Photoelectron spectrum of valence anions of uracil and first-principles calculations of excess electron binding energies

The photoelectron spectrum (PES) of the uracil anion is reported and discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of the anions of various tautomers of uracil. The PES peak maximum is found at an electron binding energy of 2.4 eV, and the width of the main feature suggests that the parent anions are in a valence rather than a dipole-bound state. The canonical tautomer as well as four tautomers that result from proton transfer from an NH group to a C atom were investigated computationally. At the Hartree-Fock and second-order Moller-Plesset perturbation theory levels, the adiabatic electron affinity (AEA) and the VDE have been converged to the limit of a complete basis set to within +/-1 meV. Post-MP2 electron-correlation effects have been determined at the coupled-cluster level of theory including single, double, and noniterative triple excitations. The quantum chemical calculations suggest that the most stable valence anion of uracil is the anion of a tautomer that results from a proton transfer from N1H to C5. It is characterized by an AEA of 135 meV and a VDE of 1.38 eV. The peak maximum is as much as 1 eV larger, however, and the photoelectron intensity is only very weak at 1.38 eV. The PES does not lend support either to the valence anion of the canonical tautomer, which is the second most stable anion, and whose VDE is computed at about 0.60 eV. Agreement between the peak maximum and the computed VDE is only found for the third most stable tautomer, which shows an AEA of approximately -0.1 eV and a VDE of 2.58 eV. This tautomer results from a proton transfer from N3H to C5. The results illustrate that the characteristics of biomolecular anions are highly dependent on their tautomeric form. If indeed the third most stable anion is observed in the experiment, then it remains an open question why and how this species is formed under the given conditions.     

Work function and negative electron affinity of ultrathin barium fluoride films

Thin films of barium fluorides with different thicknesses were deposited on GaAs substrate by electron beam evaporation. The aim of the work was to identify the best growth conditions for the production of coatings with a low work function suitable for the anode of hybrid thermionic-photovoltaic (TIPV) devices. The chemical composition and work function φ of the films with different thicknesses were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The lowest value of φ = 2.1 eV was obtained for the film with a thickness of ~2 nm. In the valence band spectra of the films at low kinetic energy, near the cutoff, a characteristic peak of negative electron affinity was present. This effect contributed to a further reduction of the film's work function.     

Electronic properties of polyoxometalates: electron and proton affinity of mixed-addenda Keggin and Wells-Dawson anions

A series of systematic DFT calculations were conducted on Keggin [SiW(9)M(3)O(40)](n-), M = Mo, V, and Nb; and Wells-Dawson anions [P(2)M(18)O(62)],(6-) M = W and Mo; [P(2)M(15)M(3)'O(62)](m-), M = W and Mo, M' = W, Mo, and V to analyze the redox properties and the basicity of the external oxygen sites in polyoxometalates with nonequivalent addenda metals. The energy and composition of the lowest unoccupied orbitals, formally delocalized over the addenda atoms, determine the redox properties of a polyoxometalate. When a Mo(6+) substitutes one W(6+) in the 1:12 tungstate, the energy of the LUMO decreases and the cluster is more easily reduced. The tungstoniobates behave differently because the niobium orbitals insert into the tungsten band and the reduction of [SiW(9)Nb(3)O(40)](7-) yields the blue species SiW(9)Nb(3) 1e and not the cluster SiW(9)Nb(2)Nb(IV). In Wells-Dawson structures, the polar and equatorial sites have different electron affinities and the reduction preferentially occurs in the equatorial sites. Inserting ions with larger electron affinities into the polar sites can modify this traditional conduct. Hence, the trisubstituted [P(2)W(15)V(3)O(62)](9-) anion is reduced in the vanadium polar sites. By means of molecular electrostatic potential maps and the relative energy of the various protonated forms of [SiW(9)V(3)O(40)](7-) and [SiW(9)Mo(3)O(40)](4-), we established the basicity scale: OV(2) > OMo(2) > OW(2) > OV > OW > OMo. Finally, a continuum model for the solvent enabled us to compare anions with different total charges.     

Estimating electron affinity from the lifetime of negative molecular ions: Cycloheptatriene derivatives

Russian Journal of Physical Chemistry A - Cycloheptatriene derivatives are studied by means of resonance electron capture negative ion mass spectrometry (REC NIMS). The average lifetimes of...     

Structure and stability of thioureate anions with water, DFT calculations

Structure and stability of thioureate anion with the water clusters CSNH₂NH⁻-(H₂O)_n = 1–7, are studied, using density functional theory. Molecular structures and the stability of the clusters are discussed based on the calculation results of the stable conformers and their relative energies. All the clusters are stable thermodynamically in gas phase with respect to separate monomers. The clusters are stabilized progressively with an increasing number of water molecules, as indicated by the increasing of the binding energies. The binding energies of CSNH₂NH⁻ and a water molecule are 14.34 and 16.36 kcal/mol for cis CSNH₂NH⁻ and trans CSNH₂NH⁻, respectively. As the reaction in aqueous solution progresses, the CS bond distance increases monotonically, indicating that the CS bond of the thioureate anion unit in the clusters is de-stabilized with an increasing number of water molecules.     

DFT study on the complexation of anions with 1,4,7,10,13,16-hexaazacyclooctodeca-2,5,8,11,14,17-hexaene

Macrocyclic compounds have been widely used as anion carriers, as they play important functions in chemical and biological systems. This work reports a theoretical study on free 1,4,7,10,13,16-hexaazacyclooctodeca-2,5,8,11,14,17-hexaene (HAC), as well as its complex with fluoride, chloride, bromide and acetate anions, with and without the presence of the sodium counterion, in the gas phase and implicit solvents (cyclohexane and acetonitrile), at the ωB97X-D/6-311G(d,p) level. The negative ?G⁰ values indicate that the crown-anion complex is prone to be formed due to hydrogen bonds in all tested media. Nevertheless, such interactions weaken as the solvent polarity increases. The ΔG⁰ _C6H12 values decrease when the counterion is taken into account, reinforcing the formation of the Na⁺?HAC?X^? complex. However, the complexation is disfavored in polar solution, since the presence of the counterion increases the HAC-anion distance. Natural bond orbital analysis, the quantum theory of atoms in molecules and non-covalent interactions methods explored the nature and strength of the hydrogen bond interactions, while spin–spin coupling constant calculations for the fluoride-based complex (^1h J _F,H(N)) gave insight into the potential of this NMR parameter to experimentally probe the complexation of HAC with fluoride.     

A DFT study of the interaction between microhydrated anions and naphthalendiimides

The characteristics of the interaction of anions with naphthalendiimides, the basic structural motif of a newly synthesized anion channel based on anion···π interactions, are studied by computational methods. Stable complexes are formed with bromide, chloride, fluoride or hydroxide anions, which exhibit strong anion···π interactions in the gas phase. Following the sequence of the polarizing power of the anions, hydroxide and fluoride complexes are the most strongly interacting. The presence of a small number of water molecules strongly affects the anion···π interactions, especially for hydroxide and fluoride complexes, so the differences in interaction strength among the anions drop significantly. The calculations suggest that a small number of water molecules can be crucial to reducing dehydration cost and contributing to stabilizing interactions with the naphthalendiimide units.     

The fluoride ion affinity of tungsten pentafluoride and the electron affinity of tungsten hexafluoride.

Enthalpies of oxidative alkaline hydrolysis of the salts LiWF₆ and NaWF₆ are −549 and −519 kj mol^-1 respectively. From these results and earlier thermochemical results for KWF₆, RbWF₆, and from crystal structure data for these salts, it has proved possible to assess the charge distribution within the WF₆^- anion in these salts, and to estimate the fluoride ion affinity of WF₅ and the electron affinity of WF₆.     

A comparative electron correlation treatment in H(2)S-benzene dimer with DFT and wavefunction-based ab initio methods

The S₀ (X¹A′), T₁ (a³A″), S₁ (A¹A″), T₂ (b³A′), and S₂ (B¹A′) states of the (trans-)HONO molecule were studied by using the CASSCF and CASPT2 methods. The CASPT2(//CASPT2) adiabatic and vertical excitation energy values are in good agreement with available experimental data. The CASPT2//CASSCF potential energy curves (PECs) calculations indicate that: (i) all the five states correlate with the products of OH (X²Π) + NO (X²Π); (ii) along each of the T₁, S₁, and T₂ PECs there is a minimum followed by a transition state (barrier); and (iii) the repulsive S₂ PEC crosses the T₂, S₁, and T₁ PECs. The geometries and relative energies for the stationary points along these PECs were calculated at the CASPT2(//CASPT2) level, and the calculations predict that the barrier height value for S₁ is negligibly small (0.0018 eV).     

On the electron affinity of perfluorocycloalkanes and perfluoroalkanes

There is an increasing body of evidence showing that perfluorocycloalkanes have a higher electron affinity than their open chain analogs, the perfluoroalkanes. A new molecular orbital model is presented to explain these results and compared with the electrostatic model of Mittal and Libby. Explicit experiments are suggested which would allow comparison of the two models.     

Azadipyrromethene-based conjugated oligomers with near-IR absorption and high electron affinity

Solution-processable conjugated oligomers incorporating red-light absorbing azadipyrromethenes (aza-DIPY) within the main chain were synthesized via palladium-catalyzed Sonogashira coupling reactions. Thin films of these compounds absorbed light up to ～1000 nm and displayed reversible reductions as ascertained by cyclic voltammetry experiments. Reactions with trifluoroboron etherate yielded materials displaying a unique combination of good solubility in organic solvents, low optical band gaps (～1.3 eV), and high electron affinity (～4.5 eV).     

Quick scheme for evaluation of atomic charges in arbitrary aluminophosphate sieves on the basis of electron densities calculated with DFT methods

It is demonstrated that unique and simple analytical functions are justified for the atomic charge dependences q of the T (T = Al, P) and O atoms of aluminophosphates (AlPOs) using DFT calculations with several basis sets, starting from STO-3G to 3-21G and 6-21G**. Three internal (bonds, angles, ...) coordinates for the charge dependences of the T atoms and four coordinates for the O are sufficient to reach a precision of 1.8% for the fitted q(Al), 1.0% for q(P), and 2.5% for q(O) relatively to the values calculated at any basis set level. The proposed strategy consists in an iterative scheme starting from charge dependences based on the neighbor's positions only. Electrostatic potential values are computed to illustrate the differences between the calculated and fitted charges in the considered AlPO models.     

Accurate evaluation of valence and low-lying Rydberg states with standard time-dependent density functional theory

Using a standard exchange-correlation functional, namely, PBE0, the basis set dependence of time-dependent density functional theory (TD-DFT) calculations has been explored using 33 different bases and five organic molecules as test cases. The results obtained show that this functional can provide accurate (i.e., at convergence) results for both valence and low-lying Rydberg excitations if at least one diffuse function for the heavy atoms is included in the basis set. Furthermore, these results are in fairly good agreement with the experimental data and with those delivered by other functionals specifically designed to yield correct asymptotic/long-range behavior. More generally, the PBE0 calculations show that a greater accuracy can be obtained for both Rydberg and valence excitations if they occur at energies below the epsilonHOMO + 1 eV threshold. This latter value is proposed as a thumb rule to verify the accuracy of TD-DFT/PBE0 applications.     

Parallel stored-integral and semidirect Hartree-Fock and DFT methods with data compression

Recent developments in magnetic disk technology have made stored-integral techniques competitive with the currently more widely used direct methods, which involve the recalculation of the basic two-electron integrals. We present efficient conventional (all integrals stored) and semidirect Hartree-Fock and DFT algorithms with data compression for single-processor and distributed memory parallel computers, and compare them with the corresponding direct algorithms. On inexpensive modern personal computer-based hardware, the stored integral method is up to three times more efficient than the direct method in terms of total elapsed job time.     

Discrete iridium pyridonate chains with variable metal valence: nature and energetics of the Ir-Ir bonding from DFT calculations

The structure of the Ir(I) complex [Ir2(mu-OPy)2(CO)4] (Opy = 2-pyridonate) has been fully characterized in its head-to-head (A) configuration as a "dimer of dimers" AA in which two binuclear complexes are connected by means of a weak, but unsupported, iridium-iridium interaction (Ir(2)...Ir(2A) 2.9808(6) A). The head-to-tail isomer, referred to as B, was found in equilibrium with A in solution. It has been shown that this complex can be oxidized by diiodine to give iridium chains with highly selective configurations and general formula I-[Ir2(mu-OPy)2(CO)4]n-I (n = 1-3). The synthesis of IAI (1), of the isomers IAAI (2AA) and IABI (2AB), and of IABAI (3) is reported. DFT calculations have been carried out on A and B and on the known isomers of 1-3, as well as on two isomers of the hypothetic chain of eight Ir(1.25) atoms corresponding to n = 4. The stability of the metal chain is assigned to a 2-electron/2n-center sigma bond delocalized along the metal backbone and supplemented with a weak attractive interaction of the metallophilic type. Calculations confirm that further oxidation of the Ir chains corresponding to n > 1 by iodine, yielding the cleavage of one or two unsupported bond(s), is a highly exothermic process. The formation of the I-[Ir2(mu-OPy)2(CO)4]n-I chains is also computed to be exothermic, either highly for n = 1 or still significantly for n = 2 and 3. At variance with these results, the formation of an octanuclear chain is predicted to be no more than marginally exothermic (DeltaG = 1.7 kcal.mol(-1)), mainly because of interligand strain induced by the steric bulk of the amidate rings.     

Gold-caged metal clusters with large HOMO-LUMO gap and high electron affinity

We report a series of isoelectronic gold-caged metal clusters, M@Au14 (M = Zr, Hf), and anion clusters, M@Au14- (M = Sc, Y), all having a calculated HOMO-LUMO gap larger than the well-known tetrahedral cluster Au20-the 3D metal cluster with a very large measured HOMO-LUMO gap (1.77 eV). The clusters M@Au14 (M = Sc, Y) also exhibit a calculated electron affinity (EA) and vertical detachment energy (VDE) not only higher than the "superhalogen" icosahedral Al13 cluster but also possibly even higher than a Cl atom which has the highest (measured) elemental EA or VDE (3.61 eV).     

Dibenzothiophene-S,S-dioxide-fluorene co-oligomers. Stable, highly-efficient blue emitters with improved electron affinity

Incorporation of dibenzothiophene-S,S-dioxide units into conjugated fluorene oligomers changes the frontier orbital energy levels and presents an effective way to increase the electron affinity of these materials, which are highly fluorescent with bright blue emission in both solution and the solid state.