/mol above its triplet ground state, the reaction with BF3 results in switching the spin state from triplet to singlet. The formation of the singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental findings.

Computational study of the structure and magnetic properties of the weakly-coupled tetranuclear square-planar complex of Cu(II) with a tetraporphyrin sheet

Owing to the fact that weakly-coupled Cu(II) complexes are potentially applicable in the fields of spintronics and single-molecule magnets we have studied one particularly conspicuous case – a tetranuclear square-planar complex of Cu(II) ions with a π-extended tetraporphyrin ligand (Tetrahedron, 2008, 64, 11433), the structural criteria of which, with very long distances between the paramagnetic centers, can sensibly define the exchange coupling constants and magnetic behaviour. We use the “broken symmetry” density functional theory formalism for various spin states (open-shell singlet, triplet and quintet states). The calculated value of the magnetic exchange integral corresponds to a very weak antiferromagnetic coupling between the neighboring Cu(II) ions (Jcalc = –1.48 cm−1(calculated) Jexp = −1.16 cm−1 (experiment)). The opposite Cu(II) ions are coupled ferromagnetically with an extremely small exchange constant (jcalc = 0.09 cm−1 (calculated) that corresponds to the open-shell singlet ground state of the following spin configuration↑↓↑↓. The exchange mechanism between the [Cu(n) + 4N] localized singly-occupied “magnetic” orbitals of the dxy(Cu) + 4sp2(N) nature indicates a diffuse non-zero overlap between them on the Carbon atoms of the organic ligand. The numerical analysis of atomic spin densities and corresponding decomposition coefficients for the singly-occupied “magnetic” orbitals confirm the proposed exchange mechanism. This superexchange pathway is mediated by the in-plane sp2-hybrid orbitals of the Carbon atoms that couple four [Cu + 4N] fragments by the σ-bond system. For more detailed characterization of the target system a topological analysis of electron density distributions in the high-spin quintet states has been carried out using the Bader’s method “quantum theory of atoms in molecules”.

Theoretical Insights into the Mechanism and Stereoselectivity of Olefin Cyclopropanation Catalyzed by Two Engineered Cytochrome P450 Enzymes.

Engineered P450s can catalyze some non-natural reactions with high efficiency and excellent selectivity, such as the carbine transfer, nitrene transfer, C-H insertion, and C-H amination, opening alternative routes for sustainable production of chemicals. Recent experiments revealed that two engineered cytochrome P450 enzymes (P450BM3-CIS and P411BM3-CIS) show different efficiencies and stereoselectivities in the olefin cyclopropanation, but key factors that affect the activity remain unclear. In this work, both quantum mechanics (QM) and QM/molecular mechanics (MM) methods were employed to explore the catalytic reactions and selectivity of these two engineered cytochrome P450 enzymes. On the basis of our results, the cyclopropanation of styrene is suggested to mainly occur on the open-shell singlet (OSS) and triplet state surfaces, which contain two elementary steps. The reactive iron(III)-porphyrin carbene (IPC) radical first attacks the terminal alkenyl group of styrene to form a C-radical intermediate, which then undergoes a cyclization reaction affording the cyclopropanation products. Importantly, it is found that the stereoselectivity of cyclopropanations is elucidated only if considering the real protein environment, and the stereoselectivity is determined by multiple factors, such as the relative orientation of IPC to styrene, the binding affinity of the substrate, and the reaction barriers of rate-limiting steps. It is the enzymatic environment that makes the reaction highly stereoselective, which provides useful clues for designing whole-cell catalysts for non-natural chemical reactions.