Structure and charge distribution of some alkynoyl cations

We have performedab initio calculations to determine the structure and charge distribution of some alkynoyl cations and their parent alkynoyl fluorides. We have used Mulliken population analysis and a new technique developed by Yáñez, Stewart and Pople. Our results indicate that the mesomeric form O⁺C–CC–R is one of the most important contributors to the structure of these cations, in agreement with experimental conclusions. We have also found that the participation of mesomeric form O=C=C=C⁺ -R is not negligible and increases with -substitution. In the 3-phenylpropynoyl cation substantial delocalization of charge into the phenyl group occurs. Calculations from YSP population analysis are in good agreement with experimental evidence.     

A Room-Temperature Stable Distonic Radical Cation

Distonic radical cations (DRCs) with spatially separated charge and radical sites have, so far, largely been observed by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Herein, we disclose the isolation of a crystalline dicarbondiphosphide-based β-distonic radical cation salt 3^.+ (BARF) (BARF=[B(3,5-(CF₃)₂C₆H₃)₄)]⁻) stable at room temperature and formed by a one-electron-oxidation-induced intramolecular skeletal rearrangement reaction. Such a species has been validated by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray diffraction, UV/Vis spectroscopy and density functional theory (DFT) calculations. Compound 3^.+ (BARF) exhibits a large majority of spin density at a two-coordinate phosphorus atom (0.74 a.u.) and a cationic charge located predominantly at the four-coordinate phosphorus atom (1.53 a.u.), which are separated by one carbon atom. This species represents an isolable entity of a phosphorus radical cation that is the closest to a genuine phosphorus DRC to date.     

Pulsradiolytische Untersuchungen der Reaktion des e aq − mit halogenierten aromatischen Verbindungen

Kinetic pulse radiolysis experiments (1 s pulse duration) on the reaction of e _aq ^– with halogenated aromatic compounds (fluoro-, chloro-, bromobenzene, benzylchloride and phenethylchloride were carried out in order to check the existence and to investigate the fate of an electron adduct. The measured absorption spectra, being identical with those previously observed for the phenyl and benzyl radical, and thepH dependence of the formation of the H-adducts, indicating no protonation of an intermediateRX ^–, can be explained simply by quantitative elimination of the halide. No evidence for the existence of an electron adduct as an intermediate could be obtained under the applied experimental conditions.

     

ESR and quantum chemical studies of the structures and thermal transformations of the radical cations of vinylcyclopropane in irradiated frozen Freon matrices. Simulation of radical processes in solids

Thermal transformations of vinylcyclopropane radical cations (VCP^.+) in X-ray-irradiated frozen Freon matrices (CFCl₂CF₂Cl and CFCl₃) were studied by ESR; radical processes involving VCP^.+ in solid VCP were simulated.Gauche- andanti-VCP ^.+ were found to be the primary radical cations, however, the former, unlike the latter, is stable only under gas-phase conditions. The thermodynamic equilibrium betweenanti-VCP^.+ and its less stable distonic form,dist(90,0)-C ₅H₈ ^.+, is established in frozen Freon matrices and the VCP host matrix; the structure of dist(90,0)****-C ₅H₈ ^.+ is stabilized by a molecule ofanti-VCP. In CFC₃, along with dist(90,0)-C₅H₈ ^.+,-dimeric resonance [anti-VCP]₂ ^.+ complex was detected. A general scheme of the transformations of VCP^.+ in the solid phase has been proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 11–21, January, 1994.     

The role of oxygen acidity on the side-chain fragmentation of ring methoxylated benzocycloalkenol radical cations

The reactivity of 2,2-dimethyl-5-methoxyindan-1-ol (1) and 2,2-dimethyl-6-methoxytetral-1-ol (2) radical cations has been studied both in acidic and basic solution. At pH≤4 both 1⁺ and 2⁺ undergo C_αH deprotonation as the exclusive reaction with k=4.6×10⁴ and 3.2×10⁴ s⁻¹, respectively. In basic solution 1⁺ and 2⁺ behave as oxygen acids undergoing ⁻OH-induced αOH deprotonation in a diffusion controlled process (k_−OH≈10¹⁰ M⁻¹ s⁻¹). An intermediate alkoxyl radical is formed which undergoes a 1,2-hydrogen atom shift in competition with CC β-scission (with 1⁺) or as the exclusive pathway (with 2⁺). A behavior which is interpreted in terms of the greater ease of ring-opening of a five membered ring as compared to a six-membered one.     

Direct ESR detection and spin trapping of radicals generated by reaction of oxygen radicals with sulfonated poly(ether ether ketone) (SPEEK) membranes

The stability of membranes under the strong oxidizing conditions in fuel cells is one of the major challenges in the development of fuel cells based on proton exchange membranes (PEMs). This study is centered on the determination of the susceptibility to degradation of SPEEK membranes exposed to OH radicals, using both direct ESR and spin trapping with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). In order to achieve a complete picture on SPEEK degradation, two types of experiments were performed: 1. UV irradiation at 77 K of SPEEK membranes swollen by aqueous solutions of H₂O₂; 2. UV irradiation of SPEEK membranes swollen by aqueous solutions of H₂O₂ in the presence of DMPO as a spin trap. UV irradiation without oxygen of SPEEK at 77 K in acid or basic form in the presence of H₂O₂/H₂O produced phenoxyl radicals as the predominant radicals detected by direct ESR or spin trapping methods. At pH 4, the oxygen radicals produced phenyl radicals as the predominant species detected by spin trapping methods. The hydroperoxyl radical, as DMPO/OOH adduct, was detected only when the DMPO/OH adduct was absent. The appearance of phenyl and phenoxyl radicals provides the evidence that OH radicals react with the aromatic ring of SPEEK or leading to the scission of its ether bridge.     

The first observation in an organic medium and DFT calculation of the TMM radical anion generated via a single electron reduction of a methylenecyclopropane

γ-Irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed 2-methyltetrahydrofuran glassy matrix at 77 K gave an intense UV/vis absorption band with λ_ab at 496 nm. This result and calculations based on density functional theory for its radical anion 3⁻ and the corresponding trimethylenemethane radical anion (2⁻) strongly suggest that single electron reduction of 3 followed by ready ring opening affords 2⁻, whose molecular geometry is largely twisted (θ = 45.5°), and the negative charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

Matrix isolation and DFT calculations of the TMM radical cation generated via the single electron oxidation of a methylenecyclopropane

A γ-irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed n-butyl chloride glassy matrix at 77 K produced an intense UV/vis absorption band with λ_ab at 432 nm. This result and calculations based on density functional theory for its radical cation 3⁺ and the corresponding trimethylenemethane radical cation (2⁺) strongly suggest that a single electron oxidation of 3 followed by ready ring opening affords 2⁺, whose molecular geometry is largely twisted (θ = 44.0°), and the positive charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

The very low-pressure pyrolysis of phenyl ethyl ether,phenyl allyl ether,and benzyl methyl ether and the enthalpy of formation of the phenoxy radical

A value of the enthalpy of formation of the phenoxy radical in the gas phase, ΔH°_,298K (?O·, g) = 11.4 ± 2.0 kcal/mol, has been obtained from the kinetic study of the unimolecular decompositions of phenyl ethyl ether, phenyl allyl ether, and benzyl methyl ether

1 Trivial names for ethoxy benzene, 2-propenoxy (allyloxy) benzene, and α-methoxytoluene, respectively

at very low pressures. Bond fission, producing phenoxy or benzyl radicals, respectively, is the only mode of decomposition in each case. The present value leads to a bond dissociation energy BDE(?O—H) = 86.5 ± 2 kcal/mol,

2 1 kcal = 4.18674 kJ (absolute)

in good agreement with recent estimates made on the basis of competitive oxidation steps in the liquid phase. A comparison with bond dissociation energies of aliphatic alcohols, BDE(RO—H) = 104 kcal/mol, reveals that the stabilization energy of the phenoxy radical (17.5 kcal/mol) is considerably greater than the one observed for the isoelectronic benzyl radical (13.2 kcal/mol). Decomposition of phenoxy radicals into cyclopentadienyl radicals and CO has been observed at temperatures above 1000°K, and a mechanism for this reaction is proposed.     

Generation, isolation, and reactivity of a kinetically stabilized diphosphene anion radical

The stable lithium diphosphene anion radical, [Li(dme)₃]⁺[TbtPPTbt]⁻ (dme: 1,2-dimethoxyethane, Tbt: 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), was readily synthesized by the one-electron reduction of the corresponding neutral diphosphene (TbtPPTbt). The molecular structure of the diphosphene anion radical was discussed in detail on the basis of its ESR, UV-Vis and Raman spectra, and theoretical calculations. The diphosphene anion radical was found to undergo ready chalcogenation reactions using elemental sulfur and selenium to afford the corresponding thiadiphosphirane and selenadiphosphirane, respectively.     

Artificial photosynthetic reaction centers with carotenoid antennas

Two reaction center-antenna models based on a purpurin macrocycle linked to a C₆₀ and to a carotenoid polyene have been synthesized. In these systems the C₆₀ moiety is the primary electron acceptor, the purpurin is the primary electron donor and the carotenoid moiety acts both as an antenna and secondary electron donor. Formation of the initial charge separated state, C-Pur⁺-C₆₀⁻, following excitation with light absorbed by either the purpurin or C₆₀ takes place on the 10 ps time scale. The final charge separated state, C⁺-Pur-C₆₀⁻, is formed in one of the compounds with a quantum yield of 32% based upon light absorbed by the carotenoid. In order to function as an antenna, the carotenoid pigment must be electronically coupled to the purpurin. The purpurin C ring provides an excellent framework for locating a carotenoid polyene in partial conjugation with the macrocycle, leading to a relatively strong electronic communication between the chromophores; functionalization of a meso position of the purpurin provides a site for the covalent attachment of C₆₀.     

C-S bond cleavage in the sensitized photooxygenation of tert-alkyl phenyl sulfides. The role of superoxide anion

The N-methylquinolinium tetrafluoroborate (NMQ⁺)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a-c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH₃CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O₂. By laser irradiation, the formation of sulfide radical cations 1a⁺-c⁺ in the monomeric form (λ_max=520 nm) and of 2⁺ in both the monomeric (λ_max=520 nm) and dimeric form (λ_max=780 nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C-S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ). A different situation was found in the presence of O₂ since steady-state photolysis produced substantial amounts of C-S bond cleavage products (alcohols, alkenes, and ketones from 1a-c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O₂⁻ generated by NMQ and O₂. For the tert-alkyl phenyl sulfides, 1a-c, these results have been interpreted by suggesting that C-S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O₂⁻. In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O₂⁻ to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O₂⁻ of 2ⁿ⁺.     

Nature of transient species formed during pulse radiolysis of 4-mercaptopyridine in aqueous solutions: Formation of a dimer radical species by one-electron reduction reaction

Reactions of one-electron reducing as well as oxidizing radicals with 4-mercaptopyridine (4-MPy) were studied in aqueous solutions at different pH values. One-electron oxidizing radicals such as N₃ and Br₂ ^–, react with 4-MPy by electron transfer reaction at pH 11 to give 4-pyridylthiyl radical. The reduction potential for the couple 4-PyS /4-PyS^– was estimated to be 0.93V vs. NHE by equilibrium reaction with I₂ ^– /2I^– couple. At pH 6.8, where the compound is predominantly present in the thione form, the transient species formed is a cation radical. OH radicals react with 4-MPy by addition to the pyridine ring at pH 6.8 and 11. At pH 0, OH radicals as well as one-electron oxidants like Cl₂ ^– and Br₂ ^– radicals react with 4-MPy to produce the protonated form of 4-pyridylthiyl radical. At pH 6.8 and 11, e_aq ^– reaction with 4-MPy gave an initial adducts which reacted with the parent molecule to give dimer radicals. Acetone ketyl radicals were unable to reduce 4-MPy at neutral pH. Reducing radicals like H-atoms and acetone ketyl radicals reacted with 4-MPy at acidic pH by H-abstraction reaction to give the same species as produced by oxidizing radicals.     

Synthesis and spectral properties of 2-methyl-5-aryl-1,3,4-oxadiazoles

By heterocyclization of 1-acyl-2-aroylhydrazines under the influence of strong dehydrating substances, a series of 2-methyl-5-aryl-1,3,4-oxadiazoles has been synthesized and their UV, IR, and PMR spectra have been investigated. Through analysis of data on the influence of substituents on the position of the maximum in the electronic absorption spectrum, together with calculated data, it has been shown that the long-wave band of 2-methyl-5-phenyl-1,3,4-oxadiazole is due to an S₀–S₁ transition of the –^* type and that it is a charge transfer band for transfer from the phenyl radical to the oxadiazole ring; the 1,3,4-oxadiazole ring as a substituent has an electron-acceptor character.Institute of Single Crystals, National Academy of Sciences of the Ukraine, Khar'kov 310001. Khar'kov State University, Khar'kov 310077. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 816–821, June, 1997.     

Structural requirements of Na+-dependent antidopaminergic agents: Tropapride,Piquindone, Zetidoline,and Metoclopramide Comparison with Na+-independent ligands

Summary Molecular graphic design coupled with PCILO and crystallographic results have been used to investigate the three-dimensional structure of Tropapride, Piquindone, Zetidoline, and Metoclopramide, four dopamine D-2 receptor antagonists showing Na⁺-dependent binding. Three putative pharmacophoric elements, a nitrogen lone pair, a phenyl ring and a carbonyl moiety, are similarly oriented in all the Na⁺-dependent drugs. Conversely, for Na⁺-independent analogs, the two latter pharmacophoric elements play a subordinate role, but two -electron regions are systematically localized on the other side of the molecule: the first is a phenyl group while the second is a carbonyl function as in butyrophenones, a cyano group as in R48455, or a phenyl ring as in diphenylbutylpiperidines or tricyclics. The presence of a benzyl ring on this side in Tropapride might explain its weak extrapyramidal effects.     

OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1⁺, 3⁺ and 4⁺, respectively) and the α-methyl derivatives 2⁺ and 5⁺ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1⁺-5⁺ under acidic and basic conditions. In acidic solution, the decay of 1⁺-5⁺ proceeds by cleavage of the C-H bond, while in the presence of ⁻OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The ⁻OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H₂O, appears to be affected by frontier orbital interactions.     

Crystal and molecular structure of a 1:1 complex of potassium perchlorotriphenylmethide and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane)

(C₆Cl₅)₃ C^– K⁺, C₁₂H₂₄O₆ is monoclinic, space groupP2₁ withZ=2,a=10.491(2),b=18.016(7),c=11.670(6) , =103.11(3)°,V=2148(1) ³, finalR=0.039 for 2339 observed reflections at room temperature. The overall shape of the perchlorotriphenyl free radical is given by the angles between the mean planes of each ring (around 70°). The K⁺ ion lies at the centre of the 18-membered macrocyclic ring. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. 82012 (19 pages).     

Determination of the hydroxyl radical by its adduct formation with phenol and liquid chromatography/electrochemical detection

An HPLC-ECD method is described for the indirect determination of the hydroxyl (OH) radical. Fenton's reaction is used to produce OH, which simultaneously attacks phenols (phenol or pyrocatechol) to form the adducts, pyrocatechol or pyrogallic acid. Thus, [OH] quantification is based on the separation and detection of pyrogallic acid and/or pyrocatechol by an isocratic HPLC-ECD method. The quantification of OH is also performed alternatively by a chronoamperometric detection in an electrochemical cell, where simultaneously formed Fe^III (Fenton's reaction) combines [Fe^II(CN)₆]⁴⁻ to produce the Prussian blue (PB) molecules (Fe₄^III[Fe^II(CN)₆]₃). Newly formed PB molecules are then immediately converted to colorless Everitts salt (K₄Fe₄^II[Fe^II(CN)₆]₃) with the reduction of the high-spin Fe^III to Fe^II at the surface of a glassy carbon electrode at +0.150 V (versus Ag/AgCl). The calculated concentration of OH during incubation (0.626 ppm) can be detected with negative errors by the HPLC-ECD (0.595 and 0.615 ppm with the errors −5.2 and −1.8%, respectively) and by the chronoamperometric method (0.552 and 0.607 ppm with the errors −11.8 and −3.0%, respectively). For the comparison of the two sets of data, HPLC-ECD method is much more promising.     

Tetrazole derivatives

It is shown that the frequencies of the long-wave band in the electronic spectra of tetrazolium salts correlate with Brown's electrophilic ⁺ constants. The color of the tetrazolium salts is due to a polar transition associated with transfer of charge from the phenyl ring attached to the N₃ atom to the central tetrazole ring.See [1] for Communication 23.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 111–114, January, 1980.