Electron Affinities Of Molecules Research Articles

Self-Interaction-Corrected Random Phase Approximation.

The short-range correlation energy of the random phase approximation (RPA) is too negative and is often corrected by local or nonlocal methods. These beyond-RPA corrections usually lead to a mixed performance for thermodynamics and dissociation properties. RPA+ is an additive correction based on density functional approximations that often gives realistic total energies for atoms or solids. RPA+ adds a moderate correction to the ionization energies/electron affinities of RPA but does not yield an improvement beyond RPA for atomization energies of molecules. This incompleteness results in severely underestimated atomization energies just like in RPA. Exchange-correlation kernels within the Dyson equation could simultaneously improve atomization, ionization energies, and electron affinities, but their implementation is computationally less feasible in localized basis set codes. In preceding work ( Phys. Rev. A 100, 2019022515), two of the authors proposed a computationally efficient generalized RPA+ (gRPA+) that changes RPA+ only for spin-polarized systems by making gRPA+ exact for all one-electron densities. gRPA+ was found to yield a large improvement of ionization energies and electron affinities of light atoms over RPA, and a smaller improvement over RPA+. Within this work, we investigate to what extent this improvement transfers to atomization energies, ionization energies, and electron affinities of molecules, using a modified gRPA+ (mgRPA+) method that can be applied in codes with localized basis functions. We thereby aim to understand the applicability of beyond-RPA corrections based on density functional approximations.

Computational approaches to dissociative chemisorption on metals: towards chemical accuracy.

We review the state-of-the-art in the theory of dissociative chemisorption (DC) of small gas phase molecules on metal surfaces, which is important to modeling heterogeneous catalysis for practical reasons, and for achieving an understanding of the wealth of experimental information that exists for this topic, for fundamental reasons. We first give a quick overview of the experimental state of the field. Turning to the theory, we address the challenge that barrier heights (Eb, which are not observables) for DC on metals cannot yet be calculated with chemical accuracy, although embedded correlated wave function theory and diffusion Monte-Carlo are moving in this direction. For benchmarking, at present chemically accurate Eb can only be derived from dynamics calculations based on a semi-empirically derived density functional (DF), by computing a sticking curve and demonstrating that it is shifted from the curve measured in a supersonic beam experiment by no more than 1 kcal mol-1. The approach capable of delivering this accuracy is called the specific reaction parameter (SRP) approach to density functional theory (DFT). SRP-DFT relies on DFT and on dynamics calculations, which are most efficiently performed if a potential energy surface (PES) is available. We therefore present a brief review of the DFs that now exist, also considering their performance on databases for Eb for gas phase reactions and DC on metals, and for adsorption to metals. We also consider expressions for SRP-DFs and briefly discuss other electronic structure methods that have addressed the interaction of molecules with metal surfaces. An overview is presented of dynamical models, which make a distinction as to whether or not, and which dissipative channels are modeled, the dissipative channels being surface phonons and electronically non-adiabatic channels such as electron-hole pair excitation. We also discuss the dynamical methods that have been used, such as the quasi-classical trajectory method and quantum dynamical methods like the time-dependent wave packet method and the reaction path Hamiltonian method. Limits on the accuracy of these methods are discussed for DC of diatomic and polyatomic molecules on metal surfaces, paying particular attention to reduced dimensionality approximations that still have to be invoked in wave packet calculations on polyatomic molecules like CH4. We also address the accuracy of fitting methods, such as recent machine learning methods (like neural network methods) and the corrugation reducing procedure. In discussing the calculation of observables we emphasize the importance of modeling the properties of the supersonic beams in simulating the sticking probability curves measured in the associated experiments. We show that chemically accurate barrier heights have now been extracted for DC in 11 molecule-metal surface systems, some of which form the most accurate core of the only existing database of Eb for DC reactions on metal surfaces (SBH10). The SRP-DFs (or candidate SRP-DFs) that have been derived show transferability in many cases, i.e., they have been shown also to yield chemically accurate Eb for chemically related systems. This can in principle be exploited in simulating rates of catalyzed reactions on nano-particles containing facets and edges, as SRP-DFs may be transferable among systems in which a molecule dissociates on low index and stepped surfaces of the same metal. In many instances SRP-DFs have allowed important conclusions regarding the mechanisms underlying observed experimental trends. An important recent observation is that SRP-DFT based on semi-local exchange DFs has so far only been successful for systems for which the difference of the metal work function and the molecule's electron affinity exceeds 7 eV. A main challenge to SRP-DFT is to extend its applicability to the other systems, which involve a range of important DC reactions of e.g. O2, H2O, NH3, CO2, and CH3OH. Recent calculations employing a PES based on a screened hybrid exchange functional suggest that the road to success may be based on using exchange functionals of this category.

Density Functional Theory for Molecule-Metal Surface Reactions: When Does the Generalized Gradient Approximation Get It Right, and What to Do If It Does Not.

While density functional theory (DFT) is perhaps the most used electronic structure theory in chemistry, many of its practical aspects remain poorly understood. For instance, DFT at the generalized gradient approximation (GGA) tends to fail miserably at describing gas-phase reaction barriers, while it performs surprisingly well for many molecule–metal surface reactions. GGA-DFT also fails for many systems in the latter category, and up to now it has not been clear when one may expect it to work. We show that GGA-DFT tends to work if the difference between the work function of the metal and the molecule’s electron affinity is greater than ∼7 eV and to fail if this difference is smaller, with sticking of O2 on Al(111) being a spectacular example. Using dynamics calculations we show that, for this system, the DFT problem may be solved as done for gas-phase reactions, i.e., by resorting to hybrid functionals, but using screening at long-range to obtain a correct description of the metal. Our results suggest the GGA error in the O2 + Al(111) barrier height to be functional driven. Our results also suggest the possibility to compute potential energy surfaces for the difficult-to-treat systems with computationally cheap nonself-consistent calculations in which a hybrid functional is applied to a GGA density.

Approximate equation-of-motion coupled-cluster methods for electron affinities of closed-shell molecules.

We investigate performance of the equation-of-motion coupled-cluster method at the single and doubles level (EOM-CCSD) and a series of approximate methods based on EOM-CCSD on electron affinities (EA) of closed-shell cations and neutral molecules with positive and negative EAs in this work. Our results confirm that P-EOM-MBPT2 can provide reasonable EAs when molecules with significant multireference character are not considered and its mean absolute error on EAs of these molecules is around or less than 0.2 eV. Its accuracy is comparable to that of the more expensive EOM-CCSD(2) method. Results of EOM-CCSD(2), P-EOM-MBPT2, and CIS(D∞) indicate that the [[H, ac +], T2] term in the 1h2p-1h block is more important on EAs than the term neglected in the 1h2p-1h2p block in P-EOM-MBPT2. We proposed an economical method where EAs from CIS(D∞) are corrected by treating this [[H, ac +], T2] term in the 1h2p-1h block perturbatively [corr-CIS(D∞)]. EAs with corr-CIS(D∞) agree very well with those of P-EOM-MBPT2 with a difference of less than 0.02 eV. Computational scaling of this method is N4 for the iterative part and N5 for some non-iterative steps. Its storage requirement is only of OV3. Corr-CIS(D∞) is an economical and reliable method on EAs, and it can be applied to EAs of large molecules.

Effect of fluorine on optoelectronic properties in DI-A-DII-A-DI type organic molecules: A combined experimental and DFT study

The impact of the substitution of fluorine atom/atoms on the optoelectronic features of organic molecules having DI-A-DII-A-DI type architecture is examined in the current work. The three synthesized organic molecules (SMD1, SMD2 and SMD3) comprise of a dithienopyrrole (DTP) derivative as a central donor (DII), which is flanked between two benzothiadiazole (BT) moieties (electron acceptors, A). The BT core on each of two ends is joined to an electron-donating benzodithiophene (BDT) derivative (DI). The SMD1, SMD2 and SMD3 are substituted with 0, 2 and 4 fluorine atoms on their BT moiety respectively. The assistance of DFT methods is taken to evaluate the influence of fluorine on reorganization energies, ionizing potential and electron affinity of molecules. The thermal stability of molecules is mapped by TGA studies. Cyclic voltammetry studies are carried out to comprehend the characteristics of highest molecular orbital, lowest unoccupied molecular orbital and the bandgap of molecules, which are also supported by DFT methods. The molecules displayed better absorption properties in the near-infrared (NIR) region, excellent solution processability in a variety of organic solvents, low bandgap and optimum thermal toughness to make them applicable in the construction of OBHJSCs to play the role of donor materials when connected with acceptors like fullerene derivatives.

The role of charge transfer at reduced graphene oxide/organic semiconductor interface on the charge transport properties

The effect of 1-pyrenesulfonicacid sodium salt (1-PSA), tetracyanoethylene (TCNE) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) on charge transport properties of reduced graphene oxide (RGO) is examined by measuring the transfer characteristics of field-effect transistors and co-planar time-of-flight photocurrent technique. Evidence of p-type doping and a reduction of mobility of electrons in RGO upon deposition of these materials is observed. Time-resolved photocurrent measurements show a reduction in electron mobility even at submonolayer coverage of these materials. The variation of transit time with different coverages reveals that electron mobility decreases with increasing the surface coverage of 1-PSA, TCNE and F4-TCNQ to a certain extent, while at higher coverage the electron mobility is slightly recovered. All three molecules show the same trend in charge carrier mobility variation with coverage, but with different magnitude. Among all three molecules, 1-PSA acts as weak electron acceptor compared to TCNE and F4-TCNQ. The additional fluorine moieties in F4-TCNQ provides excellent electron withdrawing capability compared to TCNE. The experimental results are consistent with the density functional theory calculations. • RGO FETs, exhibit a positive shift of the charge-neutrality point due to deposition of organic semiconductor layer. • The amount of shift varies with the molecule electron affinity. • The electron mobility is quenched by the presence of F4-TCNQ and TCNE, while 1-PSA don’t affect the electron mobility. • With increasing the coverage of 1-PSA, the electron mobility drops and then slightly recovers at higher coverages. • The change in the electron mobility is due to effect of teh RGO/organic-layer interface dipole field.

Интегральные характеристики оптических спектров, как новый класс дескрипторов для сложных молекулярных систем

The authors summarized the results on the study of the relationship of physicochemical properties with spectral integral characteristics of complex molecular systems: integral absorption, reflection coefficients, integral oscillator forces, color characteristics, integral autocorrelation functions, signal convolutions defined in the visible or UV spectral regions. It is shown that these integral characteristics are numerical parameters that adequately convey the physicochemical and electronic properties of molecules and their mixtures, so these values can be used as descriptors. Since electronic spectra are unique characteristics of a substance, these descriptors have a high discriminating ability. The classification of such descriptors is given. These values differ from physicochemical descriptors such as absorption maxima, since their determination requires information on the spectrum of these systems without isolating individual bands and maxima using the Fourier transform. In contrast to conventional physicochemical and quantum mechanical descriptors, the proposed descriptors can be applied to the study of multicomponent systems with unknown structure and composition. This makes it possible to use these values to determine the physicochemical properties and averaged over the composition of the electronic characteristics of a substance, such as petroleum distillates, petroleum, biological fluids, and similar mixtures. The paper provides relevant examples, confirmed by statistical data processing. The obtained patterns should be defined as the patterns "spectrum-property" by analogy with the patterns "structure-property", known in organic chemistry and chemical informatics. The consistency of the developed approach is shown, and a quantum-chemical interpretation of these laws is given. Based on the established spectrum-property dependencies, analytical methods can be developed for determining the amount of complex hydrocarbon systems, such as oil and their products, gas condensates, products of high-temperature pyrolysis of organic substances, etc. The established patterns can be applied to determine the ionization potentials and electron affinity of molecules, as well as in medical diagnostics, oil refining, petrochemistry, oil field development technology, and other fields of science and nanotechnology.

Interpretation and Simulation of Negative Ion Mass Spectra of Some Phosphorus Organoelement Compounds

Negative ion mass spectra for a series of organophosphorus compounds were obtained and negative ion fragmentation processes were treated theoretically. Using O-isopropyl and O-pinacolyl methylphosphonofluoridates as examples, electron affinities of molecules and their fragments were estimated using the UB3LYP/6-311+G(d,p) quantum-chemical approach and energetically more favorable and characteristic routes of dissociative electron attachment, including simple bond cleavage and rearrangements, were determined. Based on the obtained experimental and theoretical data, hypothetic fragmentation patterns were proposed and a special algorithm was compiled to predict negative ion mass spectra for some groups of organophosphorus compounds, such as О-alkyl methylphosphonofluoridates, О,О-alkyl phosphonodichloridates, and О,О′-dialkyl phosphonochloridates. The simulated mass spectra showed a good agreement with the experimental ones, confirming reasonable reliability of the proposed algorithm.

Size-Dependence of Nonempirically Tuned DFT Starting Points for G0W0 Applied to π-Conjugated Molecular Chains.

G0W0 calculations for predicting vertical ionization potentials (IPs) and electron affinities of molecules and clusters are known to show a significant dependence on the density functional theory (DFT) starting point. A number of nonempirical procedures to find an optimal starting point have been proposed, typically based on tuning the amount of HF exchange in the underlying hybrid functional specifically for the system at hand. For the case of π-conjugated molecular chains, these approaches lead to a significantly different amount of HF exchange for different oligomer sizes. In this study, we analyze if and how strongly this size dependence affects the ability of nonempirical tuning approaches to predict accurate IPs for π-conjugated molecular chains of increasing chain length. To this end, we employ three different nonempirical tuning procedures for the G0W0 starting point to calculate the IP of polyene oligomers up to 22 repeat units and compare the results to highly accurate coupled-cluster calculations. We find that, despite its size dependence, using an IP-tuned hybrid functional as a starting point for G0W0 yields excellent agreement with the reference data for all chain lengths.

The effect of local approximations in the ground-state coupled cluster wave function on electron affinities of large molecules

A possibility to calculate electron affinities (EAs) by a software devised for electron excitations is exploited to examine the accuracy of a partly local EA-EOM-CCSD method. In the proposed approach local approximations are applied to the ground-state coupled cluster wave function, while the EAs themselves are obtained in a full configurational space. The results of a numerical test for 14 molecules show that already with standard local settings the method reproduces the nonlocal EAs with the average error of 0.009 eV. Since the EA-EOM step of the calculation requires less computational resources than the computation of the CCSD ground state, the proposed hybrid approach can become a valuable tool for obtaining the EAs for molecules, which are too large for a canonical CCSD calculation, but still small enough for the EA-EOM step to be performed in a nonlocal way.

1H and 13C NMR spectra, structure and physicochemical features of phenyl acridine-9-carboxylates and 10-methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulphonates – alkyl substituted in the phenyl fragment

The 1H and 13C NMR spectra of twelve phenyl acridine-9-carboxylates – alkyl-substituted in the phenyl fragment – and their 10-methyl-9-(phenoxycarbonyl)acridinium salts dissolved in CD 3CN, CD 3OD, CDCl 3 and DMSO- d 6 were recorded in order to examine the influence of the structure of these compounds and the properties of the solvents on chemical shifts and 1H– 1H coupling constants. Experimental data were compared with 1H and 13C chemical shifts predicted at the GIAO/DFT level of theory for DFT(B3LYP)/6-31G** optimised geometries of molecules, as well as with values of 1H chemical shifts and 1H– 1H coupling constants, estimated using ACD/HNMR database software to ensure that the assignment was correct. To investigate the relations between chemical shifts and selected structural or physicochemical characteristics of the target compounds, the values of several of these parameters were determined at the DFT or HF levels of theory. The HOMO and LUMO energies obtained at the HF level yielded the ionisation potentials and electron affinities of molecules. The DFT method provided atomic partial charges, dipole moments, LCAO coefficients of p z LUMO of selected C atoms, and angles reflecting characteristic structural features of the compounds. It was found that the experimentally determined 1H and 13C chemical shifts of certain atoms relate to the predicted dipole moments, the angles between the acridine and phenyl moieties, and the LCAO coefficients of the p z LUMO of the C atoms believed to participate in the initial step of the oxidation of the target compounds. The spectral and physicochemical characteristics of the target compounds were investigated in the context of their chemiluminogenic ability.

Classical scaling of the quantum capacitances for molecular wires

Detailed calculations demonstrate that the quantum capacitances of nonspherical, rodlike molecular wires scale with their dimensions according to a linear law much like that obeyed by classical, macroscopic spherical capacitors. Further, this linear isoperimetric scaling law leads to a particularly simple formula that relates a molecule's electron affinity directly to its ionization potential and its spatial dimensions.

Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods

Spectroscopic and optical properties of nanosystems and point defects are discussed within the framework of Green's function methods. We use an approach based on evaluating the self-energy in the so-called GW approximation and solving the Bethe-Salpeter equation in the space of single-particle transitions. Plasmon-pole models or numerical energy integration, which have been used in most of the previous GW calculations, are not used. Fourier transforms of the dielectric function are also avoided. This approach is applied to benzene, naphthalene, passivated silicon clusters (containing more than one hundred atoms), and the F center in LiCl. In the latter, excitonic effects and the $1s \to 2p$ defect line are identified in the energy-resolved dielectric function. We also compare optical spectra obtained by solving the Bethe-Salpeter equation and by using time-dependent density functional theory in the local, adiabatic approximation. From this comparison, we conclude that both methods give similar predictions for optical excitations in benzene and naphthalene, but they differ in the spectra of small silicon clusters. As cluster size increases, both methods predict very low cross section for photoabsorption in the optical and near ultra-violet ranges. For the larger clusters, the computed cross section shows a slow increase as function of photon frequency. Ionization potentials and electron affinities of molecules and clusters are also calculated.

Substrate-mediated interactions and intermolecular forces between molecules adsorbed on surfaces.

Adsorbate interactions and reactions on metal surfaces have been investigated using scanning tunneling microscopy. The manners in which adsorbates perturb the surface electronic structure in their vicinity are discussed. The effects these perturbations have on other molecules are shown to be important in overlayer growth. Interactions of molecules with surface steps are addressed, and each molecule's electron affinity is shown to dictate its adsorption sites at step edges. Standing waves emanating from steps are demonstrated to effect transient molecular adsorption up to 40 A away from the step edge. Halobenzene derivatives are used to demonstrate how the surface is important in aligning reactive intermediates.

Gas phase ion thermochemistry based on ion-equilibria from the ionosphere to the reactive centers of enzymes

The discovery that ion-molecule equilibria can be determined in the gas phase with mass spectrometers, which was made accidentally with a high-pressure mass spectrometer constructed for other purposes, has led to the development of a vast field of gas-phase ion-equilibria determinations with mass spectrometers. Data have been obtained for many thousands of important reactions, such as the ion-solvent molecule and ion-ligand bonding interactions, basicities of bases B and acidities of acids AH in the gas phase, stabilities of carbocations, and electron affinities of molecules. The data have applications in many areas of chemistry. Examples of such applications are given.

Laser-induced crossed-beam charge transfer: energy threshold of the Na (3 2P [formula omitted]) + I 2 → Na + + I −2 channel

A crossed-beam charge-transfer study was carried out for the Na(3 2P 3 2 ) + I 2 → Na + + I − 2 system using laser excitation of the Na atom. The energy threshold and post-threshold laws were measured for the title reaction and the I 2 molecule electron affinity was determined. The value EA (I 2) = 2.69 ± 0.05 eV obtained in the present work is in accordance with previous determinations.

Electronic spectroscopy method for determining the ionization potential and electron affinity of molecules of aromatic compounds

Electronic spectroscopy method for determining the ionization potential and electron affinity of molecules of aromatic compounds

The sputtering of molecular ions from surfaces in secondary ion mass spectrometry

Recently, experimental evidence was reported that the charge state and the sputtering mechanism of molecular ions may be connected. In an attempt to understand this phenomenon, this paper discusses the excess energies and the stability towards electron ejection for molecules formed by the recombination of nonadjacent sputtered atoms. It is shown that since the electron affinities of molecules are usually small, stability of a negative molecular ion against neutralization by autodetachment is possible only when the molecule is formed by lattice fragmentation in most cases. On the otherhand, the ionization potentials of neutral and positive molecular ions are usually large and stability against electron ejection is possible even if they are formed by the recombination of nonadjacent sputtered atoms.

Electron affinity of adsorbed dye monolayers

We have calculated the electron affinity of dye molecules and compared it with experimental results from the electron beam retardation method. The calculations were performed on the dye pinacyanol adsorbed on CdS. First, we calculated the single molecule electron affinity which corresponds to the vapour state electron affinity. Next, we calculated the electron affinity of a molecule in a linear aggregate, considering the neighbouring molecules, and finally, the electron affinity of molecules in aggregates adsorbed on an ideal crystal substrate and on a substrate with defects. We prove that there is a distribution of electron affinities which is caused by random defects on the substrate surface. The experimental electron affinity for monolayers of pinacyanol on CdS corresponds to the highest electron affinities in the distribution. We propose that this is caused by electron diffusion in the dye, and present additional experimental evidence which supports our conclusion. The effects of various interactions ...