Electron correlation effects on uranium isotope fractionation in U(VI)-U(VI) and U(IV)-U(VI) equilibrium isotopic exchange systems.

Uranium isotope fractionation has been extensively investigated in the fields of nuclear engineering and geochemical studies, yet the underlying mechanisms remain unclear. This study assessed isotope fractionations in U(VI)-U(VI) and U(IV)-U(VI) systems by employing various relativistic electron correlation methods to explore the effect of electron correlation and to realize accurate calculations of isotope fractionation coefficients (ε). The nuclear volume term, ln Knv, the major term in ε, was estimated using the exact two-component relativistic Hamiltonian in conjunction with either HF, DFT(B3LYP), MP2, CCSD, CCSD(T), FSCCSD, CASPT2, or RASPT2 approaches for small molecular models with high symmetry. In contrast, chemical species studied in prior experimental work had moderate sizes and were asymmetrical. In such cases, electron correlation calculations other than DFT calculations were not possible and so only the HF and B3LYP approaches were employed. For closed-shell U(VI)-U(VI) systems, the MP2, CCSD and CCSD(T) methods yielded similar ln Knv values that were intermediate between those for the HF and B3LYP methods. Comparisons with experimental results for U(VI)-U(VI) systems showed that the B3LYP calculations gave results closer to the experimental data than the HF calculations. Because of the open-shell structure of U(IV), multireference methods involving the FSCCSD, CASPT2 and RASPT2 techniques were used for U(IV)-U(VI) systems, but these calculations exhibited instability. The average-of-configuration HF method showed better agreement with the experimental ε values for U(IV)-U(VI) systems than the B3LYP method. Overall, electron correlation improved the description of ε for the U(VI)-U(VI) systems but challenges remain with regard to open-shell U(IV) calculations because an energy accuracy of 10-6-10-7Eh is required for ln Knv calculations.

How does multi-reference computation change the catalysis chemistry? DFT and CASPT2 studies of the Cu-catalysed coupling reactions between aryl iodides and ß-diketones.

The molecular mechanism of a Cu-catalysed coupling reaction was theoretically studied using density functional theory (DFT) and the complete active space self-consistent field method followed by the second-order perturbation theory (CASSCF/CASPT2) to investigate the effects of the strong electron correlation of the Cu centre on the reaction profile. Both DFT and CASSCF/CASPT2 calculations showed that the catalytic cycle proceeds via an oxidative addition (OA) reaction, followed by a reductive elimination (RE) reaction, where OA is the rate-determining step. Although the DFT-calculated activation energies of the OA and RE steps are highly dependent on the choice of functionals, the CASSCF/CASPT2 results are less affected by the choice of DFT-optimised geometries. Therefore, with a careful assessment based on the CASSCF/CASPT2 single-point energy evaluation, an optimal choice of the DFT geometry is of good qualitative use for energetics at the CASPT2 level of theory. Based on the changes in the electron populations of the 3d orbitals during the OA and RE steps, the characteristic features of the DFT-calculated electronic structure were qualitatively consistent with those calculated using the CASSCF method. Further electronic structure analysis by the natural orbital occupancy of the CASSCF wavefunction showed that the ground state is almost single-reference in this system and the strong electron correlation effect of the Cu centre can be dealt with using the MP2 or CCSD method, too. However, the slightly smaller occupation numbers of the 3dπ orbital in the course of reactions suggested that the electron correlation effect of the Cu(III) centre appears through the interaction between the 3dπ orbital and the C-I antibonding σ* orbital in the OA step, and between the 3dπ orbital and the Cu-C antibonding σ* orbital in the RE step.

Density Functional Study on the Photopolymerization of Styrene Using Dinuclear Ru-Pd and Ir-Pd Complexes with Naphthyl-Substituted Ligands.

A density functional study was performed to investigate the mechanism of the photocatalytic reactivity of styrene polymerization using dinuclear Ru-Pd and Ir-Pd catalytic complexes. In previous experiments with these catalysts, the reactivity increased, and more polymer products were yielded compared to dimers under visible light irradiation. The best catalytic reactivity was obtained using an Ir-Pd complex containing naphthyl substituents at the phenyl ligands coordinated to Ir (Ir-Pd1). In contrast, Ir-Pd2, an isomer of Ir-Pd1, containing naphthyl substituents at the pyridine ligands, did not show good reactivity, which may be related to the stability of the excited state of the catalytic complexes. In this study, we calculated the radiative lifetimes of these catalytic complexes and Ir-Pd1 had the longest lifetime; this result was consistent with the experimental results. The longest lifetime of the Ir-Pd1 was attributed to the destabilization of the highest occupied molecular orbital (HOMO) energy by π*-π* interactions between the naphthyl and phenyl ligands. Further, this destabilization of the HOMO energy afforded a small energy gap between the HOMO and lowest unoccupied molecular orbital, enhancing the metal-to-ligand charge transfer to the bridging ligand between Ir and Pd. Additionally, we focused on the reaction of the second insertion of styrene, which was identified as the rate-determining step of the polymerization cycle in a previous study. The singlet-triplet crossing points of the intermediates were estimated, and the barrier heights of the intersystem crossing were much lower than those in the thermal paths, which explained the efficiency of the photocatalytic reactivity in the experiment.

Mechanistic Insights into the Selectivity of Norcarane Oxidation by Oxo-Manganese(V) Porphyrin Complexes.

Oxo-Mn(V) porphyrin complexes perform competitive hydroxylation, desaturation, and radical rearrangement reactions using diagnostic substrate norcarane. Initial C-H cleavage proceeds through the two hydrogen abstraction steps from the two adjacent carbon on the norcarane and then through selective reactions various products are generated. Using density functional theory calculations, we show that the hydroxylation and desaturation reactions are triggered by a rate-determining H-abstraction step, whereas the rate-determining step for the radical rearrangement is located at the rebound step (TS2). We find that the endo-2 reaction is favorable over other reactions, which is consistent with the experimental result. Furthermore, the competitive pathways for norcarane oxidation depend on the non-covalent interaction between norcarane and the porphyrin-ring, and orbital energy gaps between donor and acceptor orbitals because of stable or unstable acceptor orbital. The stereo- and regio-selectivities of norcarane oxidation are hardly sensitive to the zero-point energy and thermal free energy corrections.

Theoretical Study on the Vapochromic Ni(II)-Quinonoid Complex: One-Dimensional Stacking Structure-Based Color Switching.

Vapochromic crystals of Ni(II)-quinonoid complexes were theoretically investigated using density functional theory (DFT) calculations. Kato et al. previously reported that the purple crystals of a four-coordinate Ni(II)-quinonoid complex (1P) exhibited vapochromic characteristics upon exposure to methanol gas, resulting in orange crystals of the six-coordinate methanol-bound complex (1O) [Angew. Chem., Int. Ed.2017, 56, 2345-2349]. However, the authors did not characterize the crystal structure of 1P. In the present study, we computationally predicted the crystal structure of 1P by performing a crystal structure search with classical force-field computations followed by optimization using DFT calculations. The simulated powder X-ray diffraction pattern of the DFT-optimized structure agreed with experimental observations, indicating that our predicted crystal structure is reliable. Investigation of the optimized crystal structure of 1P revealed that its color change arose from changes in its 1D-band structure, which consists of Ni 3d orbitals and quinonoid π-orbitals. Intermolecular interactions were weakened upon the binding of methanol to the Ni(II) center in 1O. Consequently, the intermolecular 3d-π interaction in 1P lowered the band gap and induced the red-shifting of the monomeric four-coordinate Ni(II)-quinonoid complex. Meanwhile, the obtained absorption spectrum of 1O closely corresponded to that of the monomeric six-coordinate Ni(II)-quinonoid complex. Our study provides a new strategy for accurately predicting molecular crystal structures and reveals a new insight into vapochromism based on band structure color switching.

Insights into the electronic structure and mechanism of norcarane hydroxylation by OxoMn(V) porphyrin complexes: A density functional theory study.

Norcarane hydroxylation by neutral [PorMn(V)O-L] (L═OH- , F- ) and cationic [PorMn(V)O-L]+ (L═H2 O, imidazole) oxoMn(V) porphyrin complex models has been investigated by density functional theory calculations to better understand the reaction mechanism and electronic structure. We found that the energy barriers of norcarane hydroxylation by cationic oxoMn(V) porphyrin complexes are lower than those by neutral oxoMn(V) porphyrin complexes. This indicates that cationic oxoMn(V) porphyrin complexes enhance norcarane hydroxylation compared with neutral oxoMn(V) porphyrin complexes. According to electronic structure analysis, in the C─H activation step, electron transfer occurs through initial interaction between the σCH and rich-oxygen π(Mn═O) orbitals to form real donor orbitals, followed by transfer to the acceptor π*(Mn═O) orbitals. Moreover, single electron shifts from norcarane (CH) to Mn atom during C─H activation. The positive charge of the cationic complex stabilizes the acceptor orbital more than the donor orbital, reducing the energy gap between these orbitals, thus lowering the reaction barrier.

Rate-Limiting Step of Epoxidation Reaction of the Oxoiron(IV) Porphyrin π-Cation Radical Complex: Electron Transfer Coupled Bond Formation Mechanism.

Epoxidation reactions catalyzed by high-valent metal-oxo species are key reactions in various biological and chemical processes. Because the redox potentials of alkenes are higher than those of most high-valent metal-oxo species, the electron transfer (ET) from the alkene to the high-valent metal-oxo species in the epoxidation reaction is endergonic and must be coupled with another exergonic process. To reveal the mechanism of the ET, we performed a Marcus plot analysis for the epoxidation reaction of the oxoiron(IV) porphyrin π-cation radical complex (compound I) with alkene. The Marcus plots can be simulated with a linear line with the gradient of 0.50 when the redox potential of compound I varies and 0.07 when the redox potential of alkene varies. These results indicate that the ET process is involved in the rate-limiting step and coupled with the following O-C bond formation process: ET coupled bond formation mechanism. The DFT calculations support this conclusion and disclose the details of the mechanism. As the alkene comes close to the oxo ligand, the energy of the highest occupied molecular orbital (HOMO) of the alkene increases and the energy for the ET becomes small enough to allow the ET. Finally, the ET occurs from the HOMO of the alkene to the porphyrin π-radical orbital. The shift of one electron from the HOMO of the alkene by the ET simultaneously results in the O-C half bond formation between the oxo ligand and the alkene. The ET process itself is still endergonic and reversible, but the bond formation coupled with the ET changes the overall process to exergonic and irreversible. We also discuss the similarity with the aromatic hydroxylation reaction and the relevance to the epoxidation reactions of other metal-oxo complexes and peracid.

meso-Substitution Activates Oxoiron(IV) Porphyrin π-Cation Radical Complex More Than Pyrrole-ß-Substitution for Atom Transfer Reaction.

There have been two known categories of porphyrins: a meso-substituted porphyrin like meso-tetramesitylporphyrin (TMP) and a pyrrole-ß-substituted porphyrin like native porphyrins and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). To reveal the chemical and biological function of native hemes, we compare the reactivity of the oxoiron(IV) porphyrin π-cation radical complex (Compound I) of TMP (TMP-I) with that of TMTMP (TMTMP-I) for epoxidation, hydrogen abstraction, hydroxylation, sulfoxidation, and demethylation reactions. Kinetic analysis of these reactions indicated that TMP-I is much more reactive than TMTMP-I when the substrate is not sterically bulky. However, as the substrate is sterically bulkier, the difference of the reactivity between TMP-I and TMTMP-I becomes smaller, and the reactivity of TMP-I is comparable to that of TMTMP-I for a sterically hindered substrate. Since the redox potential of TMP-I is almost the same as that of TMTMP-I, we conclude that TMP-I is intrinsically more reactive than TMTMP-I for these atom transfer reactions, but the steric effect of TMP-I is stronger than that of TMTMP-I. This is contrary to the previous result for the single electron transfer reaction: TMTMP-I is faster than TMP-I. DFT calculations indicate that the orbital energies of the Fe═O moiety for TMTMP-I are higher than those for TMP-I. The difference in steric effect between TMP-I and TMTMP-I is explained by the distance from the mesityl group to the oxo ligand of Compound I. Significance of the pyrrole-ß-substituted structure of the hemes in native enzymes is also discussed on the basis of this study.

Density Functional Study on Compounds to Accelerate the Electron Capture Decay of 7Be.

In radionuclide compounds undergoing electron capture (EC) decay, the electron density at the nucleus (ρ(0)) and half-life of the nucleus are inversely proportional. Thus, the decay can be accelerated by changing the chemical or physical conditions. A previous study reported a 1.1-1.5% reduction in the half-life of 7Be encapsulated in C60 compared with 7Be metal. However, 7Be was inserted into the fullerene using the rebound energy of the nuclear reaction, which may not be a practical method. This paper elucidates the mechanism of ρ(0) change in various Be compounds from density functional calculations and attempts to propose better systems that show faster EC decay (larger ρ(0)) and/or that are easier to generate than Be in C60. In typical Be compounds, ρ(0) decreases because Be donates electrons to other atoms through chemical bonds and, thus, is not effective. Among the various Be-encapsulated fullerenes (C20-C180), the largest increase in ρ(0) was obtained for C50 fullerene, but the magnitude was almost similar to that of C60. As new systems, we propose Be-encapsulated rare gas solids, which would be generated only by applying high pressure. An increase in ρ(0) from Be metal in the range 2-10%, which depends on the lattice constant, is obtained.

Surface-enhanced Raman scattering of M2 -pyrazine-M2 (M = Cu, Ag, Au): Analysis by natural perturbation orbitals and density functional theory functional dependence.

Here, we propose a new method to analyze various electronic properties of molecules based on natural perturbation orbitals (NPOs). We applied the proposed method to chemical enhancement of the surface-enhanced Raman scattering (SERS) intensity of M2 -pyrazine-M2 (M = Cu, Ag, Au) complexes. The SERS intensity can be effectively decomposed into the contributions of four NPO pairs (1σ-1σ*, 2σ-2σ*, 1π-1π*, and 2π-2π*), so NPO analysis makes the SERS intensity much easier to understand than by conventional canonical molecular orbitals. Moreover, we analyzed the dependence of the density functional theory functional on the SERS intensity. For the Ag2 -pyrazine-Ag2 complex, the BP86 functional overestimates the Raman intensity by about 23 times compared with coupled-cluster singles and doubles level of theory, while the CAM-B3LYP functional gives moderately accurate values. This overestimation arises from the inaccuracy of the energy derivative along the normal vibrational mode.

DFT insight into axial ligand effects on electronic structure and mechanistic reactivity of oxoiron(iv) porphyrin.

A series of DFT studies on the epoxidation reactions of olefins by oxoiron(iv) porphyrin cation radical complexes are performed in this work, to elucidate the axial ligand effects on the electronic features and reaction mechanism in detail. We analyzed the molecular orbitals, spin populations, and Mulliken charges along the intrinsic reaction coordinate route. From the findings, we confirmed that the interaction between the axial ligand and the oxoiron(iv) porphyrin is strong and the initial changes in the electronic structures occur early during the reaction, which further enhances the reactivity toward olefin epoxidation. More importantly, the patterns of the electron transfer from olefin to oxoiron(iv) porphyrin were impacted by the axial ligand. The pattern of successive electron transfer from Fe-O to porphyrin and then from C[double bond, length as m-dash]C to Fe-O for oxoiron(iv) porphyrin in case of fluorine and acetate axial ligands, whereas the pattern of electron transfer occurs from C[double bond, length as m-dash]C to porphyrin for oxoiron(iv) porphyrin in case of chlorine and nitrate axial ligands during the epoxidation reaction of the olefins. We also determined the intersystem crossing between the quartet and sextet spin states occurring at the second transition state (TS2) by the analysis of the two-dimensional potential energy surface.

Time-dependent DFT study of the K-edge spectra of vanadium and titanium complexes: effects of chloride ligands on pre-edge features.

X-ray absorption near edge structures (XANES) of vanadium and titanium complexes were investigated with time-dependent density functional theory (TDDFT). In particular, observed characteristic K-edge features in the presence of chloride ligands were assigned. Although TDDFT includes a large systematic error attributed to the 1s core energy levels of transition metals, pre-edge spectral shapes could be reproduced by a simple energy shift in the calculated excitation energies. The doublet peak in the pre-edge region was assigned to dipole-allowed transitions from 1s to 3d + 4p hybridized orbitals, while a characteristic shoulder peak in the chloride complex was assigned to excitations of chloride 4p orbitals. A similar but weak absorption band was computed for the methyl complex as excitation to C-H σ* orbitals. However, because these excitations were highly dependent on the direction of the C-H bonds, the shoulder peak was not experimentally observed because of methyl free rotation. Hence, the intensity of the shoulder peak was proportional to the number of chloride ligands unless other ligands contribute to this energy region and, therefore, could be used to detect the presence or absence of chloride ligands in unknown compounds, such as reaction intermediates.

Can large active-space CASSCF calculation make sense to the reaction analysis of iron complex? A benchmark study of methane oxidation reaction by FeO.

A methane oxidation reaction by FeO+ cation was theoretically investigated based on the density functional theory (DFT) and the complete active-space self-consistent field (CASSCF) method as well as the coupled-cluster singles, doubles, and perturbative triples (CCSD(T)) to explore the active-space dependency to computational analyses in such strongly correlated reaction systems. A small active-space CASSCF(5e in 5o) calculation, which only includes five 3d orbitals of the Fe atom in the active-space, showed remarkable difference both in energy and geometry compared to those computed by the DFT and CCSD(T) methods. Interestingly, a large active-space CASSCF(17e in 17o) calculation, which includes almost all the valence orbitals gives a qualitative agreement with either the DFT or the CCSD(T) results in the first half part of the reaction, although it varies from them in the latter half part. Therefore, it is indicated that the active-space dependency is serious in some part of the reaction and the small active-space CASSCF might lead a wrong discussion. We further investigated the optimized geometry of the intermediate complex with the small and the large active-space CASSCF methods as well as the CCSD(T) method, and found that the CASSCF(5e in 5o)-optimized geometry is considerably different from the others. In consequence, a small active-space CASSCF/CASPT2 calculation does not really work for such a strongly correlated reaction system even qualitatively, and a sophisticated assessment using the large active-space CASSCF/CASPT2 method will be indispensable. © 2018 Wiley Periodicals, Inc.

Substitution effects on olefin epoxidation catalyzed by Oxoiron(IV) porphyrin π-cation radical complexes: A dft study.

The effects of peripheral fluorine atoms on epoxidation reactions of ethylene by oxoiron(IV) porphyrin cation radical complex in the quartet and sextet spin multiplicities are systematically investigated using the DFT method. The overall reaction routes are determined using a model system of ethylene and Fe(IV)OCl-porphyrin with substituted fluorine atoms. By obtaining the energy diagrams and electron- and spin-density difference contour maps of the transition states and intermediate compounds, we confirm that the electron-withdrawing by peripheral fluorine atoms enhances the reactivity as the number of fluorine atoms increases, as is observed experimentally. The intersystem crossing between the quartet and sextet spin multiplicities is discussed by means of the intrinsic reaction coordinate method. We conclude that the rate-determining step is located at the first transition state (TS1) for the activation of CC and FeO bonds, and the ground electronic state changes from quartet to sextet around the TS1. © 2019 Wiley Periodicals, Inc.

Experimental and theoretical studies of the porphyrin ligand effect on the electronic structure and reactivity of oxoiron(iv) porphyrin π-cation-radical complexes.

Oxoiron(IV) porphyrin π-cation-radical complexes (Cpd I) have been studied as models for reactive intermediates called compound I in cytochromes P450, peroxidases, and catalases. It has been well known that the electronic structure and reactivity of Cpd I are modulated by the substituted position and the electron-withdrawing ability of the substituent. However, there still remain two major questions: (1) how many electronegative halogen atoms should be introduced in the meso-phenyl group to switch the porphyrin π-cation-radical state of Cpd I? (2) How does the electron-withdrawing effect of the substituent modulate the reactivity of Cpd I? To answer these two questions, we here performed experimental and theoretical studies on the electron-withdrawing effect of the meso-substituent. We gradually increased the electron-withdrawing effect by increasing the number of fluorine atoms in the meso-phenyl group. Spectroscopic analyses of these Cpd I models reveal that the porphyrin radical state shifts from having a2u radical character to having a1u radical character with an increase in the number of the fluorine atoms in the phenyl group, and the ground state of Cpd I switches from the a2u state to the a1u state when four fluorine atoms are introduced in the meso-phenyl group. The switch of the radical state is predicted well by LC-BLYP, but not by the commonly used B3LYP. The theoretical calculations indicate that the electron-withdrawing substituent makes Cpd I more reactive by stabilizing the ferric porphyrin state (product state) more than the Cpd I state (reactant state), generating a larger free energy change in the oxygenation reaction (ΔG) of Cpd I.

Functionalization of Endohedral Metallofullerenes toward Improving Barrier Height for the Relaxation of Magnetization for Dy2@C80-X (X = CF3, C3N3Ph2).

We theoretically studied the electronic and magnetic properties of the exterior functionalized endohedral metallofullerenes (EMFs) of Gd2@ I h-C80-X (where X is the exterior functional group). Molecular orbital analysis suggests that the presence of unpaired electron on the I h-C80 cage is not favoring the observation of stable species. One of the effective strategies to address this problem is by attaching an exterior functional group to the fullerene cage. Out of the studied exterior functionalized EMFs, we were successful in finding two stable species such as Gd2@ I h-C80-CF3 and Gd2@ I h-C80-C3N3Ph2 with no unpaired spin on the cage. Further, we utilized exterior functional groups such as -CF3 (1) and -C3N3Ph2 (2) to model and to stabilize dinuclear Dy2@ I h-C80 species, and we thoroughly investigated their magnetic properties using ab initio calculations. Within the single-ion paradigm, DyIII ions in 1 and 2 are magnetically anisotropic, and their magnetization-reversal energy barriers are estimated to be ∼698 and ∼705 cm-1, respectively. Furthermore, beyond the single-ion paradigm, i.e., considering a ferromagnetic coupling (∼30 cm-1) between the lanthanide ions and the radical spin, the energy barriers of 1 and 2 are estimated to be 79.8 and 73.0 cm-1, respectively.

13C and 207Pb NMR Chemical Shifts of Dirhodio- and Dilithioplumbole Complexes: A Quantum Chemical Assessment.

Density functional theory (DFT) and zeroth-order regular approximation DFT calculations were performed to investigate the electronic structures and 13C and 207Pb nuclear magnetic resonance (NMR) chemical shifts of metal-coordinated plumboles, namely, monorhodioplumbole ([Rh-plumbole]-), dirhodioplumbole (Rh2-plumbole), and dilithioplumbole (Li2-plumbole), which have a five-membered ring containing lead. The molecular orbital correlation diagram and extended transition state-natural orbitals for chemical valence analysis of the [Rh-plumbole]- and Rh2-plumbole complexes showed that the plumbole is primarily a π-donor, with π-donation being dominant in the Rh2-plumbole complex. The present calculations show that the Pb-Cα internuclear distances are longer in the Rh2-plumbole complex than in [Rh-plumbole]- because of the combined effect of strong π-donation and weak π-back-donation in the Rh2-plumbole complex. The calculated 207Pb and 13Cα NMR chemical shifts agree with the experimental trends reasonably well. The influences of the relativistic effect, role of the functional, effect of the solvent, and dependence of the exact exchange admixture on the calculated 207Pb and 13Cα NMR chemical shifts were investigated. The NMR chemical shift trend of the 207Pb atom in the complexes originates from the paramagnetic and spin-orbit contributions. NMR component analysis revealed that the upfield shift of the 13Cα atoms of the [Rh-plumbole]- and Rh2-plumbole complexes compared to that of the Li2-plumbole complex is mainly due to the decrease in the paramagnetic term.

A theoretical study on the size-dependence of ground-state proton transfer in phenol-ammonia clusters.

Geometries and infrared (IR) spectra in the mid-IR region of phenol-(ammonia)n (PhOH-(NH3)n) (n = 0-10) clusters have been studied using density functional theory (DFT) to investigate the critical number of solvent molecules necessary to promote ground-state proton transfer (GSPT). For n ≤ 8 clusters, the most stable isomer is a non-proton-transferred (non-PT) structure, and all isomers found within 1.5 kcal mol-1 from it are also non-PT structures. For n = 9, the most stable isomer is also a non-PT structure; however, the second stable isomer is a PT structure, whose relative energy is within the experimental criterion of population (0.7 kcal mol-1). For n = 10, the PT structure is the most stable one. We can therefore estimate that the critical size of GSPT is n = 9. This is confirmed by the fact that these calculated IR spectra are in good accordance with our previous experimental results of mid-IR spectra. It is demonstrated that characteristic changes of the ν9a and ν12 bands in the skeletal vibrational region provide clear information that the GSPT reaction has occurred. It was also found that the shortest distance between the π-ring and the solvent moiety is a good indicator of the PT reaction.

Quantum Chemical Studies on Electron-Accepting Overcrowded Ethylene with a Polarizable Skeleton.

We report the quantum chemical studies on the neutral and radical anion forms of an electron-accepting overcrowded ethylene (OCE1) featuring a highly polarizable skeleton based on the density functional theory (DFT) approach using the M06-2X hybrid functional. Calculated results indicate that OCE1 (bis{4H,8H-4-(dicyanomethylene)benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazol-8-ylidene}) shows conformational behaviors and energetics similar to those of bianthrone (OCE2), a typical thermochromic overcrowded ethylene. Neutral OCE1 and its radical anion have antifolded (afOCE1) and twisted (tOCE1) isomers on their potential energy surfaces. The calculated isomerization barrier heights of OCE1 and its radical anion are considerably low, indicating that its conformation is susceptible to interactions with surrounding molecules. While two afOCE1 molecules can form a simple π-stacked dimer, tOCE1 tends to be converted to afOCE1 when the two tOCE1 molecules come close together, indicating the instability of tOCE1 in the homogeneous OCE1 solid state. The thermochromic behavior difference between OCE1 and OCE2 in solution is closely associated with the considerably small energy difference between the afOCE1 and the tOCE1 as compared with OCE2. The properties of OCE1 are also compared with other typical electron-accepting overcrowded ethylenes in terms of electronic structure, energetics, and conformational behaviors.

Vanadium NMR Chemical Shifts of (Imido)vanadium(V) Dichloride Complexes with Imidazolin-2-iminato and Imidazolidin-2-iminato Ligands: Cooperation with Quantum-Chemical Calculations and Multiple Linear Regression Analyses.

The NMR chemical shifts of vanadium (51V) in (imido)vanadium(V) dichloride complexes with imidazolin-2-iminato and imidazolidin-2-iminato ligands were calculated by the density functional theory (DFT) method with GIAO. The calculated 51V NMR chemical shifts were analyzed by the multiple linear regression (MLR) analysis (MLRA) method with a series of calculated molecular properties. Some of calculated NMR chemical shifts were incorrect using the optimized molecular geometries of the X-ray structures. After the global minimum geometries of all of the molecules were determined, the trend of the observed chemical shifts was well reproduced by the present DFT method. The MLRA method was performed to investigate the correlation between the 51V NMR chemical shift and the natural charge, band energy gap, and Wiberg bond index of the V═N bond. The 51V NMR chemical shifts obtained with the present MLR model were well reproduced with a correlation coefficient of 0.97.