Theoretical Ionization Potentials Research Articles

Photoionization, Structures, and Energetics of Na-Doped Formic Acid-Water Clusters.

The influence of formic acid on water cluster aggregation has been investigated experimentally by mass spectrometry and tunable UV laser ionization applied to Na‐doped clusters formed in the supersonic expansion of water vapors seeded with formic acid (FA) as well as theoretically using high level quantum chemistry methods. The mass spectra of Na−FA(H2O)n clusters show an enlarging of mass distribution toward heavier clusters with respect to the Na−(H2O)n clusters, suggesting similar mass distribution in neutral clusters and an influence of formic acid in water aggregation. Density functional theory and coupled‐cluster type (DLPNO‐CCSD(T)) calculations have been used to calculate structures and energetics of neutral and ionized Na−FA(H2O)n as well as neutral FA(H2O)n. Na‐doped clusters are characterized by very stable geometries. The theoretical adiabatic ionization potential values match pretty well the measured appearance energies and the calculated first six electronic excited states show Rydberg‐type characters, indicating possible autoionization contributions in the mass spectra. Finally, theoretical calculations on neutral FA(H2O)n clusters show the possibility of similarly stable structures in small clusters containing up to n=4–5 water molecules, where FA interacts significantly with waters. This suggests that FA can compete with water molecules in the starting stage of the aggregation process, by forming stable nucleation seed.

Heteroatom (N, P, As, Sb, Bi) Effects on the Resonance-Stabilized 2-, 3-, and 4-Picolyl Radicals.

Important recent experimental studies have allowed the isomer-selective identification of the 2-, 3-, and 4-picolyl radicals. The picolyl radicals and their valence isoelectronic P, As, Sb, and Bi congeners are investigated here. For the three observed parent radicals, the theoretical ionization potentials agree with experiment to within 0.02 eV. Two rules are proposed for predicting vertical ionization potentials (EVIE) and relative energies. The EVIE values for these radicals will be higher when large percentages of the SOMO orbitals are distributed on the atoms with greater electronegativities. The cations of these systems were also studied along with the closed-shell methylpyridines and their P, As, Sb, and Bi analogs. The energies for the cationic species will lie lower when high percentages of π natural localized molecular orbitals occur on the more electronegative atoms. The structures of the 2- and 4-isomers strongly depend upon the heteroatoms, with the C-C linkages adopting a single-double alternating bond manner when the heteroatoms become heavier. The 3-isomers adopt roughly equal C-C bond distances with small changes from N to Bi.

Experimental and Theoretical Soft X-ray Study of Nicotine and Related Compounds.

The valence and core electronic structure of nicotine, nicotinic acid, and nicotinamide have been studied by photoelectron and soft X-ray absorption spectroscopy, supported by theoretical calculations, which take into account conformational isomerism. The core-level photoionization spectra of all molecules have been assigned, and theory indicates that the effects of conformational differences are small, generally less than the natural line widths of the core ionic states. However, in the case of nicotinamide, the theoretical valence ionization potentials of cis and trans conformers differ significantly in the outer valence space, and the experimental spectrum is in agreement with the calculated outer valence cis conformer spectrum. In addition, the C, N, and O K edge near-edge absorption fine structure spectra are reported and interpreted by comparison with reference compounds. We find evidence at the N and O K edges of interaction between the delocalized orbitals of the pyridine ring and the substituents for nicotinic acid and nicotinamide. The strength of the interaction varies because the first is planar, while the second is twisted, reducing the extent of orbital mixing.

Investigation into reactions of methyl methacrylate and ethyl acrylate with chlorine atom

Reactions between chlorine and unsaturated esters in gas phase are examined in a slow-flow reaction tube inside the laboratory-built photoionization mass spectrometer at the energy range of 8–11 eV. 248 nm laser radiation is used to initiate the reaction. Products are distinguished, C5H8O2Cl for addition, and C5H7O2, C5H7O2Cl and C5H9O2Cl for abstraction. The direct or indirect products are detected, indicating secondary reactions. And experimental ionization potentials are procured for direct adducts of methyl methacrylate to be 8.30 eV and for that of ethyl acrylate to be 9.95 eV which are well consistent with theoretical ionization potentials of likely isomers. Theoretical reaction channels are also accounted for, optimized under M06-2X/6-31 + G(d,p) level and ionization potentials of products are calculated under M06-2X/6-31 + G(d,p) level also. Differences between experimental and theoretical details are discussed.

Green's Function Coupled-Cluster Approach: Simulating Photoelectron Spectra for Realistic Molecular Systems.

In this paper, we present an efficient implementation for the analytical energy-dependent Green's function coupled-cluster with singles and doubles (GFCCSD) approach with our first practice being computing spectral functions of realistic molecular systems. Because of its algebraic structure, the presented method is highly scalable and is capable of computing spectral function for a given molecular system in any energy region. Several typical examples have been given to demonstrate its capability of computing spectral functions not only in the valence band but also in the core-level energy region. Satellite peaks have been observed in the inner valence band and core-level energy region where a many-body effect becomes significant and the single particle picture of ionization often breaks down. The accuracy test has been carried out by extensively comparing the computed spectral functions by our GFCCSD method with experimental photoelectron spectra as well as the theoretical ionization potentials obtained from other methods. It turns out the GFCCSD method is able to provide a qualitative or semiquantitative level of description of ionization processes in both the core and valence regimes. To significantly improve the GFCCSD results for the main ionic states, a larger basis set can usually be employed, whereas the improvement of the GFCCSD results for the satellite states needs higher-order many-body terms to be included in the GFCC implementation.

Thermochemistry of icosahedral closo-dicarboranes: a composite ab initio quantum-chemical perspective

We obtained accurate thermochemical properties for the ortho-, meta-, and para-dicarborane isomers (C2B10H12) by means of explicitly correlated high-level thermochemical procedures. The thermochemical properties include heats of formation, isomerization energies, C–H and B–H bond dissociation energies (BDEs), and ionization potentials. Of these only the ionization potentials are known experimentally. Our best theoretical ionization potentials, obtained by means of the ab initio W1–F12 thermochemical protocol, was 241.50 kcal mol–1 (para-dicarborane), 238.45 kcal mol–1 (meta-dicarborane), and 236.54 kcal mol–1 (ortho-dicarborane). These values agree with the experimental values adopted by the National Institute of Standards and Technology (NIST) thermochemical tables to within overlapping uncertainties. However, they suggest that the experimental values may represent significant underestimations. For all isomers, the C–H BDEs are systematically higher than the B–H BDEs because of the relative stability of the boron-centred radicals. The C–H BDEs for the three isomers cluster within a narrow energetic interval, namely between 110.8 kcal mol–1 (para-dicarborane) and 111.7 kcal mol–1 (meta-dicarborane). The B–H BDEs cluster within a larger interval ranging between 105.8 and 108.1 kcal mol–1 (both obtained for ortho-dicarborane). We used our benchmark W1–F12 data to assess the performance of a number of lower cost composite ab initio methods. We found that the Gaussian-3 procedures (G3(MP2)B3 and G3B3) result in excellent performance with overall root-mean-square deviations (RMSDs) of 0.3–0.4 kcal mol–1 for the isomerization, ionization, and bond dissociation energies. However, the Gaussian-4 procedures (G4, G4(MP2), and G4(MP2)-6X) showed relatively poor performance with overall RMSDs of 1.3–3.7 kcal mol–1.

A DFT study on equilibrium geometries, stabilities, and electronic properties of small bimetallic Na-doped Au n (n = 1-9) clusters: comparison with pure gold clusters

A systematic study on the geometric structures, relative stabilities, and electronic properties of small bimetallic Au(n)Na (n = 1-9) clusters has been performed by means of first-principle density functional theory calculations at the PW91PW91 level. The results show that the optimized ground-state isomers adopt planar structures up to n = 5, and the Na-capped geometries are dominant growth patterns for n = 6-9. Dramatic odd-even alternative behaviors are obtained in the second-order difference of energies, fragmentation energies, highest occupied-lowest unoccupied molecular orbital energy gaps, and chemical hardness for both Au(n)Na and Au(n+1) clusters. It is found that Au(5)Na and Au(6) have the most enhanced stability. Here, the size evolutions of the theoretical ionization potentials are in agreement with available experimental data, suggesting a good prediction of the lowest energy structures in the present study. In addition, the charge transfer has been analyzed on the basis of natural population analysis.

Electro-oxidation of Biginelli Dihydropyrimidones

The electrochemical oxidation of several Biginelli compounds in acetonitrile at different pH values was carried out. The influence of the substituents at the heterocyclic ring was studied in tetrabutylammonium perchlorate by linear and sweep voltamperometry with platinum rotatory disc. Cyclic voltamperometry, controlled potential electrolysis and EPR techniques were used in order to determine the oxidation mechanisms. In acidic medium all derivatives follow an oxidation mechanism with an ECE (Electrochemical, Chemical, Electrochemical process), sequence. On the other hand, in basic media the derivatives obey a DISP1 (Disproportionation) mechanism, whereas the uptake of the proton at N1 by the OH- ion of the medium was the rate determining step. The effect of different substituents in the compounds was studied by using the theoretical ionization potential and the half wave oxidation potential correlation, as. It was found that there is an acceptable relationship between these parameters. The electrosynthesis of the para-nitro derivative was carried out in order to characterize the corresponding oxidation product.

Structures and Properties of Stable Al4, Al4+, and Al4- Comparatively Studied by ab Initio Theories

Previous high-level theoretical calculations of aluminum clusters mostly relied on density functional theory (DFT) or theories less sophisticated than it. Here, we point out that a second-order Møller-Plesset perturbation (MP2) method is more appropriate and is the minimum level of theory in the predictions of property such as geometries, electronic structures, and IR and Raman spectra of Al4, Al4+, and Al4- clusters. The theoretical electron affinities and ionization potentials predicted with MP2 geometries are closer to experimental ones than those by DFT. The p-electron characters and single valence state of aluminum atoms and IR and Raman spectra of the aluminum clusters were also reliably predicted by MP2 and could be based on for further experimental and theoretical studies.

Prediction of the photoelectron spectrum for XPY2(X = H, F, Cl; Y = O, S) type of molecules by ab initio and DFT methods

The present study reports the theoretical vertical ionization potentials (IPs) for all the valence electrons in six XPY2molecules by utilizing the corrected orbital energies calculated with three theoretical methods, namely, the ab initio Hartree–Fock (HF), and both the pure and hybrid density functional theory (DFT) methods at, respectively, the BLYP/6-311+G* and B3lYP/6-311+G* levels of theory. Evaluation of the numerical corrections to the orbital energies was achieved by comparisons with the IP values obtained via explicit computation of the energy differences between the neutral molecules and the corresponding radical cations (the ΔSCF method) and shows values from –0.9 to –1.9 eV for the HF, and positive values from 2.9 to 3.9 eV and from 1.8 to 2.4 eV for the pure and hybrid DFT methods, respectively. In contrast to the orbital energies, the ΔSCF method is shown to give consistent values among the three methods, as well as reasonable agreement with the experimental IP values.Key words: ionization potential, phosphorane, Koopmans' theorem, Janak's theorem.

Thermodynamic properties of germanium/carbon microclusters

Theoretical studies on the Ge(n)C(m) (n=1,2; m=1-3) microclusters have been performed using the state of the art calculations. Several alternative structures of these clusters were studied to locate the lowest-energy isomers. It is observed that the structures of the complexes result from the competition between ionic Ge-C, conjugated covalent C-C, and metallic Ge-Ge bonds. The ionization of the molecules enhances the ionic character of the Ge-C bond and has significant structural consequences. Using theoretically determined partition functions, thermodynamic data are computed and experimental enthalpies are enhanced. The ab initio atomization energies of germanium carbides compare well with corrected experimental functions. The experimental appearance potentials are well reproduced by the theoretical ionization potentials.

Density-functional study ofAun(n=2–20)clusters: Lowest-energy structures and electronic properties

We have investigated the lowest-energy structures and electronic properties of the Au$_n$(n=2-20) clusters based on density functional theory (DFT) with local density approximation. The small Au$_n$ clusters adopt planar structures up to n=6. Tabular cage structures are preferred in the range of n=10-14 and a structural transition from tabular cage-like structure to compact near-spherical structure is found around n=15. The most stable configurations obtained for Au$_{13}$ and Au$_{19}$ clusters are amorphous instead of icosahedral or fcc-like, while the electronic density of states sensitively depend on the cluster geometry. Dramatic odd-even alternative behaviors are obtained in the relative stability, HOMO-LUMO gaps and ionization potentials of gold clusters. The size evolution of electronic properties is discussed and the theoretical ionization potentials of Au$_n$ clusters compare well with experiments.

Quantum-chemical study of chelate ring cleavage in some alkaline and alkaline earth β-diketonates

This paper reports a theoretical study of chelate ring cleavage in the ground and electronically excited states of lithium, sodium, potassium, magnesium, and beryllium malonodialdehynates in the ab initio approximation including configuration interaction. As shown by calculations, electron excitation mostly lowers the energy barriers to rotation and the metallocycle cleavage energies. The modeling of chelate ring cleavage revealed a change in the composition of the wave function of the excited states of the complexes. In addition to analysis of the composition of wave functions, the paper discusses changes in the energies of the highest orbitals of the compounds. For magnesium malonodialdehynate, the theoretical ionization potentials are compared with experimental values.

Electronic, Structural, and Hyperfine Interaction Investigations on Rydberg Molecules: NH4, OH3, and FH2

The geometrical structures of NH4 and OH3 were optimized at the MP2 level with the aug-cc-pvdz, aug-cc-pvtz, and aug-cc-pvqz basis set plus s-type diffuse functions. The adiabatic ionization potential, barrier height, and dissociation energy of NH4 and OH3 were calculated with the above basis set series and were extrapolated to the infinite basis set limit. The theoretical ionization potential of NH4 was in very good agreement with the experimental value. The N−H bond lengths of NH4 and NH4+ at the infinite basis set limit were obtained by parabolic interpolation around the equilibrium point. FH2 was optimized at the UHF, B3LYP, and MP2 levels. However, only a dissociated ground state was found. The potential energy curves for dissociation of the above molecules were calculated with MP2. The relative stabilities of these molecules and their isotopic counterparts are discussed. Theoretical hyperfine calculations were performed in the gas phase as well as in a neon matrix for NH4 and OH3 with a single-refer...

Density-functional study of structures and electronic properties of Cd clusters

The lowest-energy structures and electronic properties of cadmium clusters are studied by density-functional theory with the generalized gradient approximation. The equilibrium structures of ${\mathrm{Cd}}_{n}$ $(n=2--21)$ clusters are determined from a number of structural isomers, which are generated from genetic algorithm simulations with a tight-binding potential. Various close-packed structures are found for cadmium clusters. ${\mathrm{Cd}}_{n}$ clusters with $n=4,9,10,15,17,20$ show relatively high stability, which can be related to the electron shell model, and agrees with experimental results. The density of states for the magic number clusters can also be associated with the electronic shell. The theoretical ionization potentials of ${\mathrm{Cd}}_{n}$ compare well with experimental values. The size evolution of electronic properties from van der Waals to covalent and bulk metallic behavior in Cd clusters is discussed.

Electronic excitation and ionization spectra of azabenzenes: Pyridine revisited by the symmetry-adapted cluster configuration interaction method

Electronic excited and ionized states of pyridine were reinvestigated by the symmetry-adapted cluster configuration interaction (SAC-CI) method using an extended basis set and a wide active space. The present SAC-CI results for the singlet and triplet excited states are greatly improved and agree well with the experimental observations, providing a firm assignment of all low-lying n→π* and π→π* valence excited states observed in the vacuum ultraviolet spectrum and electron energy-loss spectrum. The ionization potentials were reexamined by the SAC-CI general-R (R represents excitation operator) method. The first four ionization potentials are greatly improved compared with our previous results obtained by the SAC-CI single- and double-R (SD-R) method. The present theoretical ionization potentials are in good agreement with the experimental values in high-resolution synchrotron photoelectron spectrum for energy regions up to 25 eV (which contain outer- and inner-valence regions), and give a detailed theoretical assignment for the photoelectron spectra.

Sequence and Stacking Dependence of 8-Oxoguanine Oxidation: Comparison of One-Electron vs Singlet Oxygen Mechanisms

The oxidation of 7,8-dihydro-8-oxoguanine (8-oxoG)-containing oligodeoxynucleotides has been investigated using a variety of oxidants, including one-electron oxidants (Ir(IV), Fe(III), NiCR/KHSO5, and SO4-•) as well as singlet oxygen, generated both photochemically and thermally. The extents of oxidation in single-stranded and duplex oligodeoxynucleotides are compared, confirming theoretical ionization potentials of 8-oxoG in different sequence contexts in duplex DNA. As with guanine, 8-oxoG residues stacked in a duplex with a 3‘ neighboring G are more readily oxidized by one-electron oxidants than those stacked next to other bases, although the effect of stacking appears to be less pronounced for 8-oxoG than for G. Regardless of sequence, 8-oxoG is always more easily oxidized than the four natural nucleobases, even in the presence of multiple G sequences. Reactions with singlet molecular oxygen, thought to proceed through a cycloaddition mechanism, show little sequence selectivity and a 7-fold higher reactivity with single-stranded compared to duplex 8-oxoG residues. One-electron oxidants, such as Ir(IV) complexes, showed a more modest 3−4-fold higher reactivity with single-stranded DNA. In contrast, the Schiff base complex [NiCR]2+, used in conjunction with a strong oxidant, KHSO5, shows a 2-fold preference for oxidation of duplex vs single-stranded 8-oxoG, perhaps because of the high driving force and the possibility for competing G oxidation to equilibrate to 8-oxoG oxidation via hole transfer. Overall, these results point to subtle mechanistic differences in one-electron oxidation but a major distinction between one-electron and 1O2-mediated oxidation. Furthermore, they suggest an important role for 8-oxoG, not only as a product of oxidative DNA damage but also as a substrate for further oxidation.

The adiabatic and vertical ionization potentials of NH 2 to the three lowest-lying states of NH 2+

The adiabatic and vertical ionization potentials of the ground state (X̃ 2B 1) of NH 2 to the X̃ 3B 1, ã 1A 1 and b̃ 1B 1 states of NH 2 + have been investigated systematically using ab initio electronic structure theory. The SCF, CASSCF, CISD, CASSCF-SOCI, CCSD and CCSD(T) levels of theory with four basis sets have been employed to predict energy differences. The adiabatic and vertical ionization potentials of the ground state of NH 2 to the X̃ 3B 1 state of NH 2 + are determined to be 255.8 kcal/mol (11.09 eV, 89 470 cm −1) and 277.1 kcal/mol (12.01 eV, 96 900 cm −1), respectively, which are in excellent agreement with the experimental values of 256.9 kcal/mol (11.14 eV, 89 850 cm −1) and 276.7 kcal/mol (12.00 eV, 96 790 cm −1). The theoretical adiabatic and vertical ionization potentials to the ã 1A 1 state of NH 2 + are evaluated to be 285.7 kcal/mol (12.39 eV, 99 910 cm −1) and 287.3 kcal/mol (12.46 eV, 100 480 cm −1), respectively, which are also in good agreement with the experimental values of 287.0 kcal/mol (12.445 eV, 100 380 cm −1) and 287.1 kcal/mol (12.45 eV, 100 420 cm −1). Finally the adiabatic and vertical ionization potentials to the b̃ 1B 1 state of NH 2 + are predicted to be 298.1 kcal/mol (12.93 eV, 104 270 cm −1) and 324.8 kcal/mol (14.09 eV, 113 600 cm −1), respectively.

A theoretical evaluation of the ionization potentials for one-electron oxidized states of cytochrome c3

The ionization potentials of one-electron oxidized states of cytochrome c 3 were evaluated by numerical calculations. In the ionization processes from cytochrome c 3 0+ to c 3 1+, four ionization potentials were observed with respect to four hemes involved in cytochrome c 3. Ab initio SCF MO calculations were carried out for the four heme complexes to obtain their ionization potentials under vacuum (IP vacuums). The electrostatic potentials (EPs) which the apo-protein and the aqueous solvent produce at the heme irons were calculated by the particle–particle/particle–mesh (PPPM) algorithm based on the self-consistent boundary (SCB) method. Final theoretical ionization potentials of cytochrome c 3 (IP proteins) were evaluated as the sum of IP vacuums and EPs. The calculated IP proteins successfully reproduced the order of corresponding experimental values. The maximum difference among IP proteins was 130 mV which was approximately twice as large as that of experimental ones. The small differences among IP proteins were given by the delicate cancellations of IP vacuums and EPs. We also discuss the detailed electronic structures of hemes in cytochrome c 3.

UV photoelectron and ab initio quantum mechanical characterization of nucleotides: The valence electronic structure of anionic 2?-deoxyadenosine-5?-phosphate

He(I) ultraviolet (UV) photoelectron spectroscopy and ab initio, self-consistent field (SCF) calculations with the 6-31G basis set have been employed to characterize the valence electronic structures of anionic 2′-deoxyadenosine-5′-phosphate (5′-dAMP−). Theoretical ionization potentials (IPs) of 5'-dAMP-, of the neutral model compounds 9-methyladenine (9-MeA) and 3-hydroxytetrahydrofuran (3-OH-THF), and of the model anion CH3HPO4− have been obtained by applying Koopmans' theorem to ab initio SCF results. The ionization potentials predicted from the SCF calculations have been compared to He(I) photoelectron spectra of 9-MeA and 3-OH-THF. The SCF calculations predict a value (8.45 eV), for the highest occupied π orbital in 9-MeA which agrees well with the experimental vertical IP (8.39 eV). However, IPs for the highest occupied lone-pair orbitals in 3-OH-THF are predicted to be more than 1.52 eV higher than the experimental IPs. Results from recently reported [H. S. Kim and P. R. LeBreton, Proc. Natl. Sci. USA 91, 3725–3729 (1994), and N. S. Kim and P. R. LeBreton, J. Am. Chem. Soc., 118, 3694 (1996)] second-order Moller-Plesset perturbation (MP2) calculations and configuration interaction calculations using the configuration interaction singles (CIS) method indicate that configuration interaction effects strongly influence the energies of the first five ionization events arising from removal of electrons from the closed-shell model anion CH3HPO4−. Results from the 6-31G SCF calculations of 5′-dAMP−, 9-MeA, 3-OH-THF, and CH3HPO4− indicates that valence orbital electron distributions in the nucleotide and in the model compounds and anion are similar. The correspondence between the orbital structure of the nucleotide, and the model compounds and anion makes it possible, employing experimental photoelectron data and MP2/CIS computational results for the model compounds and anion, to individually correct IPs calculated for the nucleotide at the 6-31G SCF level. Here, this approach has provided values for the 13 lowest IPs of 5′-dAMP− and indicates that the first IPs of the base, sugar, and phosphate groups are 6.1, 7.8, and 5.5 eV, respectively. © 1996 John Wiley & Sons, Inc.