Ab initio MO LCAO UHF calculations of the stability in dimeric ethylene radical ions

The stability of ethylene dimer ions has been computed by ab initio methods. The positively charged dimer is stable (0.6–0.7 eV) while the neutral and the negatively charged dimers are unstable with respect to decomposition into monomers. The magnetic hyperfine coupling constants have also been evaluated, hyperfine splittings are half those in the monomer in agreement with experimental data.     

Ab initio SCF MO calculations on the CH3Br molecule

Self-consistent fieldab initio calculations have been performed for CH₃Br. The calculated equilibrium conformation is in good agreement with experiment. Valence and core level ionization potentials, Mulliken population analysis and electronic properties are presented. The ionization potentials are in good agreement with the experimental values, except for one case in which the experimental value may be wrongly assigned. The calculated dipole moment, 2.43 Debye, is 34% or 0.6 Debye larger than the experimental value.Two of us (C.W.B. and G. del C.) wish to acknowledge the hospitality of the IBM San José Research Laboratory and joint study agreements between the IBM and the Lawrence Berkeley Laboratory and the National University of Mexico respectively.     

Ab initio MO calculations of hyperfine coupling constants. A basis set study for 19F, 35Cl, 19F,and 35Cl

Hyperfine coupling constants (HFCC ) of the ¹⁹F and ³⁵Cl atoms and the ¹⁹F and ³⁵Cl radical anions have been calculated by the unrestricted Hartree–Fock (UHF ) method using polarization and diffuse functions with contracted double-zeta as well as uncontracted basis sets. The A_dip values are fairly insensitive to changes in the basis set and show good accordance with experimental and other theoretical studies. The isotropic HFCCS a_N of ¹⁹F, ¹⁹F, and ³⁵Cl show strong dependence on d functions and the state of contraction of the s, p set. Spin-projected UHF wave functions lead to better agreement with experiment.     

Ab initio calculations of the excited states of the permanganate and chromate ions

The excited states of the permanganate and chromate ions are described by better-than-minimal basis set SC FMO CI calculations. The results are compared with the experimental absorption spectra.     

Ab initio MO calculations and 17O NMR at natural abundance of para-substituted acetophenones

¹⁷O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X? C₄H₆? COCH₃, where X = NH₂, OCH₃, F, Cl, CH₃, H, COCH₃, CN, NO₂. The ¹⁷O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ_I and σ_R⁺ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ_I and ρ_R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ values are also reported, and the role of the average excitation energy, ΔE, in determining ¹⁷O SCS values is discussed. It is concluded that variations in ΔE are minor and that the local Δ-electron density is the dominant feature controlling ¹⁷O SCS values.     

Geometrical consequences of resonance-assisted intramolecular hydrogen-bond formation from Ab initio MO calculations on 2-nitroresorcinol

Ab initio orbital calculations on phenol, nitrobenzene, and 2-nitroresorcinol have been performed with the GAUSSIAN 92 series of programs. Initial RHF/6-31G^* and RHF/6-31G^** optimizations were followed by second-order MØller-Plesset MP2(FC)/6-31G^* optimizations. The general geometrical features of these molecules, and, in particular, the characteristic changes as going from phenol to 2-nitroresorcinol and from nitrobenzene to 2-nitroresorcinol are in good agreement with recent gas-phase electron diffraction studies and with the notion of resonance-assisted intramolecular hydrogen-bond formation in 2-nitroresorcinol.     

Ab initio MO calculation of hydration energies of ammonium ions and their solvation effect

The hydration energies for the NH⁺₄ and CH₃NH⁺₃ ions were calculated by an ab initio MO method. The aqueous solvation energy difference between these two ions was found to be accounted for by the interactions of the ions with a few solvent molecules.     

Ab initio calculations of the O1s XPS spectra of ZnO and Zn oxo compounds

O1s core level binding energies of oxygen atoms in bulk ZnO, at different ZnO surfaces, and in some Zn oxo compounds were calculated by means of wave function based quantum chemical ab initio methods. Initial and final state effects were obtained by Koopmans' theorem and at the DeltaSCF level, respectively. After correction for scalar relativistic effects and electron correlation, the calculated XPS peak positions are in excellent agreement with the available experimental data for all systems included in the present study. The O1s core level shifts between an isolated H2O molecule and the Zn oxo compounds or ZnO, as well as between oxygen atoms in bulk ZnO and at various ZnO surfaces, can be understood by means of Madelung potentials and electronic relaxation or screening. XPS spectra were calculated for various cluster models which are designed to describe different possibilities of stabilizing the polar O-terminated ZnO(0001) surface by the adsorption of H atoms. The experimental spectra are only compatible with the theoretical results for the fully hydroxylated H-ZnO(0001) surface exhibiting a (1x1) surface structure.     

Ab initio calculations of structure and stability of small boron nitride clusters

Clusters of boron nitride B_xN_x (x = 1–4, 12, 15, 30) were investigated by the Hartree-Fock and density functional methods using the 6-31G* basis. It was found that linear, cyclic, and shell structures are stable against minor deformations of the B_xN_x cluster. Inclusion of electron correlation in calculation markedly changes the electron density distribution and the structure of the clusters.     

Ab initio calculations on SF2 and its low-lying cationic states: anharmonic Franck-Condon simulation of the UV photoelectron spectrum of SF2

Geometry optimization calculations were carried out on the (approximate)X(1)A(1) state of SF2 and the (approximate)X(2)B(1), (approximate)A(2)A(1), (approximate)B(2)B(2), (approximate)C(2)B(2), (approximate)D(2)A(1), and (approximate)E(2)A(2) states of SF2(+) employing the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] method and basis sets of up to the augmented correlation-consistent polarized quintuple-zeta [aug-cc-pV(5+d)Z] quality. Effects of core electron (S 2s(2)2p(6) and F 1s(2) electrons) correlation and basis set extension to the complete basis set limit on the computed minimum-energy geometries and relative electronic energies (adiabatic and vertical ionization energies) were investigated. RCCSD(T) potential energy functions (PEFs) were calculated for the (approximate)X(1)A(1) state of SF2 and the low-lying states of SF2(+) listed above employing the aug-cc-pV(5+d)Z and aug-cc-pV5Z basis sets for S and F, respectively. Anharmonic vibrational wave functions of these neutral and cationic states of SF2, and Franck-Condon (FC) factors of the lowest four one-electron allowed neutral photoionizations were computed employing the RCCSD(T) PEFs. Calculated FC factors with allowance for Duschinsky rotation and anharmonicity were used to simulate the first four photoelectron bands of SF2. The agreement between the simulated and observed first bands in the He I photoelectron spectrum reported by de Leeuw et al. [Chem. Phys. 34, 287 (1978)] is excellent. Our calculations largely support assignments made by de Leeuw et al. on the higher ionization energy bands of SF2.     

Ab initio calculations on the molecular structure of fluorocyanopolyynes

The molecular structure of the first three members of the fluorocyanopolyynes was studied by ab initio Hartree-Fock calculations with a polarized double zeta basis set and at MP2 level with the same basis set. Alternating triple and single bonds were found; a theoretical estimate of rotational constants and dipole moments was performed and a comparison with the available experimental data was made. An analysis of the theoretical vibrational frequencies of the title compounds was carried out.     

Ab initio calculations on the thermodynamic properties of azaspiropentanes

Following our recent study on the usefulness of borospiropentanes as new potential high-energy materials, we propose a new series of substituted spiropentane molecules, the azaspiropentanes. Presented here are the results of our ab initio calculations at the MP2 and B3LYP levels of theory using the 6-311++G(d,p) basis set. Results include optimized structural parameters, enthalpies of formation, specific enthalpies of combustion, and proton affinities. Our results indicate that azaspiropentane gives off the most energy of any of the nitrogen-containing spiropentanes, as indicated by its specific enthalpy of combustion of -41 kJ g(-1); however, it does not give off as much energy as spiropentane itself, which gives off about 48 kJ g(-1).     

Ab initio cluster calculations for the absorption energies of F and F+ centers in bulk ZnO

We present the results of a series of ab initio calculations on the ground states and the low lying excited states of the F and F+ centers in bulk ZnO. Both types of F centers are oxygen vacancies, causing rather strong distortions of the local geometries. The calculations were performed by means of wave function based methods, mostly at the CASSCF level. Dynamic correlation was included for the first two coordination shells of the F centers. The calculated absorption energy for the F+ center (3.19 eV) is in excellent agreement with the experimental value of 3.03 eV. For the emission from the 3T2 state of the F center to the 1A1 ground state we obtained a transition energy of 2.73 eV. Experimentally, a green photoluminescence is observed at 2.38-2.45 eV. We estimated that the errors in our calculation should be even smaller in the latter case than for the F+ state, where the calculated transition energy differs by less than 0.2 eV from the experimental value. Therefore, we assume that the 3T2 to 1A1 transition is not the origin of the green luminescence.     

Ab initio calculations for the Zn 2s and 2p core level binding energies in Zn oxo compounds and ZnO

The Zn 2s and 2p core level binding energies of ZnO and a few Zn oxo compounds containing Zn in its oxidation state +2 were calculated by means of wave function based quantum chemical ab initio methods. The computations were performed at two levels of approximation. First, Hartree-Fock calculations were carried out for the ground state of the neutral systems yielding the "initial state" effects, i.e. the shifts of the core level binding energies due to the changes in the chemical environment of the Zn atom under consideration (Koopmans' theorem level, KT). In the second step, Hartree-Fock calculations were performed for the core ionized states in order to account for the relaxation effects after ionization, i.e. for the "final state" effects (DeltaSCF level). Scalar relativistic corrections and spin-orbit coupling were included in a "spin-orbit-coupling configuration interaction" (SOC-CI) treatment both at the KT and DeltaSCF levels. In all Zn oxo compounds (Zn(4)O(formate)(6), Zn(4)O(acetate)(6) and several ZnO cubanes) small negative initial state shifts between -1.0 and 0.0 eV (relative to the free Zn atom) were found which are caused by the negative charges at the surrounding O atoms. The relaxation effects vary between -1.0 and -0.5 eV, such that the calculated total shifts are moderately negative (-1.5 to -0.5 eV). Embedded ZnO clusters of increasing size, ranging from Zn(13)O(4) to Zn(69)O(38), were used as models for bulk ZnO, the Zn 2s and 2p core level shifts calculated for these clusters being extrapolated to infinite cluster size. The calculations show that bulk ZnO has a rather large negative initial state shift of -2.1 +/- 0.1 eV, due to the Madelung potential at the Zn atom, and a comparatively small relaxation contribution of -1.0 +/- 0.1 eV. This yields a total shift of -3.1 +/- 0.2 eV (both for 2s and 2p, relative to atomic Zn), which is in very good agreement with experiment, -2.9 +/- 0.2 eV. The surprising experimental observation that the Zn 2s and 2p XPS peak positions are nearly identical in Zn metal and ZnO is explained by the fact that the sum of initial and final state effects is accidentally the same for the two systems though the individual contributions differ quite significantly: the initial and final state shifts amount to +2.4 and -5.1 eV for Zn metal vs.-2.1 and -1.0 eV for ZnO.     

Ab initio calculations on the structure of the cyclopropenyl anion

Calculations at the SCF level and with inclusion of correlation were carried out on a portion of the potential ¹A′ state of the cyclopropenyl anion which was found to be nonplanar in its most stable geometry.     

Ab initio calculations on the isomerization of alkene radical cations

Ab initio calculations on the isomerization of butene and pentene radical cations indicate that, for all classical ion structures, the lowest barrier for a rearrangement to the most stable ion structure is below the dissociation limit. Isomerizations of linear butene radical cations to the isobutene structure take place via the CH₃CC₂H₅^·+ structure, whereas in the pentene case the connection between linear and branched ion structures proceeds via the 1,2-dimethylcyclopropane radical cation. From the results a qualitative model is derived which suggests that for larger alkene radical cations an isomerization to structures with four alkyl substituents on the double bond may be in close competition with dissociation.     

Ab initio analysis of pentadienyllithium, pentadienylsodium, and the pentadienyl ions

Ab initio calculations were used to determine the equilibrium geometries and rotational barriers of the pentadienyl cation, anion, and metalated pentadienes. Pentadienyllithium and pentadienylsodium are most stable in a U-shaped structure. This geometry is a higher energy local minimum for the pentadienyl anion and is not a stationary point for the pentadienyl cation. The atomic and group charges were analyzed by natural population analysis and were determined for each of the conformations studied.     

Ab initioSCF MO calculations on the reactions of hydroxyl radical with imidazole and monoprotonated imidazole

The addition reactions of hydroxyl radical with imidazole and its protonated form to yield radical adducts have been investigated by ab initio SCF MO methods using STO -3G and 4-31G basis sets. Analogous radical species are of importance in radiation damage to biological systems. Of the possible radical products, the calculations indicate that the allylic species are generally favored energetically over the nonallylic forms. On an energetic basis, the results show that the allylic adducts formed by addition at the C2 and C5 positions are about equally favorable. Although the C5 species is generally identified as the experimentally observed product in aqueous media for both protonated and unprotonated imidazole, some experimental evidence exists indicating the presence of other forms. Our results suggest that this other form is the C2 adduct. The calculations also point to the protonated form of imidazole being less reactive than imidazole, which is in accord with experimental observations.     

Ab initio MO and approximate density functional theory studies on the lowest singlet and triplet states of s and as-indacene

The lowest singlet and triplet states of s- and as-indacene have been studied by means of high-level ab initio MO and approximate density functional methods. Among the geometrical and energetical details discussed are the equilibrium structure of s-indacene (C_2h or D_2h), the structures and energies of the low-lying s-indacene triplet states, and the stability and geometry of the singlet and triplet states of as-indacene. It is shown that single-determinant-based methods, such as Hartree-Fock or MP 2, are not suited to properly describe these molecules. Instead, methods are required which explicitly take into account nondynamical and dynamical electron correlation. The results obtained by density functional theory-based methods compare very well with the most elaborate ab initio MO data and seem to provide an economical alternative even for molecules with a complicated electronic structure such as s- and as-indacene. © 1995 John Wiley & Sons, Inc.