Structure and Reactivity of the Distonic and Aromatic Radical Cations of Tryptophan

In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations—a distonic indolyl N-radical (H₃N⁺ - TrpN^?) and a canonical aromatic π (Trp^?+) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary [Cu^II(terpy)(Trp)] ^?2+ complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000–1800 cm^–1 region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.      

Formation of the Charge-Localized Dimer Radical Cation of 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA)₂^.+ is investigated by transient absorption (TA) and time-resolved resonance Raman (TR³) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA)₂^.+ by association of DMA and DMA^.+. TR³ spectra obtained from 30 ns to 300 μs time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of non-covalent (DMA)₂^.+ is localized in a single monomer unit. From DFT calculations for (DMA)₂^.+, our TR³ spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA)₂^.+, termed DT^.+, although the charge-delocalized asymmetric π-stacked face-to-face (DMA)₂^.+, termed DF3^.+, is the most stable structure of (DMA)₂^.+ according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA^.+ and DMA showed that DT^.+ is likely to be efficiently formed and contribute significantly to the TR³ spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.     

Nonempirical Study of Structure of Methylnitroxyl Radical in Gas Phase and Solution

Using quantumchemical calculation by nonrestricted Hartree-Fock method in the bases up to 6-311+G^{* *} we established that induced solvent electrostatic field causes flattening of the nitroxyl fragment of MeN(O')H radical, shifts electron density to its oxygen atom and spin density to opposite direction, increases dopole moment, decreases N-O bond length, averages spin population at nitrogen and oxygen atoms.     

正丁醇作溶剂顺酐在Cu-Zn-Ti催化剂上气相加氢制取四氢呋喃

采用共沉淀法制备Cu-Zn-Ti系列催化剂。利用X射线衍射(XRD)、X射线光电子能谱(XPS)、程序升温还原(TPR)、N2吸附等测试技术对催化剂进行了表征。在反应压力1MPa,反应温度235~280℃的条件下,进行顺酐气相加氢活性测试。顺酐转化率高于96%,四氢呋喃选择性高达92.7%。同时正丁醇发生脱氢-歧化-酯化反应,可提供顺酐加氢耗氢量15%~40%的H2气。受到ZnO和TiO2协同作用影响的CuO还原后生成的Cu0具有较高的顺酐深度气相加氢活性。Cu-Zn-Ti系列催化剂的深度加氢能力基本上随Cu0晶格畸变率的升高而增强。     

Generation and Characterization of a Distonic Biradical Anion Formed from an Enediynone Prodrug in the Gas Phase

A negatively charged biradical intermediate was successfully generated in the gas phase via cyclization of the deprotonated bicyclo[8.3.0]trideca-12-ene-2,7-diyn-1-one precursor. The inherent negative charge of this biradical allows its characterization via collision-activated dissociation and reactions with a variety of neutral substrates in an FT-ICR mass spectrometer. Although the biradical is unreactive toward reagents that usually react rapidly with positively charged biradicals, such as dimethyl disulfide, it reacts with the halogen-containing substrates carbon tetrachloride, carbon tetrabromide, and bromotrichloromethane via bromine or chlorine atom abstraction, which supports its biradical structure. The results presented in this study indicate that cyclizations commonly used in solution to form biradical intermediates from enediyne compounds may also occur in the gas phase.      

Selective Oxidative Decomposition of Pyridine on a Copper-Containing Zeolite in the Gas Phase

Copper-containing zeolites of mordenite, pentasil, and faujasite types selectively catalyze the oxidation of pyridine in the gas phase to CO₂, H₂O, and N₂. The cation-decationated CuHY samples, prepared by solid state ion exchange, were optimal (most active and selective) among the catalysts tested with respect to formation of the products named under adsorption-catalytic conditions of gas purification.     

Comparative Study of the Reactivity of Iodinating Agents in Solution and Solid Phase

The results of reactions of a series of aromatic substrates with iodine, iodine(I) chloride, and N-iodosuccinimide in solution and solid phase were compared for the first time. In all cases, the general relations holding in the iodination process were similar. Iodine(I) chloride was found to chlorinate anthracene. A high efficiency of solid-phase iodination of β-diketones was demonstrated using dibenzoylmethane as an example.__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 6, 2005, pp. 876–880.Original Russian Text Copyright © 2005 by Krasnokutskaya, Lesina, Gorlushko, Filimonov.     

Reactivity of Oxygen Radical Anions Bound to Scandia Nanoparticles in the Gas Phase: CH Bond Activation

The activation of C?H bonds in alkanes is currently a hot research topic in chemistry. The atomic oxygen radical anion (O^?.) is an important species in C?H activation. The mechanistic details of C?H activation by O^?. radicals can be well understood by studying the reactions between O^?. containing transition metal oxide clusters and alkanes. Here the reactivity of scandium oxide cluster anions toward n‐butane was studied by using a high‐resolution time‐of‐flight mass spectrometer coupled with a fast flow reactor. Hydrogen atom abstraction (HAA) from n‐butane by (Sc₂O₃)_NO^? (N=1–18) clusters was observed. The reactivity of (Sc₂O₃)_NO^? (N=1–18) clusters is significantly sizedependent and the highest reactivity was observed for N=4 (Sc₈O₁₃^?) and 12 (Sc₂₄O₃₇^?). Larger (Sc₂O₃)_NO^? clusters generally have higher reactivity than the smaller ones. Density functional theory calculations were performed to interpret the reactivity of (Sc₂O₃)_NO^? (N=1–5) clusters, which were found to contain the O^?. radicals as the active sites. The local charge environment around the O^?. radicals was demonstrated to control the experimentally observed size‐dependent reactivity. This work is among the first to report HAA reactivity of cluster anions with dimensions up to nanosize toward alkane molecules. The anionic O^?. containing scandium oxide clusters are found to be more reactive than the corresponding cationic ones in the C?H bond activation.     

Stable Radical Cation and Dication of an N-Heterocyclic Carbene Stabilized Digallene: Synthesis,Characterization and Reactivity

One- and two-electron oxidation of a digallene stabilized by an N-heterocyclic carbene afforded the first stable gallium-based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃SnH and cyclo-S₈ was studied; a digallium–hydride cation salt containing a Ga−Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder-like Ga₄S₄ core structure were obtained, respectively.