Spectroscopy of the cyano radical in an aqueous environment

The effect of bulk water on the B (2)Sigma(+) <-- X (2)Sigma(+) and A (2)Pi <-- X (2)Sigma(+) electronic transitions of the cyano radical is investigated. First, the cyano radical-water dimer is characterized to understand the nature of the interactions and parametrize molecular mechanics (MM) potentials. The carbon atom, which hosts the unpaired electron, is found to have a Lennard-Jones radius smaller than typical force fields values. Classical molecular dynamics (MD) is then used to sample water configurations around the radical, employing two sets of MM parameters for the cyano radical and water. Subsequently, vertical excitation energies are calculated using time-dependent density functional theory (TD-DFT) and equation-of-motion coupled-cluster with single and double substitutions (EOM-CCSD). The effect of water is modeled by point charges used in the MD simulations. It is found that both bands blue-shift with respect to their gas phase position; the magnitude of the shift is only weakly dependent on the method and the MM parameter set used. The calculated shifts are analyzed in terms of the solute-solvent interactions in the ground and excited states. Significant contributions come from valence repulsion and electrostatics. Consequences for experiments on ICN photodissociation in water are discussed.     

Redox reactivity of photogenerated osmium(II) complexes

Powerful reductants [Os(II)(NH(3))(5)L](2+) (L = OH(2), CH(3)CN) can be generated upon ultraviolet excitation of relatively inert [Os(II)(NH(3))(5)(N(2))](2+) in aqueous and acetonitrile solutions. Reactions of photogenerated Os(II) complexes with methyl viologen to form methyl viologen radical cation and [Os(III)(NH(3))(5)L](3+) were monitored by transient absorption spectroscopy. Rate constants range from 4.9 × 10(4) M(-1) s(-1) in acetonitrile solution to 3.2 × 10(7) (pH 3) and 2.5 × 10(8) M(-1) s(-1) (pH 12) in aqueous media. Photogeneration of five-coordinate Os(II) complexes opens the way for mechanistic investigations of activation/reduction of CO(2) and other relatively inert molecules.     

Proton transfer reactions of photogenerated cyanoaromatic-methylaromatic radical ion pairs2

The photochemical reactions of a number of cyanoaromatic (acceptor) and methylaromatic (donor) molecules have been investigated. These reactions can result in the formation of photosubstitution products or benzyl radical coupling products. A survey of our results and previously published data indicates that exergonic photostimulated electron transfer is a necessary but not sufficient condition for the observation of reaction products. The efficiency of proton transfer from the donor cation radical to the acceptor anion radical is determined by the kinetic acidity and basicity of the radical ion pair. Mechanistic evidence is presented which indicates that proton transfer requires diffusion apart and reencounter of the initially formed radical ion pair. Predominant radical pair combination is observed for anion radicals which yield electron-deficient free radicals upon protonation, whereas predominant cage escape and benzyl radical coupling is observed for anion radicals which yield electron-rich free radicals upon protonation.     

Mapping photogenerated radicals in thin polymer films: fluorescence imaging using a prefluorescent radical probe

Prefluorescent radical probes, in which fluorescence is activated by radical trapping, and photoinitiators were used to detect radical generation in polymer films using fluorescence spectroscopy and microscopy. Prefluorescent radical probes are the foundation of a fluorescence imaging system for polymer films, that may serve both as a mechanistic tool in the study of photoinitiated radical processes in polymer films and in the preparation of functional fluorescent images.     

Decarboxylative reduction of free aliphatic carboxylic acids by photogenerated cation radical

The decarboxylation of free carboxylic acids was effected by a photogenerated cation radical of phenanthrene to yield the reduction product in the presence of a thiol, which provides an alternative method to the Barton decarboxylation procedure for aliphatic acids such as N-Boc amino acids.     

Formation and reactivity of a persistent radical in a dinuclear molybdenum complex

The reactivity of the S-H bond in Cp*Mo(mu-S) 2(mu-SMe)(mu-SH)MoCp* ( S 4 MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(mu-S) 3(mu-SMe)MoCp* ( S 4 Me*), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, Delta H (double dagger) and Delta S (double dagger) were determined to be 1.54 +/- 0.25 kcal/mol and -25.5 +/- 0.8 cal/mol K, respectively, giving k abs = 1.3 x 10 (6) M (-1) s (-1) at 25 degrees C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo 2S 4 core was found to be the benzyl cross-termination product, Cp*Mo(mu-S) 2(mu-SMe)(mu-SBz)MoCp* ( S 4 MeBz), consistent with the Fischer-Ingold persistent radical effect. S 4 Me* was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(mu-S) 2(mu-SMe)MoCp*) 2(mu-S 2) ( ( S 4 Me) 2 ). The radical-dimer equilibrium constant has been determined to be 5.7 x 10 (4) +/- 2.1 x 10 (4) M (-1) from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 x 10 (3) s (-1) at 25 degrees C, based on variable temperature (1)H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 x 10 (7) M (-1) s (-1) in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for ( S 4 Me) 2 , S 4 MeBz, and the cationic Cp*Mo(mu-S 2)(mu-S)(mu-SMe)MoCp*(OTf) ( S 4 Me ( + )), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S 4 core is presented.     

Synthesis of structurally diverse and defined bivalent mannosides on saccharide scaffolding

[Structure: see text] The synthesis of bivalent mannosides by the grafting of alpha-D-mannopyranoside onto monosaccharide acceptors and conjugation to terephthalic acid or phenylenediamine is described. Computational methods were used to predict accessible orientations and distances between the mannose units.     

Formation and spectroscopy of a tryptophan radical containing peptide in the gas phase

A doubly deprotonated tryptophan containing peptide was electrosprayed and isolated in an ion trap. UV excitation on this peptide leads to electron detachment and to the formation of an indolyl radical. The photogenerated radical was fragmented by a second laser. The visible spectrum of the gas-phase neutral tryptophan radical containing peptide has been recorded and constitutes a benchmark for calculations and optical measurements.     

Spectroscopy of free-base N-confused tetraphenylporphyrin radical anion and radical cation

The radical anions and radical cations of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin have been studied using a combination of cyclic voltammetry, steady state absorption spectroscopy, and computational chemistry. N-Confused porphyrins (NCPs), alternatively called 2-aza-21-carba-5,10,15,20-tetraphenylporphyrins or inverted porphyrins, are of great interest for their potential as building blocks in assemblies designed for artificial photosynthesis, and understanding the absorption spectra of the corresponding radical ions is paramount to future studies in multicomponent arrays where electron-transfer reactions are involved. NCP 1e was shown to oxidize at a potential of E(ox) 0.65 V vs Fc(+)|Fc in DMF and reduce at E(red) -1.42 V, while the corresponding values for 1i in toluene were E(ox) 0.60 V and E(red) -1.64 V. The geometries of these radical ions were computed at the B3LYP/6-31+G(d)//B3LYP/6-31G(d) level in the gas phase and in solution using the polarizable continuum model (PCM). From these structures and that of H(2)TPP and its corresponding radical ions, the computed redox potentials for 1e and 1i were calculated using the Born-Haber cycle. While the computed reduction potentials and electron affinities were in excellent agreement with the experimental reduction potentials, the calculated oxidation potentials displayed a somewhat less ideal relationship with experiment. The absorption spectra of the four radical ions were also measured experimentally, with radical cations 1e(?+) and 1i(?+) displaying significant changes in the Soret and Q-band regions as well as new low energy absorption bands in the near-IR region. The changes in the absorption spectra of radical anions 1e(?-) and 1i(?-) were not as dramatic, with the changes occurring only in the Soret and Q-band regions. These results were favorably modeled using time-dependent density functional calculations at the TD-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level. These results were also compared to the existing data of free base tetraphenylporphyrin and free base tetraphenylchlorin.     

Production mechanism and reactivity of the SiH radical in a silane plasma

SiH free radicals in a dc multipole post-discharge plasma were analyzed using laser-induced fluorescence. The cross section for SiH formation from 40–70 eV electron impact induced dissociation of SiH₄ was measured to be (10 ± 5) × 10^?17 cm². At the instant of its formation in its X²Π ground state, the rotational temperature of the SiH radical is 950 K. This temperature subsequently relaxes by collisions with SiH at a collisional relaxation rate k = (9 ± 2) × 10¹³ cm³/mole s. Quenching of the A²Δ electronic state of SiH was found to be negligible below 0.3 Torr. The primary reaction path for destruction of the SiH radical was observed to be via: SiH + SiH₄ → products, k = 2 × 10¹² cm³/mole s.     

Spectroscopy and photochemical reactivity of cyclooctadiene platinum complexes

The spectroscopic and photochemical properties of a series of 1,5-cyclooctadiene platinum complexes of the type [(COD)Pt(R)₂] (R=alkyl, alkynyl, or aryl) were examined. The observed photoreactivity is wavelength dependent and observed reaction rates correlate with the donor-strength of the R group. For strongly donating substituents like adamantylmethyl, benzyl or iso-propyl rates were increased by factors of about 100 for a given model reaction compared to the dimethyl derivative. The products were determined by NMR spectroscopy. Different reaction pathways were found depending on the substituents R. Theoretical calculations (DFT) on the electronic structure revealed the character of optical transitions and excited states.     

Hydroxyl radical,sulfate radical and nitrate radical reactivity towards crown ethers in aqueous solutions

Reaction rate constants of crown ethers (12-crown-4, 15-crown-5, 18-crown-6) and their analogs 1,4-dioxane (6C2) with some important oxidative radicals, hydroxyl radical (OH), sulfate radical (SO₄^?) and nitrate radical (NO₃), were determined in various aqueous solutions by pulse radiolysis and laser photolysis techniques. The reaction rate constants for 6C2 and crown ethers with OH and SO₄^? increase with the number of hydrogen atoms in the ethers, indicating that the hydrogen-atom abstraction is a dominant reaction between crown ethers and these two radicals. The presence of cations in solution has negligible effect on the rate constants of crown ether towards OH and SO₄^?. However, for the NO₃, the rate constants are not proportional to the number of hydrogen atoms in ethers, and 12-crown-4 (12C4) is the most reactive compared with other crown ethers. Except 12C4 and 6C2, the cations in the aqueous solution affect the reactivities of 15-crown-5 (15C5) and 18-crown-6 (18C6). The cations with high binding stability for crown ether would improve the reactivity of 15C5. For the studied crown ethers, the reaction rate constants of these oxidative radicals have the order OH>SO₄^?>NO₃. Furthermore, the formation of radicals after the reaction of crown ethers with sulfate radical could be observed in the range of 260–280 nm using laser photolysis and pulse radiolysis. This is the first report on the kinetic behavior of crown ethers with NO₃, and it would be helpful for the understanding of stability of crown ethers in the processing of spent nuclear fuel.     

Structure and dynamics of photogenerated triplet radical ion pairs in DNA hairpin conjugates with anthraquinone end caps

A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, (1*)Aq undergoes rapid intersystem crossing to yield (3*)Aq, which is capable of oxidizing purine nucleobases resulting in the formation of (3)(Aq(-?)Gn(+?)). All (3)(Aq(-?)Gn(+?)) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the (3*)Aq precursor. The TREPR spectra of the (3)(Aq(-?)Gn(+?)) radical ion pairs depend strongly on their spin-spin dipolar interaction and weakly on their spin-spin exchange coupling. The anisotropy of (3)(Aq(-?)Gn(+?)) makes it possible to determine that the π systems of Aq(-?) and G(+?) within the radical ion pair are parallel to one another. Charge recombination of the long-lived (3)(Aq(-?)Gn(+?)) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.     

Spectroscopy of the free phenalenyl radical

After benzene and naphthalene, the smallest polycyclic aromatic hydrocarbon bearing six-membered rings is the threefold-symmetric phenalenyl radical. Despite the fact that it is so fundamental, its electronic spectroscopy has not been rigorously scrutinized, in spite of growing interest in graphene fragments for molecular electronic applications. Here we used complementary laser spectroscopic techniques to probe the jet-cooled phenalenyl radical in vacuo. Its spectrum reveals the interplay between four electronic states that exhibit Jahn-Teller and pseudo-Jahn-Teller vibronic coupling. The coupling mechanism has been elucidated by the application of various ab initio quantum-chemical techniques.     

Metal-controlled reactivity of a pincer-type, sigma-coordinated naphthyl radical anion

Reduction of the new, naphthalene-based pincer complex [(C10H5(CH2PiPr2)2)Rh(eta1-N2)] with potassium metal gave the corresponding sigma-coordinated naphthyl radical anion complex, with a ring-centered radical and no change in the formal metal oxidation state. This paramagnetic complex can be reoxidized to the diamagnetic one with [Cp2Fe][BF4], the structural integrity being retained. Unexpectedly, treatment of a THF solution of the reduced complex with water leads to the immediate evolution of dihydrogen, with reoxidation to the starting complex. This is in striking contrast with the well-known reactivity of the naphthide radical anion, which undergoes ring protonation by water to form 1,4-dihydronaphthalene and naphthalene in a 1:1 ratio.     

Recent advances in the formation of luminescent lanthanide architectures and self-assemblies from structurally defined ligands

This article gives some highlights of the recent advances in the development of novel lanthanide based complexes, conjugates and self-assembly structures formed from the use of organic ligands and organo-metallic (transition metal) complexes, that are designed with the aim of capitalising on the high coordination requirement of the lanthanide ions. The examples shown, demonstrate the versatility of the lanthanide ions as luminescent probes and sensors that emit at long wavelength either in the visible or the near infrared (NIR) part of the electromagnetic spectrum.     

Spectroscopy and reactivity of size-selected Mg+ -ammonia clusters

Photodissociation spectra for mass-selected Mg(+)(NH(3))(n) clusters for n=1 to 7 are reported over the photon energy range from 7000 to 38 500 cm(-1). The singly solvated cluster, which dissociates primarily via a N-H bond cleavage, exhibits a resolved vibrational structure corresponding to two progressions in the intracluster Mg(+)-NH(3) modes. The addition of the second, third, and fourth solvent molecules results in monotonic redshifts that appear to halt near 8500 cm(-1), where a sharp feature in the electronic spectrum is correlated with the formation of a Mg(+)(NH(3))(4) complex with T(d) symmetry and the closing of the first solvation shell. The spectra for the clusters with 5 to 7 solvent molecules strongly resemble that for the tetramer, suggesting that these solvent molecules occupy a second solvation shell. The wavelength-dependent branching-ratio measurements show that increasing the photon energies generally result in the loss of additional solvent molecules but that enhancements for a specific solvent number loss may reveal special stability for the resultant fragments. The majority of the experimental evidence suggests that the decay of these clusters occurs via the internal conversion of the initially excited electronic states to the ground state, followed by dissociation. In the case of the monomer, the selective cleavage of a N-H bond in the solvent suggests that this internal-conversion process may populate regions of the ground-state surface in the vicinity of an insertion complex H-Mg(+)-NH(2), whose existence is predicted by ab initio calculations.     

Site-selective modification of proteins for the synthesis of structurally defined multivalent scaffolds

A combination of classical site-directed mutagenesis, genetic code engineering and bioorthogonal reactions delivered a chemically modified barstar protein with one or four carbohydrates installed at specific residues. These protein conjugates were employed in multivalent binding studies, which support the use of proteins as structurally defined scaffolds for the presentation of multivalent ligands.