Substituent effects on the energies of the electronic transitions of geminally diphenyl-substituted trimethylenemethane (TMM) radical cations. Experimental and theoretical evidence for a twisted molecular and localized electronic structure

Substituent effects on the energies (Eob) of electronic transitions of geminally diphenyl-substituted trimethylenemethane (TMM) radical cations 5a-k*+ and those of structurally related 1,1-diarylethyl cations 7a-k+ were determined experimentally by using electronic transition spectroscopy. In addition, transition energies of these radical cations were determined by using density functional theory (DFT) and time-dependent (TD)-DFT calculations. The electronic transition bands of 5a-k*+ and 7a-k+ have maxima (lambdaob) that appear at 500-432 and 472-422 nm, respectively. A Hammett treatment made by plotting the Eob values relative to that of the diphenyl-TMM radical cation 5d*+ (DeltaEob) vs the cationic substituent parameter sigma+ give a favorable correlation with a boundary point at sigma+ = 0.00 and a positive rho for sigma+ < 0 and a negative rho for sigma+ > 0. A comparison of the lambdaob and rho values for 5a-k*+ and 7a-k+ suggests that the chromophore of 5*+ is substantially the same as that of 7+. The results of TD-DFT calculations, which reproduce the experimental electronic transition spectra and relationships between DeltaEob and sigma+, and suggest that the absorption band of 5*+ is associated with the SOMO-X --> SOMO transition, while that of 7+ is due to the HOMO --> LUMO transition. Another interesting observation is that Cl and Br substituents in the diphenyl-substituted TMM radical cations and 1,1-diarylethyl cations 7a-k+ act as electron-donating groups in terms of their effect on the corresponding electronic transitions. The results show that the molecular structure of 5*+ is a considerably twisted and that 5*+ has a substantially localized electronic state in which the positive charge and odd electron are localized in the respective diarylmethyl and the allyl moieties.     

Study of radical cations ofcis- andtrans-decalin in nonpolar solutions by the MARY and optically detected ESR spectroscopy

Radical cations ofcis- andtrans-decalin in nonpolar solvents were studied by optically detected ESR and magnetically affected reaction yield (MARY) spectroscopy. The observed differences in the spectra ofcis- andtrans-decalin are in agreement with the assumption of the existence of temperature-activated intramolecular dynamic transitions in the radical cation oftrans-decalin. Using MARY spectroscopy, the signals of the corresponding radical cations were detected at room temperature in diluted solutions containingcis- andtrans-decalin molecules as acceptors. Under these conditions, the total recovery of dynamic transitions in the radical cation oftrans-decalin is observed. Radical cations of bothcis- andtrans-decalin participate in the reaction of the ion-molecular charge transfer to a neutral molecule; the rate constant of this reaction is close to the diffusion-controlled one. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 968–973, May 1997.     

Probing the Electronic Structure of Bacteriochlorophyll Radical Ions—A Theoretical Study of the Effect of Substituents on Hyperfine Parameters

In reaction centers (RCs) of photosynthesis, a light‐induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria, different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron–nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication, we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data. The observed differences in the computed hyperfine data are discussed in view of a possible distinction between these species by EPR/ENDOR methods. In addition, ¹⁴N nuclear quadrupole coupling constants (nqcs) computed for BChl a, b, g, and also for Chl a in their charge neutral, radical cation and radical anion states are presented. These nqcs are compared with experimental values obtained by ESEEM spectroscopy on several different radical ions.     

The nature of transitions in electronic absorption spectra of radical cations of dipyrroles with phenylene bridging groups

The electronic absorption spectra of radical cations of dipyrroles with a phenylene bridge were studied by laser flash photolysis and quantum chemical methods. Intense absorption bands of the radical cations in the visible region (λ_max ≈ 500 nm, ε_max > 2 · 10⁴ L mol⁻¹ cm⁻¹) are caused by excitation of electrons from single occupied MOs to the LUMO. In the near IR region, calculations predict additional, relatively intense (f≈ 0.27–0.29) electronic transitions associated with excitation of electrons from low-lying MOs to the single occupied MO.     

Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: A (2)Sigma(v(') = 0)<--X (2)Pi(v(") = 0,1,2,3). Simulations of the NO rovibronic intensities for the A<--X transitions show that dissociated NO from RDX and HMX is rotationally cold (approximately 20 K) and vibrationally hot (approximately 1800 K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.     

Spectroscopic and density functional theory studies of the molecular geometry and electronic structure of classical and nonclassical radical ions derived from 7-benzhydrylidenenorbornene analogues

A spectroscopic study, using nanosecond time-resolved laser flash photolysis and gamma-irradiation of low-temperature matrices, was undertaken along with a theoretical study using density functional theory (DFT) and time-dependent (TD)-DFT calculations to gain insight into the molecular geometry and electronic structure of radical cations and radical anions of 7-benzhydrylidenenorbornene (4) and its derivatives 6-8. The radical ions 4(.+), 6(.+), 7(.+), 8(.+), 4(.-), 6(.-), 7(.-), and 8(.-) exhibited clear absorption bands in the 350-800 nm region, which were reproduced successfully from the electronic transitions calculated with TD-UB3LYP/cc-pVDZ. Radical cations 4(.+) and 8(.+) are consistent with a bent structure having a delocalized electronic state where the spin and charge are delocalized not only in the benzhydrylidene subunit but also in the residual subunit. In contrast, 6(.+) and 7(.+) have nonbent structures with a localized electronic state where their spin and charge are localized in the benzhydrylidene subunit only. Therefore, 4(.+) and 89(.+) have a nonclassical nature, with 6(.+) and 7(.+) possessing a classical nature. In contrast, in the radical anion system, 7(.-) and 8(.-) are considered nonclassical, and 4(.-) and 6(.-) are classical. Orbital interaction theory and DFT calculations can account fully for the spectroscopic features, molecular geometries, and electronic structures of the radical ions. For example, the shift of the absorption bands and the nonclassical nature of 4(.+) are due to the antibonding character of the highest occupied molecular orbital (HOMO) of 4, and those of 7(.-) arise from the bonding character of the lowest unoccupied molecular orbital (LUMO) of 7. A topological agreement of p-orbitals at C-2, C-3 (or C-5, C-6), and C-7 produces strong electronic coupling with an antibonding or a bonding character in the frontier orbitals. It is the ethylene and butadiene skeleton at C-2-C-3 (or C-5-C-6), with its contrasting topology in the HOMO and LUMO of the neutral precursor, that holds the key to deducing the nonclassical nature of the 7-benzhydrylidenenorbornene-type radical cation and radical anion systems.     

A study of photoelectron spectra in the framework of the semiempirical AM1 method

The characteristics of photoelectron (PE) spectra of 2-phenylpyrroles and biphenyls have been correlated with the UV spectroscopy data for their radical cations. The geometric and electronic structures of the compounds and radical cations have been calculated using the AM1 method. The absorption bands of the radical cations observed in the visible region of spectrum have been found to pertain to the quasi-Koopmans type,i.e., they are realized between the doubly and partially occupied -MO, but their energies differ strongly from those calculated on the basis of PE spectra. This is explained by the distinction between the spatial structure of a molecule and that of the respective radical cation. The second cause of the discrepancy between the data of the two spectral methods consists in orbital — interactions, which are most conspicuous in the case of 4(4)-halogen-substituted compounds.For Part 4 see ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 869–874, May, 1993.     

Electronic transitions and intramolecular hydrogen bonding in anthralin. UV-VIS linear dichroism spectroscopy and quantum chemical calculations

The electronic and molecular structure of the antipsoriatic drug anthralin (1,8-dihydroxy-9(10H)-anthracenone) is investigated by UV-VIS linear dichroism (LD) spectroscopy in stretched polyethylene and by quantum chemical model calculations. Seven individual electronic transitions are resolved below 47?000 cm⁻¹ and assigned to calculated π–π* transitions. The low-energy region is characterized by a relatively broad band around 28?000 cm⁻¹ that can be assigned to two overlapping, differently polarized electronic transitions involving a considerable degree of intramolecular charge transfer from the phenolic moieties to the carbonyl group. Computed wavenumbers for these transitions depend significantly on the assumed geometrical parameters for the intramolecular H-bonds in anthralin; best agreement with observed data is obtained with a geometry corresponding to strong H-bonding. The calculations also indicate that excited state intramolecular proton transfer (ESIPT) is likely to occur, leading to the prediction of a very large Stoke's shift.     

Structure-reactivity relationships in reactions of alkane radical cations: Study of the proton transfer from alkane radical cations to alkane molecules in γ-irradiated disordered CCl3F/alkane systems by ESR spectroscopy at cryogenic temperatures

A summary is presented of ESR results obtained in γ-irradiated disordered CCl₃F/alkane systems at cryogenic temperatures, with respect to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The nature of the alkyl radicals formed by proton transfer is indicative for the site of proton donation and is derived unambiguously from ESR results by comparison with powder spectra of authentic isomeric alkyl radicals, obtained by γ-irradiation of various chloro and bromoalkanes in perdeuterated cis-decalin. The experiments can be divided into two main classes. (i) Experiments on n-alkane radical cations in the extended all-trans conformation, i.e. ESR results on the system CCl₃F/heptane. The ESR spectrum of γ-irradiated CCl₃F/heptane consists of a triplet due to heptane radical cations in the extended all-trans conformation. In this conformation, the unpaired electron is delocalized over the carbon-carbon σ-bonds as well as the two chain-end carbon-hydrogen bonds that are in the plane of the C---C skeleton. Superimposed on the ESR triplet is a low-intensity spectrum due to heptyl radicals, which increases drastically with increasing heptane concentration. The formation of these heptyl radicals can be attributed unambiguously to proton transfer from heptane radical cations to heptane molecules, taking place in small heptane clusters to which positive-hole transfer still occurs efficiently. At the onset of proton transfer with increasing heptane concentration only primary heptyl radicals are present, clearly showing that the proton transfer takes place selectively from a chain-end position, in accordance with the electronic structure of the reacting radical cations. At higher heptane concentration secondary heptyl radicals also appear as a result of intermolecular radical-site transfer, i.e. the nature of the heptyl radicals becomes governed by their thermodynamic stability. (ii) Experiments on n-alkane radical cations in the gauche-at-C₂ conformation, i.e. ESR results on the system CCl₃F/octane. The ESR spectrum of γ-irradiated CCl₃F/octane indicates that octane radical cations are largely in the gauche-at-C₂ conformation in this matrix, with large unpaired-electron (and positive-hole) density on one planar chain-end C---H bond and one planar penultimate C---H bond at the other side of the radical cation. Careful investigation of ESR spectra with increasing octane concentration clearly reveals that in this case secondary octyl radicals are present from the very onset of proton transfer, in accordance with the electronic structure of the reacting radical cations. The results clearly point to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules and to a strict dependence of the site of proton donation on the electronic structure and conformation of the reacting radical cations.     

Theoretical Characterisation of Phosphinyl Radicals and Their Magnetic Properties: g Matrix

The g matrices (g tensors) of various phosphinyl radicals (R₂P^.) were calculated using the DFT and multireference configuration interaction (MRCI) methods. The g matrices were distinctly dependent on the molecular structure of the radical. To thoroughly examine this dependence, the contributions from individual atoms and excited states were calculated. The former revealed the gain from the phosphorus atom to be preeminent unless P?O or P?S bonds are present in the radical molecule. The contributions owing to excited states arising from electronic transitions between doubly occupied molecular orbitals and the SOMO were clearly positive, as in the case of semiquinone and niroxide radicals. The transitions from the phosphorus lone pair were of paramount importance. Surprisingly, unlike for semiquinones and nitroxides, a significant negative contribution was observed from excitations from the SOMO to unoccupied molecular orbitals. For radicals with P?O bonds, this contribution to the g₂ component was dominant.     

Theoretical study of electronically excited radical cations of naphthalene and anthracene as archetypal models for astrophysical observations. Part I. Static aspects

Motivated by the recent discovery of new diffuse interstellar bands and results from laboratory experiments, ab initio quantum chemistry calculations are carried out for the lowest six electronic states of naphthalene and anthracene radical cations. The calculated adiabatic electronic energies are utilized to construct suitable diabatic electronic Hamiltonians in order to perform nuclear dynamics studies in Part II. Complex entanglement of the electronic states is established for both the radical cations and the coupling surfaces among them are also derived in accordance with the symmetry selection rules. Critical examination of the coupling parameters of the Hamiltonian suggests that 29 (out of 48) and 31 (out of 66) vibrational modes are relevant in the nuclear dynamics in the six lowest electronic states of naphthalene and anthracene radical cations, respectively.     

Spectroscopic characterization of alkane radical cations—I. Electronic absorption spectra of 3-methylalkane radical cations

Radical cations of various 3-methylalkanes (C₆-C₁₄) have been produced and stabilized by γ-irradiation of the corresponding neutral compounds in saturated chloroflourocarbon (1,1-diflourotetra-chloroethane and 1,1,2-trichlorotriflouroethane) and perflourocarbon (perflourohexane and perfluoro-methylcyclohexane) matrices at 77 K. The perfluorocarbon matrices appeared more suitable for studies of the lighter radical cations, whereas the chlorofluorocarbon matrices were more suited for studies of the heavier radical cations; intermediary cations could be studied in both types of matrices. After irradiation, electronic absorptions associated with both the matrix and the alkane additive were observed. Pure spectra of the 3-methylalkane radical cations were obtained by difference spectrometry, after selective elimination of these cations by illumination. The electronic absorption spectra of the 3-methylalkane radical cations consist in all cases of a single broad absorption band. The spectral position of this band shifts to longer wavelengths with increasing chain length; the maximum of the absorption band was found to be situated at 490 nm for 3-methylpentane radical cations and at 940 nm for 3-methyltridecane radical cations. The results are most interesting because they give direct information on the electronic absorption of 3-methylpentane radical cations. It was found that the molar extinction coefficients of these cations are not very much smaller than those of other 3-methylalkane radical cations and thus must be of the order of 10³dm³·mol^-1·cm^-1. From this it is deduced that the majority of positive ions trapped in irradiated pure 3-methylpentane glasses at 77 K are not parent cations.     

Experimental and computational studies of structure and bonding in parent and reduced forms of the azo dye Orange II

The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.     

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.     

Bridge-Length-Dependent Intramolecular Charge Transfer in Bis(dianisylamino)-Terminated Oligo(p-phenylene)s

Radical cations of bis(dianisylamino)-terminated oligo(p-phenylene)s (OPPs) with up to five phenyl moieties were characterized by means of UV/Vis-NIR and variable-temperature ESR spectroscopy to investigate the bridge-length-dependence on intramolecular charge/spin self-exchange between two nitrogen redox-active centers. Additionally, a comparative study between bis(dianisylamine)-based mixed-valence (MV) radical cations connected by p-terphenylene and hexa-peri-hexabenzocoronene (HBC) π-bridging units also provided information on the influence of extended π-conjugation over the OPP-bridge due to the planarization between adjacent phenylene units on the strength of electronic coupling. The present study on a homologous series of organic MV systems clarifies the attenuation factor through the OPP-bridge and the linear relationship between the electrochemical potential splitting and the electronic coupling in the region of intermediate-to-weak electronic coupling regime.     

Raman spectroscopy of short-lived terthiophene radical cations generated by photochemical and chemical oxidation.

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-capped 2,2':5',2'-terthiophene (3T) and 3'-[(E)-2-(4-nitrophenyl)ethenyl]-2,2':5',2'-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5'-dimethyl-3'-[(E)-2-phenylethenyl]-2,2':5',2'-terthiophene (DM-pe3T) is stable for up to five minutes as a result of the two alpha end caps and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bands in the low-frequency region. These bands have been assigned, by density functional theory methods, to C-S stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the sigma-dimer of NO2-pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe3T)2]. This observation is attributed to a smaller change in the C--S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the laser excitation wavelength.     

Isomerisierung von Methoxy- und Carbomethoxysubstituierten Cycloalkyl- und Alken-radikalkationen

The very complex isomerization patterns of methoxy and carbomethoxy substituted cycloalkanes (3- to 7-membered rings) have been investigated using collisional activation, metastable ion characteristics and field ionization kinetics. The extent of isomerization depends on both the ring size and the substituent. Irrespective of the electronic properties of the substituent, ring opening involves exclusively the C-1? C-2 bond whereby linear alkene radical cations are formed. In the case of OCH₃- and COOCH₃ substituents the position of the resulting double bond (terminal or α,β-unsaturated) is determined more by the ring size of the precursor molecules and less by the electronic properties of the substituents. Contrary to these findings alklyl substituted cycloalkanes (3- to 5-membered rings) rearrange exclusively to terminal alkene radical cations. The barrier for double bond isomerization seems to be substantially influenced by substituents.     

Electron-transfer-induced tautomerization in methylindanones: electronic control of the tunneling rate for enolization.

The radical cations generated from 4-methyl- and 4,7-dimethylindanone, as well as their deuterated isotopomers, isolated in Argon matrices, were found to undergo enolization to the corresponding enol radical cations at rates that differ by orders of magnitude. It is shown by quantum chemical calculations that the effect of the remote methyl group in the 4-position is of purely electronic nature in that it stabilizes the unreactive pi-radical relative to the reactive sigma-radical state of the 7-methylindanone radical cation. The observed kinetic behavior of the two compounds can be reproduced satisfactorily on the basis of calculated height and width of the thermal barrier for enolization, using the Bell model for quantum mechanical tunneling. High-level calculations on the methylacrolein radical cation show that barriers for enolization in radical cations are overestimated by B3LYP/6-31G.     

Calculations of the optical spectra of hydrocarbon radical cations based on Koopmans' theorem

The first few bands in the optical spectra of radical cations can often be interpreted in terms of A-type transitions that involve electron promotions from doubly occupied to the singly occupied molecular orbital (SOMO) and/or B-type transition which involve electron promotion from the SOMO to virtual molecular orbitals. We had previously demonstrated that, by making use of Koopmans' theorem, the energies of A-type transitions can be related to orbital energy differences between lower occupied MOs and the highest occupied MO (HOMO) in the neutral molecule, calculated at the geometry of the radical cation. We now propose that the energies of B-type transitions can be related similarly to energy differences between the lowest unoccupied MO (LUMO) and higher virtual MOs in the dication, also calculated at the geometry of the radical cation, by way of an extension of Koopmans' theorem to virtual MOs similar to that used sometimes to model resonances in electron scattering experiments. The optical spectra of the radical cations of several polyenes and aromatic compounds, the matrix spectra of which are known (or presented here for the first time), and for which CASSCF/CASPT2 calculations are available, are discussed in terms of these Koopmans-based models. Then the spectra of five poly(bicycloalkyl)-protected systems and that of hexabenzocoronene, compounds not amenable to higher level calculations, are examined and it is found that the Koopmans-type calculations allow a satisfactory interpretation of most of the features in these spectra. These simple calculations therefore provide a computationally inexpensive yet effective way to assign optical transitions in radical ions. Limitations of the model are discussed.     

Cations M2(O2CtBu)4+, where M = Mo and W, and MoW(O2CtBu)4+. Theoretical, spectroscopic, and structural investigations

With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the delta --> delta* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM delta orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(micro-O2C-X-CO2)+ (X = conjugated spacer).