Spectroscopic constants of SiH2, GeH2, SnH2, and their cations and anions from density functional computations

Local (LSD ) and nonlocal (NLSD ) spin density calculations using different exchangecorrelation functionals have been performed to determine equilibrium geometries, harmonic vibrational frequencies (ωe), ionization potentials (IP ), electron affinities (EA ), dipole moments (μ), and singlet-triplet energy gaps (Δ E_ST) of SiH₂, GeH₂, and SnH₂. Geometrical structures as well as vibrational frequencies are in agreement with the available experimental data and compare favorably with the most sophisticated postHartree-Fock computations performed until now. Both computed IPS (9.15 and 9.25 eV for SiH₂ and GeH₂, respectively) and EA of SiH₂ (1.17 eV) compare favorably with experimental data (9.17, 9.21, and 1.2 eV). Accurate values are obtained also for singlet-triplet energy gaps. We report for the first time the electron affinities of all neutral systems and the spectroscopic constants of the cations and anions. © 1995 John Wiley & Sons, Inc.     

Ab initio molecular orbital calculations of the cobalt porphine complex. I. LCAO SCF MO calculation of low-spin,high-spin,and π-ionized states of Co-porphine

Ab initio LCAO SCF MO calculations are carried out on planar Co-porphine with a basis set of roughly double zeta quality for Co and N and of single zeta quality for C and H. The net charge on Co and N and the overlap population between them are 1.78, ?0.57, and 0.06, respectively, in the ²A_1g, state, which is known to be the ground state by experiment. The bonding in this complex is thus largely ionic. The first and second calculated ionization potentials are 6.51 and 6.77 eV, respectively, and are in reasonable agreement with the observed ionization potentials of 6.44 and 6.62 eV for Ni-tetraphenylporphine. CI calculations within the framework of the ligand field theory are also performed. The calculated order of the five lowest states is ⁴B_2g ≈ ⁴E_g, ⁴A_2g, ²A_1g, ⁴E_g from below and is not in agreement with the semiempirical order of ²A_1g ≈ ⁴B_2G, ⁴A_2g, ²E_g, ⁴E_g determined by Lin.     

Applications of spectral-Representation model as a potential method for Cu clusters

We applied the spectral-representation technique developed by Katsuki and Huzinaga as a model potential in calculating the electronic structure of Cu clusters. The characteristics of this potential were closely investigated in Cu and Cu₂. For Cu, Cu₂, Cu₅, Cu₉, and Cu₁₃, we performed all-electron ab initio self-consistent field calculations and model-potential calculations where 3p, 3d, and 4s electrons, and 3d and 4s electrons are treated as valence electrons. The ionization potentials (IPs) given by the all-electron calculations were 6.26, 5.55, 4.52, 4.02, and 4.08 eV for Cu, Cu₂, Cu₅, Cu₉, and Cu₁₃, respectively. The IPs given by the model-potential calculations were 6.25, 5.56, 4.62, 4.09, and 4.23 eV for the 3p-, 3d-, and 4s-valence electrons, and 6.26, 5.68, 4.71, 4.07, and 4.19 eV for the 3d- and 4s-valence electrons. The IPs given by the model-potential calculations agree well with those of the all-electron calculations. We also performed model-potential calculations where only the 4s electrons were treated as valence electrons. The calculated IPs were 6.47, 5.98, 5.38, 4.63, and 4.88 eV for Cu, Cu₂, Cu₅, Cu₉, and Cu₁₃, respectively. These are ca. 0.8 eV higher than the IPs by the all-electron calculation for the larger clusters of Cu₅, Cu₉, and Cu₁₃. The higher IPs originate from the expulsion of the 3d electrons from the valence electrons. We also performed model-potential calculations with 4s electrons for Cu₇₄. The calculated IP is 4.61 eV, which is estimated to be 0.8 eV larger than that obtained by the all-electron calculation. The IPs with correlation corrections are 7.7, 7.4, 6.3, 5.8, 5.9, and 5.6 eV for Cu, Cu₂, Cu₅, Cu₉, Cu₁₃, and Cu₇₄, respectively. Experimental values are 7.73, 7.37, 6.30, 5.37, 5.67, and 5.26 eV. The agreement between the two is fairly good. The electron affinities are also discussed. © 1996 by John Wiley & Sons, Inc.     

Ground states of molecules. 53.MNDO calculations for molecules containing chlorine

MNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (Cl^I) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl₂O, Cl₂, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF₃ are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.     

A contracted bromine basis set for use in calculation of molecular energies

A contracted [9s6p2d] basis set derived from Dunning's (14s11p5d) primitive Gaussian set for bromine has been used in ab initio molecular orbital calculations of the dissociation energies of HBr, CH₃Br, and Br₂, the ionization potentials of Br and HBr, and the electron affinity of Br. The calculated energies are within 0.1 eV of the experimental values. This is similar to the accuracy obtained in a previous study, also using a contracted [9s6p2d] basis set, of the dissociation and ionization energies of the GeH_n, AsH_n, and SeH_n hydrides.     

Core polarization and relativistic effects competition in the first ionization potentials for some systems in the Cu,Ag, and Au isoelectronic sequences

Single-configuration relativistic Hartree–Fock values of the first ionization potentials for Cu through Kr⁷⁺, Ag through I⁶⁺, and Au through Pb³⁺ are computed in “frozen” and “relaxed core” approximations with and without allowance for core polarization. Effects of polarization of the atomic core by the valence electron are included by introducing a polarization potential in the one-electron Hamiltonian of the valence electron. The core polarization potential depends on two parameters, the static dipole polarizability of the core α and the cut-off radius r₀, which are chosen independently of the ionization potential data. It is demonstrated that by including the core polarization potential with α and r₀ parameters, which are simply chosen instead of being empirically fitted, it is still possible to account, on the average, for at least 70% of the discrepancy between the single-configuration relativistic Hartree–Fock ionization potentials and the experiment, a discrepancy usually ascribed to the contribution of valence-core electron correlations, and to bring the theoretical ionization potentials to an average agreement with experiment of around 1%. It can be concluded from this study that for low and medium Z elements the core polarization dominates for neutral systems or systems in low ionization stages, whereas for highly ionized systems the relativistic effects prevail. For heavy elements, however, the core polarization influence is comparable to the relativistic one only for neutral systems, whereas for ions the relativistic effects are overwhelmingly predominant.     

Latest Findings on the Photoionization of Met-Cars

An overview of our latest findings on the photoionization of metallocarbohedrene clusters (Met-Cars) is presented. The ionization potentials (IPs) of titanium and zirconium Met-Cars of the stoichiometry Ti_8–x Zr _x C₁₂ (x=0–4, 8) were determined by studying their photoionization efficiency spectra near threshold. It was found that the IPs are about 4 eV and decrease smoothly as the Ti/Zr ratio decreases. Comparison of the determined IPs with those predicted by theoretical calculations by other groups suggests that the geometry of the Met-Cars is that of the tetrahedral cage structure with T _d symmetry, assuming a high degree of accuracy in the IP calculations. In another experiment, we investigated the mechanism of the delayed ionization of both single- and binary-metal Met-Cars. Using a reflectron time-of-flight mass spectrometer with a modified Wiley–McLaren double acceleration region, we measured ionization rates arising from the electron emission of Met-Cars composed of various transition metals under different photoionization conditions, which led to the conclusion that a thermionic emission mechanism dominantly contributes to the delayed ionization.     

The assignment of the ionization potentials of formaldehyde by an (e, 2e) experiment

The angular variation of the binding energy spectrum in an electron-electron coincidence experiment indicates that the third and fourth ionization potentials of formaldehyde are due to the 5a₁ and 1b₂ orbitals respectively. The vertical ionization potentials of the 4a₁ and 5a₁ orbitals are found to be 21.15 ± 0.15 and 34.2 ± 0.2 eV respectively.     

A theoretical study of the photoelectron spectra of dichloroketene with accurate computation of ionization energies via complete basis set limit extrapolation

We calculated the equilibrium geometries and harmonic vibrational frequencies of the ground state and five cationic states of dichloroketene using (TD-)B3LYP, PBE0, and M06/M06-2X approaches. The photoelectron spectra of dichloroketene were simulated by computing Franck-Condon factors. The ionization energies were computed using the CCSD(T) approach with extrapolation to the complete basis set (CBS) limit. We propose two new CBS energy formulas (E = E_CBS + Aexp(-x) + B/(x−1) ⁿ, n = 2 or 3) and compare the performance of different CBS approaches. A new ionic state of dichloroketene belonging to the C_s point group is reported. This state is identified as the first excited state of Cl₂CCO⁺ having a double-well potential-energy curve along the CCO bending mode with a barrier height of 1.335 eV. The simulated photoelectron spectra are in agreement with the experiment. The vertical ionization energies calculated via spectral simulation are more accurate compared with those obtained at the ground-state structure. Among the CBS formulas used, the proposed ansatz with n = 2 performs best, with a mean absolute error of 0.021 and 0.012 eV for the adiabatic and vertical ionization energies, respectively.     

Charge transfer reactions in alkane and cycloalkane systems. Estimated ionization potentials

Rate constants of change transfer reactions k_CT, involving C₃? C₉ alkanes and cycloalkanes, have been determined in an ion cyclotron resonance mass spectrometer. The rate constants are significantly lower than the corresponding rate constants for collision when the reaction is less than about 0.5 eV exothermic for linear alkane ions, or less than about 0.2 eV exothermic for cycloalkane ions. In this region of low reaction efficiency, the efficiency of reaction with linear or branched alkanes seems to depend primarily on reaction exothermicity. (The efficiencies of reaction of a given ion with cyclic alkanes also depend on ΔH_rn, but are higher than for reactions with other compounds). Although the lowered reaction efficiencies probably result, at least in part, from unfavorable Franck–Condon factors in the energy range near the ionization onset, quantitative correlations between reaction efficiency and estimated relative Franck–Condon factors were not observed. When the enthalpy of reaction is small (less than about ?0.15 eV), it is seen that the reverse charge transfer can also occur, and equilibrium is established under the conditions of these experiments. From the observed equilibrium constants, values for the standard free energy change are derived, and assuming that ΔS is small for electron transfer equilibria, values of ΔH_rn are estimated. In the case of the equilibria involving cyclohexane ion, these values of ΔH_rn lead to estimates of the ionization potentials of methylcyclopentane, 3-methylpentane, n-octane, 2,2-dimethylbutane, and 2,3-dimethylbutane, which are lower than the ionization potentials of cyclohexane, that is, <9.88 eV, although all these compounds had previously been reported to have ionization potentials above 10.03 eV. That the ionization potentials are indeed lower than 10.03 eV is confirmed by determining the quantum yields of ionization with 10.03-eV photons. It is pointed out that the conclusions reached here apparently also apply to the charge transfer reactions of alkane ions in the liquid phase.     

Theoretical study of the vertical electron affinity and ionization potentials of C3

The electron affinity and first three ionization potentials of C₃ are calculated using the multiconfigurational SCF and configuration interaction methods and by Möller-Plesset perturbation theory. Whereas Koopmans' theorem and SCF calculations indicate that the first cation state is ²Π_u, upon inclusion of correlation effects both the ²Σ_u and ²Σ_g cation states are found to lie lower in energy. CI calculations indicate that the ground state (²Π_g) anion is stable by 1.74 eV. Allowing for the error in the calculated electron affinity of the carbon atom, C₃^? is estimated to be stable by 2.0 eV, in excellent agreement with the 2.05 eV value determined from recent photodetachment measurements. No excited anion states are found to be bound at the equilibrium geometry of the neutral molecule.     

Highly symmetrical phthalocyanines and perfluorophthalocyanines: The quantum-chemical study

By the quantum-chemical method (U)B3LYP/6-31G(d ₅,p) are determined point symmetry group (D _4h ) and equilibrium structure of phthalocyanine (PcH₂), phthalocyaninates PcBe, PcMg, PcCo, PcNi, PcCu, PcZn, perfluorophthalocyaninates ^FPcNi, ^FPcCu, ^FPcZn, cations Pc⁺Mt, ^FPc⁺Mt and anion PcCo⁻. In the approximation (U)B3LYP/6-311++G(3d ₅ f ₇,p)//6-31G(d ₅,p) is achieved the satisfactory accuracy of the calculation of ionization potentials of the studied molecules. Effect of nuclear relaxation at the ionization is 0.07±0.06 eV; correction for “zero” vibrations does not exceed 0.01 eV. Perfluorination increases ionization potentials by 0.7–0.8 eV.     

Theoretical study of positron scattering by group 14 tetra hydrides: A quantum mechanical approach

The present work provides a comprehensive set of positron impact scattering cross sections for group 14 tetrahydrides, namely, SiH₄, GeH₄, SnH₄, and PbH₄. The well‐established spherical complex optical potential and complex scattering potential‐ionization contribution methods are modified to incorporate positron scattering in the present work to calculate various cross sections. The positronium formation channel is adequately included through an improved inelastic threshold. The energy range chosen for the direct ionization cross section is from the respective ionization potential (I) of the molecule to 5 keV. Likewise, positronium formation and total ionization cross sections are reported from the positronium formation threshold to 300 eV and 5 keV, respectively, and the total cross section is computed for an extensive energy range of 1 eV to 5 keV. The positron impact total cross section for stannane molecule is computed for the first time. A characteristic valley is observed in the total cross sections with minima close to the positronium formation threshold. Further increase of cross section signifies the opening of inelastic channels especially positronium formation. In general, a reasonable agreement is found between the present results and other comparisons, wherever available. Furthermore, this is the first report of the inelastic cross sections (direct ionization, positronium formation, and total ionization) for the present set of targets.     

Repulsive potentials for the interaction of oxygen atoms with the noble gases and atmospheric molecules

Repulsive potentials between about 0.2 eV and 20 eV are determined for interactions of oxygen atoms with the noble gas atoms and with H₂, N₂, O₂, NO, CO, CO₂ and H₂O molecules from incomplete total scattering cross sections. Good agreement with theory is obtained for the noble gases. The potentials for the interaction with molecules obtained by assuming spherically symmetric potentials are compared with the same potentials computed by averaging the cross sections for different orientations of the molecules. By introducing reduced variables, the problem of finding two isotropic potential parameters is simplified to a search for one, the other being determined by a least-squares constraint.     

Ab initio calculations on porphin

Ab initio SCF calculations are reported for the porphin molecule. The positions of the central protons have been optimized, and the equilibrium geometry is found to be a linear NH ? HN arrangement. The NH vibrational frequencies have been computed and are compared to experimentally measured quantities. Several low ionized states have also been studied in separate spin-restricted SCF calculations. The lowest state is found to have B_1u symmetry with an ionization potential of 8.0 eV.     

Density functional theory calculation of 2p spectra of SiH4, PH3, H2S,HCl, and Ar

The method developed recently for prediction of 1s electron spectra is now extended to the 2p spectra of SiH₄, PH₃, H₂S, HCl, and Ar. The method for X‐ray absorption spectra involves the use of ΔE for the excitation and ionization energies, and application of time‐dependent density functional theory using the exchange‐correlation potential known as statistical average of orbital potentials for the intensities. Additional assumptions and approximations are also made. The best exchange‐correlation functional E_xc for the earlier calculation of ΔE in 1s spectra of C to Ne (namely Perdew–Wang 1986 exchange, combined with Perdew–Wang 1991 correlation) is no longer used in this work on 2p spectra of Si to Ar. Instead, recently tested E_xc good for 2p core‐electron binding energies (known as OPTX) for exchange and LYP for correlation, plus scalar zeroth‐order regular approximation is adopted here for the ΔE calculations. Our calculated X‐ray absorption spectra are generally in good agreement with experiment. Although the predictions for the higher excitations suffer from basis set difficulties, our procedure should be helpful in the interpretation of absorption spectra of 2p electrons of Si to Ar. In addition, we report calculated results for other kinds of electron spectra for SiH₄, PH₃, H₂S, HCl, and Ar, including valence electron ionizations and excitations as well as X‐ray emission. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

The electronic structure of all-trans-polyenes C2nH2n+2, n = 3–7

The electronic structure of the C_2nH_2n+2 trans-polyenes, n = 3–7, is calculated by the Discrete Variational X_α method (DVM -X_α). The valence ionization potentials (IP ) calculated using the Clementi double zeta basis agree with the known experimental data within several tenths an electron volt. However, the DVM energies of the π → π* optical excitations are systematically underestimated by 0.8–1.0 eV. For polyenes with equal C—C bond lengths, the computed energies of the first optical transitions are smaller than those of polyenes with alternating C—C bond lengths. The charge distribution in polyenes is analyzed in the framework of a Mulliken scheme. The composition of the frontier molecular orbitals (MO ) is analyzed.     

Electronic,optical, and charge transfer properties of porphyrin and metallated porphyrins in different media

Porphyrin and M-Porphyrin (M = Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) complexes were designed to examine their organic light-emitting diode (OLED) properties. All calculations were performed in different media, which are gas, benzene, DMSO, and water phases. The calculations of both porphyrin and its metal complexes as a monomer form were performed at B3LYP/6-31G(d) level by using the Gaussian 16 and GaussView 6 package programs. On the other hand, emission calculations for the monomer form and dimer form computations of the studied compounds were carried out at PBE0/TZP and B3LYP/TZP levels, respectively, by using Amsterdam density functional (ADF) 2019 package program. The OLED tensors of the mentioned molecules, which are emission energies, reorganization energies (λ_e and λ_h), the ionization potentials and the electron affinities (adiabatic and vertical), the effective transfer integrals (V_e and V_h), and the charge transfer rates (W_e and W_h), were calculated to evaluate the OLED behaviors and determine the best OLED structure.     

The Ionization Energies of Di-n-Alkyl Diacetylenes

The He(Iα) photoelectron spectra and the ionization energies of symmetrically substituted di-n-alkyl-diacetylenes R-(C?C)₂-R (with R ? CH₃, C₂H₅, n-C₃H₇, n-C₄H₉) are presented. The effect of the alkyl substitutents is that the two acetylenic ionization energies, I_v,1 and I_v,2, shift by the same amount, i.e. their difference I_v,2 – I_v,1 remains constant (2.45 ± 0.05 eV). Between 12.5 eV and 17 eV the band system in the photoelectron spectrum of R-(C?C)₂-R is superimposable with that in the spectrum of the corresponding alkane, RH, with the exception of a uniformly small shift of all the bands to higher ionization energy.     

Study of photoelectron spectra of some XPY3 molecules (X = O,S and Y = Cl,Br) by the SCF-Xα-SW method

In this paper we extend our previous studies to investigate the ionization energies of some XPY₃ molecules (X = O, S and Y = Cl, Br). The calculated orbital energies agree very well with reported experimental ionization energies. The molecular orbital orderings obtained coincide with recent experimental orbital assignments. The results are also compared with previous ab initio and semiempirical calculations for OPCl₃, OPBr₃, SPCl₃, and SPBr₃ molecules. The comparison indicates that the present results show improved agreement with experiment and clarify certain ambiguities in the earlier assignments.