Communication: stabilization of radical anions with weakly bound electron in condensed media: a case study of diacetonyl radical anion

The radical anion resulting from electron capture by diacetonyl molecule has been characterized by EPR and optical absorption spectroscopy in glassy ether matrices at 77 K. In non-polar alkane glasses this species was not observed under the same conditions, which confirms the crucial role of matrix interactions in stabilizing this species. Calculations at the MP2 level show the vertical detachment energy to increase gradually from roughly zero for a bare anion to ～1 eV for the complex involving six ether molecules.     

Odd carbon long linear chains HC2n+1H (n = 4-11): properties of the neutrals and radical anions

The optimized geometries, adiabatic electron affinities, vertical electron affinities, vertical electron detachment energies (for the anions), and IR-active vibrational frequencies have been predicted for the long linear carbon chains HC(2)(n)()(+1)H (n = 4-11). The B3LYP density functional combined with DZP and TZ2P basis sets was used in this theoretical study. These methods have been extensively calibrated versus experiment for the prediction of electron affinities (Chem. Rev. 2002, 102, 231). The computed physical properties are discussed and compared with the even carbon chains HC(2)(n)()H. The predicted electron affinities form a remarkably regular sequence: 2.12 eV (HC(9)H), 2.42 eV (HC(11)H), 2.66 eV (HC(13)H), 2.85 eV (HC(15)H), 3.01 eV (HC(17)H), 3.14 eV (HC(19)H), 3.25 eV (HC(21)H), and 3.35 eV (HC(23)H). These electron affinities are as much as 0.4 eV higher than those for analogous even carbon chains. The predicted structures display an intermediate cumulene-polyacetylene type of bonding, with the inner carbons appearing cumulenic and the outer carbons polyacetylenic.     

Benzoylnitrene radical anion: A new reagent for the generation of M-2H anions

Benzoylnitrene radical anion, formed in high abundance by electron ionization of benzoylazide, is found to be a useful reagent for the formation of ionized reactive intermediates, such as diradicals and carbenes. The reactivity of the ion is similar to what is observed with atomic oxygen anion, occurring in many instances by H(2)(+) transfer. However, because benzoylnitrene radical anion is less basic than oxygen anion, it undergoes H(2)(+) transfer with substrates that react with oxygen anion by only proton transfer and therefore can be used to generate reactive ions not easily prepared by other methodologies.     

Valence anions of N-acetylproline in the gas phase: computational and anion photoelectron spectroscopic studies

We report the photoelectron spectrum of anionic N-acetylproline, (N-AcPro)(-), measured with 3.49 eV photons. This spectrum, which consists of a band centered at an electron binding energy of 1.4 eV and a higher energy spectral tail, confirms that N-acetylproline forms a valence anion in the gas phase. The neutrals and anions of N-AcPro were also studied computationally at the B3LYP∕6-31++G(d,p) level. Based on the calculations, we conclude that the photoelectron spectrum is due to anions which originated from proton transfer induced by electron attachment to the π* orbital localized at the acetyl group of N-AcPro. We also characterized the energetics of reaction paths leading to pyrrolidine ring opening in the anionic N-AcPro. These data suggest that electron induced decomposition of peptides/proteins comprising proline strongly depends on the presence of proton donors in the close vicinity to the proline residue.     

The electronic structure of all-trans-polyenes C2nH2n+2, n = 3–7

The electronic structure of the C_2nH_2n+2 trans-polyenes, n = 3–7, is calculated by the Discrete Variational X_α method (DVM -X_α). The valence ionization potentials (IP ) calculated using the Clementi double zeta basis agree with the known experimental data within several tenths an electron volt. However, the DVM energies of the π → π* optical excitations are systematically underestimated by 0.8–1.0 eV. For polyenes with equal C—C bond lengths, the computed energies of the first optical transitions are smaller than those of polyenes with alternating C—C bond lengths. The charge distribution in polyenes is analyzed in the framework of a Mulliken scheme. The composition of the frontier molecular orbitals (MO ) is analyzed.     

Microsolvation pattern of the hydrated radical anion of uracil: U-(H2O)n (n = 3-5)

The microsolvation patterns of the uracil radical anion in water clusters U-(H2O)n with n ranging from 3 to 5 were investigated by the density functional theory approach. The electron detachment energies (VDE) of the stable anionic complexes with different numbers of hydration water are predicted. The linear dependence of the VDE value of the most stable anionic complexes with respect to the hydration number suggests the importance of the clustered waters in the microsolvation of the radical anion of the nucleobases. The formation of the water clusters is found to be necessary in the most stable conformers of the tri-, tetra-, and pentahydrated radical anion of uracil. The microsolvation pattern with three or more well-separated hydration water molecules in the first hydration layer is less stable than the arrangement with the waters in tight clusters. The charge transfer between the anionic uracil and the hydration water is high. Good agreement between the experimental and the theoretical vertical detachment energy yield in this study further demonstrates the practicability of the B3LYP/DZP++ approach in the study of radical anions of the DNA subunits.     

Structures of tetrafluorocyclopropene, hexafluorocyclobutene, octafluorocyclopentene and related perfluoroalkene radical anions revealed by electron spin resonance spectroscopic and computational studies

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C(4)F(6)(-)), octafluorocyclopentene (c-C(5)F(8)(-)) and perfluoro-2-butene (CF(3)CF=CFCF(3)(-)) in gamma-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d(12) matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C(4)F(6)(-) and c-C(5)F(8)(-) are characterized by three different sets of pairs of (19)F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C(4)F(6)(-) and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C(5)F(8)(-). By comparison with the results of ab initio quantum chemical computations, the large triplet (19)F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the C=C bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C(3)F(4)(-): C(2) symmetry ((2)A state) <-- C(2)(v) ((1)A(1)), c-C(4)F(6)(-): C(1) ((2)A) <-- C(2)(v) ((1)A(1)) and c-C(5)F(8)(-): C(1) ((2)A) <-- C(s) ((1)A'). The structural distortion arises from a mixing of the pi* and higher-lying sigma* orbitals at the C=C carbons similar to that previously found for CF(2)=CF(2)(-) with a C(2)(h) distortion. The isotropic (19)F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C(4)F(6)(-), c-C(5)F(8)(-) and CF(2)=CF(2)(-) were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of (19)F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF(2)=CF(2)(-), c-C(3)F(4)(-), c-C(4)F(6)(-) and c-C(5)F(8)(-) radical anions were computed for the first time by TD-DFT methods.     

Effect of acidity on microstructure and spectroscopic properties of Y2O3:Eu3+ powders and ceramics

Y₂O₃:Eu³⁺ nano-powders were synthesized by homogeneous precipitation and the influence of solution acidity was investigated. IR and TG–DTA analysis showed that in different acidic solutions different kinds of sediments were generated. TEM images revealed distinct morphologies of obtained powders. The resultant powders were sintered into ceramics in vacuum and then in N₂ atmosphere without any additives. SEM images indicated that the morphologies of powders have significant impact on the microstructures of sintering-derived ceramics. Low-agglomerated and uniform powders were in favor of production of ceramics with homogeneous microstructures. Excitation and emission spectra of both powders and ceramics were measured and some changes were observed.     

Selective intercalation of Cs+ in the "V"-shaped cavity of a bichromophoric anion radical: Cs+ assisted pi-s-pi-delocalization of an electron

EPR studies in tetrahydrofuran, reveal that the one electron reduction of 1-(9-methyl-9H-fluoren-9-yl)-4-methylbenzene via electron transfer from cesium metal produces an anion radical that has a large affinity for the cesium cation. The affinity of this anion radical for Cs+ is so great that it will actually "suck" the Cs+ (but not Na+ or K+) right out of the grasp of 18-crown-6, leading to a cation-assisted pi-stacked complex, where the s-orbital of the metal cation is simultaneously overlapped with the pi-clouds of the phenyl and fluorenyl moieties. At ambient temperature, proton- and cesium-electron coupling constants are rapidly (on the EPR time scale) modulated as a result of the simultaneous existence of two interconverting conformers having an averaged cesium splitting (a(Cs)) of about 1.6 G. The pi-s-pi-electronic coupling can be turned on or off via the addition or removal of cesium cations. Analogous pi-s-pi-electronic coupling is observed in the 1,4-bis(9-methyl-9H-fluoren-9-yl)benzene-cesium system.     

Microhydration of cytosine and its radical anion: cytosine.(H2O)n (n=1-5)

Microhydration effects on cytosine and its radical anion have been investigated theoretically, by explicitly considering various structures of cytosine complexes with up to five water molecules. Each successive water molecule (through n=5) is bound by 7-10 kcal mol(-1) to the relevant cytosine complex. The hydration energies are uniformly higher for the analogous anion systems. While the predicted vertical detachment energy (VDE) of the isolated cytosine is only 0.48 eV, it is predicted to increase to 1.27 eV for the lowest-lying pentahydrate of cytosine. The adiabatic electron affinity (AEA) of cytosine was also found to increase from 0.03 to 0.61 eV for the pentahydrate, implying that the cytosine anion, while questionable in the gas phase, is bound in aqueous solution. Both the VDE and AEA values for cytosine are smaller than those of uracil and thymine for a given hydration number. These results are in qualitative agreement with available experimental results from photodetachment-photoelectron spectroscopy studies of Schiedt et al. [Chem. Phys. 239, 511 (1998)].     

The 2'-deoxyadenosine-5'-phosphate anion, the analogous radical, and the different hydrogen-abstracted radical anions: molecular structures and effects on DNA damage

The 2'-deoxyadenosine-5'-phosphate (5'-dAMP) anion and its related radicals have been studied by reliably calibrated theoretical approaches. This study reveals important physical characteristics of 5'-dAMP radical related processes. One-electron oxidation of the 5'-dAMP anion is found on both the phosphoryl group and the adenine base with electron detachment energies close to that of phosphate. Partial removal of electron density from the adenine fragment leads to an extended pi system which includes the amine group of the adenine. Although the radical-centered carbon increases the extent of bonding with its adjacent atoms, it usually weakens the chemical bonds between the atoms at the alpha- and beta-positions. This tendency should be important in predicting the reactivity of the sugar-based radicals. The overall stability sequence of the H-abstracted 5'-dAMP anionic radicals is consistent with the analogous results for the H-abstracted neutral radicals of the adenosine nucleoside: aliphatic radicals > aromatic radicals. The negatively charged phosphoryl group attached to atom C(5)' of the ribose does not change this energetic sequence. All the H-abstraction produced 5'-dAMP radical anions are distonic radical anions. Studies have shown that the charge-radical-separating feature of the distonic radical anions is biologically relevant. This result should be important in understanding the reactive properties of these H-abstraction-produced anion radicals.     

Asymmetrically solvated anion with both kinetic and thermodynamic stabilities: Theoretical studies on the cluster anions (HF)n - (n=3-6)

At the level of MP2 with the aug-cc-pVDZ and aug-cc-pVTZ basis sets supplemented with diffuse bond functions, the authors searched the potential energy surfaces of (HF)(n) (-) (n=3-6). In accordance with the literature, they found that the symmetrically solvated-electron anion (3(FH){e}) possesses the largest vertical detachment energy (VDE), while the dipole-bound anion ((FH)(3){e}) is the lowest isomer in energy for (HF)(3) (-). Their calculations demonstrated that, with the increase of the cluster size, the asymmetric (FH)(a){e}(HF)(b) cluster is stabilized with a simultaneously increased VDE. Thus they predicted that, for (HF)(6) (-), the (FH)(4){e}(HF)(2) cluster is both kinetically and thermodynamically most stable, possessing the largest VDE and being the global minimum at the same time.     

Reactions of O+ with CnH2n+2, n=2-4: a guided-ion beam study

We have measured absolute reaction cross sections for the interaction of O(+) with ethane, propane, and n-butane at collision energies in the range from near thermal to approximately 20 eV, using the guided-ion beam (GIB) technique. We have also measured product recoil velocity distributions using the GIB time-of-flight (TOF) technique for several product ions at a series of collision energies. The total cross sections for each alkane are in excess of 100 A(2) at energies below approximately 2 eV, and in each case several ionic products arise. The large cross sections suggest reactions that are dominated by large impact parameter collisions, as is consistent with a scenario in which the many products derive from a near-resonant, dissociative charge-transfer process that leads to several fragmentation pathways. The recoil velocities, which indicate product ions with largely thermal velocity distributions, support this picture. Several product ions, most notably the C(2)H(3) (+) fragment for each of the alkanes, exhibit enhanced reaction efficiency as collision energy increases, which can be largely attributed to endothermic channels within the dissociative charge-transfer mechanism.     

Theoretical study of spectroscopic constants and molecular properties of diatomic anions using B3LYP method

Structure, spectroscopic constants and molecular properties of selected diatomic anions in their ground states have been studied in detail using HF/DF B3LYP method. The consistency of the calculated values of spectroscopic constants and molecular properties has been tested using four basis sets with improved quality. The spectroscopic constants and molecular properties of these diatomic ions agree well with the experimental and theoretical values wherever available. Most of the spectroscopic constants and molecular properties of these ions, in particular the spectroscopic constants of SiO⁻, CS⁻ and the molecular properties of SiN⁻, CP⁻, SiO⁻ are first reported.     

Oxidative properties of FeO2+: electronic structure and solvation effects

An electronic structure analysis is provided of the action of solvated FeO(2+), [FeO(H(2)O)(5)](2+), as a hydroxylation catalyst. It is emphasized that the oxo end of FeO(2+) does not form hydrogen bonds (as electron donor and H-bond acceptor) with H-bond donors nor with aliphatic C-H bonds, but it activates C-H bonds as an electron acceptor. It is extremely electrophilic, to the extent that it can activate even such poor electron donors as aliphatic C-H bonds, the C-H bond orbital acting as electron donor in a charge transfer type of interaction. Lower lying O-H bonding orbitals are less easily activated. The primary electron accepting orbital in a water environment is the 3sigma*alpha orbital, an antibonding combination of Fe-3d(z(2)) and O-2p(z), which is very low-lying relative to the pi*alpha compared with, for example, the sigma* orbital in O(2) relative to its pi*. This is ascribed to relatively small Fe-3d(z(2)) with O-2p(z) overlap, due to the nodal structure of the 3d(z(2)).The H-abstraction barrier is very low in the gas phase, but it is considerably enhanced in water solvent. This is shown to be due to strong screening effects of the dielectric medium, leading to relative destabilization of the levels of the charged [FeO(H(2)O)(5)](2+) species compared to those of the neutral substrate molecules, making it a less effective electron acceptor. The solvent directly affects the orbital interactions responsible for the catalytic reaction.     

Study on structures and electron affinities of small potassium–silicon clusters Si n K (n = 2–8) and their anions with Gaussian-3 theory

The neutral Si _n K (n = 2–8) clusters and their anions have been systematically studied by means of the higher level of Gaussian-3 schemes. Equilibrium geometries and electron affinities have been calculated and are discussed for each considered size. For neutral Si _n K clusters, the ground state structure is found to be “attaching structure”, in which the K atom is bound to Si _n clusters. The most stable isomer for their anions, however, is found to be “substitutional structures”, which is derived from Si_(n+1) by replacing the Si atom with a K. The dissociation energies of K atom from the lowest energy structures of Si _n K have also been estimated to examine relative stabilities.     

2,3,5,6-Tetrafluorophenylnitren-4-yl: electron paramagnetic resonance spectroscopic characterization of a quartet-ground-state nitreno radical

2,3,5,6-Tetrafluorophenylnitren-4-yl (5) was synthesized in argon at 4 K via the photolysis of 2,3,5,6-tetrafluoro-4-iodo-phenyl azide (6). Electron paramagnetic resonance (EPR) spectroscopy allows us to observe triradical 5 in its quartet state with the zero-field splitting (ZFS) parameters |D/hc| = 0.285 and |E/hc| = 0.043 cm-1. The quartet ground state of 5 is in accordance with our previous infrared (IR) spectroscopic investigation, in which the high-spin quartet state, but no low-spin doublet state, of 5 was observed in solid argon at 4 K [Wenk, H. H.; Sander, W. Angew. Chem., Int. Ed. 2002, 41, 2742-2745]. Because annealing of the matrix at temperatures of >10 K results in the rapid recombination of the highly reactive species 5 with I atoms produced during the photolysis of 6, the Curie-Weiss behavior could not be investigated. However, the absence of low-spin states in the IR investigations, as well as the results of ab initio and density functional theory (DFT) calculations, strongly suggest that 5 has a robust quartet ground state that is best-described as an unprecedented sigma,sigma,pi-triradical. The ZFS of 5 has been successfully reproduced by DFT calculations, which furthermore provide qualitative insight into the origin of the observed EPR parameters.     

四氯合锌酸烷基铵层形结构配合物的Raman光谱研究

测定了[n-CnH2n+1NH3]2ZnCl(n=7～12, 16)配合物在碳氢伸缩、碳氢弯曲、碳碳伸缩和低频区的Raman光谱。发现它们的频率和相对强度与分子中烷基链的链长有关, 并且表现出碳原子数的奇偶效应。从不同的分子堆积结构和分子间相互作用的角度讨论了光谱变化。