Microwave spectra of transient species — cyclopentadienone

The microwave spectrum of the main isotopic species of cyclopentadienone has been measured and assigned. The rotational constants imply an essentially planar geometry and that the ring is rather “fatter” than the analogous ring in fulvene.The dipole moment, derived from Stark-effect measurements, is 10.4 × 10^?30 C m (3.13 D). A vibrational satel- lite spectrum, which, from its inertial defect, corresponds to an out-of-plane vibration has also been assigned.The life time of cyclopentadienone under our spectroscopic conditions is approximately 15 sec.     

Ultrafast 2D-IR spectroscopy of transient species.

Multidimensional spectroscopic experiments offer fascinating insights into molecular structure and dynamics in the field of NMR spectroscopy. With the introduction of ultrafast two-dimensional infrared spectroscopy (2D-IR), multidimensional concepts have entered the optical domain, measuring couplings and correlations between molecular vibrations with femtosecond time resolution. In the transient 2D-IR (T2D-IR) experiments described in this minireview we exploit the high time resolution of 2D-IR to study transient species during fast nonequilibrium processes in real time. Information on molecular structure and dynamics is obtained that is not available from one-dimensional spectroscopy. We discuss examples from chemistry, physics and biophysics.     

Novel gaseous transient species:generation and characterization

Due to special properties of transient species,such as short-lived,unstable,reactive,and even explo-sive,the generation and subsequent characterization is a great challenge for experimental chemists. In our laboratory,systematic researches have been carried out to investigate novel transient species:reactive halogen species(RHS) ,short-lived free radicals,and metastable pseudohalogen compounds,based on the successive technical improvements on the HeI Photoelectron spectroscopy(PES) . In this review,the topic mainly focuses on innovative methods of generating novel transient species,and subsequent geometric and electronic structure characterizations of these species combined with quantum chemical calculations.     

Photoelectron spectroscopy of transient species: The CS molecule

The vacuum ultraviolet photoelectron spectrum of the CS molecule shows the first two ionization potentials to be 11.33 eV and 12.79 eV. The associated vibrational fine structure indicates that the lowest ionization potential (IP) involves removal of an electron from the essentially non-bonding (xσ) orbital whereas the second IP involves removal of an electron from the bonding (wπ) orbital.     

Structures and electronic states of paramagnetic species in Al-NTCDA co-deposit films

Structures and electronic states of paramagnetic species in co-deposit film composed of 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) and aluminum (Al) have been investigated by means of hybrid density functional theory (DFT) calculations to determine the species in detail. Al-NTCDA 1:1 complex, 1:3 Al₃-NTCDA complex and an Al metal bridged dimer Al-(NTCDA)₂ were examined as paramagnetic species of Al-NTCDA complexes. The simulated electron paramagnetic resonance (EPR) spectra of the 1:3 complex and the dimer were in reasonable agreement with experiment reported previously by Tachikawa et al. [Tachikawa et al., J. Phys. Chem. B 109 (2005) 3139]. It was found that the contribution from the 1:1 complex to the EPR spectra was very small. From the comparison with theoretical and experimental UV and EPR spectra, it was found that several paramagnetic and diamagnetic species exist in the co-deposit film of Al/NTCDA. The structures and electronic states were discussed on the basis of theoretical results.     

Investigation of transient species in pulse-irradiated poly(ethylene terephthalate) film

A pulse radiolysis study of poly(ethylene terephthalate), PET, film has been carried out with the main aims of investigating charge trapping. In PET, pulse radiolysis gives electron-positive hole pairs. Both charges can be stabilized by the reaction with polymeric matrix. In the first step, the PET radical anions and cations are formed (transient absorption maxima at ∼ 370 and 530 nm). During the second step, the electron is transferred from a PET radical anion to an ester group, followed by formation of an ester radical anion (transient absorption maximum at 430 nm). The recombination of these ionic species leads to an excited state formation observed during and after the 1-μs pulse. Spectral distribution of luminescence observed for pulse-irradiated PET (emission bands at ∼ 340, ∼ 370, and 400–410 nm) was similar to the one obtained for photoexcited PET. The detailed mechanism of ionic reactions in PET is proposed and discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2853–2862, 1999     

Ionic transient species in irradiated poly(vinyl chloride) film

The studies of transient species in irradiated poly(vinyl chloride) (PVC) film have been carried out with the main aim of investigating the charge trapping in a pure system. In PVC, pulse radiolysis gives electron-positive hole pairs. The electron can generate a matrix anion due to the presence of chlorine atom in the PVC macromolecules followed by Cl⁻ detachment. The positive hole may be stabilized as a short-lived matrix cation characterized by visible absorption band with wide maximum in the 350–650 nm range. The positive holes can be scavenged by additives like pyrene, Py and respective radical-ions of Py can be detected. The rate of Py radical cations decay at ∼450 nm was found to be temperature dependent. Two linear parts of the Arrhenius plot were observed which intersected in the 200–240 K range, which is close to the mechanical loss peak connected with the β relaxation in PVC matrix. The activation energies calculated for two parts of Arrhenius plot were equal to 0.40 kJ mol⁻¹ for T < 200 K and 52.6 kJ mol⁻¹ for T > 240 K. The novel mechanism of the ionic and radical reactions in PVC is proposed and discussed.     

Time-resolved thermodynamics: heat capacity change of transient species during photoreaction of PYP

Heat capacity changes of short-lived transient species in different time ranges were measured for the first time by using the thermal component of the transient grating and transient lens signals at various temperatures. This method was applied to the transient intermediates of Photoactive Yellow Protein (PYP). The temperature dependence of the enthalpy change shows that the heat capacity of the short-lived intermediate pR2 (also called I1 or PYP(L)) species is the same as that of the ground state (pG) species within our experimental accuracy, whereas that of the long-lived intermediate pB (I2 or PYP(M)) is much larger (2.7 +/- 0.4 kJ/mol K) than that of pG. The larger heat capacity is interpreted in terms of the conformational change of the pB species such as melted conformation and/or exposure of the nonpolar residues to the aqueous phase. This technique can be used for photochemical reactions in general to investigate the conformational change and the hydrophobic interaction in a time domain.     

Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Nature of the transient species formed during pulse radiolysis of thioacetamide in aqueous solutions

Thioacetamide (TA) is an organic compound having thioamide group similar to that in thiourea derivatives. Its reactions with e_aq⁻, H-atom and OH radicals were studied using the pulse radiolysis technique at various pHs and the kinetic and spectral characteristics of the transient species were determined. The initial adduct formed by the reaction of TA with OH radicals at pH 7 does not absorb light in the 300–600 nm region but reacts with the parent compound to give a transient species with an absorption maximum around 400 nm. At pH 0, the reaction of OH radicals with TA directly gives a similar transient species with absorption maximum at 400 nm. Transient species formed by H-atom reaction with TA and pH 0 has no absorption in the 300–600 nm region but at higher acidity a new transient species is formed which has absorption maximum at 400 nm. This transient absorption observed in the case of both OH and H atom reaction with TA is ascribed to the formation of a resonance stabilized radical similar to that obtained in the case of thiourea derivatives. The species produced by electron reaction viz. electron adduct was found to be a strong reductant and could reduce MV²⁺ with a high rate constant. H₂S was produced as a stable product in the reaction of e_aq⁻ and its G-value was determined to be about 0.8.     

Characterization of transient species in laser photolysis of aromatic amino acids using acetone as photosensitizer

The photochemical processes of aromatic amino acids were investigated in aqueous solution using acetone as photosensitizer by KrF (248 nm) laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments. The intermediates recorded were assigned to the excited triplet state of tryptophan, the radicals of tryptophan and tyrosine. The excited triplet state of tryptophan produced via a triplet-triplet excitation transfer and the radicals arising from electron transfer reaction has been identified. Neither electron transfer nor energy transfer between triplet acetone and phenylalanine can occur in photolysis of phenylalanine aqueous solution which contains acetone. Furthermore, triplet acetone-induced radical transformation: Trp/N-Tyr→Trp-Tyr/O was observed directly in photolysis of dipeptide (Trp-Tyr) aqueous solution containing acetone, and the transformation resulting from intramolecular electron transfer was suggested.     

He(l) Photoelectron spectroscopy of transient species: The SF2 Molecule

The He(I) photoelectron spectrum of^SF2, formed in the reaction between fluorine atoms and flowers of sulphur, is presented. The spectral interpretation is based on the results of Hartree-Fock-Slater calculations, using STO basis sets of double zeta quality with and without a 3d-polarization function in the sulphur basis set. For comparison similar computations are performed for ^SH2. Calculated dipole moments and population analyses are presented. The ^SF2 photoelectron spectrum is compared with spectra of similar molecules, i.e. SCl₂, OCl₁₂ and OF₂. A discussion in terms of Walsh's predictions for AB₂ molecules with twenty valence electrons is given. Sum rule considerations are found to be especially useful for the assignment of π-type orbitals, and the perfluoro effect is illustrated.     

Structures of ionic species and electronic interactions in radical ions; ketyls with divalent cations

EPR and optical spectra of magnesium and calcium ketyls were investigated in MTHF and mixtures of DMF and MTHF at various concentrations. It was shown that these ketyls form aggregates in ethereal solvents and dissociate into biradicals in more polar solutions. Models of ion aggregate to explain the observed EPR and optical spectra are suggested.     

An EPR characterisation of stable and transient reactive oxygen species formed under radiative and non-radiative conditions

Research on Chemical Intermediates - Electron paramagnetic resonance (EPR) spectroscopy is the ideal method of choice when detecting and studying the wide variety of paramagnetic oxygen-centred...     

UV photoelectron spectroscopy of transient species: germanium dhodide (GeI2)

Valence ionization energies of the transient species GeI₂, obtained with He(I) photoelectron (PE) spectroscopy, are presented. The interpretation of the results is aided by Hartree-Fock-Slater calculations in which relativistic effects have been taken into account through perturbation theory. A comparison with PE data of the other germanium dihalides is made.     

UV photoelectron spectroscopy of transient species: germanium difluoride (GeF2)

The He(1) photoelectron spectrum of germanium difluoride (GeF₂) is reported. Results are analyzed on the basis of Hartree-Fock-Slater quantum-chemical calculations. A comparison is made with previous data for GeCl₂, GeBr₂, CF₂ and SiF₂.     

Characterization of the transient species generated by the photoionization of Berberine: A laser flash photolysis study

Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO₄⁻ radical oxidation method, the assignment of radical cation of BBR was further confirmed, the pK_a value of it was calculated, and the related set up rate constant was also determined.     

Nature of transient species formed during pulse radiolysis of 4-mercaptopyridine in aqueous solutions: Formation of a dimer radical species by one-electron reduction reaction

Reactions of one-electron reducing as well as oxidizing radicals with 4-mercaptopyridine (4-MPy) were studied in aqueous solutions at different pH values. One-electron oxidizing radicals such as N₃ and Br₂ ^–, react with 4-MPy by electron transfer reaction at pH 11 to give 4-pyridylthiyl radical. The reduction potential for the couple 4-PyS /4-PyS^– was estimated to be 0.93V vs. NHE by equilibrium reaction with I₂ ^– /2I^– couple. At pH 6.8, where the compound is predominantly present in the thione form, the transient species formed is a cation radical. OH radicals react with 4-MPy by addition to the pyridine ring at pH 6.8 and 11. At pH 0, OH radicals as well as one-electron oxidants like Cl₂ ^– and Br₂ ^– radicals react with 4-MPy to produce the protonated form of 4-pyridylthiyl radical. At pH 6.8 and 11, e_aq ^– reaction with 4-MPy gave an initial adducts which reacted with the parent molecule to give dimer radicals. Acetone ketyl radicals were unable to reduce 4-MPy at neutral pH. Reducing radicals like H-atoms and acetone ketyl radicals reacted with 4-MPy at acidic pH by H-abstraction reaction to give the same species as produced by oxidizing radicals.