Closed-shell Singlet Research Articles

Direct spectroscopic observation of closed-shell singlet, open-shell singlet, and triplet p-biphenylyloxenium ion.

The photophysics and photochemistry of p-biphenylyl hydroxylamine hydrochloride was studied using laser flash photolysis ranging from the femtosecond to the microsecond time scale. The singlet excited state of this photoprecursor is formed within 350 fs and partitions into three different transients that are assigned to the p-biphenyloxy radical, the open-shell singlet p-biphenylyloxenium ion, and the triplet p-biphenylyloxenium ion, having lifetimes of 40 μs, 45 ps, and 1.6 ns, respectively, in CH3CN. The open-shell singlet p-biphenylyloxenium ion predominantly undergoes internal conversion to produce the closed-shell singlet p-biphenylyloxenium ion, which has a lifetime of 5-20 ns. The longer-lived radical is unambiguously assigned by nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy, and the assignment of the short-lived singlet and triplet oxenium ion transient absorptions are supported by matching time-dependent density functional theory (TD-DFT) predictions of the absorptions of these species, as well as by product studies that implicate the intermediacy of charged electrophilic intermediates. Product studies from photolysis give p-biphenylol as the major product and a chloride adduct as the major product when NaCl is added as a trap. Thermolysis studies give p-biphenylol as a major product, as well as water, ammonium, and chloro adducts. These studies provide a rare direct look at a discrete oxenium ion intermediate and the first detection of open-shell singlet and triplet configurations of an oxenium ion, as well as providing an intriguing example of the importance of excited state dynamics in governing the electronic state population of reactive intermediates.

Rotational constants of linear and/or bent Cn+1H+ and CnN+(n = 1–6): A DFT study

The geometries, dipole moments, and rotational constants for the linear and/or bent cations, Cn+1H+ and CnN+(n=1–6), were studied by the B3LYP method with the modest basis sets. For CnH+(n=odd; 3, 5, 7) and CnN+(n=even; 2, 4, 6), the theoretical rotational constants (Bes) of closed-shell singlet C3H+, C5H+, C7H+, CCN+, C4N+, and C6N+ were calculated to be about 11,244, 2420, 885.2, 11,970, 2439, and 880.8MHz, respectively. By contrast, the triplets are stable than the corresponding singlets for CnH+(n=odd; 2, 4, 6) and CnN+(n=even; 3, 5) except CN+.

Quantum-Chemical Ab Initio Calculations on Ala-(C5H5Al) and Galabenzene (C5H5Ga)

1 Quantum-chemical ab initio and time-dependent density functional theory (TD-DFT) calculations employing various basis sets were used to elucidate the spatial as well as the electronic structure of C5H5Al () and C5H5Ga (2) (ala- and galabenzene). The lowest closed shell singlet states of both compounds were found to have a non-planar structure of C S symmetry with C-X-C bond angles of about 116° (MP2/6-311++G**) and 125° (CCSD/aug-cc-pVDZ). At approximately 103°, the corresponding angles of the lowest triplets are significantly smaller. The lowest triplet state of alabenzene is also non-planar (C S) at the MP2 level while optimization with the CCSD and the CASPT2 method resulted in planar structures with C 2v symmetry. The corresponding state of galabenzene has C 2v symmetry at all levels of optimization. The relative stability of the lowest closed shell singlet and the lowest triplet (ΔE(T 1-S 0)) state is small and its sign even strongly method-dependent. However, according to the highest levels of theory applied in this study the singlet states of both molecules are slightly lower in energy than the corresponding triplets with singlet/triplet gaps between about 0.5 and 5.8 kcal/mol in favour of the singlet states. Most of the applied methods give a slightly smaller splitting for ala- than for galabenzene. Independent of the applied method (TD-DFT/CAM-B3LYP/6-311++G(3df,3pd)//MP2/6- 311++G** or SAC-CI/6-31++G(3df,3pd)//MP2/6-311++G**), the general shape of the calculated UV/VIS spectral curves are quite similar for the lowest singlet states of ala- and galabenzene, and the same applies to the spectra of the normal modes. The calculated UV/VIS spectra of C5H5Al and C5H5Ga are featured by long wavelength bands of moderate intensity around 900 nm at the TD-DFT and between 1300 and 1500 nm at the SAC-CI level. According to both methods these bands are predominantly due to HOMO(π)→LUMO(σ*) transitions. The results of isodesmic bond separation reactions for the singlet states indicate some degree of stabilization due to delocalization in both of the title compounds. With our best values between 29 and 32 kcal/mol this stabilization appears to be only slightly less than the previously reported value for borabenzene (∼38 kcal/mol).

Weakening of the π*–π* dimerisation in 1,2,3,5-dithiadiazolyl radicals: structural, EPR, magnetic and computational studies of dichlorophenyl dithiadiazolyls, Cl2C6H3CNSSN

A series of dichlorophenyl dithiadiazolyls (DCP-DTDA) have been prepared and structurally characterized by X-ray diffraction (1 = 2,3-DCP-DTDA; 2 = 2,4-DCP-DTDA; 3 = 2,5-DCP-DTDA, 4 = 3,4-DCP-DTDA and 5 = 3,5-DCP-DTDA). All the radicals adopt cis-cofacial π*–π* dimers with intra-dimer S⋯S contacts in the range 3.119(8)–3.300(5) A. The propensity for chloro-aromatics to adopt a β-sheet structure assists formation of lamellar structures in 1–5 with intra-stack Cl⋯Cl contacts in the range 3.44(1)–3.82(1) A. Perpendicular to the stacking direction, the packing is dominated by formation of intermolecular S⋯Cl contacts. Variable-temperature magnetic susceptibility measurements on 1–5 in the range 5–300 K reveal the onset of paramagnetism around ca. 200 K. Solid-state EPR studies reveal that this paramagnetism is associated with thermal population of a triplet excited state. A good fit to the Bleaney–Bowers model determined the exchange interactions (2J) between the radicals in the dimers to be in the range −904 to −1529 cm−1. Computational studies of the dimerisation process indicate that at intra-dimer S⋯S distances in excess of 3.2 A the closed-shell singlet becomes destabilized with respect to both the open-shell singlet and triplet state configurations.

Heterogeneously substituted radicals and carbenes: photoelectron imaging of the FC(H)CN(-) and FCCN(-) anions.

This work represents the next step in the studies of heterogeneous substitution effects in cyanohalo radicals and carbenes. Negative-ion photoelectron imaging was used to investigate the substituted radical and carbene derivatives of fluoroacetonitrile. We report a closed-shell singlet ground state for cyanofluorocarbene, FCCN, with a directly measured adiabatic electron affinity EA = 2.081 ± 0.002 eV and a singlet-triplet gap of ΔE(S-T) = 0.42 ± 0.04 eV, estimated through a combination of experimental and theoretical results. The open-shell singlet (1)A″ state was also observed experimentally. The cyanofluoromethyl radical, FC(H)CN, was similarly estimated to have an EA of 1.53 ± 0.08 eV. This value was used to estimate the C-H bond dissociation energy (BDE) of fluoroacetonitrile, DH298 = 90.7 ± 2.8 kcal mol(-1). The results are discussed in comparison with results for other fluoro- and cyano-substituted radicals and carbenes, and in light of our recent work on the radical and carbene derivatives of chloroacetonitrile. The estimated ΔE(S-T) of FCCN agrees well with the general trend of similar carbenes. We also find that, similar to chloroacetonitrile, the low C-H BDE of fluoroaceotnitrile indicates a synergistic stabilization of the corresponding radical by a π donor (halogen) and π acceptor (CN).

Multiconfigurational character of the ground states of polycyclic aromatic hydrocarbons. A systematic study.

A systematic study of the electronic structure of polycyclic hydrocarbons from naphthalene to a system containing 80 fused benzene has been carried out. Geometries were optimized for closed shell singlet, open shell singlet, triplet and multiplet states at B3LYP/cc-pVDZ level of theory, D1 (second order Møller-Plesset) and D1 (second-order approximate coupled-cluster) diagnostics have been calculated for studied molecules. Complete active space self-consistent field (10,10)/6-31G(d) single point energy calculations have been carried out for all optimized structures. Multireference character of the ground state becomes important when the number of atoms in the polycyclic hydrocarbon exceeds 40-50. At this point, D1 diagnostics reaches 0.04-0.05 and the squared configuration interaction expansion coefficient for dominant configuration drops to about 0.6. However, only for the three largest systems predominantly polyradicalic ground states have been detected. All other polycyclic hydrocarbons showing significant multiconfigurational character of singlet ground state have only two dominant configurations which are closed shell singlet and doubly excited singlet, respectively. Thus, small polycyclic hydrocarbons have mostly single reference singlet ground state, the medium size systems have notably multireference ground state (singlet or triplet) with only moderate polyradicalic character. The ground state of largest systems is singlet polyradical.

Twisted Triplet Ethylene: Anharmonic Frequencies and Spectroscopic Parameters for C2H4, C2D4, and 13C2H4

Ethylene is an exceptional example of a stable closed-shell singlet molecule with a low-lying triplet state of very different symmetry. Triplet C2H4 (the ã (3)A1 state), which has a twisted D2d geometry, is studied herein with high-level theoretical methods, namely, CCSD(T) (coupled cluster theory with single, double, and perturbative triple excitations) and Dunning's correlation-consistent quadruple-ζ basis set (cc-pVQZ). Geometric parameters, including equilibrium (re) and vibrationally corrected (rg) values, are reported for C2H4, C2D4, and (13)C2H4. Harmonic and anharmonic vibrational frequencies are also predicted using second-order vibrational perturbation theory (VPT2). Challenges encountered for the wagging vibrational features are discussed.

Computational design of the magnetism-tunable oligobenzylic carbanion complexes

Unrestricted density functional theory calculations in combination with the broken symmetry approach have been employed to study several benzylic carbanions. As a free anion, 2-(3,5-dinitrophenyl)-1,3-dithiane carbanion has near-degenerate singlet and triplet states and appears to be a promising magnetism-tunable species. In this work, we computationally design some of its derivatives in two ways: expanding the π-conjugated structures and introducing Lewis acids (Li+, Na+, and K+, and polar molecules are considered here) with different acidities. Calculations reveal that ring expansion does not change its open-shell broken symmetry singlet diradical ground state and antiferromagnetic character, but decreases its magnetism, whereas introduction of Lewis acids can lead to different ground states (triplet vs. singlet) and different magnetism, depending on the binding sites of the Lewis acid. That is, they show closed-shell singlet ground states without magnetism when a cation locates near the anionic center of the 1,3-dithiane ring, but convert to triplet as their ground states with ferromagnetic character when the cation moves to one nitro group of the 3,5-dinitrophenyl-based π-conjugation-expanded fragment. These findings regarding modulation through ring expansion and Lewis acid-binding ways make the magnetisms of 2-(3,5-dinitrophenyl)-1,3-dithiane-based carbanions tunable, and thus provide prospects of a new extension of the results from the previous study for designing magnetism-tunable building blocks for novel electromagnetic materials.

Cover Picture: Tuning Ground States of Bis(triarylamine) Dications: From a Closed-Shell Singlet to a Diradicaloid with an Excited Triplet State (Angew. Chem. Int. Ed. 11/2014)

Cover Picture: Tuning Ground States of Bis(triarylamine) Dications: From a Closed-Shell Singlet to a Diradicaloid with an Excited Triplet State (Angew. Chem. Int. Ed. 11/2014)

Remarkable Solvatochromic Color Change via Proton Tautomerism of a Phenol-Linked Imidazole Derivative

A unique solvatochromic 2-phenyl-4,5-diarylimidazole derivative linked with a phenol moiety and a p-quinonemethide moiety at the 4- and 5-positions of a imidazole ring, which shows remarkable color change via proton tautomerism, was synthesized and the mechanism of the solvatochromic color change was investigated. The yellow-colored OH tautomeric form (1_OH) exists as a dominant species in nonpolar solvents, whereas the blue-colored NH tautomeric form (1_NH) is stabilized in polar solvents. The molecular structures of these tautomers were determined by X-ray crystallographic analysis. The p-quinonemethide moiety and the imidazole ring of 1_OH are coplanar to one another and possess a planar quinoidal structure. On the other hand, 1_NH has a nonplanar twisted quinoidal structure causing large bathochromic shift in the visible absorption spectrum. Moreover, the X-ray crystallographic analysis and the DFT calculations support the closed-shell singlet character of 1_OH. In contrast 1_NH possesses partial single bond character that leads to the open-shell singlet biradical character and the decrease in the singlet-triplet energy gap. The twisting of the π-conjugated electron system induced by the proton tautomerization was found to be the origin of the open-shell biradical character of 1_NH and the enhanced solvatochromic color change.

Ruthenium, Rhodium, Osmium, and Iridium Complexes of Osazones (Osazones = Bis-Arylhydrazones of Glyoxal): Radical versus Nonradical States

Phenyl osazone (L(NHPh)H2), phenyl osazone anion radical (L(NHPh)H2(•-)), benzoyl osazone (L(NHCOPh)H2), benzoyl osazone anion radical (L(NHCOPh)H2(•-)), benzoyl osazone monoanion (L(NCOPh)HMe(-)), and anilido osazone (L(NHCONHPh)HMe) complexes of ruthenium, osmium, rhodium, and iridium of the types trans-[Os(L(NHPh)H2)(PPh3)2Br2] (3), trans-[Ir(L(NHPh)H2(•-))(PPh3)2Cl2] (4), trans-[Ru(L(NHCOPh)H2)(PPh3)2Cl2] (5), trans-[Os(L(NHCOPh)H2)(PPh3)2Br2] (6), trans- [Rh(L(NHCOPh)H2(•-))(PPh3)2Cl2] (7), trans-[Rh(L(NHCOPh)HMe(-))(PPh3)2Cl]PF6 ([8]PF6), and trans-[Ru(L(NHCONHPh)HMe)(PPh3)2Cl]Cl ([9]Cl) have been isolated and compared (osazones = bis-arylhydrazones of glyoxal). The complexes have been characterized by elemental analyses and IR, mass, and (1)H NMR spectra; in addition, single-crystal X-ray structure determinations of 5, 6, [8]PF6, and [9]Cl have been carried out. EPR spectra of 4 and 7 reveal that in the solid state they are osazone anion radical complexes (4, gav = 1.989; 7, 2.028 (Δg = 0.103)), while in solution the contribution of the M(II) ions is greater (4, gav = 2.052 (Δg = 0.189); 7, gav = 2.102 (Δg = 0.238)). Mulliken spin densities on L(NHPh)H2 and L(NHCOPh)H2 obtained from unrestricted density functional theory (DFT) calculations on trans-[Ir(L(NHPh)H2)(PMe3)2Cl2] (4(Me)) and trans-[Rh(L(NHCOPh)H2)(PMe3)2Cl2] (7(Me)) in the gas phase with doublet spin states authenticated the existence of L(NHPh)H2(•-) and L(NHCOPh)H2(•-) anion radicals in 4 and 7 coordinated to iridium(III) and rhodium(III) ions. DFT calculations on trans-[Os(L(NHPh)H2)(PMe3)2Br2] (3(Me)), trans-[Os(L(NHCOPh)H2)(PMe3)2Br2] (6(Me)), and trans-[Ru(L(NHCONHPh)HMe(-))(PMe3)2Cl] [9(Me)](+) with singlet spin states established that the closed-shell singlet state (CSS) solutions of 3, 5, 6, and [9]Cl are stable. The lower value of M(III)/M(II) reduction potentials and lower energy absorption bands corroborate the higher extent of mixing of d orbitals with the π* orbital in the case of 3 and 6. Time-dependent (TD) DFT calculations elucidated the MLCT as the origin of the lower energy absorption bands of 3, 5, and 6 and π → π* as the origin of transitions in 4 and 7.

Tuning Ground States of Bis(triarylamine) Dications: From a Closed‐Shell Singlet to a Diradicaloid with an Excited Triplet State

Three bis(triarylamine) dications were isolated by using weakly coordinating anions. Their electronic structures in the ground state were investigated by various experiments in conjunction with theoretical calculations. The ground-state electronic structures of these species were tunable by substituent effects, with two of them as closed-shell singlets and one of them as an open-shell singlet in the solid state. The excited state of the latter is thermally accessible, indicated by EPR and SQUID measurements. The work provides a new and stable diradicaloid structure motif with an excited triplet sate.

Ruthenium, osmium and rhodium complexes of 1,4-diaryl 1,4-diazabutadiene: radical versus non-radical states.

Ruthenium, osmium and rhodium complexes of 1,4-di(3-nitrophenyl)-1,4-diazabutadiene (LDAB) of types trans-[RuII(LDAB)(PPh3)2Cl2] (1), trans-[OsII(LDAB)(PPh3)2Br2] (2) and trans-[Rh(LDAB)(PPh3)2Cl2] (3) are isolated and characterized by elemental analyses, IR, mass and 1H NMR spectra including the single crystal X-ray structure determination of 1·2toluene. The α-diimine fragment of the LDAB ligand in 1·2toluene is deformed, showing a relatively longer -C=N- bond, 1.320 Å, and a shorter =CH–CH= bond, 1.395 Å. Density functional theory (DFT) calculations on trans-[Ru(LDAB)(PMe3)2Cl2] (1Me) and trans-[Os(LDAB)(PMe3)2Br2] (2Me) with singlet spin states authenticated that the closed shell singlet state (CSS) solutions of 1 and 2 are stable and no perturbation occurs because of the diradical open shell singlet (OSS) state. The EPR spectra of 3 and the Mulliken spin density distribution obtained from the DFT calculation on trans-[Rh(LDAB)(PMe3)2Cl2] (3Me) imply that the ground electronic state of 3 can be defined by the [RhIII(LDAB˙−)(PPh3)2Cl2] (3RhL˙) ↔ [RhII(LDAB)(PPh3)2Cl2] (3Rh˙L) resonating states. In solid, the contribution of 3RhL˙ is higher and the gav is 2.018 with Δg = 0.10, whereas in frozen glasses the contribution of 3Rh˙L is higher and the gav is 2.026 with Δg (frozen glasses) = 0.13. The g parameters of the electrogenerated [1]+ (g1 = 2.456, g2 = 2.128 and g3 = 1.624, Δg = 0.824), [2]+ (g1 = 2.599, g2 = 2.041 and g3 = 1.965, Δg = 0.634), [1]− (g1 = 2.138, g2 = 2.109, g3 = 1.978 and Δg = 0.160) and [2]− (g1 = 2.168, g2 = 2.097, g3 = 1.987 and Δg = 0.181) ions and the spin density distributions obtained from the DFT calculations on [1Me]+, [2Me]+, [1Me]− and [2Me]− reveal that the reversible anodic peaks of 1 and 2 at 0.11 and 0.34 V, referenced versus Fc+/Fc couple, are due to the M(III)/M(II) redox couple, while the reversible cathodic waves at −1.27 V and −0.82 V of 1 and 2 are caused by the LDAB/LDAB˙− redox couple. Both [MII(LDAB˙−)(PPh3)2Br2]− and [MI(LDAB)(PPh3)2Br2]− tautomers contribute to the ground electronic states of [1]− (g = 2.075) and [2]− (g = 2.084) ions, which are isoelectronic to 3. Time dependent (TD) DFT calculations and spectroelectrochemical measurements elucidated that lower energy absorption bands of 1 and 2 are caused by the metal to ligand charge transfer (MLCT) that disappears upon oxidation or reduction.

Theoretical investigation on nonlinear optical properties of carbon nanotubes with Stone–Wales defect rings

Based on (6, 0) zigzag carbon nanotubes (ZCNTs), a Stone–Wales defect ring ([5.7]3) is constructed at one end of the ZCNTs, forming a novel nanostructure [5.7]3ZCNT. The introduction of [5.7]3 breaks the centrosymmetry of the nanotube and remarkably changes electronic and magnetic properties of the nanotube. Unlike the (6, 0)ZCNT which has an open-shell singlet ground state, the [5.7]3ZCNT has a closed-shell singlet ground state with a large dipole moment, polarizability, and first hyperpolarizability. Interestingly, the [5.7]3ZCNT itself has a donor–π–acceptor framework, in which the Stone–Wales defect ring serves as an electron donor (D) while the zigzag nanotube works as an electron acceptor (A) and a conjugated bridge (π). Moreover, increasing both the tube diameter and tube length could enhance second-order nonlinear optical (NLO) responses, with the former being more effective than the latter.

5-Azido-2-aminopyridine, a New Nitrene/Nitrenium Ion Photoaffinity Labeling Agent That Exhibits Reversible Intersystem Crossing between Singlet and Triplet Nitrenes

The photochemistry of a new photoaffinity labeling (PAL) agent, 5-azido-2-(N,N-diethylamino)pyridine, was studied in aprotic and protic solvents using femtosecond-to-microsecond transient absorption and product analysis, in conjunction with ab initio multiconfigurational and multireference quantum chemical calculations. The excited singlet S1 state is spectroscopically dark, whereas photoexcitation to higher-lying singlet excited S2 and S3 states drives the photochemical reaction toward a barrierless ultrafast relaxation path via two conical intersections to S1, where N2 elimination leads to the formation of the closed-shell singlet nitrene. The singlet nitrene undergoes intersystem crossing (ISC) to the triplet nitrene in aprotic and protic solvents as well as protonation to form the nitrenium ion. The ISC rate constants in aprotic solvents increase with solvent polarity, displaying a "direct" gap effect, whereas an "inverse" gap effect is observed in protic solvents. Transient absorption actinometry experiments suggest that a solvent-dependent fraction from 20% to 50% of nitrenium ions is generated on a time scale of a few tens of picoseconds. The closed-shell singlet and triplet nitrene are separated by a small energy gap in protic solvents. As a result, the unreactive triplet state nitrene undergoes delayed, thermally activated reverse ISC to reform the reactive closed-shell singlet nitrene, which subsequently protonates, forming the remaining fraction of nitrenium ions. The product studies demonstrate that the resulting nitrenium ion stabilized by the electron-donating 4-amino group yields the final cross-linked product with high, almost quantitative efficiency. The enhanced PAL function of this new azide with respect to the widely applied 4-amino-3-nitrophenyl azide is discussed.

Density functional theory studies on the structure and electron distribution in the peroxide intermediate of the catalytic cycle of multicopper oxidases

The peroxide intermediate (PI) is obtained in the first step of the reduction of O2 by multicopper oxidases. Earlier density functional theory (DFT) studies as well as spectral and structural comparison to a fully oxidized structural analogue of the PI known as the peroxide adduct (PA) reveal that O2 bridges all three copper atoms of the trinuclear cluster in the PI. This orientation of the oxygen moiety has been discussed as a result of the influence from the second coordination sphere. In the present study, we investigate by DFT and quantum mechanics/molecular mechanics (QM/MM) the potential energy surface (PES) of the PI as a function of the orientation of O2 within the copper cluster to examine the influence of the second coordination sphere on the structure of the PI. We use the second order spin-flip constricted variational DFT method to probe a possible multideterminantal nature of the PI and to devise a computational strategy for its treatment. Our results suggest that the PI can be approximated to a closed shell singlet. Additionally, for the determination of the oxidation states of the three copper atoms in the PI, the electron redistribution upon the formation of the PI has been investigated with the extended transition state–natural orbitals for chemical valence method. We observe a flat PES on which oxygen can easily rotate between the copper atoms. The fully bridged PI structure emerges in the absence of atoms from the second coordination sphere and has been attributed to the coordination unsaturation of the copper atoms in the cluster. The good Cu–O overlap leads to the participation of all copper atoms in the reduction of O2.

Aromaticity Effects on the Profiles of the Lowest Triplet‐State Potential‐Energy Surfaces for Rotation about the CC Bonds of Olefins with Five‐Membered Ring Substituents: An Example of the Impact of Baird’s Rule

A density functional theory study on olefins with five-membered monocyclic 4n and 4n+2 π-electron substituents (C4H3X; X=CH(+), SiH(+), BH, AlH, CH2, SiH2, O, S, NH, and CH(-)) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet-state (T1) potential-energy surface (PES) for twisting about olefinic C=C bonds. It exploited both Hückel's rule on aromaticity in the closed-shell singlet ground state (S0) and Baird's rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH2=CH(C4H3X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C4H3X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S0 -antiaromatic/T1 -aromatic (C5H4 (+)) to strongly S0 -aromatic/T1- antiaromatic (C5H4(-)). Our hypothesis is that the shapes of the T1 PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T1 [ΔE(T1)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T1) and the (anti)aromaticity changes of the C4 H3 X groups, as measured by the zz-tensor component of the nucleus-independent chemical shift ΔNICS(T1;1)zz , are found both for sets A and B separately (linear fits; r(2) =0.949 and 0.851, respectively) and for the two sets combined (linear fit; r(2) =0.851). For sets A and B combined, strong correlations are also found between ΔE(T1) and the degree of S0 (anti)aromaticity as determined by NICS(S0,1)zz (sigmoidal fit; r(2) =0.963), as well as between the T1 energies of the planar olefins and NICS(S0,1)zz (linear fit; r(2) =0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T1 PESs suitable for adiabatic Z/E photoisomerization.

Oxidovanadium Catechol Complexes: Radical versus Non-Radical States and Redox Series

A new family of oxidovanadium complexes, [(L1(R))(VO)(L(R(')))] (R = H, R' = H, 1; R = H, R' = -CMe3, 2; R = H, R' = Me, 3; R = -CMe3, R' = H, 4 and R = -CMe3, R' = -CMe3, 5), incorporating tridentate L1(R)H ligands (L1(R)H = 2,4-di-R-6-{(2-(pyridin-2-yl)hydrazono)methyl}phenol) and substituted catechols (L(R('))H2) was substantiated. The V-Ophenolato (cis to V═O), V-OCAT (cis to V═O) and V-OCAT (trans to V═O) lengths span the ranges, 1.894(2)-1.910(2), 1.868(2)-1.887(2), and 2.120(2)-2.180(2) Å. The metrical oxidation states (MOS) of the catechols in 1-5 are fractional and vary from -1.43 to -1.60. The (51)V isotropic chemical shifts of solids and solutions of 1-5 are deshielded ((51)V CP MAS: -19.8 to +248.6; DMSO-d6: +173.9 to +414.55 ppm). The closed shell singlet (CSS) solutions of 1-5 are unstable due to open shell singlet (OSS) perturbations. The ground electronic states of 1-5 are defined by the resonance contribution of the catecholates (L(R('))CAT(2-)) and L(R('))SQ(-•) coordinated to the [VO](3+) and [VO](2+) ions. 1-5 are reversibly reducible by one electron at -(0.58-0.87) V, referenced vs ferrocenium/ferrocene, to VO(2+) complexes, [(L1(R-))(VO(2+))(L(R('))CAT(2-))](-) [1-5](-). 1-5 display another quasi-reversible or irreversible reduction wave at -(0.80-1.32) V due to the formation of hydrazone anion radical (L1(R2-•)) complexes, [(L1(R2-•))(VO(2+))(L(R('))CAT(2-))](2-), [1-5](2-), with S = 1 authenticated by the unrestricted density functional theory (DFT) calculations on 1(2-) and 3(2-) ions. Frozen glasses electron paramagnetic resonance (EPR) spectra of [1-5](-) ions [e.g., for 2, g|| = 1.948, g⊥ = 1.979, A|| = 164, A⊥ = 60] affirmed that [1-5](-) ions are the [VO](2+) complexes of L(R')CAT(2-). Spectro-electrochemical measurements and time-dependent DFT (TD DFT) calculations on 1, 3, 1(-), 3(-), and 1(2-) disclosed that the near infrared (NIR) absorption bands of 1-5 at 800 nm are due to the CSS-OSS metal to ligand charge transfer which are red-shifted in the solid state and disappear in [1-5](-) and [1-5](2-) ions.

Electronic structure and ligand vibrations in FeNO, CoNO, and FeOO porphyrin adducts.

The gaseous ligands, CO, NO, and O2 interact with the Fe ion in heme proteins largely via backbonding of Fe electrons to the π* orbitals of the XO (X = C, N, O) ligands. In these FeXO adducts, the Fe-X stretching frequency varies inversely with the X-O stretching frequency, since increased backbonding strengthens the Fe-X bond while weakening the X-O bond. Inverse frequency correlations have been observed for all three ligands, despite differing electronic and geometric structures, and despite variable composition of the "FeX" vibrational mode, in which Fe-X stretching and Fe-X-O coordinates are mixed for bent FeXO adducts. We report experimental data for 5-coordinate Co(II)(NO) porphyrin adducts (isoelectronic with Fe(II)(OO) adducts), and the results of density functional theory (DFT) modeling for 5-coordinate Fe(II)(NO), Co(II)(NO), and Fe(II)(OO) adducts. Inverse ν(MX)/ν(XO) correlations are obtained computationally, using model porphyrins with graded electron-donating and -withdrawing substituents to modulate the backbonding. Computed slopes agree satisfactorily with experiment, provided nonhybrid functionals are used, which avoid overemphasizing high-spin states. The BP86 functional gives correct ground states, a closed-shell singlet for Co(II)(NO) and an open-shell singlet for the isoelectronic Fe(II)(OO), as corroborated by structural data for Co(II)(NO), and the ν(MX)/ν(XO) slope agreement with experiment for both adducts. However, for Fe(II)(OO) adducts, the computed inverse ν(MX)/ν(XO) correlation applies only to porphyrins with electron-donating and withdrawing substituents of moderate strength. For substituents more donating than -CH3, a direct correlation is obtained, the Fe-O and O-O bonds weakening in concert. This effect is ascribed to the dominance of σ bonding via the in-plane dxz(+dz(2))-π* orbital, when electron-donating substituents raise the d orbital energies sufficiently to render backbonding (dyz-π*) unimportant.

Binuclear pentafluorocyclopentadienyl metal carbonyls of iron, cobalt, and nickel: Effect of fluorine substitution

Density functional theory studies on the binuclear pentafluorocyclopentadienyl metal carbonyls (C5F5)2M2(CO)n (M=Fe, n=4, 3, 2; M=Co, n=3, 2, 1; M=Ni, n=2, 1) using the B3LYP and BP86 methods find pentahapto η5-C5F5 ligands in all of the low energy structures. In general, the low energy structures of these η5-C5F5 derivatives parallel rather closely those of their hydrocarbon analogs with relatively little change upon complete substitution of the hydrogen atoms with fluorine atoms. The lowest energy (C5F5)2M2(CO)n (M=Fe, n=4; M=Co, n=3; M=Ni, n=2) structures are predicted to be closed-shell singlets with metal–metal distances of ∼2.6Å, ∼2.5Å, and ∼2.4Å, respectively, corresponding to the single bonds required to give each metal atoms the favored 18-electron configuration and similar to their experimentally known hydrocarbon analogs. For (C5F5)2Fe2(μ-CO)2(CO)2 the cis and trans isomers are predicted to have similar energies within 2kcal/mol similar to the well-known (C5H5)2Fe2(μ-CO)2(CO)2 except the relative energies of the cis and trans isomers are reversed. For the unsaturated derivatives (C5F5)2M2(CO)n (M=Fe, n=3, 2; M=Co, n=2, 1; M=Ni, n=1), the lowest energy structures have shorter metal–metal multiple bonds of orders 2 and 3 corresponding to the favored 18-electron configurations for the metal atoms.