Adiabatic Electron Affinity Values Research Articles

Integrating redox-response in crown ethers by disulfide incorporation: a computational approach

Reducing properties are investigated for the minimum energy conformers of selected thiacrown systems with redox-responsive disulfide linkage. The adiabatic electron affinity values of the investigated host systems in the gas phase lie in a range that makes them capable to undergo reversible redox reactions. In water, these show a high propensity to get reduced. Their binding energy with different alkali metal ions (Li+, Na+, and K+) is also determined under gas and aqueous phase conditions. The computational analysis reveals the weak metal binding interactions of disulfide systems than the parent thiacrown ethers. Such weak interaction energies in host-guest complexes indicate the possibility of reversible complexation-decomplexation. This assumably assists the fast release of guest molecules from the disulfide systems in response to the redox stimuli. Possibilities of incorporating such disulfide incorporated moieties in thiacalixarenes are also discussed. The results discussed in the present work are valuable for further development in the practical applications of crown ether–based host materials.

Efficient EOM-CC-based Protocol for the Calculation of Electron Affinity of Solvated Nucleobases: Uracil as a Case Study.

We present an explicit solvation protocol for the calculation of electron affinity values of the solvated nucleobases. The protocol uses a quantum mechanics/molecular mechanics (QM/MM) approach based on the newly implemented domain-based pair natural orbital EOM-CCSD (equation-of-motion coupled-cluster single-double) method. The stability of the solvated nucleobase anion is sensitive to the local distribution of the water molecules around the nucleobase, and the calculated electron affinity values converge slowly with respect to the number of snapshots and the size of the water box. The use of nonpolarizable water molecules leads to an overestimation of the electron affinity and makes the result sensitive to the size of the QM region in the QM/MM calculation. The electron affinity values, although sensitive to the size of the basis set, lead to an almost constant blue shift of the electron affinity upon the increase in the basis set. The present protocol allows for a controllable description of the various parameters affecting the electron affinity value, and the calculated adiabatic electron affinity values are in excellent agreement with experimental results.

The Studies on Structure and Stability of CaBn Clusters.

Calcium-boron systems have excellent properties of hardness, strength, and chemical stability, and we studied a series of CaBn clusters to investigate their structures and relative stability. The results showed the most stable structures of CaBn clusters are not planar. The B atoms tend to get together and form the planar ring to stabilize the structure, and the Ca atoms are coordinated to the periphery of the formations. The average binding energy (Eb), fragmentation energy (EF), second-order energy difference (Δ2E), adiabatic detachment energy (ADE), and adiabatic electron affinity (AEA) of the CaBn clusters were calculated to investigate the relative stability and the ability of removing or obtaining an electron. As shown by the results, EF and Δ2E values had obvious odd-even alteration as n increased, which indicated that the formations CaB4, CaB6, and CaB8 were more stable. The ADE values for CaBn clusters with even values of n were higher than those with odd values of n, which indicated CaBn clusters with even values of n had difficultly removing an electron. The AEA values of CaB3 and CaB7 were larger than the others, which meant CaB3 and CaB7 easily obtained an electron. These results provide a useful reference for understanding the formation mechanism and stability of the alkaline earth metal boride as well as guidance for synthesizing the CaBn clusters.

IcMRCI+Q Study of the Spectroscopic Properties of the 14 Λ-S and 49 Ω States of the SiN- Anion in the Gas Phase.

This paper calculates the potential energy curves of the 14 Λ-S and 49 Ω states, which come from the first three dissociation channels of the SiN− anion. These calculations are conducted using the valence internally contracted multireference configuration interaction and the Davidson correction approach. Core-valence correlation and scalar relativistic corrections are taken into account. The potential energies are extrapolated to the complete basis set limit. The spin-orbit coupling is computed using the state interaction approach with the Breit–Pauli Hamiltonian. We found that the X1Σ+ (υ′′ = 0–23) and a3Σ+ (υ′ = 0–2) states of SiN− are stable at the computed adiabatic electron affinity value of 23,262.27 cm−1 for SiN. Based on the calculated potential energy curves, the spectroscopic parameters and vibrational levels were determined for all stable and metastable Λ-S and Ω states. The computed adiabatic electron affinity of SiN and the spectroscopic constants of SiN− (X1Σ+) are all in agreement with the available experimental data. The d3Σ+, 25Σ+, 15Δ, and 15Σ− quasi-bound states caused by avoided crossings were found. Calculations of the transition dipole moment of a3Σ+1 to X1Σ+0+ are shown. Franck-Condon factors, Einstein coefficients, and radiative lifetimes of the transition from the a3Σ+1 (υ′ = 0–2) to the X1Σ+0+ state are evaluated.

Electron affinity calculation for selected PAHs using DFT: Effect of cyclopenta ring fusion and aromaticity

Quantities like adiabatic electron affinity (AEA), electron density distribution and geometry based aromaticity index are calculated for a set of polycyclic aromatic hydrocarbon (PAH) molecules. The calculated AEA values are compared with experimental values. Presence of a cyclopenta ring is observed to enhance the ability of the molecule to accommodate the extra electron in the anionic form and hence increasing the electron affinity (EA). Electrostatic potential maps and electron spin density distribution are used to analyze electron density distribution over the molecular volume in case of anions. A detailed comparison is made with the help of aromaticity index to quantify the correlation of AEA with the π-electron delocalization. A clear correlation between AEA and the aromaticity index is demonstrated. The effect of sp3 hybridized carbon atom on AEA values in PAHs is studied for the first time.

The Influence of the Terminal Phosphorothioate Diester Bond on the DNA Oxidation Process. An Experimental and Theoretical Approach.

In this study, the influence of the terminal phosphorothioate (PT) internucleotide bond in ds-DNA on the oxidation process was taken into consideration. The interaction of UV with the targeted oligonucleotide leads to an electron ejection and radical cation “hole” migration through the ds-DNA until it is trapped irreversibly in a suitable place. Phosphorothiate internucleotide bonds were detected in the bacterial genome; however, their role is still unclear. In this study a PAGE analysis of irradiated ds-DNA showed that the degradation rea ction was slowed down by the presence PT next to the anthraquinone moiety. Further, theoretical study shows that [RP] AQ-PS-dG can adopt a slightly lower ionisation potential energy and triplet excited state with a subsequent slightly higher adiabatic electron affinity value in comparison with [SP] AQ-PS-dG and AQ-PO-dG. Moreover, the energy gap between HOMO and LUMO, indicated the radical stabilisation properties of [RP] AQ-PS-dG, which can hinder the charge transfer through ds-DNA.

Electron attachment to DNA and RNA nucleobases: An EOMCC investigation

We report a benchmark theoretical investigation of both vertical and adiabatic electron affinities of DNA and RNA nucleobases: adenine, guanine, cytosine, thymine, and uracil using equation of motion coupled cluster method. The vertical electron affinity (VEA) values of the first five states of the DNA and RNA nucleobases are computed. It is observed that the first electron attached state is energetically accessible in gas phase. Furthermore, an analysis of the natural orbitals exhibits that the first electron attached states of uracil and thymine are valence‐bound in nature and undergo significant structural changes on attachment of an extra electron, which reflects in the deviation of the adiabatic electron affinity (AEA) than that of the vertical ones. Conversely, the first electron attached states of cytosine, adenine, and guanine are in the category of dipole‐bound anions. Their structure, by and large, remain unaffected on attachment of an extra electron, which is evident from the observed small difference between the AEA and VEA values. VEA and AEA values of all the DNA and RNA nucleobases are found to be negative, which implies that the first electron attached states are not stable rather quasi bound. The results of all previous theoretical calculations are out of track and shows large deviation with respect to the experimentally measured values, whereas, our results are found to be in good agreement. Therefore, our computed values can be used as a reliable standard to calibrate new theoretical methods. © 2015 Wiley Periodicals, Inc.

Combined photoelectron, collision-induced dissociation, and computational studies of parent and fragment anions of N-paranitrophenylsulfonylalanine and N-paranitrophenylalanine

After synthesizing the compounds N-paranitrophenylsulfonylalanine (NPNPSA) and N-paranitrophenylalanine (NPNPA), the photoelectron spectrum of the valence anion of N-paranitrophenylsulfonylalanine (NPNPSA)(-), was measured and the collision-induced dissociation (CID) pathways of deprotonated N-paranitrophenylsulfonylalanine (NPNPSA-H)(-) and deprotonated N-paranitrophenylalanine (NPNPA-H)(-) were determined. Pertinent calculations were conducted to analyze both sets of experimental data. From the valence anion photoelectron spectrum of (NPNPSA)(-), the adiabatic electron affinity (AEA) of NPNPSA was determined to be 1.7 ± 0.1 eV, while the vertical detachment energy (VDE) of (NPNPSA)(-) was found to be 2.3 ± 0.1 eV. Calculations for four low lying conformers of (NPNPSA)(-) gave AEA values in the range of 1.6-2.1 eV and VDE values in the range of 2.0-2.4 eV. These calculations are in very good agreement with the experimental values. While the NPNPA anion (NPNPSA)(-) was not observed experimentally it was studied computationally. The six low lying (NPNPSA)(-) conformers were identified and calculated to have AEA values in the range of 0.7-1.2 eV and VDE values in the range of 0.9-1.6 eV. CID was used to study the fragmentation patterns of deprotonated NPNPA and deprotonated NPNPSA. Based on the CID data and calculations, the excess charge was located on the delocalized π-orbitals of the nitrobenzene moiety. This is made evident by the fact that the dominant fragments all contained the nitrobenzene moiety even though the parent anions used for the CID study were formed via deprotonation of the carboxylic acid. The dipole-bound anions of both molecules are studied theoretically using the results of previous studies on nitrobenzene as a reference.

Photoelectron Spectroscopy of the 6-Azauracil Anion

We report the photoelectron spectrum of the 6-azauracil anion. The spectrum is dominated by a broad band exhibiting a maximum at an electron binding energy (EBE) of 1.2 eV. This spectral pattern is indicative of a valence anion. Our calculations were carried out using ab initio electron propagator and other many-body methods. Comparison of the anion and corresponding neutral of 6-azauracil with those of uracil shows that substituting a nitrogen atom for C-H at the C6 position of uracil gives rise to significant changes in the electronic structure of 6-azauracil versus that of uracil. The adiabatic electron affinity (AEA) of the canonical 6-azauracil tautomer is substantially larger than that of canonical uracil. Among the five tautomeric, 6-azauracil anions studied computationally, the canonical structure was found to be the most stable. The vertical detachment energies (VDE) of the canonical, valence-bound anion of 6-azauracil and its closest "very-rare" tautomer have been calculated. Electron propagator calculations on the canonical anion yield a VDE value that is in close agreement with the experimentally determined VDE value of 1.2 eV. The AEA value of 6-azauracil, assessed at the CCSD(T) level of theory to be 0.5 eV, corresponds with the EBE value of the onset of the experimental spectrum.

Interaction of gold nanoclusters of different size with adenine: A density functional theory study of neutral, anionic and cationic forms of [adenine + (Au)n=3,6,9,12] complexes

In the present work, we have studied the interaction of adenine and gold nanoclusters of different size at B3LYP level of theory using 6-31G (d) basis set except for gold atoms. For the gold atoms, LANL2DZ effective core potential (ECP) was used. In order to understand the binding interaction, neutral, anionic and cationic forms of [adenine+(Au)n=3,6,9,12] complexes have been selected. We found that the adenine has high binding affinity at their N1, N3 and N7 atomic sites for each size of gold nanoclusters. The distance between gold clusters (Au, Au2, Au3 and Au4) and adenine (N1, N3 and N7) was found to be larger than 2Å. Thus, it favors the formation of non-covalent interaction between adenine and gold clusters. The distance between adenine to gold nanoclusters is found to decrease by increasing the cluster size. Mulliken charge analysis was performed for all three forms of [adenine+(Au)12] complexes for determining the nature of charge transfer between gold nanoclusters to adenine and vice versa. It was observed that the gold nanoclusters acquire a large amount of negative charge in anionic form of [adenine+(Au)12] complex. However, in case of cationic form of the complex, adenine gains a substantial amount of positive charge. The adiabatic electron affinity (AEA) of [adenine+(Au)n=3,6,9,12] complexes was also calculated. The AEA values of [adenine+(Au)n=3,6,9,12] complexes were found to be less than that of the isolated adenine.

Electron Attachment to a Hydrated DNA Duplex: The Dinucleoside Phosphate Deoxyguanylyl-3′,5′-Deoxycytidine

The minimal essential section of DNA helices, the dinucleoside phosphate deoxyguanylyl-3',5'-deoxycytidine dimer octahydrate, [dGpdC](2), has been constructed, fully optimized, and analyzed by using quantum chemical methods at the B3LYP/6-31+G(d,p) level of theory. Study of the electrons attached to [dGpdC](2) reveals that DNA double strands are capable of capturing low-energy electrons and forming electronically stable radical anions. The relatively large vertical electron affinity (VEA) predicted for [dGpdC](2) (0.38 eV) indicates that the cytosine bases are good electron captors in DNA double strands. The structure, charge distribution, and molecular orbital analysis for the fully optimized radical anion [dGpdC](2)(·-) suggest that the extra electron tends to be redistributed to one of the cytosine base moieties, in an electronically stable structure (with adiabatic electron affinity (AEA) 1.14 eV and vertical detachment energy (VDE) 2.20 eV). The structural features of the optimized radical anion [dGpdC](2)(·-) also suggest the probability of interstrand proton transfer. The interstrand proton transfer leads to a distonic radical anion [d(G-H)pdC:d(C+H)pdG](·-), which contains one deprotonated guanine anion and one protonated cytosine radical. This distonic radical anion is predicted to be more stable than [dGpdC](2)(·-). Therefore, experimental evidence for electron attachment to the DNA double helices should be related to [d(G-H)pdC:d(C+H)pdG](·-) complexes, for which the VDE might be as high as 2.7 eV (in dry conditions) to 3.3 eV (in fully hydrated conditions). Effects of the polarizable medium have been found to be important for increasing the electron capture ability of the dGpdC dimer. The ultimate AEA value for cytosine in DNA duplexes is predicted to be 2.03 eV in aqueous solution.

Photoelectron Spectroscopy of Lanthanide−Silicon Cluster Anions LnSin− (3 ≤ n ≤ 13; Ln = Ho, Gd, Pr, Sm, Eu, Yb): Prospect for Magnetic Silicon-Based Clusters

Photoelectron spectroscopy was utilized to study a variety of LnSi(n)(-) cluster anions (Ln = Yb, Eu, Sm, Gd, Ho, Pr; 3 <or= n <or= 13). For a particular size n, the measured valence electronic transitions of all these systems fall into either one of two categories, reflecting the influence of the different oxidation states of the lanthanide atoms involved. In one, the spectra of YbSi(n)(-) and EuSi(n)(-) are nearly identical to each other, while in the other the spectra of GdSi(n)(-), HoSi(n)(-), and PrSi(n)(-) are essentially identical. SmSi(n)(-) clusters exhibit an intermediate behavior with smaller clusters resembling the former category and larger clusters resembling the latter category. In the intermediate size range, 7 <or= n <or= 10, for SmSi(n)(-) both categories appear to be present, with one matching the EuSi(n)(-)-like systems and the other HoSi(n)(-)-like clusters. The distinction between LnSi(n)(-) categories strongly correlates with the oxidation state of the particular lanthanide as usually found in its compounds. On the basis of this observation, we conclude that, among the Ln-silicon clusters studied herein, Yb, Eu, and in case of Sm, sizes n >or= 10, adopt a nominal +2 oxidation state while Ho, Pr, Gd, and in case of Sm, sizes n <or= 7, exhibit a nominal +3 oxidation state. Furthermore, dramatic increases in adiabatic electron affinity values observed at n = 10 for the Ln(III)Si(n) series and at n = 12 for the Ln(II)Si(n) series were attributed to an inherent electronic stabilization of those particular clusters, rather than to the lanthanides' encapsulation. The observed limited effect of f-electrons on the valence electronic structure and thus on bonding in LnSi(n)(-) clusters may leave these electrons available for inducing magnetism. Consequently, Ln@Si(n) clusters may hold promise as building blocks of silicon-based cluster materials with magnetic properties.

Density functional studies on the electron affinities of DNA and RNA bases

Influence of basis sets on electron affinities (EAs) of DNA and RNA bases has been investigated using density functional method (B3LYP functional) with different basis sets (6-31G, TZVP and 6-311+ + G**). Effect of some PBE functionals namely, PBEOP, PBELYP and PBEVWN, on EA values of the nucleobases was studied using basis set which predicted the most reliable values with B3LYP functional. Observation of the trends in the values of EA and dipole moment of the molecules enable us to identify the features of a basis set that shows the presence of dipole-bound state of some of the nucleobases. The vertical electron affinities with B3LYP and PBEOP functionals are close to the experimental values. The adiabatic electron affinities of uracil and thymine were found to be positive for basis set with diffuse functions using B3LYP functional. Adenine does not have a stable covalently bound anion at all levels of basis sets and functionals. The sign of adiabatic electron affinity value of cytosine is inconsistent with that of experimental value but in agreement with previous theoretical results. For guanine the adiabatic electron affinity value with 6-311+ + G** basis set was found to be very high as comparison with other two basis sets confirming the formation of mixed covalent-dipole character.

Microhydration of cytosine and its radical anion: Cytosine∙(H2O)n (n=1–5)

Microhydration effects on cytosine and its radical anion have been investigated theoretically, by explicitly considering various structures of cytosine complexes with up to five water molecules. Each successive water molecule (through n=5) is bound by 7-10 kcal mol(-1) to the relevant cytosine complex. The hydration energies are uniformly higher for the analogous anion systems. While the predicted vertical detachment energy (VDE) of the isolated cytosine is only 0.48 eV, it is predicted to increase to 1.27 eV for the lowest-lying pentahydrate of cytosine. The adiabatic electron affinity (AEA) of cytosine was also found to increase from 0.03 to 0.61 eV for the pentahydrate, implying that the cytosine anion, while questionable in the gas phase, is bound in aqueous solution. Both the VDE and AEA values for cytosine are smaller than those of uracil and thymine for a given hydration number. These results are in qualitative agreement with available experimental results from photodetachment-photoelectron spectroscopy studies of Schiedt et al. [Chem. Phys. 239, 511 (1998)].

Valence and Dipole-Bound Anions of the Most Stable Tautomers of Guanine

Anionic states of guanine, which is the only nucleic acid base of which the anions have not yet been studied in either photoelectron spectroscopic (PES) or Rydberg electron transfer (RET) experiments, have been characterized for the four most stable tautomers of neutral guanine using a broad spectrum of electronic structure methods from the density functional theory, with the B3LYP exchange-correlation functional, to the coupled-cluster method, with single, double, and perturbative triple excitations. Both valence and dipole-bound anionic states were addressed. We identified some of the difficulties facing future PES or RET experiments on the anion of guanine. Even if guanine is successfully transferred to the gas phase without thermal decomposition, it is critical to have the canonical amino-oxo (G) and both amino-hydroxy (GH and GHN7H) tautomers in the beam, not only the most stable, a noncanonical, amino-oxo tautomer (GN7H), as the latter does not support an adiabatically bound anionic state. We also suggested a scheme for enrichment of gas-phase guanine with the canonical tautomer, which is not the most stable in the gas phase, but which is of main interest due to its biological relevance. The tautomers G, GN7H, and GHN7H support vertically bound valence anionic states with the CCSD(T) value of vertical detachment energy of +0.58, +0.21, and +0.39 eV, respectively. These anionic states are, however, adiabatically unbound and thus metastable. The vertical electronic stability of these valence anionic states is accompanied by serious "buckling" of the molecular skeleton. The G and GHN7H tautomers support dipole-bound states with the CCSD(T) values of adiabatic electron affinity of 65 and 36 meV, respectively. A contribution from higher-than-second-order correlation terms represents, respectively, 48 and 68% of the total vertical electron detachment energy determined at the CCSD(T) level.

Structure of the hexafluorobenzene anion

The D 6 h , D 2 h (two), C 2 v , and D 2 structures of the hexafluorobenzene radical anion (C 6F 6 −) were studied by using density functional B3LYP method in conjunction with 6-31+G(d) and 6-311+G(d) basis sets. The B3LYP geometry optimization and frequency analysis calculations indicate that the C 2v( 2 A 1) structure has the lowest energy and represents an energy minimum. Adiabatic electron affinity value of the C 6F 6 molecule with respect to the anionic ground state (C 2v( 2 A 1)) is predicted to be 0.72–0.76 eV, in good agreement with the experimental data. Isotropic hyperfine coupling constants on the F- and 13C-centers ( a(F) and a(C)) in the C 2v( 2 A 1) structure were calculated by using ab initio MP2 as well as B3LYP methods, and averages of the calculated a(F) and a(C) values are in good agreement with the experimental coupling values. All these results indicate that the C 6F 6 − anion has a C 2 v structure and is in the 2A 1 ground state. The B3LYP calculations confirm that the D 2( 2 A) structure be the transition state for the pseudo-rotation process of the C 2v( 2 A 1) structure.

Photoelectron spectroscopy of naphthalene cluster anions

Mass spectrometric and anion photoelectron spectroscopic studies of homogeneous naphthalene cluster anions, (Nph)n=2–7−, were conducted to characterize the nature of their anionic cores. The smallest stable species in this series was found to be the naphthalene dimer anion. The vertical detachment energies of naphthalene clusters, n=2–7, were determined and found to increase smoothly with cluster size. By extrapolation, the vertical detachment energy of the isolated naphthalene molecule was found to be −0.18 eV, in agreement with its adiabatic electron affinity value from literature. The strong similarity between the spectral profiles of (Nph)2− and (Nph)1−(H2O)1 implied that (Nph)2− possesses a solvated monomeric anion core. All of the naphthalene cluster anions studied here were interpreted as having monomer anion cores.

Electron-capture detector and multiple negative ions of aromatic hydrocarbons

Multiple electron affinities are identified in the temperature dependence of the electron-capture detector: naphthalene, 0.16, 0.13±0.01; anthracene, 0.69, 0.60, 0 53±0.01; tetracene 1.1, 0.88,±0.03, 0.53±0.05; pyrene, 0.61, 0.50±0.02; azulene 0.90, 0.80, 0.70±0.02, 0.65, 0.55±0.05; acenaphthylene, 0.80, 0.69, 0.60, 0.50±0.05; and c-C 8H 8, 0.80, 0.70, 0.55±0.02; (all in eV) These are obtained from a rigorous least squares procedure incorporating literature values and uncertainties. The adiabatic electron affinities for about 40 hydrocarbons listed in the US National Institute of Standards and Technology (NIST) tables are evaluated. The adiabatic electron affinity values not listed in NIST are biphenylene, 0.45±0.05 eV and coronene, 0.8±0.05 eV. Morse potential energy curves in the C–H dimensions illustrate multiple states for benzene and naphthalene.

Quantum Monte Carlo calculations of molecular electron affinities: First-row hydrides

Very accurate energies can be computed by the fixed-node diffusion Monte Carlo method. They are affected only by the nodal error due to the approximate description of the nodal surfaces by the trial wave function. We examine the cancellation of nodal errors in molecular electron affinity calculations. Ground state energies of the anions of first-row hydrides AH (A=Li–O) have been computed using the fixed-node diffusion Monte Carlo method with a determinant times a correlation factor as the trial wave function. The energies are among the lowest to date. Using the energy values for the neutral molecules computed by Luchow and Anderson [A. Luchow and J. B. Anderson, J. Chem. Phys. 105, 7573 (1996)] we computed adiabatic electron affinity values and found them in agreement with the experimental data. As a consequence, the values of the anion dissociation energies are also correctly evaluated.

Adiabatic states of ozone using Fock space multireference coupled cluster method

In this paper we present the Fock space multireference coupled cluster theory suitable for calculation of low-lying adiabatically excited or electron attached states. Low-lying adiabatic as well as vertical excited states of ozone are calculated using this theory in singles and doubles approximation. The calculated adiabatic excitation energies are compared with the experimental values. We also report the adiabatic electron affinity value of ozone.