ESR.-Messungen an Radikal-Anionen Trimethylsilyl-substituierter π-Elektronensysteme

The ESR spectra of the radical anions of several trimethylsilyl substituted polyenes, benzenes and naphthalenes have been recorded; most of them are surprisingly stable under the conditions of measurement. Accurate values could be obtained for the splitting parameters – not only of the protons, but also of the ²⁹Si isotopes in natural abundance. The coupling constants of the ²⁹Si nuclei in trimethylsilyl substituents are of the same order of magnitude as the corresponding values of the protons attached to centers of comparable π-spin population. The results indicate some Si ← C_π delocalization.     

ESR. Spectra of the Radical Anion of Dimethyl-phenyl-phosphine

ESR. spectra of the radical anion (I^?) produced from dimethyl-phenyl-phosphine (I) both by electrolysis and reaction with alkali metals have been studied upon variation of temperature. The coupling constant assigned to the ³¹P nucleus depends strongly on temperature, whereas the coupling constants attributed to protons do not exhibit such a dependence. The π-spin populations at the benzene ring of I^? give evidence - in accordance with other experimental data [1] [2] – that the dimethylphosphino substituent is electron-attracting. This effect is thought to be due mainly to P ← C_π delocalization, which is analogous to the Si? C_π interaction in trimethylsilyl-substituted π-systems [3]. The ESR. spectrum previously [4] ascribed to I^? is shown to arise from a secondary radical. The formation and structure of this radical are briefly discussed.     

[2+2]-Cycloadditionen und -Cycloreversionen in Radikalanionen. Eine ESR.-spektroskopische Beschreibung am Beispiel 2,2′-disubstituierter Biphenyle

[2+2]-Cycloadditions and -cycloreversions in radical anions. An ESR. spectroscopic study for 2,2′-disubstituted diphenyl derivatives The radical anions derived from the polycyclic olefins 1, 2 and 3 are shown by ESR. spectroscopy to undergo [_π2+_π2]-cycloaddition reactions even at low temperatures. Similarly, facile cleavage by [_σ2+_σ2]-cycloreversion processes is observed for the radical anions of the corresponding cyclobutane species. This reactivity, which is in marked contrast with the thermal stability of the neutral parent compounds, is discussed taking into account the molecular geometry and the spin density distribution.     

Synthesis,structural characterization and luminescence properties of 1‐carboxymethyl‐3‐ethylimidazolium chloride

A microcrystalline carboxyl‐functionalized imidazolium chloride, namely 1‐carboxymethyl‐3‐ethylimidazolium chloride, C₇H₁₁N₂O₂⁺·Cl⁻, has been synthesized and characterized by elemental analysis, attenuated total reflectance Fourier transform IR spectroscopy (ATR‐FT‐IR), single‐crystal X‐ray diffraction, thermal analysis (TGA/DSC), and photoluminescence spectroscopy. In the crystal structure, cations and anions are linked by C—H…Cl and C—H…O hydrogen bonds to create a helix along the [010] direction. Adjacent helical chains are further interconnected through O—H…Cl and C—H…O hydrogen bonds to form a (10) layer. Finally, neighboring layers are joined together via C—H…Cl contacts to generate a three‐dimensional supramolecular architecture. Thermal analyses reveal that the compound melts at 449.7 K and is stable up to 560.0 K under a dynamic air atmosphere. Photoluminescence measurements show that the compound exhibits a blue fluorescence and a green phosphorescence associated with spin‐allowed (¹π←¹π*) and spin‐forbidden (¹π←³π*) transitions, respectively. The average luminescence lifetime was determined to be 1.40 ns for the short‐lived (¹π←¹π*) transition and 105 ms for the long‐lived (¹π←³π*) transition.     

Chemistry of alkali metal-unsaturated hydrocarbon adducts : X. Electron-transfer reactions of α, β-unsaturated organosilanes

The chemical behavior of alkali metal adducts of α,β-unsaturated organosilanes was investigated by generating the adducts with lithium or potassium metal in donor solvents at ?78°C and studying the ensuing reduction, bimolecular coupling, cleavage and isomerization processes. Chosen for study were the triphenylvinyl derivatives of silicon, germanium and tin, as well as other vinyl-, phenyl-, allyl-, 1,3-alkadienyl-, 1-alkynyl- and cyclopropyl-silanes. By assessing gradations in substituent reactivity toward alkali metal, an empirical ordering of electron affinity for these substituents on silicon was determined: 1,3-alkadienyl > 1-alkynyl > 1-alkenyl > phenyl > > 2-alkenyl or cyclopropyl. Where available, ESR data were used to explain the reactivity and the reaction pathway observed for radical anion intermediates. Where such information was unobtainable, the behavior of unstable radical anions was ascribed to variations in p_π → d_π stabilization.     

Radical Anions of Sterically Protected Polyenes: An ESR and ENDOR Study

Owing to the steric protection by four bulky substituents in the terminal positions 1 and n, several conjugated polyenes could be reduced with K or Cs metal in 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF) to fairly persistent radical anions. These compounds, denoted here as 2 , 3 ,…︁ 7 (which corresponds to the number, \2 n=2, 3, …︁7, of their formal double bonds) are 1,1,n,n-tetra(tert-butyl) derivatives of buta-1,3-diene, hexa-1,3,5-triene, octa-1,3,5,7-tetraene, deca-1,3,5,7,9-pentaene, dodeca-1,3,5,7,9,11-hexaene, and tetradeca-1,3,5,7,9,11,13-heptaene. In addition to the six polyenes 2 – 7 with all-trans-configuration, the studies comprised an isomer of 3 , the trans,cis,trans-triene, c -3 . The radical anions 2 ^.− – 7 ^.− and c -3 ^.− were characterized by their hyperfine data acquired with ESR, ENDOR, and TRIPLE-resonance spectroscopy. The ¹H-coupling constants comply with the spin distribution predicted for the radical anions of such `linear' π-systems by simple MO models. Ion pairs formed with K⁺ in DME were loose but became tighter with Cs⁺ in THF. Propensity to ion pairing decreased with the lengthening of the π-system on going from 2 ^.− to 3 ^.− – 7 ^.−. Hyperfine data are likewise reported for the radical anions of all-trans-polyenes 8 and 9 , in which two tert-butyl substituents in one terminal position of 2 and 3 , respectively, were replaced by CN groups.     

The Radical Ions of Indolizino [6,5,4,3-aij]quinoline and Some of its Derivatives

Indolizino [6,5,4,3-aij]quinoline (1) and its 3,9-dimethyl- (2) , di-tert-butyl- (3) and diphenyl- (4) derivatives yield persistent radical cations which have been studied by ESR and ENDOR spectroscopy. The corresponding radical anions have also been prepared, but only that of 3 was amenable to a complete characterization by hyperfine data. The π-spin populations in the radical anions are essentially localized on the 13-membered C-perimeter, and thus both the HOMO and LUMO of 1 exhibit an ‘annulenyl character’. The radical anions of 1 and 2 have gradually been converted into those of the corresponding 3,4-dihydro derivatives ( 1 -H₂ and 2 -H₂) which have been unambiguously identified by a combination of preparative and spectroscopic methods.     

Time-resolved four-level double resonance in the microwave spectrum of formaldehyde

The technique of transient double resonance has been applied to the (2₁₁ ← 2₁₂)_s-(3₁₂ ← 3₁₃)_p system in the microwave spectrum of formaldehyde. An effective broadening parameter γ*/2πp=22.96±0.3 MHz Torr^?1 was measured and theory is presented to relate this to other measured multipole relaxation rates.     

Acene Size-Dependent Transition of The Radical Centers From the Metal to The Acene Parts In Monocationic Dinuclear (Diethynylacene)diyl Complexes

Controlling radical localization/delocalization is important for functional materials. The present paper describes synthesis and results of electrochemical, spectroscopic, and theoretical studies of diruthenium (p-diethynylacene)diyl complexes, Me₃Si-(C≡C)₂-Ru(dppe)₂-C≡C−Ar-C≡C−Ru(dppe)₂-(C≡C)₂-SiMe₃ ( 1–6 ) (dppe: 1,2-bis(diphenylphosphino)ethane), and their monocationic radical species ([ 1 ]⁺–[ 6 ]⁺). The HOMO-LUMO energy gaps can be finely tuned by the acene rings in the bridging ligands installed, as indicated by the absorption maxima of the electronic spectra of 1–6 ranging from the UV region even to the NIR region. The cationic species [ 1 ]⁺–[ 6 ]⁺ show two characteristic NIR bands, which are ascribed to the charge resonance (CR) and π-π* transition bands, as revealed by spectroelectrochemistry. Expansion of the acene rings in [ 1 ]⁺–[ 6 ]⁺ causes (1) blue shifts of the CR bands and red shifts of the π-π* transition bands and (2) charge localization on the acene parts as evidenced by the ESR, DFT and TD-DFT analyses. Notably, the monocationic complexes of the larger acene derivatives are characterized as the non-classical acene-localized radicals.     

Some electrochemically generated elemento-organicradical ions and metal complexes as studied by esr

ESR spectra have been obtained for the radical anions Ph_nEl^XXX , where El = P, Si or As, and also for substituted nitrobenzenes. Radical anions were generated by electrochemical reduction within a microwave cavity. The nature and stability of radical ions and other paramagnetic species were established by the investigation of the electrochemical behaviour of the substances. The spin density distribution shows that atoms of the elements in Ph_nEl^XXX are not to be taken as “isolating bridges”. Spin density distributions have also been established for other radical ions.Paramagnetic complexes Mo^V and W^V have been prepared by electrochemical reduction of appropriate molybdenum and tungsten compounds and their structure and magnetic parameters have been established.     

ESR spectra of endohedral metallofullerene Ce@C82 radical anions in dimethylformamide and pyridine

Endohedral metallofullerene Ce@C₈₂ dissolved in dimethylformamide or pyridine is reduced to the radical anion. Analysis of hyperfine coupling with the topologically different ¹³C nuclei indicates the electronic structure with bivalent cerium and the paramagnetic carbon framework Ce²⁺@C₈₂ ^·3–. The ESR spectra of the radical anions of the functionalized Ce@C₈₂ derivatives are detected.     

Radical Anions of Cyclic Azoalkanes: An ESR,ENDOR, and TRIPLE-Resonance Study

Radical anions often monocyclic and bicyclic azoalkanes containing the azo group in (Z)-conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general-TRIPLE-resonance spectroscopy. These azoalkanes are represented by 3,3,5,5-tetramethyl-1-pyrazoline ( 1 ), 2,3-diaza-bicyclo[2.2.1]hept-2-ene ( 4 ), and 2,3-diazabicyclo[2.2.2]oct-2-ene ( 9 ), as well as by their derivatives 2 , 3 , 5 – 8 , and 10 . For all radical anions 1 ″– 10 ″, the ¹⁴N-coupling constant, a_N, is in the range of +0. 83 to +0. 97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The ¹⁴N-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of ⁷Li-, ²³Na-, ³⁹K-, and ¹³³Cs-nuclei point to a close association of the radical anions with their alkali-metal counterions. With the exception of ³⁹K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of 1 ″– 10 ″ are discussed.     

Ab initio Study on Formation Mechanism of Spiro-Si-Heterocyclic Ring Compound Involving Ge from H2Ge=Si: and Formaldehyde

H₂Ge=Si: and its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) are new species. Its cycloaddition reactions are new area for the study of silylene chemistry. The cycloaddition reaction mechanism of singlet H₂Ge=Si: and formaldehyde has been investigated with the MP2/aug-cc-pVDZ method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rule is that two reactants firstly form a four-membered Ge-heterocyclic ring silylene through the [2+2] cycloaddition reaction. Because of the 3p unoccupied orbital of Si: atom in the four-membered Ge-heterocyclic ring silylene and the π orbital of formaldehyde forming a π→p donor-acceptor bond, the four-membered Ge-heterocyclic ring silylene further combines with formaldehyde to form an intermediate. Because the Si: atom in the intermediate undergoes sp³ hybridization after transition state, then the intermediate isomerizes to a spiro-Si-heterocyclic ring compound involving Ge via a transition state. The result indicates the laws of cycloaddition reaction between H₂Ge=Si: or its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) and asymmetric π-bonded compounds are significant for the synthesis of small-ring involving Si and Ge and spiro-Si-heterocyclic ring compounds involving Ge.     

ESR.-Untersuchungen an Radikal-Anionen des 1, 6-Imino-[10]annulens und seines N-Methyl-Derivats

The ESR.-spectrum of the radical anion of 1,6-imino-[10]annulene (II) has been recorded. Its hyperfine structure reflects the reduced symmetry (C_s) of the molecule, as compared with that (C_2v) of 1,6-methano- and 1,6-oxido-[10]annulenes (I and III, resp.). The coupling constants of the ring protons in II^? are intermediate between the corresponding values of I^? and III^?. The ESR.-spectrum of the radical anion of 1,6-methylimino-[10]annulene (IV) has also been obtained, but not analysed in detail. The relative stabilities of the radical anions of the four bridged [10]annulenes are: I^??II^? > III^? > IV^?. The main secondary product identified by ESR.-spectroscopy after the decay of II^?, III^? and IV^? is the naphthalene radical anion. A remarkable exception is IV, when reduced with sodium in 1,2-dimethoxyethane: in this case the ESR.-spectrum of the azulene radical anion is observed.     

Electron transfer and paramagnetic products of reactions of haloquinones with aliphatic amines

The reactions of chloranil (Cl₄Q) and bromanil (Br₄Q) with aliphatic amines in a DMF : H₂O (5 : 1, vol/vol) mixture were studied. The radical anions of 2,5-didimethylamino-3,6-chloro-p-benzoquinone and 2,5-didimethylamino-3,6-bromo-p-benzoquinone were identified by ESR spectra. The reaction rate constant of the replacement of two chlorine atoms by the amino groups in the radical anion of Cl₄Q at 288 K was estimated.     

Lithium and sodiumtris(trimethylsilyl)silanolates. Synthesis and reactivity

Lithium and sodium tris(trimethylsilyl)silanolates were obtained by the reaction of tris(trimethylsilyl)silanol with BuⁿLi or PrⁱONa in hexane. The degree of association of silanolates in benzene solution was found to be 2 and 4 for the sodium and lithium derivatives, respectively. (Me₃Si)₃SiONa is noticeably more active than the lithium derivative in the reaction with Me₃SiCl. Tris(trimethylsilyl)silanol reacts with trimethylchlorosilane to give (Me₃Si)₃SiCl. The hydrolysis of (Me₃Si)₃SiONa (Li) in benzene and hexane yields the corresponding silanol, whereas in HMPA the splitting of Si-Si bonds and hydrogen evolution were observed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1146–1149, June, 1995.This work was carried out with financial support from the International Scientific and Technical Center (Project No 015-94).     

The singlet nπ* states of para-benzoquinone

The vibronic nπ* singlet spectra of p-benzoquinone-h₄ and p-benzoquinone-d₄ have been observed in a supersonic jet and some as yet unknown excited state fundamentals in the vapor phase have been assigned. The electric dipole forbidden, magnetic dipole allowed origin of the ¹B_1g ← ¹A_g transition is observed at 20045 cm^?1. The origin of the¹A_u ← ¹A_g, transition has been indirectly determined at 19991 cm^?1 from the vibronic excitation spectra. Neither shows a deuterium shift.     

Electron spin resonance studies of several anion radicals of nitro-substituted thioacylpiperidines and morpholines

The radical anions of m- and p- nitro-substituted derivatives of several classes of N-(thioacyl)-piperidines and -morpholines have been studied by ESR spectroscopy. The anion radicals were found to be centred on the nitro group, and the distribution of the unpaired electron was found to be dependent on the extent of conjugation between the thiocarbonyl group and the aromatic moiety of the molecule. The hfs constants were discussed in terms of the spin densities calculated by the McLachlan procedure.     

The Radical Anions of N,N′-Dicyanoquinone Diimines,a New Class of Electron Acceptors

The radical anions of 12 N,N′-dicyanoquinone diimines, a new class of electron acceptors, hace been characterized by their hyperfine data with the use of ESR and ENDOR spectroscopy. The largest coupling constant (0.30–0.45 mT), due to the two ¹⁴N nuclei in the exocyclic positions, gives rise to a conspicuous broadening of the peripheral ESR lines by an incomplete averaging of the hyperfine anisotropy. The most plausible interpretation of the experimental results for the radical anions of N,N′-dicyano- 1,4-benzoquinone diimine ( 1 ) and N,N′-Dicyano-9,10-anthraquinone diimine ( 9 ) is in terms of both ‘syn’- and ‘anti’-configurations contributing to the ESR and ENDOR spectra and having equal proton- and ¹⁴N-coupling constants. The π-spin distribution in the radical anions of N,N′-dicyanoquinone diimines is compared with those in the analogous ions of tetracyanoquinodimethanes and quinones.     

SYNTHESIS AND NMR SPECTROSCOPIC INVESTIGATION OF PHENYLPHOSPHORYL DERIVATIVES

Abstract

The syntheses of 16 phenyl- (1 and 3) and phenylthiophosphoryl (2) derivatives are described. The ¹³C and ¹⁷O NMR data sensitively reflect electron density changes in the phosphoryl substituents caused by p^π–d^π back-donation from X/Y to P. Taft constants σ₁ and σ_R ⁰ were derived for 16 phosphoryl substituents.