Parent Dication Research Articles

On the two-body dissociation of dications of water isotopologues

Multiple ionization of water isotopologues is studied using near-IR fs laser beams and asymmetric ω/2ω phase-locked fields, in order to elucidate the involved dissociation mechanics and isotopic effects. The double ionization mechanism of water is identified by the laser polarization dependence of the released, via two body fragmentation, ions. The angular distribution of the H+/D+ fragments, reveal the participation of different molecular orbitals in the ionization process, indicating excitation of both the ground and first few excited states of the dications. The rate of the dissociative processes of the dication is found to be different for the two isotopologues, highlighting an isotopic effect. Inspection of the H2+/D2+ fragments formed prior to dissociation, reveals that they may have been generated from parent cations and parent dications. In the case of dication fragmentation, we find evidence of autoionization. Finally, the formation of the H2+/D2+ ions, is completed within the duration of the pulse.

Production of Long-Lived Benzene Dications from Electron Impact in the 20–2000 eV Energy Range Combined with the Search for Global Minimum Structures

In this work, we report a systematic search of metastable C6Hn2+ (n = 1-6) dications from electron impact time-of-flight measurements of several benzene derivatives in combination with global minimum search based on the genetic algorithm. Our theoretical calculations reveal that the C6Hn2+ (n < 6) global minimum structures are completely different from that of the benzene dication, featuring linear carbon chains and/or cyclopropenylium moieties. Experimentally, the doubly charged species were investigated for a wide range of electron impact energies, from 20 to 2000 eV, for benzene and several monosubstituted compounds containing either electron-withdrawing or -donating groups. Furthermore, the C6Hn2+ production, evaluated from the yields of the dications with respect to that of the parent ion (or parent dication), was compared to those obtained from charge exchange in the doubly charged 2E spectra and electron impact experiments available in the literature. The yields of the long-lived benzene dications were contrasted to those analogues formed in chlorobenzene. Moreover, the formation of C6Hn2+ species is strongly dependent on the nature of substituent groups, with electron-withdrawing ones favoring the dication formation.

Time-resolved dynamics of thiophene dication – probing parent molecule survival times and multi-step dissociation processes of cyclic molecules by free-electron-laser experiments combined with theoretical simulations

SynopsisThiophene is a cyclic molecule that becomes unstable as a molecular dication. We followed the multi-step dissociation dynamics triggered by soft x-ray FEL pulses from SACLA, core-ionizing the S 2p orbital and creating parent dications by the Auger decay. The time-evolution was probed by optical laser pulsesthat reveal the survival lifetime of the parent dication and the build-up of fragmentation proceeding by ring opening and rupture, accompanied by hydrogen ejection and secondary dissociation. Experimental results are compared to statistical analysis of molecular dynamics simulations.

Absolute ionization and dissociation cross sections of tetrahydrofuran: Fragmentation-ion production mechanisms

We report an experimental and theoretical investigation on the absolute ionization and fragmentation cross sections of tetrahydrofuran (THF, C4H8O), a base molecule of the DNA-backbone. The measurements enabled the identification of 50 ionic species produced by 12 eV up to 2000 eV electron impact which allowed us to make a close inspection of the mechanisms of both the primary vacancy production and the postcollisional fragmentation pathways which lead to the fragment ions. The experimental cross sections of the ionic species were examined in the framework of the fragmentation matrix model to attribute from which molecular orbitals (MOs) the electron is removed and find out the relative contribution of up to 15 outer and inner valence MOs in the fragment-ion production. A comparison between measured and calculated dissociative cross sections relative to the molecular parent ion cross section allowed us to clearly identify the prevalence of the single or double ionization in the fragment-ion production. Due to these different production mechanisms, the relative proportion of some ejected fragment-ions strongly depends on the impact energy. The single ionization of the MOs leading to selected fragment ions was compared to those measured at a low electron-impact energy using a reaction microscope with good agreement between the two techniques. The formation of the dications C4HnO2+, excluding the molecular parent dication, is also presented.

Fragmentation Dynamics of Doubly Charged Methionine in the Gas Phase

The dependence of the fragmentation of doubly charged gas-phase methionine (C5H11NO2S) on the electronic-state character of the parent ion is studied experimentally by energy-resolved electron ion-ion coincidence spectroscopy. The parent dication electronic states are populated by Auger transitions following site-specific sulfur 2p core ionization. Two fragmentation channels are observed to be strongly dependent on the electronic states with vacancies in weakly bound molecular orbitals. All-electron calculations are applied to assign doubly charged final states of sulfur 2p core ionized methionine. In addition, the Car-Parrinello method is applied to model fragmentation dynamics of doubly charged methionine molecules with various initial temperatures to understand the typical characteristics of the molecular dissociation and partly to support the interpretation of experimental data.

2,2′-Bis-azonia-Cope Rearrangements of 2,3-Homo-6H-1,4-diazepinium Dications

The condensation of cis-1,2-cyclopropanediamines with 1,3-dicarbonyl compounds and 1,5-diazapentadienium salts afforded 2,3-homo-1H-1,4-diazepinium salts, which were protonated by very strong acids to yield 2,3-homo-6H-1,4-diazepinium dications. As measures of basicity, the mole fractions of water in trifluoromethanesulfonic acid at half-protonation were determined for 2,3-homo- and 2,3-dihydro-1H-1,4-diazepinium salts. 2,3-Homo-6H-1,4-diazepinium dications underwent 2,2′-bis-azonia-Cope rearrangements, as can be inferred from the product structures, or H/D exchange involving the methylene group of the cyclopropane rings. Apparently, in the Cope rearrangement, this group is interchanged with the methylene group in the seven-membered ring (C-6), which is susceptible to H/D exchange in strong deuteriated acids. Rapid, degenerate, as well as very slow, strongly biased Cope rearrangements have been uncovered in this way. 1H NMR spectra, recorded at elevated temperatures for a solution of the parent dication in trifluoromethanesulfonic acid, reveal broadening of the interchanging 2-H, 3-H and 5-H, 7-H signals and eventually the onset of their coalescence. Surprisingly, the estimated free enthalpy of activation is higher (ΔG‡110 °C ≈ 73 ± 2 kJ/mol) than that of 3,4-homotropilidene (ΔG‡110 °C = 61 kJ/mol), not lower, as would be expected on the basis of the well-known charge-acceleration of Cope rearrangements.

Experimental investigation of the reaction window in double charge transfer process involving biomolecule target FeTPPCl

In the study of ion induced fragmentation of gas phase biomolecule FeTPPCl, a reaction window was experimentally determined. The populations of individual fragmentation channels of parent dications FeTPPCl2+ were found to be modulated by such a reaction window. A shift to lower energy was observed using projectile ions H+ comparing to the case of F+ impact.

Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO2 Nanoparticles

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V(1) and V(2), interconnected by a rigid terphenylene bridge. In their parent dication states, V(1)(2+) and V(2)(2+) can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side-to-side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO(2) nanoparticle, viologen V(2)(2+) is reduced at significantly more negative potentials (-0.3 V) than in flexible analogues that contain aliphatic bridges between V(1) and V(2). These overpotentials are analysed in terms of electron-transfer rates and a donor-bridge-acceptor (D-B-A) formalism, in which D is the doubly reduced viologen, V(1)(0), adjacent to the TiO(2) surface (TiO(2)-V(1)(0)), B is the terphenylene bridge and A is viologen V(2)(2+). We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V(2)(2+) by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution-based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO(2) surface.

Synchrotron radiation VUV double photoionization of some small molecules

The VUV double photoionizations of small molecules (NO, CO, CO2, CS2, OSC and NH3) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO2+, CO2+, CO2+2, CS2+2, OSC2+ and NH2+3) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then, the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.

Helquats, helical extended diquats, as fast electron transfer systems

Helicene-viologen structural hybrids, like [5]helquat, 6,7,10,11-tetrahydrodipyrido[2,1-a:1',2'-k][2,9]phenanthrolinediium, and its four methylated derivatives, are characterized by electrochemical admittance and EPR spectroscopy. All compounds are reversibly reduced in two one-electron steps. Formal redox potentials correlate with the calculated LUMO energies. The electron transfer is coupled with a weak adsorption of the reactants. The analysis of the frequency dependence of the electrode admittance is used for the separation of Faradaic and double layer contributions and finally to the estimation of heterogeneous rate constants. Heterogeneous rate constants determined this way are in the range 0.1 to 3 cm s(-1). In all cases the second electron transfer is faster than the first redox step by a factor of three. The Frumkin correction for the acceleration by the double layer potential further amplifies this difference. The heterogeneous rate constants of derivatives correlate with the solvent reorganization energy estimated from the Marcus model. EPR spectra confirm the radical cation formation. The radical of [5]helquat participates in an extremely fast self-exchange process with the parent dication characterized by the self-exchange rate constant k(ET) = (2.4 +/- 0.5) x 10(9) M(-1) s(-1).

Synchrotron radiation VUV double photoionization of CHF 2Cl

VUV double photoionization of CHF 2Cl in an energy region 32–40 eV was investigated with photoionization mass spectroscopy by using synchrotron radiation. The double ionization energy of CHF 2Cl and appearance energies for its main fragment dications (CHCl 2+, CF 2 2+ and CHFCl 2+), were determined with photoionization efficiency spectroscopy for the first time. The single point energies of CHF 2Cl and its parent dication (CHF 2Cl 2+) were calculated using Gaussian 03 program and density functional theory (DFT and B3LYP functional). The vertical double ionization energy of CHF 2Cl was predicted by using B3LYP method and empirical equation. According to our research results, the experimental double ionization energy of CHF 2Cl is in good agreement with the theoretically calculated vertical double ionization energy. The mechanism of double photoionization of CHF 2Cl was discussed based on the comparison of our experimental results with those predicted theoretically.

Antiaromatic Spacer-Bridged Bisfluorenyl Dications Generated by Superacid Induced Ionization

Derivatives of the dication of tetrabenzo[5.5]fulvalene were prepared with phenyl and ethynyl spacers through ionization of the appropriate bis-methylethers. The antiaromaticity shown by the parent dication was demonstrated for these dications with spacers, although it was attenuated by the presence of the spacer. It was substantially greater than that of fluorenyl monocations with similar substituents. Antiaromaticity was evaluated through comparison of (1)H NMR shifts with those of acyclic analogues, through nucleus independent chemical shifts, and through magnetic susceptibility exaltation. Although the fluorenyl systems are separated by spacers, the antiaromaticity of one system is affected by the other remote fluorenyl system. An explanation for this interaction may lie in the ability of a remote cationic substituent to attenuate delocalization in the spacer. The use of spacers is designed to prevent side reactions in less stable antiaromatic dications, allowing exploration of a number of species that have previously been inaccessible.

Radical cations from dipyridinium derivatives: A combined EPR and DFT study

The monoelectronic reduction of 1,1′-dimethyl-2,2′-dicyano-4,4′-bipyridinium (DCMV ++) bis-methylsulphate, conducted directly in the cavity of the electron paramagnetic resonance (EPR) spectrometer at room temperature and in DMSO solution, gave the signal of the corresponding radical cation (DCMV +) whose interpretation has been carried out with the aid of density functional theory (DFT) calculations run at different levels. The model chemistries considered yielded in general hyperfine coupling constants (hfcc) in good agreement with the experimental ones, except for the methyl protons directly bonded to the pyridinium nitrogens. The use of various computational methods accounting for solvent–solute interactions did not give significant improvements with respect to the gas phase results, while the geometry optimizations performed showed that the two pyridinium rings are coplanar in the radical cation but staggered in the parent dication, although the corresponding energy barrier involved is very low.

Redox-Induced Indenyl Slippage in [IndCpMoL2]2+/+/0 Complexes

The dicationic complexes [IndCpMoL2]2+ (Ind = indenyl; Cp = cyclopentadienyl; 1, L2 = dppe; 2, L = PMe3; 4, L2 = Bipy; 5, L2 = tBu2bipy; 6, L2 = H2biim; 7, L = CO and CNMe; 8, L = CNMe; 9, L = CNtBu; 10, L = NCMe and NMF; 11, L = NMF) were synthesized from [IndCpMoL‘2]2+ (L‘ = CO, NCMe), [IndCpMo(NCMe)Cl]BF4, or IndCpMoCl2 by ligand substitution. The neutral complexes (η3-Ind)CpMoL2 (13, L2 = dppe; 14, L = PMe3; 15, L2 = Bipy; 16, L = CO and CNMe; 17, L = CNtBu) were obtained upon reduction of the respective dications with Cp2Co. (η3-Ind)CpMo(dppe) (13) was also prepared by deprotonation of the cyclopentadiene cation [IndMo(η4-C5H6)(dppe)]BF4 (12) with NEt3. The cyclic voltammograms of the dications present one reversible 2e reduction step from Mo(IV) to Mo(II) except for 1, 2, and 6, in which cases two reversible 1e waves are observed. The ring-slipped complexes (η3-Ind)CpMoL2 and the parent dications present similar CV's. The X-ray crystal structures of [IndCpModppe][BF4]2 and (η3-Ind)CpMo(dppe) confirm the indenyl ring planar η5 coordination and slip-folded η3, respectively.

Reactions of Pyridine Coenzyme Dimers and Monomers with Viologens

DCMV++(1,1′-dimethyl-2,2′-dicyano-4,4′-bipyridinium,bis-methylsulfate) promotes the aerobic oxidation of the NAD(P) dimers (NADP)2and (NAD)2with the formation of 2 mol of NADP+or NAD+per mole of dimers. The reaction appears to follow a pseudo-first-order kinetics with respect to the dimer concentration. One mole of oxygen was consumed in the reaction per mole of NAD(P) dimer oxidized and hydrogen peroxide was produced. The monomers NADPH and NADH under the same reaction conditions were not oxidized by DCMV++. In anaerobiosis NAD(P) dimers but not NAD(P)H rapidly reduced DCMV++to its radical cation DCMV•+, which was rapidly back-oxidized by air to its parent dication. Paraquat (MV++) was also able to catalyze the aerobic oxidation of NAD(P) dimers and, at a much lower extent, NADPH and NADH, but only under light irradiation. In anaerobiosis and upon light irradiation all the above nucleotides were able to convert paraquat to its radical cation MV•+, reoxidized to MV++by air admission. This study shows the different ability of NAD(P) dimers and NAD(P)H to undergo one-electron and two-electron oxidation reactions, with different viologens.

Dynamics of the dissociation of thes-tetrazine dication produced by one photon excitation of the neutral molecule

The dissociation of thes-tetrazine dication has been investigated by the photoelectron photoion photoion coincidence method, after photoexcitation in the far ultraviolet (25 to 56 eV) of the neutral molecule. Three body dissociations are found much more abundant than two body ones even close to threshold. From the measurement of kinetic energy release and appearance energies of two body and three body reactions, we find that all channels requires an activation energy. The main three body dissociation processes always involve a N2 ejection. After simulation of the coincidence peak shape, those processes are described by stepwise processes (ejection of a neutral fragment followed by charge separation or the reverse). The intermediate cation (doubly or singly charged) is found either long or short lived compared to its rotational motion. Autoionization of highly excited states of the monocation is suspected to populate the parent dication in rearranged geometry or even in some specific charge separation reactions.

ChemInform Abstract: Crystal Structure, Raman and Resonance Raman Spectroscopic Studies of the Dimethyl Diquaternary Salt of 2,2′‐Dicyano‐4,4′‐bipyridine and Its Radical Monocation

Abstract The crystal structure of the dimethyl diquaternary salt of 2,2′-dicyano-4,4′-bipyridine is reported. Crystal data: C14H12Br2N4, Mr = 396.1, a = 7.1934(9), b = 10.192(1), c = 10.715(1) A, β = 103.89(1)°, V = 762.61 A3, monoclinic, space group P21/n, Z = 2, Dm = 1.70, Dc = 1.73 g cm−3, Mo Kα, λ = 0.71069 A, μ = 56.24 cm−1, F(000) = 388, T = 293 ± 1 K, R = 0.028, Rw = 0.029 for 1156 (I > 2.50σ(I)) reflections. The cation straddles a crystallographic centre of symmetry with the two pyridine rings being coplanar. The bromide ion is in a general position. Bond lengths and angles are normal. Raman spectra of the parent dication and resonance Raman spectra of the coloured radical monocation are also reported. The CN stretching frequency for the radical monocation is only weakly enhanced and is displaced only 12 cm−1 from the corresponding feature for the parent dication.

Crystal structure, Raman and resonance Raman spectroscopic studies of the dimethyl diquaternary salt of 2,2′-dicyano-4,4′-bipyridine and its radical monocation

The crystal structure of the dimethyl diquaternary salt of 2,2′-dicyano-4,4′-bipyridine is reported. Crystal data: C 14H 12Br 2N 4, M r = 396.1, a = 7.1934(9), b = 10.192(1), c = 10.715(1) Å, β = 103.89(1)°, V = 762.61 Å 3, monoclinic, space group P2 1/ n, Z = 2, D m = 1.70, D c = 1.73 g cm −3, Mo Kα, λ = 0.71069 Å, μ = 56.24 cm −1, F(000) = 388, T = 293 ± 1 K, R = 0.028, R w = 0.029 for 1156 ( I > 2.50σ(I)) reflections. The cation straddles a crystallographic centre of symmetry with the two pyridine rings being coplanar. The bromide ion is in a general position. Bond lengths and angles are normal. Raman spectra of the parent dication and resonance Raman spectra of the coloured radical monocation are also reported. The CN stretching frequency for the radical monocation is only weakly enhanced and is displaced only 12 cm −1 from the corresponding feature for the parent dication.

Resonance Raman spectra of the N,N ‐ dihydro‐4,4′‐bipyridinium radical cation

AbstractThe Raman spectrum of diprotonated 4,4′‐bipyridine in the solid state as the dibromide salt is reported. The resonance Raman spectrum of the radical cation derived by one‐electron reduction of the dication is also reported. Viscous glycerol was employed as a solvent in order to protect the radical by inhibiting oxygen diffusion. The spectra are interpreted on the basis of a comparison with those of related compounds. A large shift in frequency (+70 cm−1) of the inter‐ring stretching vibration between the parent dication and radical monocation is observed. This has been interpreted in terms of a major contribution to the delocalized radical structure of a quinoid‐type form.

Bridged diquaternary salts of di-2-pyridyl ketone and their radical cations: crystal structure, raman and resonance raman studies

Raman spectra are reported for two bridged diquaternary salts of di-2-pyridyl ketone. The crystal structure of one parent dication is also reported. Crystal data: C 13H 12Br 2N 2O, M r = 372.05, a = 9.369(1), b = 11.036(4), c = 7.115(1) Å, α = 97.60(2), β = 90.12(1), γ = 67.79(1)°, V = 674.20 Å 3, triclinic, space group P 1 ; Z = 2, D m = 1.83, D x = 1.832 g cm −3, MoKα, λ = 0.71069, Å, μ = 63.54 cm −1, F(000) = 364, T = 292 ± 1 K, R = 0.033, R w = 0.032 for 1330 ( I &> 2.5σ) reflections. Resonance Raman spectra of the coloured radical cations obtained from the diquaternary salts by one-electron reduction are also reported. Changes in the frequency of the carbonyl stretching vibration of the parent dications are correlated with changes in the conformation of the conjugated system. Changes in the Raman spectrum of a parent dication between the solid state and aqueous solution appear to arise from a modification of molecular geometry in solution. The decrease in frequency of the carbonyl stretching mode on radical formation is attributed to a decrease in the carbonyl bond order, due to delocalisation of the odd electron.