1H CIDNP study of the kinetics and mechanism of the reversible photoinduced oxidation of tryptophyl-tryptophan dipeptide in aqueous solutions

The pulsed method of chemically induced dynamic nuclear polarization (CIDNP) with the microsecond time resolution was used to study the nuclear polarization kinetics of Trp-Trp (tryptophyl-tryptophan) dipeptide protons in the photoreaction between the dipeptide and 2,2??-dipyridyl in aqueous solutions at pH from 2 to 10. The dependence of the selectivity of the reversible photoinduced oxidation of the dipeptide by 2,2??-dipyridyl in the triplet-excited state was found. When the pH of the solutions was below pK _a of the terminal amino group, polarization was observed only for the protons of the C-terminal residue. The threshold effect of protonation of the terminal amino group of the dipeptide on the selectivity of both the deactivation of the 2,2??-dipyridyl triplet state and the degenerate intermolecular electron exchange was revealed. No intramolecular electron migration in the oxidized dipeptide was detected. At the pH exceeding pK _a of the terminal amino group of the dipeptide, the quenching of the triplet excited state of 2,2??-dipyridyl is not selective. This is manifested as equal intensities of the nuclear polarization signals of the both residues of tryptophan in the 1H NMR spectrum of the dipeptide containing the CIDNP signals of the geminate reaction products. The rate constant of the degenerate electron exchange and nuclear paramagnetic relaxation times of the protons of the neutral radical and radical cation of the dipeptide were determined by the numerical simulation of the nuclear polarization kinetics.     

Effects of paramagnetic lanthanide ions on decay process of photogenerated radical pairs in porphyrin-viologen linked compounds at reversed micellar surfaces

Radical pairs were generated in reversed micelles by laser excitation of zinc tetraphenylporphyrin-viologen linked compounds (ZPnV) with a polymethylene spacer (?(CH₂)_n?; n=4, 6 and 8). The optical transient-absorption and chemically induced dynamic electron polarization (CIDEP) spectra indicated that paramagnetic lanthanide ions invariably accelerated the decay process of the radical pairs at 0.3 T. The typical E/A/E/A spectra for the ZP6V and ZP8V systems were explained as due to S-T₀ mixing and electron-spin relaxation between Zeeman-splitted triplet sublevels. In the case of ZP4V, the paramagnetic Gd³⁺ ion accelerated the radical decay process even at zero-magnetic field, and strong emissive CIDEP spectra evolved with the elapsed time after laser excitation. Appreciable contribution of S-T level crossing in the radical decay process was suggested to account for the novel features of the ZP4V system.     

Line dependence of CIDEP polarizations for the p-benzosemiquinone,radical

Time-resolved steady state experiments were performed on six prominent hf-lines of the p-benzosemiquinone radical (PBQH) dissolved in ethylene glycol using an improved experimental technique. For the same six lines T₁ and _T2 were determined experimentally. Initial and radical pair polarizations as well as the second order chemical decay constant β were obtained by simulating each of the time-resolved curves using a new procedure. It was found that both the initial polarization P_a^∞(I) and β were the same within experimental accuracy for all six lines in four different experiments. The radical pair polarization P_a*, however, varied within the experimental accuracy in the manner predicted qualitatively from the theory for a second order termination reaction. Light attenuation experiments with both pulsed and continuous light confirmed these results. The magnitude of P_a^∞ (I) may be interpreted in terms of the microscopic theory for the photo-activated triplet mechanism, whereas the magnitude of P_a* appears to be too high by a factor of four to six as compared to the microscopic theory for the radical pair mechanism.     

Electron spin polarization in a three-electron spin system. An application to bacterial photosynthesis

Calculations on a system consisting of three electron spins and one nuclear spin are presented and their implications for bacterial photosynthesis discussed. Comparison with experimental measurements of electron spin polarization in pre-reduced photosynthetic reaction centres leads to conclusion that the exchange interaction within the primary radical pair is positive and less than 0.8 mT when the g values of the photoinduced radicals are taken to be those measured for the isolated radical species.     

Photoinduced electron transfer between C60 and N,N,N′,N′-tetramethylbenzidine (NTMB). Fourier transform electron paramagnetic resonance study

Fourier transform EPR spectroscopy was employed in studying the electron transfer (ET) reaction and the quenching mechanisms of the photoexcited triplet state of C₆₀ as electron acceptor and N,N,N′,N′-tetramethylbenzidine (NTMB) as electron donor in benzonitrile solution. The ET reaction product, the cation radical NTMB^*+, interacts with ^3*C₆₀, leading to photoinduced electron polarization of NTMB^*+ via triplet-doublet mixing mechanism combined with triplet mechanism. The quenching of ^3*C₆₀ and the polarization behavior of NTMB^*+ are discussed.     

Analysis of guest—host and intra-guest transitions for hydronaphthyl radicals in naphthalene crystals

High resolution spectroscopic studies of optical absorption and emission around 539 nm and 636 nm for 1- and 2-hydronaphthyl radicals in naphthalene crystals have been made. For each radical several lines with the same polarization as the intra-guest OO vibrational lines were assigned as the vibrational progression, the separation being explained in terms of the intramolecular vibrations of naphthalene. About 20 other lines for each radical, including completely a- or b-polarized lines, were ascribed to the guest—host charge transfer transitions. Each line was assigned to the transition to specific molecules around the radical taking the polarization into consideration. The energy of the transition to a molecule at a distance r can be described by a coulombic relation with the dielectric constant ?: E = E_∞ ?e²/?r, where E_∞ is a constant. The phonon sidebands of the absorption and emission lines of the most intense charge transfer transition in the 1-hydronaphthyl radical was also analyzed and compared with phonon singularities in naphthalene crystals.     

A simple chemical model for the electron polarization of cyclopentyl radicals produced in radiolysis

The electron polarization of cyclopentyl radicals was observed by Smaller . This paper presents an attempt to account for this observation by an initial polarization mechanism. The mechanism is based on the addition of hydrogen atoms to the product olefin to give the alkyl radicals. The electron polarization of the alkyl radicals arises from the polarized hydrogen atoms. It should be emphasized that the processes leading to polarized H atoms are not established but it is assumed that they are formed initially with the populations in αa_eα_N and α_eβ_N being greater than β_eα_N and β_eβ_N, respectively.     

Electron Capture Dissociation of Hydrogen-Deficient Peptide Radical Cations

Hydrogen-deficient peptide radical cations exhibit fascinating gas phase chemistry, which is governed by radical driven dissociation and, in many cases, by a combination of radical and charge driven fragmentation. Here we examine electron capture dissociation (ECD) of doubly, [M + H]^2+?, and triply, [M + 2H]^3+?, charged hydrogen-deficient species, aiming to investigate the effect of a hydrogen-deficient radical site on the ECD outcome and characterize the dissociation pathways of hydrogen-deficient species in ECD. ECD of [M + H]^2+? and [M + 2H]^3+? precursor ions resulted in efficient electron capture by the hydrogen-deficient species. However, the intensities of c- and z-type product ions were reduced, compared with those observed for the even electron species, indicating suppression of N?CC_?? backbone bond cleavages. We postulate that radical recombination occurs after the initial electron capture event leading to a stable even electron intermediate, which does not trigger N?CC_?? bond dissociations. Although the intensities of c- and z-type product ions were reduced, the number of backbone bond cleavages remained largely unaffected between the ECD spectra of the even electron and hydrogen-deficient species. We hypothesize that a small ion population exist as a biradical, which can trigger N?CC_?? bond cleavages. Alternatively, radical recombination and N?CC_?? bond cleavages can be in competition, with radical recombination being the dominant pathway and N?CC_?? cleavages occurring to a lesser degree. Formation of b- and y-type ions observed for two of the hydrogen-deficient peptides examined is also discussed.     

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a₂, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D_3h to C_2v at 4 K in CFCl₃, and the SOMO is likely to be 6b₂, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C₆F₁₂ as well as in CFCl₂CF₂Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.     

Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor–acceptor chromophore via two different meta-phenylene bridges in (bpy)Pt(CAT-m-Ph-NN) (mPh-Pt) and (bpy)Pt(CAT-6-Me-m-Ph-NN) (6-Me-mPh-Pt) (bpy = 5,5′-di-tert-butyl-2,2′-bipyridine, CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy˙⁻, CAT⁺˙ = semiquinone (SQ), and NN), charge-separated doublet ²S₁ (S = chromophore excited spin singlet configuration) excited state. Following excitation, the ²S₁ state rapidly decays to the ground state by magnetic exchange-mediated enhanced internal conversion via the ²T₁ (T = chromophore excited spin triplet configuration) state. This process generates emissive ground state ESP in 6-Me-mPh-Pt while for mPh-Pt the ESP is absorptive. It is proposed that the emissive polarization in 6-Me-mPh-Pt results from zero-field splitting induced transitions between the chromophoric ²T₁ and ⁴T₁ states, whereas predominant spin–orbit induced transitions between ²T₁ and low-energy NN-based states give rise to the absorptive polarization observed for mPh-Pt. The difference in the sign of the ESP for these molecules is consistent with a smaller excited state ²T₁ – ⁴T₁ gap for 6-Me-mPh-Pt that derives from steric interactions with the 6-methyl group. These steric interactions reduce the excited state pairwise SQ-NN exchange coupling compared to that in mPh-Pt.

A change in the sign of the ground state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor–acceptor chromophore via two different meta-phenylene bridges.     

Electron affinity and substituent effects in radical anions

The first vertical electron affinities EA of 13 series of molecules and free radicals D(X _i ) _n are related to the inductive (σ _I ), resonance (σ _R ^? ), and polarization (σ_α) parameters of substituents X _i by the dependences EA = EA _H + aΣσ _I + bΣσ _R/? + cΣσ_α: In radical anions D^+·(X _i ) _n , compared to radical cations D^+·(X _i ) _n , the polarization interaction is weaker or similar in magnitude but has an opposite sign. The previously unknown resonance parameters σ _R ^? of substituents SiMe₃ and CH₂SiMe₃ bound to the radical anion center H₂C=CH^?· were calculated.     

Chemically induced dynamic nuclear polarization in the dehalogenation of <Emphasis Type="Italic">gem</Emphasis>-dihalocyclopropanes under the action of <Emphasis Type="Italic">i</Emphasis>-Bu<Subscript>2</Subscript>AlH in the presence of Zr(acac)<Subscript>4</Subscript>

The integral and multiplet effects associated with the chemically induced dynamic nuclear polarization in the catalytic dehalogenation of alkyl-and aryl-gem-dibromocyclopropanes under the action of diisobutylaluminum hydride in the presence of Zr(acac)₄ have been discovered and investigated. The main products of the reaction are monobromocyclopropanes and cyclopropanes. The other reaction products are isobutane, isobutylene, and isobutyl bromide. Integral polarization is observed for the isobutylene protons; multiplet polarization, for the isobutyl bromide protons. Integral polarization arises in a geminal singlet radical pair between an isobutyl radical and a Zr(III) paramagnetic ion that have resulted from the homolysis of an i-Bu-Zr bond.     

CIDNP in the radiolysis of aqueous methanol indication of methoxy radical involvement

The unusual high-field polarization in ethylene glycol observed in the NMR spectra of irradiated aqueous methanol implies that an encounter of a hydroxymethyl radical with a radical having a higher g-factor precedes dimerization. Arguments are presented supporting identification of CH₃O· as the glycol polarizing species. Participation of e^?_aq in the formation of CH₃O· in a spur process is considered.     

Electrochemical reduction of organic and organometallic compounds. Polarization effect in radical anions

Half-wave potentials of electrochemical reduction and electron affinities of X-B-R_C compounds belonging to 23 reaction series depend not only on the inductive and resonance effects but also on the polarization effect of the X substituent, which was not taken into account previously. In some cases, the contribution of the polarization effect reaches 50% of the overall substituent effect. The main factors responsible for the magnitude of polarization effect in X-B-R _C ^? radical anions are the natures of reaction center R_C and bridging moiety B and the distance between the substituent X and reaction center R_C.     

ESR study of the photo-induced decomposition of polypropylene in the presence of N,N,N′,N′-tetramethyl-p-phenylenediamine

The mechanism of uv (λ > 325 nm) photodegradation of polypropylene (PP) containing N,N,N′,N′-tetramethyl-p-phenylenediamine (T₄MPD) has been investigated by means of ESR spectroscopy. The observed spectra after uv irradiation of both isotactic-PP (IPP) and stereoblock-PP (SPP) samples in vacuum at 77 K consisted principally of a broad singlet which was assigned to a T₄MPD cation radical (T₄MPD^+·). On the other hand, the spectrum observed after irradiation of an atactic polypropylene (APP) sample at 77 K in vacuum was resolved into several components which decayed almost up to ca. 263 K to give rise to the broad singlet of T₄MPD^+·. One component was a sharp quartet which was assigned to a methyl radical, ·CH₃·. The other component, a singlet, was attributed to a trapped electron, e_t^?.By comparison of the ESR spectrum of deuterated T₄MPD with that of the normal compound it was found that 60 ～ 70% of the methyl radicals arose from the added T₄MPD due to β-scission, which also formed the N,N,N′-trimethyl-p-phenylenediamine radical, T₃MPD·. The T₃MPD· radical presumably captures an electron at lower temperatures to become a carbanion, T₃MPD^?, which releases the electron to reproduce the T₃MPD· radical at elevated temperatures. This production of the radical T₃MPD· due to the liberation of an electron provides an explanation for the observed increase in intensity of the decay curve in the temperature range from ? 168 K to 185 K. The remaining fraction, 30 ～ 40%, of the total methyl radicals was produced from the PP matrix by an energy transfer from the excited T₄MPD¹ to the PP matrix. The broad singlet which appeared in the temperature range near 195 K was attributed to an acyl radical ～CH₂CH(CH₃)CH₂?O from the observed g-value. By photoillumination of this sample this broad singlet was converted reversibly into the quartet which was assigned to the radical ～CH₂CH(CH₂·)CH₂CHO.     

Fourier transform-EPR studies of photoinduced electron transfer in homogeneous and micellar solutions

Fourier transform (FT) EPR was used to study the pulsed-laser-induced electron transfer from porphyrins to quinones in homogeneous and micellar solutions. By monitoring the EPR signal of the quinone anion radicals as a function of delay time (τ_d) between laser and microwave pulses, with τ_d ranging from nanoseconds to 1 millisecond, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. FT-EPR spectra of electron transfer products generated in micellar solution provide evidence for the generation of long-lived spincorrelated radical pairs.     

Chemically induced dynamic electron polarization. The isotropic hyperfine coupling and the CIDEP of radicals produced by pulse radiolysis

Time resolved electron paramagnetic resonance spectroscopy was used to examine the electron spin polarization in several radicals produced in pulse radiolysis of aqueous alcoholic solutions. The observation of the distribution of polarized intensities in the various hyperfine lines in an EPR spectrum provides yet another indication of the S-T₀ mixing of the electron-nuclear states in radical pairs.     

Radical cations of cyclopentadiene dimers—facets of an intriguing energy surface

The radical cations of various cyclopentadiene dimers can be generated by photoinduced electron transfer to strong electron acceptors. Nuclear spin polarization effects observed during these reactions provide insight into the radical cation structures. Three of the systems studied, endo-[4 + 2]-(1), anti-[2+2]-(2), and exo-(4+2]-dicyclopentadiene (5) give rise to unusual singly linked, delocalized radical cations, whereas the anti-[4 + 4]-dimer (3) and 1,3-bishomocubane (4) give rise to more conventional radical cations. The reactions of spiroheptadiene (9) and di(spiroheptadiene) (10) provide evidence for two different dimer radical cations, a doubly linked (D) and a singly linked (S) species. This finding supports a stepwise mechanism for the radical cation Diels-Alder reaction of 9.     

Cross sections and spin polarizations of electrons elastically scattered from oriented molecules (CH3I)

Elastic differential cross sections and spin polarizations for electrons elastically scattered from CH₃I are calculated using the independent atom model. Three molecular orientations with respect to the incident electron wavevector are considered — first, the molecule is oriented randomly, second, the electron wave front and molecular bond are parallel, and third, the wavefront and the bond axis are perpendicular. It will be seen to what extent orientational averaging weakens features of the cross section and spin polarization. The calculations show that cross section and spin polarization measurements are a possible tool for determining the degree of molecular orientation. There is no degeneracy betweenI–C andC–I in cross section and spin polarization measurements. The results presented here for 200 eV and 600 eV electrons scattered by CH₃I should be considered as a case study and it should be possible to find molecules and electron energies for which even more dramatic differences between the various orientations between the molecules and the electrons can be expected.     

The kinetics of radical polymerization—XXVII. Investigation of the radical reactivity of 3,3′- and 4,4′-disubstituted azobenzenes in the polymerization of vinylacetate

A study was made on the inhibition by some 3,3′- and 4,4′-disubstituted azobenzenes of the polymerization of vinylacetate initiated by azoisobutyronitrile at 50°. The inhibitory effects of these substances can be attributed to their ability to engage in radical addition giving a less reactive hydrazyl type radical. The mechanism of the inhibition has been established by ESR and kinetic (stoichiometric) measurements. The value of k₅/k₂ (characteristic of the reactivity of an inhibitor) was determined for 9 substituents. The radical reactivity of the aromatic azo group was decreased by electron donor substituents and increased by electron acceptors. The substituent effect can be well interpreted by the Hammett equation; the value of the reaction constant was ? = +0.53.