Pyridyne radical cations produced by photodissociation of Mg*+ (multifluoro-pyridine) complexes: a combined experimental and theoretical study

Gas phase complexes Mg*+ (2,6-difluoropyridine) (1) and Mg*+ (pentafluoropyridine) (2) have been subjected to photodissociation in the spectral range of approximately 230-440 nm. Except for the evaporative photofragment Mg*+ , the primary photoproduct for is C(5)H(3)N*(+), which is associated with the rupture of two C-F bonds by the photoexcited Mg*+ , forming very stable MgF(2). In contrast, the direct loss of MgF(+) is more favorable for due to fluorine substitution. Given enough energy, C(5)H(3)N*(+) can undergo decomposition to form C(4)H(2)*(+) and HCN. These results are very different from those for Mg*+ (2-fluoropyridine), highlighting the significance of the additional F at C6 of and . Density functional theory (DFT) calculations have been employed to examine the geometries and energetics of the complexes as well as relevant reaction mechanisms. All of the complexes feature the direct attachment of Mg*+ to the N atom. The key intermediate is found to be FMg(+) (C(5)H(x)F(4-x)N) (x = 3 or 0), which can lead to the formation of MgF(+) directly or MgF(2) through activation of another C-F bond adjacent to N, producing the pyridyne radical cations. However, hydrogen-transfer prior to the rupture of the second C-F bond followed by ring-opening of C(5)H(3)N*(+) may result in the formation of chain forms of C(5)H(3)N*(+). The influence of the fluorine substitution on the competition of the two routes have been demonstrated.     

Variable-temperature ion mobility time-of-flight mass spectrometry studies of electronic isomers of Kr2+ and CH3OH*+ radical cations

Preliminary results from a liquid nitrogen-cooled ion mobility (IM) orthogonal-time-of-flight (o-ToF) mass spectrometer applied to the separation of electronic isomers of Kr2+ and methanol radical cations (conventional and distonic) are presented. Ab initio calculations were used to estimate the energies and energy barriers to interconversion between conventional (CH3OH*+) and distonic (CH2*OH2+) radical cations. In addition, computations and experiments are used to compare ion-neutral collision cross-sections for CH3OH*+ and CH2*OH2+ radical cations and suggest that the mobility separation is achieved by ion-neutral interactions between ions and neutral buffer gas.     

Gas phase dissociation energies of saturated AHn*+ radical cations and AHn neutrals (A = Li-F,Na-Cl): dehydrogenation,deprotonation, and formation of AHn-2*+ - H2 complexes

The dissociation energies corresponding to the two possible A-H cleavages of A (A = Li-F and Na-Cl) radical cations (loss of a H(+) and loss of a H(.)) have been computed at the CCSD(T)/ 6-311++G(3df,2pd) level of theory and compared to those of their neutral precursors. Removing an electron from AH(n)() decreases dramatically its deprotonation energy, especially for the A molecules (C and ), which become one of the most acidic species of the row, their acid character being only exceeded by FH(.+) and ClH(.+), respectively. However, dehydrogenation energies only decrease for the systems on the left side of the row (up to C and SiH(4)(.+)) for which the electron is removed from a A-H bonding orbital. Nevertheless, the loss of hydrogen is the more favorable cleavage in all cases except FH(.+). Ionization of SiH(4) leads to a Jahn-Teller distorted structure that corresponds to a Si - H(2) complex. Other - eta(2)H(2) complexes in the doublet spin state have also been found to be stable for A = Be, Mg, Al, and P, the hydrogen molecule complexes being more stable than their corresponding radical cations, for Be, Mg, and Al.     

Hydrogen bonding interactions of pyridine*+ with water: stepwise solvation of distonic cations

The solvation energies of the pyridine*+ radical cation by 1-4 H2O molecules were determined by equilibrium measurements in a drift cell. The binding energies of the pyridine*+(H2O)n clusters are similar to the binding energies of protonated pyridine-water clusters, (C5H5NH+)(H2O)n, which involve NH+..OH2 bonds and different from those of the solvated benzene radical cation-water clusters, C6H6*+(H2O)n, which involve CHdelta+..OH2 bonds. These relations indicate that the observed pyridine*+ ions have the distonic *C5H4NH+ structures that can form NH+..OH2 bonds. The observed thermochemistry and ab initio calculations show that these bonds are not affected significantly by an unpaired electron at another site of the ion. Similar observations also identify the 2-fluoropyridine*+ distonic ion. The distonic structure is also consistent with the reactivity of pyridine*+ in H atom transfer, intra-cluster proton transfer and deprotonation reactions. The results present the first measured stepwise solvation energies of distonic ions, and demonstrate that cluster thermochemistry can identify distonic structures.     

Dimer radical cations of indole and indole-3-carbinol: localized and delocalized radical cations of diindolylmethane

Extending our previous study on the title species (J. Phys. Chem. A2010, 114, 6787), we investigated the dimer cations that are formed on oxidation of the glucobrassin derivatives indole-3-carbinol (I3C) and diindolylmethane (DIM) and of parent indole (I). Radiolysis in ionic liquid and Ar matrices shows that, at sufficiently high concentrations and/or on annealing the solid glasses, intense intermolecular charge-resonance (CR) absorption bands in the NIR herald the formation of sandwich-type dimer cations. The molecular and electronic structure of these species is modeled by calculations with the double-hybrid B2-PLYP-D density functional method which yields predictions in good accord with experiment. The radical cation of DIM also shows a CR band, but unlike in the case of I and I3C, its occurrence is not dependent on the concentration but instead on the solvent: in ionic liquid the CR band is initially absent and arises only on annealing, whereas in Ar matrices it is present from the outset and undergoes blue shifting and sharpening on annealing. These puzzling findings are rationalized on the basis of B2-PLYP-D calculations which predict that neutral DIM exists in the form of two conformers, present in different relative amounts in the two experiments, which on vertical ionization form distinct radical cations, a nonsymmetric one where the odd electron is largely localized on one of the two indole moieties and one with C(2) symmetry where charge and spin are completely delocalized over both halves of the molecule, thus giving rise to an intramolecular CR transition. On annealing, the nonsymmetric cation relaxes to a similarly delocalized structure with C(s) symmetry, thus explaining the observed increase and the shift of the CR band. We believe that DIM(?+) represents the first example of a radical cation which can exist under the same conditions as a localized and a delocalized complex cation.     

Toward a more general synthetic route to paramagnetic solids containing RCNSSS*+ radical cations. A structure-property correlation for RCNSSS*+ (R = F5C2, Cl3C)

Reaction of Cl3CN and F5C2CN with a 1:1 mixture of S4(AsF6)2 and S8(AsF6)2 affords the paramagnetic solids Cl3CNSSSAsF6 (1CCl3AsF6) and F5C2CNSSSAsF6 (1C2F5AsF6). Isotropic electron paramagnetic resonance spectra of 1CCl3AsF6 and 1C2F5AsF6 in SO2 consist of a single line with g = 2.01675 and 2.01580, respectively. The structure of 1CCl3AsF6 contains chains of radical cations with relatively close interchain interactions. In contrast, chains are isolated in 1C2F5AsF6. The magnetic behavior of both compounds was interpreted as that of 1D Heisenberg antiferromagnetic chains (1CCl3AsF6, J = -34 cm(-1), theta = -9 cm(-1), TIP = 0.00082, rho = 0.012; 1C2F5AsF6, J = -21 cm(-1), theta = -4.2 cm(-1), TIP = 0.00092, rho = 0.065). Density functional theory calculated and experimental magnetic coupling constants were in good agreement. The correlation between intermolecular S...S contacts and the strength of magnetic couplings was established.     

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°.     

Trends in the reactions of gaseous phenyl pnictogen radical cations C6H5EH2*+ (E = N, P, As)

In context of an analysis of the effect of the central atom E of gaseous radical cations of phenyl pnictogens C(6)H(5)EH(2), E = N (1), P (2), and As (3), the mass spectrometric reactions of phenyl phosphane 2 have been re-investigated by D-labeling and by using methods of tandem mass spectrometry. The 70 eV mass spectrum of 2 shows the base peak for ion [M-2H](*+) and significant peaks for ions [M-H](+), [M-(2C,3H)](+), [M-PH] (*+), and [M-(C,P,2H)](+). Metastable 2(*+) fragments exclusively by loss of H(2), and the investigation of deuterated 2-d(2) shows that excessive H/D migrations occur before fragmentation. Other significant fragment ions in the mass spectrum of 2 arise by losses of C(2)H(2,) P, or HCP from the ion [M-H](+). This mass spectrometric behavior puts the radical cation 2(*+) in between the fragmentation reactions of aniline radical cation 1(*+) (loss of H and subsequent losses of C(2)H(2,) or HCN) and phenyl arsane radical cation 3(*+) (elimination of H(2) and loss of As from ion [M-H](+)). The fragmentation mechanisms of the radical cations 1(*+) -3(*+) and of related ions were analyzed by calculations of the enthalpy of relevant species at the stationary points of the minimum enthalpy reaction pathways using the DFT hybrid functionals UBHLYP/6-311+G(2d,p)//UBHLYP/6-311+G(d). The results show that, in contrast to ionized aniline 1(*+), the reactions of the derivatives 2(*+) and 3(*+) of the heavier main group elements P and As are characterized by an easy elimination of H(2)via a reductive elimination of group C(6)H(5)-E (E = P, As) and by a special stability of bicyclic isomers of 2(*+) and 3(*+). Thus, while 1(*+) rearranges by ring expansion and formation an 7-aza-tropylium cation by loss of H., the increased stability of bicyclic intermediates in the rearrangement of 2(*+) and in particular of 3(*+) results in separate rearrangement pathways. The origin of these effects is the more extended and diffuse nature of the 3p and 4p AO of P and As.     

Substituent effects in pericyclic reactions of radical cations: the ring opening of 3-substituted cyclobutene radical cations

The substituent effects on the ring-opening reaction of cyclobutene radical cations have been studied at the Becke3LYP/6-31G* level of theory. The effect on the reaction energies and activation energies of the concerted and stepwise pathways of electron-donating substituents such as methyl and methoxy as well as electron-withdrawing substituents such as nitrile and carboxaldehyde in the 3-position of the cyclobutene is discussed. The exothermicity of the reaction correlates well with the ability of the substituent to stabilize the 1,3-butadiene radical cation by electron donation or conjugation. The relative stability of the (E) and (Z) isomers of the resulting 1,3-butadiene radical cations depends largely on steric effects. Similarly, steric effects are responsible for the relative energies of the different diastereomeric transition structures. The cyclopropyl carbinyl intermediate of the stepwise pathway resembles the nonclassical carbocation and is stabilized by electron-donating substituents. In the case of electron-donating substituents, this species becomes a minimum on the potential energy hypersurface, whereas unstabilized or destabilized cyclopropyl carbinyl radical cations are not minima on the hypersurface. The stabilization of the cyclopropyl carbinyl radical cation by substituents correlates qualitatively with the Brown-Okamoto substituent parameter sigma+. However, in all cases studied here, the concerted mechanism is the lowest energy pathway.     

Towards persistent cyclobutadiene radical cations? The tetra-1-adamantyl-, and tetraneopentylcyclobutadiene radical cations.

The persistence of alkyl substituted cyclobutadiene radical cations strongly depends on the method of generation and the size of the alkyl substituents used. Hindered rotation, the consequence of bulky substituents, is observed in the title compounds.     

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.     

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.     

Formation and reactions of aryldiazomethane radical cations

Dicyanoanthracene-sensitized photolysis of aryldiazomethanes affords stilbenes with cis/trans ratios of ～3 via free radical cations; anodic oxidation gives similar results but via radical cations adsorbed on electrode surface.     

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.     

The formation and structure of radical cations

ions generated from a number of different presursors have been studied by high kinetie energy ion—molecule reations. It has been shown that at least four distinct stable species oeeur, of which acetonitrile and methyl isoeyanide retain their original structure. With imidazole or pyrazole as precursors, a mixture of open thain radical cations, not identical to the above species and probably interconvertible via the 1H-azirine radocal cation, is formed. From butrynitrile, pyrrole, crotonitrile, allyl interconvertible via the and cyanocyopropane a fourth species, probably the vinylidenimine ion, is formed.