Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices

Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O(2)) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M(3)(+)) produced via the corresponding dimer radical cation (M(2)(+)) is clearly seen in the solution containing O(2) as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M(3)(+) CR absorption is observed; this is due to the large M(3)(+)/M(2)(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at approximately 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M(3)(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M(3)(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.     

Formation of intramolecular poly(4-hydroxystyrene) dimer radical cation

Poly(4-hydroxystyrene) (PHS) has been used in lithography as a backbone polymer and is also a promising material for extreme-ultraviolet or electron beam lithography. The dynamics of PHS radical cations generated upon exposure to electron beam were investigated. The transient absorption of PHS was observed in the near-infrared region in p-dioxane solutions by pulse radiolysis. Charge resonance (CR) bands that represent pi-pi interaction between the two chromophores of the intramolecular PHS dimer radical cation were observed, whereas p-cresol shows no distinct CR band. Although the radical cations of phenol derivatives are known to be easily deprotonated, it was found that the dimer radical cation formation leads to less deprotonation by its charge resonance stabilization.     

Stable gas-phase radical cations of dimeric tryptophan and tyrosine derivatives

Stable radical cations of dimeric amino acid derivatives of tryptophan and tyrosine were generated by collision-induced dissociation of [Cu(II)(diethylenetriamine)(amino acid derivative)2]*2+. The yields of the dimer radical cations were dependent on both the auxiliary ligand and the tryptophan or tyrosine derivatives used. Amino acid derivatives with an unmodified carboxylic acid group did not generate dimer radical cations. For the amino acid derivatives Ac-Trp-OMe and Ac-Trp-NH2 (Ac is N-acetyl; OMe and NH2 are the methyl ester and amide modifications of the C-terminal carboxylic group), no auxiliary ligand was required for generating the dimer radical cations. Collision-induced dissociation of the [Cu(II)(amino acid derivative)4]*2+ precursor generated the dimer radical cation [(amino acid derivative)2]*+. Stabilizing interactions, most likely involving hydrogen bonding, between the two amino acid derivatives are proposed to account for observation of the dimer radical cations. Dissociation of these ions yields protonated or radical cationic amino acid derivatives; these observations are consistent with the expectation of proton competition between monomeric units, whose proton affinities were calculated using density functional theory.     

Infrared spectroscopy of gas phase C3H5+ : the allyl and 2-propenyl cations

C3H5+ cations are probed with infrared photodissociation spectroscopy in the 800-3500 cm(-1) region using the method of rare gas tagging. The ions and their complexes with Ar or N2 are produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers are characterized, namely, the allyl (CH2CHCH2+) and 2-propenyl (CH3CCH2+) cations. The infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of the C(2nu) charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is consistent with a C(s) symmetry structure having a nearly linear CCC backbone and a hyperconjugatively stabilizing methyl group.     

ESR spectra and structures of radical cations of chlorotrifluoroethylene and bromotrifluoroethylene

Radical cations of both chlorotrifluoroethylene and bromotrifluoroethylene produced by γ-irradiation of solid solutions of halocarbon matrices were detected and identified by ESR. The experimental spin density distributions strongly suggest that these radical cations have a planar structure with the unpaired electron in the bonding π orbital of a″ in C, symmetry.     

Reactivity of triarylphosphine peroxyl radical cations generated through the reaction of triarylphosphine radical cations with oxygen

One-electron oxidation of triarylphosphines (Ar3P, Ar = phenyl and substituted phenyl) in benzonitrile (PhCN) has been studied using pulse radiolysis technique. One-electron oxidation of Ar3P occurred to yield the radical cation (Ar3P*+) which showed an intense absorption with a peak at 360-370 nm together with a broad band at 500-600 nm. The addition of molecular oxygen (O2) to the phosphorus atom of Ar3P*+ took place at the second-order rate constant of 10(7)-10(9) dm(3) mol(-1) s(-1) to yield the peroxyl triarylphosphinyl radical cation (Ar3P+OO*). It is found that the electron-releasing substituents on the para position of the phenyl ring of Ar3P influence the rate constants of the reaction of Ar3P*+ with O2 and that o-methyl substituents on the phenyl ring influence the reactivity of Ar3P+OO*.     

DFT study on the cycloreversion of thietane radical cations

The molecular mechanism of the cycloreversion (CR) of thietane radical cations has been analyzed in detail at the UB3LYP/6-31G* level of theory. Results have shown that the process takes place via a stepwise mechanism leading to alkenes and thiobenzophenone; alternatively, formal [4+2] cycloadducts are obtained. Thus, the CR of radical cations 1a,b(?+) is initiated by C2-C3 bond breaking, giving common intermediates INa,b. At this stage, two reaction pathways are feasible involving ion molecule complexes IMCa,b (i) or radical cations 4a,b(?+) (ii). Calculations support that 1a(?+) follows reaction pathway ii (leading to the formal [4+2] cycloadducts 5a). By contrast, 1b(?+) follows pathway i, leading to trans-stilbene radical cation (2b(?+)) and thiobenzophenone.     

Triphenylamine (TPA) radical cations and related macrocycles

Triphenylamine (TPA) derivatives and their radical cation counterparts have successfully demonstrated a great potential for applications in a wide range of fields including organic redox catalysis, organic semiconductors, magnetic materials, etc., mainly because of their excellent redox activity. The stability of TPA radical cation has significant effect on the properties of the TPA-based functional materials, especially in relation to their electronic properties. Considering the instability of parent TPA radical cation, many efforts have been devoted to the development of stable TPA radical cations and related materials. Among them, TPA radical cation-based macrocycles have attracted particular attention because their large delocalized structures can stabilize the TPA radicals, thus endow them with outstanding redox behaviors, multiple resonance structures, and wide application in various optoelectronic devices. In this review, we give a brief introduction of organic radicals and the documented stable TPA radicals. Subsequently, a number of TPA radical cation-based macrocycles are comprehensively surveyed. It is expected that this minireview will not only summarize the recent development of TPA radical cations and their macrocycles, but also shed new light on the prospect of the design of more sophisticated radical cation-based architectures and related materials.     

Spectral properties and reactivity of diarylmethanol radical cations in aqueous solution. Evidence for intramolecular charge resonance

Spectral properties and reactivities of ring-methoxylated diarylmethane and diarylmethanol radical cations, generated in aqueous solution by pulse and gamma-radiolysis and by the one-electron chemical oxidant potassium 12-tungstocobalt(III)ate, have been studied. The radical cations display three bands in the UV, visible, and vis-NIR regions of the spectrum. The vis-NIR band is assigned to an intramolecular charge resonance interaction (CR) between the neutral donor and charged acceptor rings, as indicated by the observation that the relative intensity of the vis-NIR band compared to that of the UV and visible bands does not increase with increasing substrate concentration and that the position and intensity of this band is influenced by the ring-substitution pattern. In acidic solution (pH = 4), monomethoxylated diarylmethanol radical cations 1a.(+ -)1e.(+) decay by C(alpha)-H deprotonation [k = (1.7-1.9) x 10(4)s(-1)] through the intermediacy of a ketyl radical, which is further oxidized in the reaction medium to give the corresponding benzophenones, as evidenced by both time-resolved spectroscopic and product studies. With the dimethoxylated radical cation 2.(+), C(alpha)-H deprotonation is instead significantly slower (k = 6.7 x 10(2)s(-1)). In basic solution, 1a.(+)-1e.(+) undergo (-)OH-induced deprotonation from the alpha-OH group with k(OH.)approximately equal to 1.4 x 10(10)M(-1)s(-1), leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding benzophenone.     

Optical spectra of protected diamine 10-bond-bridged intervalence radical cations related to N,N,N'N'-tetraalkylbenzidine

The optical absorption spectra of the delocalized intervalence radical cations of seven o,o'-linked benzidine derivatives that have the nitrogens protected as 9-(9-aza-bicyclo[3.3.1]nonan-3-one) derivatives are discussed and compared with that of the p-phenylene radical cation. The linking units are CH2, CH2CH2, NMe, S, SO2, and C=O, and we also studied H,H (the unlinked benzidine). The lowest-energy absorption band is assigned as the transition from the antibonding combination of symmetrical N and aromatic orbitals to the antibonding combination of the antisymmetric N and aromatic orbitals using TD-DFT calculations, and a good correlation between the observed transition energies and those calculated using the simple Koopmans theorem-based "neutral in-cation geometry" calculations on the UB3LYP/6-31G* structures is found. The use of the two-state model that equates the electronic interaction through the bridge between the amino groups with half of the lowest transition energy is seriously incorrect for these and other delocalized intervalence compounds. The problem of extracting the electronic interactions that actually are involved from calculated transition energies is discussed.     

Photochemical transformations of 1,3,5-trioxane radical cations in freonic matrices at 77 K

It was found that the principal photochemical reaction of 1,3,5-trioxane radical cations in freonic matrices at 77 K is their cycle-opening dissociation yielding the distonic radical cation in which the unpaired electron is preferentially localized on the oxygen atom. The dissociation of the trioxane radical cations at 77 K is characterized by high quantum yields, which vary from 0.24 to 0.36 in different matrices. The distonic radical cations produced during photolysis are unstable at 77 K and undergo further transformations, which occur at different rates in freonic matrices. The structure of the intermediates produced and a possible mechanism of the processes are discussed with the use of quantum-chemical calculation data.     

Ionization of aniline and its N-methyl and N-phenyl substituted derivatives by (free) electron transfer to n-butyl chloride parent radical cations

The electron transfer from aniline and its N-methyl as well as N-phenyl substituted derivatives (N-methylaniline, N,N-dimethylaniline, diphenylamine, triphenylamine) to parent solvent radical cations was studied by electron pulse radiolysis in n-butyl chloride solution. The ionization results in the case of aniline (ArNH2) and the secondary aromatic amines (Ar2NH, Ar(Me)NH) in the synchronous and direct formation of amine radical cations, as well as aminyl radicals, in comparable amounts. Subsequently, ArNH2*+ deprotonates in a delayed reaction with the present nucleophile Cl-, and forms further ArNH*. In contrast, tertiary aromatic amines such as triphenylamine and dimethylaniline yield primarily the corresponding amine radical cations Ar3N*+ or Ar(Me2)N*+, only. The persistent Ar3N*+ forms a charge transfer complex (dimer) with the parent amine molecule, whereas Ar(Me2)N*+ deprotonates to carbon-centered radicals Ar(Me)NCH2*.     

ESR spectra and structure of the n-butane and n-hexane radical cations

Well-resolved ESR spectra of the n-butane and n-hexane radical cations have been generated, each spectrum showing a 1:2:1 triplet and a detailed substructure. The results are interpreted in terms of an a_g singly occupied MO (^σC—C, δC–H_ax) which is delocalized over the extended carbon chain (C_2h symmetry) and two axial hydrogens.     

Denitration of nitroaromatic compounds by arylnitrile radical cations

Substituted nitrobenzenes react with substituted benzonitrile radical cations in an ion trap mass spectrometer by a novel ion/molecule reaction involving NO2 elimination. Formation of an arylated nitrile, Ar1+N identical to CAr2 (where Ar1, Ar2 = aryl), is indicated by collision induced dissociation and comparison with the behavior of the authentic ion. Ab initio calculations (MP2/6-31G*/ /HF/6-31G*) show the reaction of the unsubstituted compounds (Ar1, Ar2 = phenyl) to be exothermic by 48 kcal/mol, consistent with the experimental observation that the reaction rate decreases as the collision energy is increased. Electron withdrawing and donating substituents on either the ionic or the neutral reagent have little effect on the relative amount of product observed, pointing to a radical mechanism. Related denitration reactions were found to occur, between nitrobenzene and its radical cation and between phenylisonitrile and ionized nitrobenzene. These reactions are suggested to yield Ar1+N(= O)OAr2 and Ar2+N identical to CAr1, respectively. The denitration reaction was applied to trinitrotoluene (TNT) as a possible diagnostic reaction for the presence of nitroaromatic explosives.     

Delocalizations in sigma-radical cations: the intriguing structures of ionized [n]rotanes

Highly symmetric aliphatic hydrocarbons such as D(4h)-[4]rotane do not necessarily have degenerate HOMOs. According to our predictions based on high-level computations, its radical cation should display a highly delocalized D(4h)-symmetric structure, in contrast to its Jahn-Teller distorted cousin, the radical cation of [3]rotane, which exists in two distonic localized forms with C(2v) and C(s) symmetry.     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

EPR studies of amine radical cations. Part 2. Thermal and photo-induced rearrangements of propargylamine and allylamine radical cations in low-temperature freon matrices

Matrix EPR studies and quantum chemical calculations have been used to characterize the consecutive H-atom shifts undergone by the nitrogen-centered parent radical cations of propargylamine (1b*+) and allylamine (5*+) on thermal or photoinduced activation. The radical cation rearrangements of these unsaturated parent amines occur initially by a 1,2 H-atom shift from C1 to C2 with pi-bond formation at the positively charged nitrogen; this is followed by a consecutive reaction involving a second H-atom shift from C2 to C3. Thus, exposure to red light (lambda > 650 nm) converts 1b*+ to the vinyl-type distonic radical cation 2*+ which in turn is transformed on further photolysis with blue-green light (lambda approximately 400-600 nm) to the allene-type heteroallylic radical cation 3*+. Calculations show that the energy ordering is 1b*+ > 2*+ > 3*+, so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5*+ undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t1/2 of approximately 1 h to yield the distonic alkyl-type iminopropyl radical cation 6*+; this thermal reaction is attributed largely to quantum tunneling, and the rate is enhanced on concomitant photobleaching with visible light. Subsequent exposure to UV light (lambda approximately 350-400 nm) converts 6*+ by a 2,3 H-shift to the 1-aminopropene radical cation 7*+, which is confirmed to be the lowest-energy isomer derived from the ionization of either allylamine or cyclopropylamine. Although the parent radical cations of N, N-dimethylallylamine (9*+) and N-methylallylamine (11*+) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9*+ leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10*+. The first step of this transformation now involves the migration of a hydrogen atom to C2 of the allyl group from one of the methyl groups (rather than from C1); the reaction is then completed by the cyclization of the generated MeN + (=CH2) CH2CH2CH2* distonic radical cation, possibly in a concerted overall process. In contrast to the ubiquitous H-atom transfer from carbon to nitrogen that occurs in the parent radical cations of saturated amines, the alternate rearrangements of either 1b*+ or 5*+ to an ammonium-type radical cation by a hypothetical H-atom shift from C1 to the ionized NH2 group are not observed. This is in line with calculations showing that the thermal barrier for this transformation is much higher (approximately 120 kJ mol-1) than those for the conversion of 1b*+ --> 2*+ and 5*+--> 6*+ (approximately 40-60 kJ mol-1).     

Mimicking the silicon surface: reactivity of silyl radical cations toward nucleophiles

Radical cations of selected low molecular-weight silicon model compounds were obtained by photoinduced electron transfer. These radical cations react readily with a variety of nucleophiles, regularly used in monolayer fabrication onto hydrogen-terminated silicon. From time-resolved kinetics, it was concluded that the reactions proceed via a bimolecular nucleophilic attack to the radical cation. A secondary kinetic isotope effect indicated that the central Si-H bond is not cleaved in the rate-determining step. Apart from substitution products, also hydrosilylation products were identified in the product mixtures. Observation of the substitution products, combined with the kinetic data, point to an bimolecular reaction mechanism involving Si-Si bond cleavage. The products of this nucleophilic substitution can initiate radical chain reactions leading to hydrosilylation products, which can independently also be initiated by dissociation of the radical cations. Application of these data to the attachment of organic monolayers onto hydrogen-terminated Si surfaces via hydrosilylation leads to the conclusion that the delocalized Si radical cation (a surface-localized hole) can initiate the hydrosilylation chain reaction at the Si surface. Comparison to monolayer experiments shows that this reaction only plays a significant role in the initiation, and not in the propagation steps of Si-C bond making monolayer formation.     

Aryl sulfoxide radical cations. Generation, spectral properties, and theoretical calculations

Aromatic sulfoxide radical cations have been generated by pulse radiolysis and laser flash photolysis techniques. In water (pulse radiolysis) the radical cations showed an intense absorption band in the UV region (ca. 300 nm) and a broad less intense band in the visible region (from 500 to 1000 nm) whose position depends on the nature of the ring substituent. At very low pulse energy, the radical cations decayed by first-order kinetics, the decay rate increasing as the pH increases. It is suggested that the decay involves a nucleophilic attack of H(2)O or OH(-) (in basic solutions) to the positively charged sulfur atom to give the radical ArSO(OH)CH(3)(*). By sensitized [N-methylquinolinium tetrafluoborate (NMQ(+))] laser flash photolysis (LFP) the aromatic sulfoxide radical cations were generated in acetonitrile. In these experiments, however, only the band of the radical cation in the visible region could be observed, the UV band being covered by the UV absorption of NMQ(+). The lambda(max) values of the bands in the visible region resulted almost identical to those observed in water for the same radical cations. In the LFP experiments the sulfoxide radical cations decayed by second-order kinetics at a diffusion-controlled rate, and the decay is attributed to the back electron transfer between the radical cation and NMQ(*). DFT calculations were also carried out for a number of 4-X ring substituted (X = H, Me, Br, OMe, CN) aromatic sulfoxide radical cations (and their neutral parents). In all radical cations, the conformation with the S-O bond almost coplanar with the aromatic ring is the only one corresponding to the energy minimum. The maximum of energy corresponds to the conformation where the S-O bond is perpendicular to the aromatic ring. The rotational energy barriers are not very high, ranging from 3.9 to 6.9 kcal/mol. In all radical cations, the major fraction of charge and spin density is localized on the SOMe group. However, a substantial delocalization of charge and spin on the ring (almost 50% for the 4-methoxy derivative and around 30% for the other radical cations) is also observed. This suggests some conjugative interaction between the MeSO group and the aromatic system that may become very significant when a strong electron donating substituent like the MeO group is present. The ionization energies (IE) of the 4-X ring substituted neutral aromatic sulfoxides were also calculated, which were found to satisfactorily correlate with the experimental E(p) potentials measured by cyclic voltammetry.     

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.