Spin density distributions in some radical anions exhibiting restricted rotation about a carbon-carbon bond

In the radical anions of styrene, 4-vinyl pyridine and p-fluoro-styrene it has been found earlier by ESR experiments that the two ortho protons, as well as the two meta protons of the six-membered ring are not equivalent. We have made a systematic theoretical study of these systems by using the unrestricted Hartree-Fock method. Satisfactory agreement with the experimental results has been obtained without invoking the so called effect or effect.     

Spin density distributions in radical anions of heterocyclic amine N-oxides

The unrestricted Hartree-Fock (UHF) method of Snyder and Amos is used to calculate, in the -electron approximation, the spin density distributions in radical anions of heterocyclic amine N-oxides. The computed spin densities are observed to be in good agreement with the experimental values. The computed spin density distribution of the radical anion of pyridine N-oxide is consistent with the greater susceptibility of pyridine N-oxide relative to pyridine to electrophilic nitration. Also, the calculations are consistent with the lower basicity of the N-oxides relative to the parent bases.     

Electronic structures of some olefins and their radical cations

CNDO/2 calculations of the electronic properties of the series of isomeric butenes and pentenes yield ionization potentials which reproduce the observed trends. These are interpreted in terms of the charge distribution effects of the alkyl groups.

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Zusammenfassung CNDO/2 Rechnungen für die Butan- und Penten-Isomeren ergeben die richtigen Trends für die Ionisierungspotentiale. Sie werden im Zusammenhang mit der Änderung der Ladungsverteilung infolge Methylsubstitution diskutiert.

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Résumé Des calculs CNDO/2 des propriétés électroniques des séries d'isomères du butène et du pentène fournissent des potentiels d'ionisation qui reproduisent les tendances observées. Les distributions de charge calculées pour les molécules sont utilisées pour interpréter les propriétés observées en termes d'effet du substituant alkyle.

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Presented in part at the Eighth Theoretical Chemistry Symposium held at the Batelle Institute, Geneva, Switzerland; March 26–30, 1972.     

Unrestricted Hartree-Fock spin density distributions in nitro aromatic radical anions

The spin density distributions in the radical anions of nitrobenzene and some para substituted nitrobenzenes are calculated using the unrestricted Hatree-Fock formalism of Amos and Snyder. A parameter scheme which gives satisfactory account of the electronic transitions and molecular ionisation potentials of substituted benzenes (nitrobenzene, toluene, aniline, benzaldehyde and benzonitrile) is used for spin density calculations of the nitro radical anions. The importance of spin densities on neighbouring atoms on the hyperfine splitting of nitrogen atom is discussed. It is seen that although Karplus-Fraenkel theory gives a better estimate of¹⁴N splitting constant, the simple McConnell-type of relation is approximately valid for the nitro group-nitrogen. The--parameters for use with unrestricted Hartree-Fock spin densities for the prediction of nitrogen hyperfine couplings in nitro, amino and cyano groups are summarised.

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Zusammenfassung Die Spindichteverteilungen in den Radikalanionen des Nitrobenzols und einiger parasubstituierter Nitrobenzole werden mit Hilfe des UHF-Formalismus von Amos und Snyder berechnet. Zur Ermittlung der Spindichte der nitroaromatischen Radikalanionen wird ein Parameterschema benutzt, das zufriedenstellende Darstellung der Elektronenübergänge und der molekularen Ionisationspotentiale von substituierten Benzolen (Nitrobenzol, Toluol, Anilin, Benzaldehyd, Benzonitril) gestattet. Der Einfluß der Spins von benachbarten Atomen auf die Hyperfeinaufspaltung des Stickstoffatoms wird diskutiert. Es zeigt sich, daß zur näherungsweisen Beschreibung des Stickstoffs der Nitrogruppe eine einfache Gleichung des McCornell-Typs ausreicht, wenn auch die Karplus-Fraenkel-Theorie eine bessere Abschätzung der¹⁴N-Aufspaltungskonstanten ermöglicht. Die --Parameter, die zur Voraussage der Stickstoff-Hyperfeinkopplung in Nitro-, Amino- und Cyanogruppen mit Hilfe der UHF-Spindichten erforderlich sind, werden zusammengestellt.

     

Photoexcitation of dinucleoside radical cations: a time-dependent density functional study

The excited states of dinucleoside phosphates (dGpdG, dApdA, dApdT, TpdA, and dGpdT) in their cationic radical states were studied with time-dependent density functional theory (TD-DFT). The ground-state geometries of all the dinucleoside phosphate cation radicals considered, in their base stacked conformation, were optimized with the B3LYP/6-31G(d) method. Further, to take into account the effect of the aqueous environment surrounding the dinucleoside phosphates, the polarized continuum model (PCM) was considered and the excitation energies were computed by using the TD-B3LYP/6-31G(d) method. From this study, we find that the first transition in all the dinucleoside molecules involves hole transfer from base to base. dG*+pdG and dApdA*+ were found to have substantially lower first transition energies than others with two different DNA bases. Higher energy transitions involve base to sugar as well as base to base hole transfer. The calculated TD-B3LYP/6-31G(d) transition energies are in good agreement with previous calculations with CASSCF/CAS-PT2 level of theory. This TD-DFT work supports the experimental findings that sugar radicals formed upon photoexcitation of G*+ in gamma-irradiated DNA and suggests an explanation for the wavelength dependence found.     

Polyfluorinated radical cations

The synthesis of the first trifluoromethanesulfonate esters of the type CF₃SO₃(CH₂)_nO₃SCF₃ (n=1,2,3) are reported. The new compounds are prepared from Cl(CH₂)_nCl by substitutive electrophilic dehalogenation reactions with CF₃SO₂OX (x=Cl,Br). The extension of this reaction to HCCl₃ results in HC(O₃SCF₃)₃ but the compound is unstable at 22°.     

Catalysis in hydrogen-bridged radical cations

Hydrogen-bridged radical cations (HBRCs) are an intriguing subclass of ion-molecule complexes. They may act as key intermediates of remarkable stability in both association and dissociation reactions of heteroatom-containing molecular ions. The H-bridge of such an HBRC can promote isomerization of its ionic component by H-transfer. Proton-transport catalysis (PTC) is a prime example. Here, a neutral molecule promotes the smooth transformation of an ion into its H-shift isomer by consecutive proton-transfer reactions. A celebrated case is the water catalyzed isomerization of CH(3)OH(?+) into its more stable distonic isomer (?)CH(2)OH(2)(+). Other early studies of PTC also deal with catalyzing 1,2-H shifts in association reactions. This short review focuses on more recent combined experimental and computational studies of catalysis in HBRCs. Mechanisms involving both proton and H atom transfers have been proposed for a variety of systems of H-shift isomers. It has also become clear that PTC is not confined to bimolecular reactions. It also features in the unimolecular chemistry of heteroatom- containing ions, which have a tendency to isomerize to HBRCs en route to their dissociation.     

A theoretical study of electronic spectra of radical cations of some dihydroxynaphthalenes

Electronic transition energies of radical cations of 1.2-, 1.3-, 1.6-, and 1.7?dihydroxynaphthalenes are calculated using an open-shell SCF method with configuration interaction. The results are critically analyzed and a correlation diagram is given that shows the energy-shift and intensity variation in the electronic transitions when moving from one system to another, thus revealing the characteristic behavior of the transitions depending on the positions of the hydroxyl substituents. An interesting relation connecting the electronic spectroscopy with the UV photoelectron spectroscopy is suggested on the basis of which the first ionization potentials (IPS ) of the substituted aromatic systems can be inferred from the calculated energy of the A-type (HOMO → LUMO ) transitions for their radical cations. Furthermore, the predictability of the IP s is found to be considerably increased with the incorporation of “molecular size” in the regression.     

Unrestricted Hartree-Fock calculations of spin density distributions in the radical anions of some heterocycles derived from thiophene,furan and pyrrole

Spin density distributions in the radial anions of some heterocycles derived from thiophene, furan and pyrrole have been obtained using the unrestricted Hartree-Fock method. Good agreement between the calculated results and experimental data has been observed.     

Fragmentation pathways of some benzothiophene radical cations formed by atmospheric pressure chemical ionisation

Polycyclic aromatic sulfur‐containing compounds (PASHs) are commonly found in fossil fuels and are of considerable importance in environmental studies. This work presents detailed studies on the fragmentation patterns of radical cations formed from four representative PASHs, benzo[b]thiophene, dibenzothiophene, 4‐methyldibenzothiophene and 4,6‐dimethyldibenzothiophene, using tandem atmospheric pressure chemical ionization mass spectrometry (APCI‐MS/MS). Understanding these fragmentation patterns can be a useful aid in the analysis of PASHs employing APCI or electron ionization (EI‐MS/MS), either alone or in conjunction with liquid or gas chromatography. Copyright © 2009 John Wiley & Sons, Ltd.     

Enantioselective alkene radical cations reactions

The reaction of enantiomerically enriched 2-methyl-2-nitro-3-(diphenylphosphatoxy)alkyl radicals with tributyltin hydride and AIBN in benzene at reflux results in the formation of alkene radical cation/anion pairs, which are trapped intramolecularly by amine nucleophiles, leading to pyrrolidine and piperidine systems with memory of stereochemistry. The scope and limitations of the system are explored with respect to nucleophile, leaving group, and substituents within the substrate backbone.     

Charge-localized naphthalene-bridged bis-hydrazine radical cations

Electron transfer (ET) in four symmetrically substituted naphthalene-bridged bis-hydrazine radical cations (1,4; 1,5; 2,6; and 2,7) is compared within the Marcus-Hush framework. The ET rate constants (k(ET)) for three of the compounds were measured by ESR; the 2,7-substituted compound has an intramolecular ET that is too slow to measure by this method. The k(ET) values are significantly dependent upon the substitution pattern of the hydrazine units on the naphthalene bridge but do not correlate with the distance between them. This is contrary to an assumption that is frequently made about intervalence compounds that the bridge serves only as a spacer that fixes the distance between the charge-bearing units. The internal vibrational and solvent portions (lambda(v) and lambda(s)) of the total reorganization energy (lambda) have been separated using solvent effects on the intervalence band maximum, resulting in a lambda(v) that is the same, 9900 cm(-1), for the differently substituted naphthalenes. This is in accord with the general assumption that lambda(v) is primarily dependent upon the charge bearing unit and not the bridge. However, the trends in lambda(s) cannot be explained by dielectric continuum theory.     

Oxidation of aminoindole radical cations

The varying stabilities of certain aminoindole radical cations toward oxidation with molecular oxygen have been studied. Oxidation leads to different products depending upon the environment around the N-amino nitrogen. A plausible reaction mechanism is proposed based on electronic and magnetic resonance spectroscopy.     

Spin density distribution in paramagnetic azafullerene

Hyperfine coupling (HFC) constants for ¹⁴N and ¹³C nuclei in azafullerene C₅₉N (1) were calculated. The HFC constants for the ¹H and ¹³C nuclei in the ^·CH₃ radical were calculated as functions of the pyramidal distortion of the angles at the carbon atom. Using this angular dependence, the spin density distribution of the unpaired -electron in 1 was determined. The spin density of the unpaired -electron in 1 is mainly localized around the nitrogen atom.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2372–2374, November, 2004.     

Polarization effect in radical cations and H-complexes

The effect of substituents X on the ionization potentials IP (process DX + hν ? D^+·X + e) and shifts in vibration frequencies Δν of ν(OH) in the IR spectra of phenol complexes PhO-H + DX ? PhO^δ?-H…D^δ+X for nine series of DX molecules were studied. On compiling with three conditions (a constant donor center D; the electron density donation only from D and not from X; a constant sampling size within each series) it was possible to compare the polarization effect in D^+·X and D^δ+X. In the radical cations D^+·X the polarization effect is on the average 2.2 times larger than in the systems D^δ+X. The systems D^+·X and D^δ+X are virtually indistinguishable with respect to the external delocalization of the positive charge.     

Putting aniline radical cations in a bottle

Salts containing aniline radical cations have been isolated and characterized by electron paramagnetic resonance(EPR)spectroscopy, UV-Vis absorption spectroscopy and single crystal X-ray diffraction. The EPR spectra and theoretical calculations indicate the unpaired electron is delocalized on phenyl rings and nitrogen atoms. Both radical cations feature a quinoidal geometry with a partially double C–N bond, but are distinct in that the C–N bond is coplanar to the phenyl plane in one cation while deviates from the plane in the other due to steric crowding. The work provides the first unequivocal examples of stable aniline radical cations.