Electron photodetachment spectroscopy of solvated anions. A probe of structure and energetics

Electron photodetachment of solvated anions of the form ROHF^- has been undertaken. Observation or non-observation of photodetachment provides information about the location of the bridging proton, i.e. whether the complex has the structure ROH·F^- or RO^-·HF. Where photodetachment is observed, the experimental detachment threshold energy yields the solvation energy for the corresponding free anion.     

Electron photodetachment dissociation of DNA anions with covalently or noncovalently bound chromophores

Double stranded DNA multiply charged anions coupled to chromophores were subjected to UV-Vis photoactivation in a quadrupole ion trap mass spectrometer. The chromophores included noncovalently bound minor groove binders (activated in the near UV), noncovalently bound intercalators (activated with visible light), and covalently linked fluorophores and quenchers (activated at their maximum absorption wavelength). We found that the activation of only chromophores having long fluorescence lifetimes did result in efficient electron photodetachment from the DNA complexes. In the case of ethidium-dsDNA complex excited at 500 nm, photodetachment is a multiphoton process. The MS³ fragmentation of radicals produced by photodetachment at λ = 260 nm (DNA excitation) and by photodetachment at λ > 300 nm (chromophore excitation) were compared. The radicals keep no memory of the way they were produced. A weakly bound noncovalent ligand (m-amsacrine) allowed probing experimentally that a fraction of the electronic internal energy was converted into vibrational internal energy. This fragmentation channel was used to demonstrate that excitation of the quencher DABSYL resulted in internal conversion, unlike the fluorophore 6-FAM. Altogether, photodetachment of the DNA complexes upon chromophore excitation can be interpreted by the following mechanism: (1) ligands with sufficiently long excited-state lifetime undergo resonant two-photon excitation to reach the level of the DNA excited states, then (2) the excited-state must be coupled to the DNA excited states for photodetachment to occur. Our experiments also pave the way towards photodissociation probes of biomolecule conformation in the gas-phase by Förster resonance energy transfer (FRET).     

N-butyl-,N-iso-butyl-andN-sec-butyl-2-picolinamineN-oxide complexes derived from copper(II) salts of polyatomic anions

Summary N-butyl-,N-iso-butyl- andN-sec-butyl-2-picolinamineN- oxide complexes have been prepared from copper(II) perchlorate, tetrafluoroborate and nitrate. Violet solids were isolated from the first two salts while complexes isolated from the nitrate salt were blue green or blue. The ligands coordinatevia both theN-oxide oxygen and the amine nitrogen to give bis (ligand) complexes and only in the case of the nitrate solids is there interaction between the copper(II) centre and the polyatomic anion. Resolution of the g feature in the powder spectra of several of these complexes results from the bulkiness of the butyl groups and therefore dilution of the copper(II) centers. In comparison to the previously studied 2 picolinamineN-oxides with lower alkyl substituents, the complexes have stronger in-plane bonding by the ligands.NATO Fellow on leave from Istanbul Medical Faculty, Istanbul University.     

Electron photodetachment from gas phase peptide dianions. Relation with optical absorption properties

Electron detachment from peptide dianions is studied as a function of the laser wavelength. The first step for the detachment is a resonant electronic excitation of the dianions. Electronic excitation spectra are reported for three peptides, including gramicidin. A comparative study of the detachment yield for 13 peptides was performed at 260 nm and at 220 nm. At 260 nm, the detachment yield is mainly driven by the sum of the absorption coefficients of the aromatic amino acids that are contained in the peptide. At 220 nm, no direct relation is observed between the electron photodetachement yields and the sum of absorption efficiencies. At this wavelength, the sequence and the structure of the peptide may have an influence on the photodetachment process.     

Cimetidine complexes derived from cobalt(II), nickel(II) and copper(II) salts of polyatomic anions

Summary Cobalt(II), nickel(II) and copper(II) complexes [M(CM)₂]X₂ (X = BF₄ or NO₃) have been prepared with cimetidine (CM). Powder data, molar conductivities, magnetic moments, i.r. and electronic spectra support apseudo-octahedral stereochemistry with MN₂N ₂ S₂ chromophore and tentative structures are proposed for the complexes. Their properties are compared with these of previously prepared complexes.     

Theoretical study of photodetachment processes of anionic boron clusters. II. Dynamics

Photodetachment bands of anionic boron clusters, B(n) (n = 4,5) are theoretically examined here. The model Hamiltonians developed through extensive ab initio quantum chemistry calculations in Paper I are employed for the required nuclear dynamics study. While the precise location of vibronic lines and progression of vibrational modes within a given electronic band is derived from time-independent quantum mechanical studies, the broadband spectral envelopes and the nonradiative decay rate of electronic states are calculated by propagating wave packets in a time-dependent quantum mechanical framework. The theoretical results are in good accord with the experiment to a large extent. The discrepancies between the two can be partly attributed to the inadequate energy resolution of the experimental results and also to the neglect of dynamic spin-orbit interactions and computational difficulty related with detachment channels involving multi-electron transitions in the theoretical formalism.     

Photochemical formation of perchlorate from aqueous oxychlorine anions

Evidence of atmospherically produced perchlorate is being accumulated, yet information regarding its formation process is largely unknown. For the first time, the present study demonstrates that perchlorate can be generated as an end-product of photochemical transformation reactions of chlorine precursors such as aqueous salt solutions of hypochlorite, chlorite, and chlorate upon exposure to ultraviolet (UV) radiation. For example, under exposure to UV light from photochemical reactor lamps at a peak wavelength of 253.7 nm for 7 days, the observed perchlorate concentrations were 5, 25, and 626 μg/L at initial chlorite concentrations of 100, 1000, and 10,000 mg/L, respectively. In addition, perchlorate was generated within 7 days from aqueous chlorite solutions at mid-latitude (33°59′N, 101°89′W) spring and summer solar radiation. Via UV radiation from the artificial lamps and sunlight, chlorite was converted to chloride (68%) and chlorate (32%) as end-products on the basis of molar percentage. However, perchlorate was not detected from aqueous chloride solutions at initial concentrations up to 10,000 mg/L under the experimental conditions. Relevant mechanistic pathways were proposed based on the fact that chlorine dioxide (as a primary intermediate) may play a significant role in phototransformation of the precursors leading to perchlorate.     

Electron spin emission spectra from pairs of radicals

Electron spin emission from both radicals of a pair has been observed. It is demonstrated that it is possible to explain the polarization by the radical pair mechanism if nuclear relaxation is fast enough. The extent of ST_?1 mixing induced by an anisotropic g-factor is deduced.     

Ionic vs pericyclic transition states for the cyclization reactions of C alpha-N anions derived from amides and phosphinamides

[structure: see text] Ab initio and DFT calculations indicate that the anionic cyclization of vinyl- and phenyl-carboxamides and phosphinamides metalated at the C(alpha)-N carbon fits better to a Michael-type ionic addition than to an electrocyclic ring closure. The lithium atom has no influence on the type of reaction mechanism.     

Decompositions of odd- and even-electron anions derived from deoxy-polyadenylates

Radical anions have been formed via electron transfer from multiply charged 5′-d(AAA)-3′ and 5′-d(AAAA)-3′ anions to CCl₃ ⁺. These ions have been isolated in a quadrupole ion trap operated with helium bath gas at a pressure of 1 mtorr and subjected to resonance excitation (i. e., conventional ion trap collisional activation). Collisional activation of the even-electron species of the same charge state formed directly via electrospray was also performed by using essentially identical conditions. The collisional activation data can be compared directly without ambiguity arising from differences in parent ion internal energies and/or dissociation time frames. Both the odd- and even-electron anions yield extensive sequence-informative fragmentation but show significant differences in the extent of nucleobase loss and in the relative contributions from the various sequence diagnostic dissociation channels. The results of this study indicate that radical anions derived from multiply deprotonated oligo-deoxynucleotides that survive the electron transfer process are stable with respect to fragmentation in the ion trap environment under normal storage conditions and that the unimolecular dissociation behavior of these ions differs from the even-electron anions of the same charge state. These findings suggest, therefore, that odd- and even-electron anions might be used to provide complementary sequence information in cases in which neither ion type provides the full sequence.     

Tritium enrichment from aqueous solutions using cryosublimation of mono- and polysaccharides

Cryosublimation is one technique, which allows the accumulation of tritium from aqueous solutions using certain chemical compounds. After studying several inorganic compounds such as zeolites and metal salts,¹ as well as some humic substances,² we have now investigated several mono-and polysaccharides, such as glucose, maltose, galactose, starch, agar, and gelatine. Except for starch all of the above mentioned compounds showed a clear enrichment of tritium. The highest value was reached for Agartine, which gave an enrichment factor of 6.2. Since mono-and polysaccharides form weak hydrogen bonds, these results prove again our theory that tritium is preferably accumulated in exchangeable hydrogen bonds.     

Modeling quantum dynamics of photodetachment from closed‐shell anions: Static versus fluctuating well‐depth models

The electronic states of halide ions are modeled by a one‐dimensional Hamiltonian with a potential V(x)=−V₀e. The two parameters V₀ and σ are fixed by requiring V(x) to reproduce the experimentally observed ground‐state ionization potentials of the halide ions concerned. The potentials so generated are shown to support only one bound state in each case. The time‐dependent Fourier grid Hamiltonian method is used to follow the ionization dynamics in monochromatic light of fairly high intensities. The total Hamiltonian, in the most general case, reads H(t)=P/2m−V₀e−ϵ₀s(t)ex sin(ωt). For pulsed fields [s(t)=sin²(πt/t_p)], the computed ionization rate constants are seen to increase with increase in the peak intensity (ϵ₀) of the electric field of light. The possibility of additional transient bound states being generated at the high intensities of light and its possible consequences on the observed ionization rates are explored. The environmental effects on the dynamics are sought to be modeled by allowing the well depth (V₀) to fluctuate randomly [V₀(t)=V₀+ΔVR(t); R(t) randomly fluctuates between +1 and −1 with time, ΔV is fixed]. The ionization rate constants (k_ϵ) are shown to be significantly affected by fluctuations in V₀ and pass through a well‐defined minimum in each case for a certain specified frequency of fluctuation. An alternative model potential V(x)=−V₀e^−σx is also shown to yield similar results. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 469–478, 1999     

Comparative modelling of mono- and dinitrophenols sorption on yellow bentonite from aqueous solutions

Equilibrium removal of three substituted nitrophenols, namely 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), and 2,4-dinitrophenol (2,4-DNP), by adsorption on yellow bentonite was tested. The batch kinetic data are described by the pseudo-first order, pseudo-second order, and intraparticle diffusion models. Results show that chemisorption processes could be rate limiting in the sorption step. The adsorption equilibrium was modelled by the Langmuir and Freundlich equations. The Langmuir model better represents the equilibrium isotherm data for 2-NP, 4-NP, and 2,4-DNP uptake on yellow bentonite. 4-NP is adsorbed in larger amounts than the disubstituted nitrophenol. Uptake of nitrophenols increases in the order 2-NP < 2,4-DNP < 4-NP.     

Generalized oscillator strengths of polyatomic molecules. II. NH3

Generalized oscillator strengths for a number of singlet transitions of the NH₃ molecule, evaluated according to the random-phase approximation approach, are presented and discussed so as to provide characterization of some portions of the Bethe surface of the molecule. © 1996 John Wiley & Sons, Inc.     

Vibrational dynamics of some anions in aqueous solutions from isotropic Raman scattering

From the I_VV and I_VH Raman spectra of a totally symmetric vibration, isotropic scattering profiles have been obtained for several anions in dilute aqueous solutions at various temperatures. The Raman spectrometer was connected to an A/D converter and a small computer, which allowed multiple scanning, data accumulation, and numerical processing of these profiles with good precision over a frequency range of about 20 times their full width at half peak height.The theoretical analysis of the isotropic Raman profiles was accomplished by assuming vibrational phase relaxation based on a MARKOV-GAUSS mechanism. Fitting procedures to the experimental data permitted the determination of the vibrational frequency distribution and its modulation time. Temperature effects are discussed in relation to the distribution and motional characteristics of the immediate environment of water molecules of the anions.     

Phase morphology of hydrolysable polyurethanes derived from aqueous dispersions

Biodegradable polyurethanes are an interesting alternative to many applications that involve plastics since they can minimize environmental problems caused by the low rates of natural degradation of synthetic polymers. In addition, since waterborne polyurethanes are based on aqueous dispersions, they restrict the use of organic solvents during processing and application of the polymer, thus contributing furthermore to reduce environmental damage. In this work, aqueous anionic polyurethane dispersions (PUD) with tailorable susceptibility for hydrolysis were synthesized by progressively replacing polypropylene glycol (PPG) with a biodegradable polycaprolactone diol (PCL) as soft segments. The hard segments were formed by extending isophorone diisocyanate (IPDI) with hydrazine (HZ). Dimethylol propionic acid (DMPA) was used as ionic center and triethyl amine (TEA) as neutralizer. The degree of phase separation was evaluated mainly by infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS). The results indicated that phase separation between hard and soft segments of poly(ester-urethane) is more significant than that of poly(ether-urethane). Data obtained from SAXS experiments indicated that phase separation within soft domains can also be present in samples containing both polyester and polyether soft segments. Hydrolytic degradation of the polymers in buffer solution of pH 7.4 and alkaline solution was performed as an initial test. The results showed that the fraction of polyester soft segments in the polyurethanes can be used to tailor the susceptibility of the materials to hydrolytic attack. Polyurethanes having higher contents of polyester were more promptly hydrolytically degraded than polyurethanes containing only polyether segments.     

Spectrothermodynamic relationship of cationic vs anionic species derived from protonation vs deprotonation of pyrrolo-aza-aromatic bases in homologous series.

Theoretical research on the spectroscopy and protonation energies of ionic species related to the neutral pyrrolo-aza-aromatic bases has been carried out, using Density Functional Theory (DFT) and its time dependent form (TDDFT). In the ionic species the skeleton localized (+) and (-) charge in the protonated and deprotonated species is shown to have a strong perturbation of the pi-electronic states. The lowest electronic S(0) --> S(1) (pi,pi*) transitions are shown to have near-coincidence for each cation and anion for the whole homologous series, in agreement with the Valle-Kasha-Catalán rule previously stated. It is further demonstrated that simultaneous dramatic changes, upon electronic excitation, in acidity and basicity at the pyrrolo- and aza-positions of the molecular skeleton are the driving force for the biprotonic phototransfer processes in these bases. This constitutes confirmation of the proton-transfer rather than H-atom transfer as the reaction mechanism.