Spatial shape of electron delocalization: structure of the laplacian of the negative exchange-correlation density

The Laplacian of the negative exchange-correlation density (with respect to coordinate r(2)), nabla<(r)2>(2)[-Gamma(sigma1)(sigma2)(XC) (r(1),r(2))] = nabla(r)2(2)X(sigma1)(sigma2)(r(1),r(2)), is proposed as an instrument for the analysis of electron delocalization in real space. It determines local concentrations in the amount of electrons that are delocalized from a reference point r(1) over space. Integration of the reference coordinate r(1) over an atomic basin Omega(n) gives the function nabla(2)X(sigma1)(sigma2)(Omega(n);r), which contains detailed information about the spatial shape of the delocalization that originates from an atom in a molecule. Its isosurface representations are richly structured and resemble molecular orbitals in their complexity and partly also in their shape. The sum over all nabla(2)X(sigma1)(sigma2)(Omega(n);r) functions of a molecule equals the Laplacian of the electron density nabla(2)rho(r), for which it provides a meaningful partitioning into atomic contributions.     

Study on the surface density of surface-active substances through total-reflection X-ray absorption fine structure measurement

The total-reflection X-ray absorption fine structure (XAFS) method previously employed for the adsorption of dodecyltrimethylammonium bromide (DTAB) at the air/water interface was applied to that in the presence of NaBr. The surface concentration of the bromide ions Gamma(X)(B) of DTAB and NaBr was evaluated by using the Br K-edge absorption jump values of the total-reflection XAFS spectra and was compared to the corresponding value Gamma(H)(B) estimated from the dependence of surface tension on the bulk concentrations of DTAB m(1) and NaBr m(2). The Gamma(X)(B) values trace almost perfectly the Gamma(X)(B) versus m(1) curve up to a concentration near the critical micelle concentration (cmc) and deviate gradually above the concentration. This behavior is basically similar to that of the single DTAB system and ensures that the XAFS method is also applicable to the DTAB system, even in the presence of NaBr. In addition, this method was extended to the single nonionic amphiphile with covalently bonded bromine, and the surface concentrations of 6-bromo-1-hexanol (BrC6OH), Gamma(X)(1) and Gamma(H)(B), were evaluated and compared with each other. It was found that the Gamma(X)(1) value almost perfectly traces the Gamma(H)(1) versus m(1) curve, even at high surface concentrations. The excellent coincidence confirmed that the total-reflection XAFS method can be applied to the nonionic amphiphile system as well as a cationic surfactant with or without an added salt system. Finally, the difference between the Gamma(X)(B) and Gamma(H)(B) values observed in the DTAB with and without an added salt system is briefly described.     

Structure, bonding, electronic and energy aspects of a new family of early lanthanide (La, Ce and Nd) complexes with phosphoric triamides: insights from experimental and DFT studies

A new family of isostructural early lanthanide(III) complexes (LnXPA) of the general formula Ln(XPA)(2)Cl(3)(solv)(2), where Ln = La, Ce and Nd, XPA = (4-X-C(6)H(4)NH)P(O)(NC(4)H(8)O)(2), X = H, F, Cl and Br, and solv = H(2)O and CH(3)OH, is introduced. X-ray crystallography shows that the replacement of the coordinated water by a methanol molecule may reduce the symmetry level of the unit cell from the orthorhombic crystal system and the space group Fdd2 to monoclinic and C2/c. DFT calculations, at B3LYP, PBE and B3PW91 levels, have been carried out to get a better insight into the structural, electronic and energy aspects of the compounds. The large cation attraction energy (-ΔE) values in the range 269-273 kcal mol(-1), at the B3PW91/ECP/6-311+G** level for the model complexes XPA-La(3+) with stoichiometry 1 : 1, represent new ligands XPA as efficient complexant agents for lanthanides. The electronic nature of para substituent X has no significant effect on the Ln-ligand bonding and cation affinity of the ligands XPA. The results of atoms in molecules (AIM) analysis reveal a partial covalent contribution of the Ln-ligand interaction for the models XPA-La(3+) in the absence of counterions and coordinated solvents. In the real complexes LnXPA, a closed-shell Ln-ligand interaction is established. Increasing the charge difference between nitrogen and phosphorus atoms (by ~0.06 e) associated with a weakening of the Lp(O(P))→σ*(P-N) electronic delocalization (Lp(O(P)) being the lone pair of the phosphoryl oxygen atom) may lead to an increase in partial multiple bond character of the P-N bonds in coordinated ligands, agreeing with the increase in ν(P-N) and (2)J(PH) coupling constant values. The changes in electron density (ρ) and electronic energy density (H(r)) values confirm these structural reorganizations upon complexation.     

Electron delocalization patterns in models of distorted (D2d) mixed-valence cubanes

Low-symmetry distortions are present in cubanes such as Fe(4)S(4), but their effects on electron delocalization properties are not well-understood. Mixed-valence cubanes often exhibit experimentally measurable "pair delocalization" of a delocalizable electron. An important question is, what is the role of physical interactions (vibronic, electronic, exchange) and symmetry distortions in determining the electron delocalization pattern? Semiclassical models are used to explore the electron delocalization patterns of S=1/2 tetragonally (D(2d)) distorted mixed-valence cubanes comprising four metal centers with bridging ligands, a single delocalizable "excess" electron, and either closed-shell or open-shell ion cores. Phase diagrams show that distorted S=1/2 ground state cubanes with antiferromagnetic exchange (as found in nature) have delocalization patterns qualitatively similar to those of an S=1/2 model with no Heisenberg exchange, suggesting that exchange is not necessarily a dominant factor in determining electron delocalization properties. The open-shell model reveals two types of pair delocalization for the S=1/2 ground state, with differing dimer subunit spins for compressed and elongated geometries. Previous studies emphasize the importance of exchange interactions for pair delocalization. Here, it is shown that electron exchange is not always necessary for pair delocalization and that it can be achieved with relatively small tetragonal distortions from tetrahedral (T(d)) symmetry. The results contradict those of an earlier theoretical study of distorted Fe(4)S(4) clusters, which concluded that distortions of lower symmetry than D(2d) are necessary to induce a transition to pair delocalization.     

Spin densities in a ferromagnetic bimetallic chain compound: polarized neutron diffraction and DFT calculations

The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound [MnNi(NO(2))(4)(en)(2)] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction experiments, and the results compared with those from theoretical estimates obtained via calculations based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom due to the more covalent character of the Ni-N bonds compared to the Mn-O ones. Also a nearly isotropic spin distribution is observed for the more ionic d(5) Mn(2+) ion and a clearly anisotropic distribution for the d(8) Ni(2+) ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant between Ni and Mn provide upper and lower limits (+17.6 and -4.2 cm(-)(1)) for the experimentally determined value (+1.3 cm(-)(1)), depending on how the missing part of the chain is simulated, but yield essentially the same spin distribution. The Mn(II)-Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.     

Atoms-in-molecules analysis of extended hypervalent five-center, six-electron (5c-6e) C(2)Z(2)O interactions at the 1,8,9-positions of anthraquinone and 9-methoxyanthracene systems

To clarify the nature of five-center, six-electron (5c-6e) C(2)Z(2)O interactions, atoms-in-molecules (AIM) analysis has been applied to an anthraquinone, 1,8-(MeZ)(2)ATQ (1 (Z=Se), 2 (Z=S), and 3 (Z=O)), and a 9-methoxyanthracene system, 9-MeO-1,8-(MeZ)(2)ATC (4 (Z=Se), 5 (Z=S), and 6 (Z=O)), as well as 1-(MeZ)ATQ (7 (Z=Se), 8 (Z=S), and 9 (Z=O)) and 9-MeO-1-(MeZ)ATC (10 (Z=Se), 11 (Z=S), and 12 (Z=O)). The total electronic energy density (H(b)(r(c))) at the bond critical points (BCPs), an appropriate index for weak interactions, has been examined for 5c-6e C(2)Z(2)O and 3c-4e CZO interactions of the n(p)(O)sigma*(Z--C) type in 1-12. Some hydrogen-bonded adducts were also re-examined for convenience of comparison. The total electronic energy densities varied in the following order: OO (3: H(b)(r(c))=0.0028 au)=OO (6: 0.0028 au)>OO (9: 0.0025 au)> or =NNHF (0.0024 au)> or =OO (12: 0.0023 au)>H(2)OHOH (0.0015 au)>SO (8: 0.0013 au)=SO (2: 0.0013 au)> or =SO (11: 0.0012 au)=SO (5: 0.0012 au)>HFHF (0.0008 au)=SeO (10: 0.0008 au)=SeO (4: 0.0008 au)> or =SeO (1: 0.0007 au)> or =SeO (7: 0.0006 au)>HCNHF (-0.0013 au). H(b)(r(c)) values for SO were predicted to be smaller than the hydrogen bond of H(2)OHOH and H(b)(r(c)) values for SeO are very close to or slightly smaller than that for HFHF in both the ATQ and 9-MeOATC systems. In the case of Z=Se and S, H(b)(r(c)) values for 5c-6e C(2)Z(2)O interactions are essentially equal to those for 3c-4e CZO if Z is the same. The results demonstrate that two n(p)(O)sigma*(Z--C) 3c-4e interactions effectively connect through the central n(p)(O) orbital to form the extended hypervalent 5c-6e system of the sigma*(C--Z)n(p)(O)sigma*(Z--C) type for Z=Se and S in both systems. Natural bond orbital (NBO) analysis revealed that n(s)(O) also contributes to some extent. The electron charge densities at the BCPs, NBO analysis, and the total energies calculated for 1-12, together with the structural changes in the PhSe derivatives, support the above discussion.     

Diruthenium(III,III) bis(alkynyl) compounds with donor/acceptor-substituted geminal-diethynylethene ligands

Reported in this contribution are the preparation and characterization of a series of Ru(2)(DMBA)(4) (DMBA = N,N'-dimethylbenzamidinate) bis(alkynyl) compounds, trans-Ru(2)(DMBA)(4)(X-gem-DEE)(2) [gem-DEE = σ-geminal-diethynylethene; X = H (1), Si(i)Pr(3) (2), Fc (3); 4-C(6)H(4)NO(2) (4), and 4-C(6)H(4)NMe(2) (5)]. Compounds 1-5 were characterized by spectroscopic and voltammetric techniques as well as the single-crystal X-ray diffraction studies of 2 and 3. Both the single-crystal structural data of compounds 2 and 3 and the spectroscopic/voltammetric data indicate that the gem-DEE ligands are similar to simple acetylides in their impact on the molecular and electronic structures of the Ru(2)(DMBA)(4) core. Furthermore, density functional theory calculations revealed more extensive π delocalization in aryl-donor-substituted gem-DEEs and that the hole-transfer mechanism will likely dominate the charge delocalization in Ru(2)-gem-DEE-based wires.     

Density functional investigation of metal-metal interactions in d4d4 face-shared [M2Cl9]3 - (M = Mn, Tc, Re) systems

The molecular and electronic structures of the d(4)d(4) face-shared [M(2)Cl(9)](3)(-) (M = Mn, Tc, Re) dimers have been calculated by density functional methods in order to investigate metal-metal bonding in this series. The electronic structures of these systems have been analyzed using potential energy curves for the broken-symmetry and other spin states arising from the various d(4)d(4) coupling modes, and closed energy cycles have been utilized to identify and quantify the parameters which are most important in determining the preference for electron localization or delocalization and for high-spin or low-spin configurations. In [Tc(2)Cl(9)](3)(-) and [Re(2)Cl(9)](3)(-), the global minimum has been found to be a spin-triplet state arising from the coupling of metal centers with low-spin configurations, and characterized by delocalization of the metal-based electrons in a double (sigma and delta(pi)) bond with a metal-metal separation of 2.57 A. In contrast, high-spin configurations and electron localization are favored in [Mn(2)Cl(9)](3)(-), the global minimum for this species being the ferromagnetic S = 4 state with a rather long metal-metal separation of 3.43 A. These results are consistent with metal-metal overlap and ligand-field effects prevailing over spin polarization effects in the Tc and Re systems, but with the opposite trend being observed in the Mn complex. The ground states and metal-metal bonding observed for the d(4)d(4) systems in this study parallel those previously found for the analogous d(2)d(2) complexes of V, Nb, and Ta, and can be rationalized on the basis that the d(4)d(4) dimer configuration is the hole equivalent of the d(2)d(2) configuration.     

Chemical bonding and aromaticity in trinuclear transition-metal halide clusters

Trinuclear transition-metal complexes such as Re(3)X(9) (X = Cl, Br, I), with their uniquely featured structure among metal halides, have posed intriguing questions related to multicenter electron delocalization for several decades. Here we report a comprehensive study of the technetium halide clusters [Tc(3)(μ-X)(3)X(6)](0/1-/2-) (X = F, Cl, Br, I), isomorphous with their rhenium congeners, predicted from density functional theory calculations. The chemical bonding and aromaticity in these clusters are analyzed using the recently developed adaptive natural density partitioning method, which indicates that only [Tc(3)X(9)](2-) clusters exhibit aromatic character, stemming from a d-orbital-based π bond delocalized over the three metal centers. We also show that standard methods founded on the nucleus-independent chemical shift concept incorrectly predict the neutral Tc(3)X(9) clusters to be aromatic.     

Structural and spectroscopic studies of valence-delocalized diiron(II,III) complexes dupported by carboxylate-only bridging ligands

The synthesis, molecular structures, and spectroscopic properties of a series of valence-delocalized diiron(II,III) complexes are described. One-electron oxidation of diiron(II) tetracarboxylate complexes afforded the compounds [Fe(2)(mu-O(2)CAr(Tol))(4)L(2)]X, where L = 4-(t)BuC(5)H(4)N (1b), C(5)H(5)N (2b), and THF (3b); X = PF(6)(-) (1b and 3b) and OTf(-) (2b). In 1b-3b, four mu-1,3 carboxylate ligands span relatively short Fe...Fe distances of 2.6633(11)-2.713(3) A. Intense (epsilon = 2700-3200 M(-1) cm(-1)) intervalence charge transfer bands were observed at 620-670 nm. EPR spectroscopy confirmed the S = (9)/(2) ground spin state of 1b-3b, the valence-delocalized nature of which was probed by X-ray absorption spectroscopy. The electron delocalization between paramagnetic metal centers is described by double exchange, which, for the first time, is observed in diiron clusters having no single-atom bridging ligand(s).     

Evidence for d orbital aromaticity in square planar coinage metal clusters

Quantitative evidence for the existence of aromaticity involving the d orbitals of transition metals is provided for the first time. The doubly bridged square planar (D(4)(h)()) coinage metal clusters (M(4)Li(2), M = Cu (1), Ag (2), and Au (3)) are characterized as aromatic by their substantial nucleus independent chemical shifts (NICS) values in the centers (-14.5, -14.1, and -18.6, respectively). Nevertheless, the participation of p orbitals in the bonding (and cyclic electron delocalization) of 1-3 is negligible. Instead, these clusters benefit strongly from the delocalization of d and to some extent s orbitals. The same conclusion applies to Tsipis and Tsipis' H-bridged D(4)(h)() Cu(4)H(4) ring (4). Canonical MO-NICS analysis of structures 1-3 shows the total diatropic d orbital contributions to the total NICS to be substantial, although the individual contributions of the five sets of filled d orbitals vary. The d orbital aromaticity of Cu(4)Li(2) also is indicated by its atomization energy, 243.2 kcal/mol, which is larger than Boldyrev's doubly (sigma and pi) aromatic Al(4)Li(2) (215.9 kcal/mol).     

Analysis of the delocalization in the topological theory of chemical bond

The topological analysis of the gradient field of the electron localization function provides a convenient theoretical framework for the partition of the molecular space into basins of attractors having a clear chemical meaning. The basin populations are evaluated by integrating the one-electron density over the basins. The variance of the basin population provides a measure of the delocalization. The behavior of the core C(X) and protonated valence basins V(X, H) populations were investigated. The analysis of the population variance in terms of cross-contributions is presented for aromatic and antiaromatic systems, hypervalent molecules and hydrogen-bonded complexes. For hypervalent molecules this analysis emphasizes the importance of the ionic resonance structures.     

Local hybrid functionals based on density matrix products

We present a novel similarity metric comparing exact and semilocal density functional theory (DFT) exchange holes in real space. This metric is obtained from the product of the one-particle density matrix and the uniform electron gas model density matrix. The metric is bound between 0 and 1, 1 in the uniform electron gas, 0 in regions asymptotically far from finite systems, and can detect delocalization of the exact exchange hole and effective fractional occupations. We also present a parameter-free local hybrid functional that uses this similarity metric to locally mix exact and semilocal DFT exchange energy densities. The resulting functional gives better thermochemistry and reaction barrier heights than our original local hybrids [Jaramillo et al., J. Chem. Phys. 118, 1068 (2003)], while retaining moderate accuracy for symmetric radical cation dimers.     

The D Parameter (EPR Zero-Field Splitting) of Localized 1,3-Cyclopentanediyl Triplet Diradicals as a Measure of Electronic Substituent Effects on the Spin Densities in Para-Substituted Benzyl-Type Radicals

The zero-field splitting parameters D of the symmetrically disubstituted and unsymmetrically monosubstituted 1,3-diaryl-1,3-cyclopentanediyl triplet diradicals 1, 2 (X = p-MeO, p-Me, p-Cl, p-NH(2), p-CO(2)Me, p-CN, p-NO(2)), and 5 were determined in 2-methyltetrahydrofuran glass at 77 K. The linear plot (m = 0.558, r(2) = 0.993) of the experimental D values for the symmetrically disubstituted derivatives versus the corresponding monosubstituted ones reveals that the electronic substituent effects are additive and implies (except for the magnetic dipolar interaction) that each benzyl-type radical site acts independently in the localized diradicals. This additivity permits us to view these triplet diradicals as a composite of the two separate monoradical components and allows us to assess valuable electronic properties of benzyl-type monoradicals from the D parameter of the triplet diradical species. A theoretical analysis shows that the D parameter is a measure of the spin density rho at the benzylic positions and the inter-radical distance d in localized diradicals. A good correlation exists between the D parameter of these triplet diradicals (constant inter-radical distance d) and the EPR hyperfine coupling constants of the corresponding benzyl-type monoradicals, which establishes that the observed electronic substituent effects reflect changes in the spin densities at the radical sites. The novel DeltaD scale allows us to quantify spectroscopically the para substituent effect on the spin delocalization at the benzylic position.     

熔融CaF2的径向分布函数

熔融CaF_2是一种典型的离子液体,又是一种重要的冶金熔体.径向分布函数不仅是描述熔体结构的重要物理量,而且是计算熔体热力学性质的基础.实验上可以通过X 射线、中子衍射测得结构因子经Fourier 变换得到径向分布函数.已有使用X 射线衍射方法实验测定熔融CaF_2结构因子的报导.但由于实验上分解三种离子对Ca~(2+)-Ca~(2+),Ca~(2+)-F~-、F~--F~-偏结构因子的困难,未能给出相应的三种径向分布函数g++(r)、g+-(r)、g--(r),仅估计出三种径     

Application of AIM parameters at ring critical points for estimation of pi-electron delocalization in six-membered aromatic and quasi-aromatic rings

The AIM parameters at the ring critical point (the electron density and its Laplacian, the total electron energy density and both its components, potential and kinetic electron energy densities), have been intercorrelated with aromaticity indices: the geometry-based HOMA and the magnetism-based NICS, NICS(1), and NICS(1)(zz). A set of 33 phenylic rings having possibly a diversified aromatic character, and a set of 20 quasi-rings formed by intramolecular hydrogen and lithium bonds, have been taken into consideration. It has been found that the density of total electron energy, H, may serve as a new quantitative characteristic of pi-electron delocalization. The dependences between H values and aromaticity indices are correlated (cc(H/HOMA)=0.99, cc(H/NICS(1)zz)=0.95).     

Spin delocalization over type zero copper

Hard-ligand, high-potential copper sites have been characterized in double mutants of Pseudomonas aeruginosa azurin (C112D/M121X (X = L, F, I)). These sites feature a small A(zz)(Cu) splitting in the EPR spectrum together with enhanced electron transfer activity. Due to these unique properties, these constructs have been called "type zero" copper sites. In contrast, the single mutant, C112D, features a large A(zz)(Cu) value characteristic of the typical type 2 Cu(II). In general, A(zz)(Cu) comprises contributions from Fermi contact, spin dipolar, and orbital dipolar terms. In order to understand the origin of the low A(zz)(Cu) value of type zero Cu(II), we explored in detail its degree of covalency, as manifested by spin delocalization over its ligands, which affects A(zz)(Cu) through the Fermi contact and spin dipolar contributions. This was achieved by the application of several complementary EPR hyperfine spectroscopic techniques at X- and W-band (～9.5 and 95 GHz, respectively) frequencies to map the ligand hyperfine couplings. Our results show that spin delocalization over the ligands in type zero Cu(II) is different from that of type 2 Cu(II) in the single C112D mutant. The (14)N hyperfine couplings of the coordinated histidine nitrogens are smaller by about 25-40%, whereas that of the (13)C carboxylate of D112 is about 50% larger. From this comparison, we concluded that the spin delocalization of type zero copper over its ligands is not dramatically larger than in type 2 C112D. Therefore, the reduced A(zz)(Cu) value of type zero Cu(II) is largely attributable to an increased orbital dipolar contribution that is related to its larger g(zz) value, as a consequence of the distorted tetrahedral geometry. The increased spin delocalization over the D112 carboxylate in type zero mutants compared to type 2 C112D suggests that electron transfer paths involving this residue are enhanced.     

NMR Investigation of beta-Substituted High-Spin and Low-Spin Iron(III) Tetraphenylporphyrins

The NMR spectra of a series of beta-substituted iron(III) tetraphenylporphyrin (2-X-TPP) complexes have been studied to elucidate the relationship between the electron donating/withdrawing properties of the 2-substituent and the (1)H NMR spectral pattern. The electronic nature of the substituent has been significantly varied and covered the -0.6 to 0.8 Hammett constant range. Both high-spin and low-spin complexes of the general formula (2-X-TPP)Fe(III)Cl and [(2-X-TPP)Fe(III)(CN)(2)](-) have been investigated. The (1)H NMR data for the following substituents (X) have been reported: py(+), NO(2), CN, CH(3), BzO (C(6)H(5)COO), H, D, Br, Cl, CH(3), NH(2), NH(3)(+), NHCH(3), OH, and O(-). The (1)H NMR resonances for low-spin dicyano complexes have been completely assigned by a combination of two-dimensional COSY and NOESY experiments. In the case of selected high-spin complexes, the 3-H resonance has been identified by the selective deuteration of all but the 3-H position. The pattern of unambiguously assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as an unique (1)H NMR spectroscopic probe to map the spin density distribution. The pyrrole isotropic shifts of [(2-X-TPP)Fe(III)(CN)(2)](-) are dominated by the contact term. In order to quantify the substituent effect, the dependence of isotropic shift of all low-spin pyrrole resonances and 3-H high-spin pyrrole resonance versus Hammett constants has been studied. The electronic effect is strongly localized at the beta-substituted pyrrole. The major change of the isotropic shift has also been noted for only one of two adjacent pyrrole rings, i.e., at 7-H and 8-H positions. These neighboring protons, located on a single pyrrole ring, experienced opposite shift changes when electron withdrawing/donating properties were modified. Two other pyrrole rings for all investigated derivatives revealed considerably smaller, substituent related, isotropic shift changes. A long-range secondary isotopic shift has been observed for [(2-D-TPP)Fe(III)(CN)(2)](-). The effect is consistent with a general spin density distribution mechanism due to beta-substitution. A fairly good correlation between the 3-H isotropic shift of (2-X-TPP)Fe(III)Cl and the Hammett constant has been found as well. The observed contact shift pattern of [(2-X-TPP)Fe(III)(CN)(2)](-) reflects spin pi delocalization into the highest filled MO equivalent to the unsubstituted porphyrin 3e(pi) orbital. To account for the substituent contribution, the semiquantitative Fenske-Hall LCAO method has been used to determine the molecular orbitals involved in the spin density delocalization. For low-spin complexes, (13)C pyrrole resonances of carbons bearing a proton have been identified by means of a (1)H-(13)C HMQC experiment. The reversed order of (13)C resonance patterns as compared to their (1)H NMR counterparts has been determined, e.g., the largest isotropic shift of 3-H has been accompanied by the smallest measured (13)C isotropic shift. Analysis of the isotropic shifts in (2-X-TPP)Fe(III)Cl and [(2-X-TPP)Fe(III)(CN)(2)](-) suggests that the observed regularities of the electronic structure modification due to the beta-substitution should apply to iron(III) natural porphyrin or geoporphyrin complexes.     

Persistent Borafluorene Radicals

N‐Heterocyclic carbene (NHC)‐ and cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single‐crystal X‐ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC–borafluorene radical ( 2 ) and the NHC–borafluorene radical ( 4 ) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4 ). In compound 2 , the unpaired electron is also partly delocalized over the CAAC ligand ^carbeneC and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π‐system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials‐relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid‐state and solution stability.