Mononucleotide gas-phase proton affinities as determined by the kinetic method

The goal of this work is to determine the proton affinities of (deoxy)nucleoside 5'- and 3'-monophosphates (mononucleotides) using the kinetic method with fast atom bombardment mass spectrometry. The proton affinities of the (deoxy)nucleoside 5'- and 3'-monophosphates yielded the following trend: (deoxy)adenosine monophosphates > (deoxy)guanosine monophosphates > (deoxy)cytidine monophosphates > deoxythymidine/uridine monophosphates. In all cases the proton affinity decreases or remains the same with the addition of the phosphate group from those values reported for nucleosides. The proton affinity is dependent on the location of the phosphate backbone (5'-vs. 3'-phosphates): the 3'-monophosphates have lower proton affinities than the 5'-monophosphates except for the thymidine/uridine monophosphates where the trend is reversed. Molecular modeling was utilized to determine if multiple protonation sites and intramolecular hydrogen bond formation would influence the proton affinity measurements. Semiempirical calculations of the proton affinities at various locations on each mononucleotide were performed and compared to the experimental results. The possible influence of intramolecular hydrogen bonding between the nucleobases and the phosphate group on the measured and calculated proton affinities is discussed.     

Electron affinities of the lanthanides

The electron affinities (EA's) of the lanthanides (La through Tm) have been determined from the expression EA = IP₁ ? C)r^?1)_n1, where IP₁ is the first ionization potential and (r^?1)_n1 is the relativistic radial integral of an electron in the unfilled shell, the constant C includes the quantum numbers n, 1 of the partly filled shells and the atomic number Z of each element. The EA's vary from +0.5 eV (La) to –0.2eV (Tm) which is consistent with other semi-empirical estimates for certain lanthanide elements.     

Electron affinities of the lanthanides

The electron affinities of the lanthanides (La through Lu) are estimated by considering the energy variations associated with changes in the 4f orbital population. The ground-state electron affinities are all predicted to be within the range +0.5 eV to ?0.3 eV.     

Electron affinities of double bond π* orbitals determined by means of electron transmission spectroscopy

The electron transmission spectra of small molecules containing C=C, C=N, C=O, C=S and N=N double bonds are reported. The electron affinities of these functional groups, associated with electron capture into their empty π^* orbitals, are discussed in terms of heteroatom electronegativities, geometrical variations and localization properties of the π^* orbitals. The largest electron acceptor properties were observed in the thioketone derivative, which generates a stable π anion state. The ionization energy values relating to the heteroatom lone pair and the filled π orbitals are also reported.     

Orientation in nylon 6 films as determined by the three-dimensional polarized infrared technique

A three-dimensional polarized infrared technique was used to obtain information about molecular orientation in both uniaxially and biaxially drawn nylon 6 films. The 835 and 930 cm^?1 bands were used to describe the orientation of the A (extended chain) conformation while absorptions at 1175 cm^?1, and 1120 cm^?1 and 1075 cm^?1 were used to give some information about orientation of the B (twisted chain) conformation. On the basis of the 835 and 930 cm^?1 bands, it was shown that the hydrogen-bonded sheets made up of chains in the A conformation are parallel to the film surface in the biaxially drawn film. Uniaxially drawn films obtained by drawing both at 100 and 150°C showed a high degree of chain alignment in the draw direction for the A conformation at draw ratios greater than 2.5. Some planar orientation was also observed in these uniaxially drawn films for both the A and B conformations at high draw ratios.     

Electron propagator calculations of molecular electron affinities

An approximate electron propagator method for predictive calculations of molecular electron affinities is proposed. The self-energy accounts for relaxation effects to all orders Additional correlation effects are treated using a diagonal approximation with shifted denominators. Applications to CN, NH₂, and PH₂ are reported.     

Electron inity of 1,3,5,7-cyclooctatetraene determined by the kinetic method

The kinetic method is used to determine the electron inity (EA) of 1,3,5,7-cyclooctatetraene (COT), a compound that undergoes a significant structural change upon electron attachment. Collision-induced dissociation of anionic clusters of COT with a set of reference compounds (Ref), [COT·Ref]^?·, at various collision energies, allowed deconvolution of the relative enthalpies and entropies of the competitive reactions. The adiabatic EA of COT is determined to be 0.58±0.10 eV, in good agreement with the value, 0.58±0.04 eV, of Wentworth and Ristau (J. Phys. Chem. 1969, 73, 2126) determined by thermal electron detachment as well as the more recent value, 0.55±0.02 eV, of Kato et al. (J. Am. Chem. Soc. 1997, 119, 7863) determined by equilibrium electron transfer with molecular oxygen. A large entropy difference, 25.6±10.0 e.u. (J mol^?1 K^?1), is observed between the two dissociation channels. This entropy difference corresponds to a negative 14.7±13.0 e.u. change for the dissociation of the dimer to give COT^?· and the neutral reference compound and a positive 10.9±8.4 e.u. entropy change for the dissociation of the dimer to give Ref^?· and neutral COT.     

Electron affinities of the DNA and RNA bases.

Adiabatic electron affinities (AEAs) for the DNA and RNA bases are predicted by using a range of density functionals with a double-zeta plus polarization plus diffuse (DZP++) basis set in an effort to bracket the true EAs. Although the AEAs exhibit moderate fluctuations with respect to the choice of functional, systematic trends show that the covalent uracil (U) and thymine (T) anions are bound by 0.05-0.25 eV while the adenine (A) anion is clearly unbound. The computed AEAs for cytosine (C) and guanine (G) oscillate between small positive and negative values for the three most reliable functional combinations (BP86, B3LYP, and BLYP), and it remains unclear if either covalent anion is bound. AEAs with B3LYP/TZ2P++ single points are 0.19 (U), 0.16 (T), 0.07 (G), -0.02 (C), and -0.17 eV (A). Favorable comparisons are made to experimental estimates extrapolated from photoelectron spectra data for the complexes of the nucleobases with water. However, experimental values scaled from liquid-phase reduction potentials are shown to overestimate the AEAs by as much as 1.5 eV. Because the uracil and thymine covalent EAs are in energy ranges near those of their dipole-bound counterparts, preparation and precise experimental measurement of the thermodynamically stable covalent anions may prove challenging.     

The solubility of alkali-metal fluorides in non-aqueous solvents with and without crown ethers, as determined by flame emission spectrometry

The solubilities of LiF, NaF, KF, RbF and CsF in acetonitrile, acetone, tetrahydrofuran, dimethylformamide, benzene and cyclohexane have been determined with and without a crown ether (usually 0.1 M 18-crown-6) present. Flame emission spectrometry was the determination method. Three procedures, selected according to the miscibility of the solvent with water, and the solubility of the fluoride, are described. Samples, standards and blanks were matrix-matched. The precision varied between 1 and 10% RSD. Although extensive drying procedures were applied, moisture present in the solvents and salts had some effect on the results.     

Electron affinities of metals computed by density functional theory and ab initio methods

An extensive computational study of the meal electron affinity was performed using the ab initio and density functional theory (DFT) methods. HF, MP2, MP3, MP4, QCISD, and QCISD(T) was used as computational methods, while the hybrid, local, and nonlocal DFT methods with the LYP, P86, PW91, and VWN correlation functionals were used. Two basis sets, one small and applicable to almost all metals (LanL2DZ) and one large [6-311 + + G(3df, 3 pd)] used only for small metals, were employed. The computed results were compared with the experimental data and the capabilities of the DFT methods to perform this study were discussed. © 1997 John Wiley & Sons, Inc.     

Open-shell coupled-cluster method: Electron affinities of Li and Na

The open-shell coupled-cluster method and the diagrams needed for its implementation are described. The method is applied to the electron affinities of Li and Na, which are calculated in two ways: as the ionization potential of the anions or as the energy of adding the second electron to the cations. The two schemes give essentially the same results, in very good agreement (<0.02 eV) with experiment. Three-body effects are negligible.     

The possible dissociation of the ions of some higher fluorides on X-ray Fls ionization

The nonempirical SCF method in the approximation of equivalent cores has been used to demonstrate the possibility of the dissociation process AF_n ⁺ AF_n–1 ⁺+F(1s¹2s²2p⁶) for the ions CF₄ ⁺, PF₅ ⁺, and SF₆ ⁺, formed by Fls ionization of the uncharged molecules. It is concluded that the interpretation of the gas-phase FK spectra based on the assignment of bands to transitions from the molecular levels of stationary states may not be applicable to these fluorides. Suggestions are made concerning the dissociation of the ions WF₆ ⁺ and MoF₆ ⁺ on Fls ionization.Translated from Teoreticheskie i Éksperimental'naya Khimiya, Vol. 26, No. 6, pp. 729–732, November–December, 1990.The authors thank A. I. Razgonov and N. V. Dobrodev for taking part in discussion of the results and in calculations of the line strengths of the FK transitions by the X-SW method.     

Electron self-exchange kinetics determined by MARY spectroscopy: theory and experiment

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the fluorophores pyrene, pyrene-d(10) and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene) in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive recombination of the radical pair under the influence of the external magnetic field and electron self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally. The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination of these simulations with high resolution, low noise experimental spectra makes the MARY technique a novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a line width vs concentration plot, the simulation method yields more reliable and accurate results. The correctness of the results obtained by the MARY method is proved by a comparison with corresponding data from the well-established EPR line broadening technique. With its less stringent restrictions on radical lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular for systems involving short-lived and unstable radicals.     

Electron attachment and detachment: electron affinities of isomers of trifluoromethylbenzonitrile

Rate constants for electron attachment to the three isomers of trifluoromethylbenzonitrile [(CF(3))(CN)C(6)H(4), or TFMBN] were measured over the temperature range of 303-463 K in a 133-Pa He buffer gas, using a flowing-afterglow Langmuir-probe apparatus. At 303 K, the measured attachment rate constants are 9.0 x 10(-8) (o-TFMBN), 5.5 x 10(-8) (m-TFMBN), and 8.9 x 10(-8) cm(3) s(-1) (p-TFMBN), estimated accurate to +/-25%. The attachment process formed only the parent anion in all three cases. Thermal electron detachment was observed for all three anion isomers, and rate constants for this reverse process were also measured. From the attachment and detachment results, the electron affinities of the three isomers of TFMBN were determined to be 0.70(o-TFMBN), 0.67(m-TFMBN), and 0.83 eV (p-TFMBN), all +/-0.05 eV. G3(MP2) [Gaussian-3 calculations with reduced M?ller-Plesset orders (MP2)] calculations were carried out for the neutrals and anions. Electron affinities derived from these calculations are in good agreement with the experimental values.     

Electron affinities of uracil: microsolvation effects and polarizable continuum model

We present adiabatic electron affinities (AEAs) and the vertical detachment energies (VDEs) of the uracil molecule interacting with one to five water molecules. Credibility of MP2 and DFT/B3LYP calculations is supported by comparison with available benchmark CCSD(T) data. AEAs and VDEs obtained by MP2 and DFT/B3LYP methods copy trends of benchmark CCSD(T) results for the free uracil and uracil-water complexes in the gas phase being by 0.20 - 0.28 eV higher than CCSD(T) values depending on the particular structure of the complex. AEAs and VDEs from MP2 are underestimated by 0.09-0.15 eV. For the free uracil and uracil-(H(2)O)(n) (n = 1,2,3,5) complexes, we also consider the polarizable continuum model (PCM) and discuss the importance of the microsolvation when combined with PCM. AEAs and VDEs of uracil and uracil-water complexes enhance rapidly with increasing relative dielectric constant (ε) of the solvent. Highest AEAs and VDEs of the U(H(2)O)(5) complexes from B3LYP with ε = 78.4 are 2.03 and 2.81 eV, respectively, utilizing the correction from CCSD(T). Specific structural features of the microsolvated uracil-(H(2)O)(n) complexes and their anions are preserved also upon considering PCM in calculations of AEAs and VDEs.     

Spectrophotometric determination of molybdenum by extraction as the molybdenum-quinoline-thiocyanate mixed complex

Summary Molybdenum (VI) is reduced to the pentavalent state very rapidly in the presence of thiocyanate and quinoline (or similar bases) in 2.5M hydrochloric acid medium. It distributes into nitrobenzene as [MoO(Qn)₂(SCN)₃] giving a orange red extract with maximum absorbance at 470 nm and molar absorbance 18 000 ±200. The system obeys Beer's law up to 5g of Mo per ml. Cu(II), Fe(III), Ti(IV) and W(VI) interfere; but methods have been developed for the elimination of interference.

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Zusammenfassung Molybdän (VI) wird in Gegenwart von Thiocyanat und Chinolin (oder ähnlichen Basen) in 2,5-m Salzsäure sehr rasch zu Mo(V) reduziert. Dieses wird als [MoO(Chin)₂(SCN)₃] mit Nitrobenzol extrahiert. Die orangerote Lösung hat ihr Absorptionsmaximum bei 470 nm; die molare Extinktion beträgt 18 000±200. Bis 5g Mo/ml entspricht die Lösung dem Beerschen Gesetz. Cu(II), Fe(III), Ti(IV) und W(VI) stören, doch läßt sich diese Störung beseitigen.

     

Electron transfer through clay monolayer films fabricated by the Langmuir-Blodgett technique

Hybrid films composed of amphiphilic molecules and clay particles were constructed by the modified Langmuir-Blodgett (LB) method. Clays used were sodium montmorillonite (denoted as mont) and synthetic smectite containing Co(II) ions in the octahedral sites (denoted as Co). Two kinds of amphiphilic molecules were used-[Ru(dC(18)bpy)(phen)2](ClO4)2 (dC(18)bpy = 4,4'-dioctadecyl-2,2'-bipyridyl and phen = 1,10-phenanthroline) (denoted as Ru) and octadecylammonium choloride (ODAH+Cl- or denoted as ODAH). Three kinds of hybrid films (denoted as Ru-mont, Ru-Co, and ODAH-Co films) were prepared by spreading an amphiphilic molecule onto an aqueous suspension of a clay. Atomic force microscopy (AFM) analyses of the films deposited on silicon wafers indicated that closely packed films were obtained at 20 ppm for all the above three cases. Cyclic voltammetry (CV) was measured on an ITO electrode modified with a hybrid film or a monolayer film of pure Ru(II) complex salt (denoted as Ru film). The Ru(II) complexes incorporated in the Ru-mont film lost their redox activity, indicating that montmorillonite layers acted as a barrier against electron transfer. In contrast, the same complexes in the Ru-Co film were electrochemically active with the simultaneous appearance of the redox peaks due to the Co(II)/Co(III) (or Co(II)/Co(IV)) couple. The results implied that electron transfer through cobalt clay layers was possible via mediation by Co(II) ions in a clay sheet. For an aqueous solution containing nitrite ions (NO2-) at pH 3.0, a large catalytic oxidation current was observed for both the electrodes modified with the Ru-mont and Ru-Co films. The results were interpreted in terms of the mechanisms that the charge separation of an incorporated Ru(II) complex took place to produce a pair of a Ru(III) complex and an electron and that the generated Ru(III) complex was reduced by a nitrite ion before it recombined with the electron.