Density functional theory study of the beta-carotene radical cation and deprotonated radicals

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.     

Pulsed EPR and DFT characterization of radicals produced by photo-oxidation of zeaxanthin and violaxanthin on silica-alumina

Pulsed electron nuclear double resonance (ENDOR) and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) studies in combination with density functional theory (DFT) calculations revealed that photo-oxidation of natural zeaxanthin (ex Lycium halimifolium) and violaxanthin (ex Viola tricolor) on silica-alumina produces the carotenoid radical cations (Car*+) and also the neutral carotenoid radicals (#Car*) as a result of proton loss (indicated by #) from the C4(4') methylene position or one of the methyl groups at position C5(5'), C9(9'), or C13(13'), except for violaxanthin where the epoxide at positions C5(5')-C6(6') raises the energy barrier for proton loss, and the neutral radicals #Car*(4) and #Car*(5) are not observed. DFT calculations predict the largest isotropic beta-methyl proton hyperfine couplings to be 8 to 10 MHz for Car*+, in agreement with previously reported hyperfine couplings for carotenoid pi-conjugated radicals with unpaired spin density delocalized over the whole molecule. Anisotropic alpha-proton hyperfine coupling tensors determined from the HYSCORE analysis were assigned on the basis of DFT calculations with the B3LYP exchange-correlation functional and found to arise not only from the carotenoid radical cation but also from carotenoid neutral radicals, in agreement with the analysis of the pulsed ENDOR data. The formation of the neutral radical of zeaxanthin should provide another effective nonphotochemical quencher of the excited state of chlorophyll for photoprotection in the presence of excess light.     

ESR/DFT study of bis-iminophosphorane cation radicals

Bis-iminophosphoranes containing various types of linkers between two R3P==N moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their 1H, 14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph3P==N moieties separated by a --C(CN)==C(CN)--group was obtained from its crystal structure.     

Spin‐Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

A potentially biocompatible class of spin‐labeled macromolecules, spin‐labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser‐type dynamic nuclear polarization (DNP). All presented SL‐heparins show high ¹H DNP enhancement factors up to E=?110, which validates that effectively more than one hyperfine line can be saturated even for spin‐labeled polarizing agents. The parameters for the Overhauser‐type DNP are determined and discussed. A striking result is that for spin‐labeled heparins, the off‐resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non‐negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin‐nuclear spin relaxation (T_1ne). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.     

Through‐Space Conjugated Molecular Wire Comprising Three π‐Electron Systems

A [2.2]paracyclophane‐based through‐space conjugated oligomer comprising three π‐electron systems was designed and synthesized. The arrangement of three π‐conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Förster mechanism. The energy transfer efficiency and rate constants were estimated to be >0.999 and >10¹² s^?1, respectively. The key point for the efficient energy transfer is the orientation of the transition dipole moments. The time‐dependent density functional theory (TD‐DFT) studies revealed the transition dipole moments of each stacked π‐electron system; each dipole moment was located on the long axis of each stacked π‐electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).     

给、吸电子基团对吡嗪衍生物电子结构影响的DFT研究

采用B3LYP方法在6—31G^#基组水平上优化了对位取代吡嗪衍生物的几何构型，利用TD—DFT方法计算了它们的前线分子轨道能级和电子光谱．结果表明，带有给、吸电子基团对吡嗪衍生物与苯、吡啶相比，也具有很好的共轭性；随着分子共轭链的增长，分子的偶极矩增大，前线分子轨道能级差减小，最大吸收波长发生红移．对于具有相同共轭链的同分异构体，推电子基团与具有给电子性质的共轭链相连，则分子的电荷转移明显，导致偶极矩增大，前线分子轨道能级间的电子跃迁更容易；吸电子基团与具有给电子性质的共轭链相连，情况正好相反，这些结果对分子设计有重要意义。     

Three‐Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton–electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi‐step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric‐like arrangement.     

Investigation of organosilicon radicals : II. Methylsilyl radicals

CNDO/2 calculations for a series of Me_xH₃?_xSi (where x = 0,1,2,3) free radicals confirm the suggestion, based on electron spin resonance measurements and chemical evidence, that these radicals are not planar. The deviation from planarity decreases with increasing number of methyl groups, but the angle between the carbonsilicon bond and the plane of an assumed planar configuration is the same for methylsilyl, dimethylsilyl, and trimethylsilyl radicals.     

Aromatic Radical Anions in Neat Aromatic Hydrocarbons as Solvents. Direct evidence of through space spin-density transfer to the ligand of the counter ion

Contact ion pairs of aromatic radical anions, with a crown ether complex of potassium as cation in a neat aromatic hydrocarbon, can be obtained by reducing the aromatic hydrocarbon in which a small amount of crown ether is dissolved. The unpaired electron stays attached to one aromatic molecule during a time interval which is long on the ESR. time scale. The radicals are stabilized by ion-pair formation in the low polarity solvent. As a consequence of this stabilization, radicals of compounds with low electron affinities, e.g. mesitylene, can be prepared. Mesitylene, m-xylene, and toluene show additional hyperfine splitting in the ESR. spectra of their anion radical pairs of the order of 18 μT. The proton ENDOR. spectra have signals at the corresponding frequencies, indicating a hyperfine coupling with protons of the crown ether ligand. Using mixtures of two aromatic compounds, their relative electron affinities can be determined by studying the temperature dependence of the radical concentrations.     

The effect of axial Mg ligation on the geometry and spin density distribution of chlorophyll and bacteriochlorophyll cation free radical models: a density functional study.

Density functional calculations are performed on models of chlorophyll and bacteriochlorophyll to examine the effect of Mg ligation on the geometry and spin density distribution of the cation free radicals formed. It is shown that, whereas the properties of the bacteriochlorophyll model can be explained on the basis of the electron density distribution of the highest occupied molecular orbital (HOMO), for the chlorophyll model the geometry and spin density properties of the ligated species do not follow this trend. For the ligated chlorophyll models it is shown that, due to the closeness in energy of the HOMO and HOMO-1 orbitals, a Jahn-Teller distortion occurs on one-electron oxidation, leading to an admixed hybrid orbital for the cation radical form. Orbital mixing is shown to lead to significant changes in the geometry and spin density distribution of the cation free radical formed. It is also shown that orbital mixing does not lead to an increase in the magnitude of the (14)N hyperfine couplings thereby invalidating reports in the literature which have dismissed mixed orbital states for the primary donor cation radicals of photosynthetic reaction centers based on this criterion.     

甲苯基重氮盐及其与冠醚络合物光解活泼自由基的ESR研究

本文以2,3,5,6-四甲基亚硝基苯(ND)和苯亚甲基叔丁基氮氧化物(PBN)作自由基捕捉剂研究了甲苯基重氮盐及共冠醚络合物光解的自由基历程。ESR结果表明光解可产生相应的甲苯基自由基,并能被ND或PBN所捕获。由共ESR谱可得到甲基取代苯环上的甲基质子通过超共轭作用产生的超精细裂分,共裂分值与苯环上相同位置质子与未偶电子间极化偶合作用引起的裂分值相近。即:α_(p-)~H≈α_(P-)~(CH):α_(o-)~H≈α_(o-)~(CH)3;α_(m-)~H≈α_(m-)~(CH)3.同时还存在下列关系式:α_(p-)~H≈α_(o-)~H>α_(m-)~H.     

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.     

Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.     

Magnetic interactions in supramolecular N-O...H-C[triple bond]C- type hydrogen-bonded nitronylnitroxide radical chains

Two paramagnetic building blocks, 2-(4-ethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (3) and 2-(5-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (4) were synthesized and crystallized. Single crystal X-ray studies of 3 and 4 show the formation of supramolecular head-to-tail one-dimensional H-bonded (N-O...H-C[triple bond]C- type) chain structures with O...C distances of 3.181 and 3.155 A, respectively. High-resolution isotropic liquid state (c相似文献   

Intrinsic minimal atomic basis representation of molecular electronic wavefunctions

The problem of finding an effective minimal atomic basis that spans the exact occupied wavefunctions of a mean‐field theory at a given molecular geometry, which has a number of special properties, is studied and a new general procedure is developed that (1) solves for a raw minimal set of strongly atom‐centered functions—products of spherical harmonics and molecule‐optimized radial parts—that approximately span the occupied molecular wavefunctions and minimize the sum of their energies, (2) uses projection operators to get a new set of deformed atom‐centered functions that exactly span the occupied space and fall into core and valence subsets, (3) applies a new zero‐bond‐dipole orthogonalization scheme to the core‐orthogonalized valence subset so that for each two‐center product of these functions the projection of its dipole moment along the line going through the two centers is zero. The resulting effective minimal atomic basis is intrinsic to the molecular problem and does not need a free‐atoms input. Some interesting features of the zero‐bond‐dipole orthogonalization are showing up in the atomic population analysis of a diverse set of molecules. The new procedure may be useful for the interpretation of electronic structure, for the construction of model Hamiltonians in terms of transferable molecular integrals, and for the definition of active valence space in the treatment of electron correlation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

NMR spin–spin coupling constants in hydrogen‐bonded glycine clusters

The influence of the hydrogen bond formation on the NMR spin–spin coupling constants (SSCC), including the Fermi contact (FC), the diamagnetic spin‐orbit, the paramagnetic spin‐orbit, and the spin dipole term, has been investigated systematically for the homogeneous glycine cluster, in gas phase, containing up to three monomers. The one‐bond and two‐bond SSCCs for several intramolecular (through covalent bond) and intermolecular (across the hydrogen‐bond) atomic pairs are calculated employing the density functional theory with B3LYP and KT3 functionals and different types of extended basis sets. The ab initio SOPPA(CCSD) is used as benchmark for the SSCCs of the glycine monomer. The hydrogen bonding is found to cause significant variations in the one‐bond SSCCs, mostly due to contribution from electronic interactions. However, the nature of variation depends on the type of oxygen atom (proton‐acceptor or proton‐donor) present in the interaction. Two‐bond intermolecular coupling constants vary more than the corresponding one‐bond constants when the size of the cluster increases. Among the four Ramsey terms that constitute the total SSCC, the FC term is the most dominant contributor followed by the paramagnetic spin‐orbit term in all one‐bond interaction.     

Isolable silyl and germyl radicals lacking conjugation with pi-bonds: synthesis,characterization, and reactivity

The one-electron oxidation reaction of tris[di-tert-butyl(methyl)silyl]silyl and -germyl anions with dichlorogermylene-dioxane complex results in the formation of stable tris[di-tert-butyl(methyl)silyl]silyl and -germyl radicals 1 and 2, representing the first isolable radical species of heavier Group 14 elements lacking stabilization by conjugation with pi-bonds. The crystal structures of both silyl and germyl radicals 1 and 2 showed a completely planar geometry around the radical centers. The ESR spectra of 1 and 2 showed strong signals with characteristic satellites due to the coupling with the 29Si and 73Ge nuclei. The small values of the hyperfine coupling constants a(29Si) and a(73Ge) clearly indicate the pi-character of both radicals, corresponding to a planar geometry and sp2 hybridization of the radical centers. Both 1 and 2 easily undergo halogenation reactions with carbon tetrachloride, 1,2-dibromoethane, and benzyl bromide to form the corresponding halosilanes and halogermanes.     

Cooperativity in ionic liquids

Cooperativity in ionic liquids is investigated by means of static quantum chemical calculations. Larger clusters of the dimethylimidazolium cation paired with a chloride anion are calculated within density functional theory combined with gradient corrected functionals. Tests of the monomer unit show that density functional theory performs reasonably well. Linear chain and ring aggregates have been considered and geometries are found to be comparable with liquid phase structures. Cooperative effects occur when the total energy of the oligomer differs from a simple sum of monomer energies. Cooperative effects have been found in the structural motifs examined. A systematic study of linear chains of increasing length (up to nine monomer units) has shown that cooperativity plays a more important role than expected and is stronger than in water. The Cl...H distance of the chloride to the most acidic proton increases with an increasing number of monomer units. The average bond distance approaches 218.9 pm asymptotically. The dipole moment grows almost linearly and the dipole moment per monomer unit reaches the asymptotic value of 16.3 D. The charge on the chloride atoms decreases with an increasing chain length. In order to detect local hydrogen bonding in the clusters a new parametrization of the shared-electron number method is introduced. We find decreasing hydrogen bond energies with an increasing cluster size for both the first hydrogen bond to the most acidic proton and the average hydrogen bond.     

Dynamic nuclear polarization studies on deuterated nitroxyl spin probes

Detailed dynamic nuclear polarization and electron spin resonance studies were carried out for 3‐carbamoyl‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl, 3‐carboxy‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl,3‐methoxycarbonyl‐2,2,5,5‐tetramethy pyrolidine‐1‐oxyl nitroxyl radicals and their corresponding deuterated nitroxyl radicals, used in Overhauser‐enhanced magnetic resonance imaging for the first time. The dynamic nuclear polarization parameters such as dynamic nuclear polarization (DNP) factor, longitudinal relaxivity, saturation parameter, leakage factor and coupling factor were estimated for deuterated nitroxyl radicals. DNP enhancement increases with agent concentration up to 3 mm and decreases above 3 mm . The proton spin–lattice relaxation time and the longitudinal relaxivity parameters were estimated. The leakage factor increases with increasing agent concentration up to 3 mm and reaches plateau in the region 3–5 mm . The coupling parameter shows the interaction between the electron and nuclear spins to be mainly dipolar in origin. DNP spectrum exhibits that the full width at half maximum values are higher for undeuterated nitroxyl radicals compared with deuterated nitroxyl radicals, which leads to the increase in DNP enhancement. The ESR parameters such as, the line width, line shape, signal intensity ratio, rotational correlation time, hyperfine coupling constant and g‐factor were calculated. The narrow line width was observed for deuterated nitroxyl radicals compared with undeuterated nitroxyl radicals, which leads to the higher saturation parameter value and DNP enhancement. The novelty of the work permits clear understanding of the DNP parameters determining the higher DNP enhancement compared with the undeuterated nitroxyl radicals. Copyright © 2017 John Wiley & Sons, Ltd.     

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton–electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.