Wolfgang P. Kraemer

SCF-CI studies of correlation effects on hydrogen bonding and ion hydration

Previous single-determinant Hartree-Fock studies on the equilibrium structures and stabilities of H2 O, H3 O+ as well as of the monohydrated ionic systems Li+ · H2O, F− · H2O and the hydrogen bonded water dimer, H2 O · HOH, are extended by large scale configuration interaction calculations including all the possible single and double excitations arising from the canonical set of Hartree-Fock molecular orbitals. The correlation energy effects on the equilibrium geometrical parameters of the systems under consideration are found to be quite small. The contributions of the correlation energy to the total binding energies of the weakly interacting composed systems are obtained to be of the order of 1 kcal/mole, leading to a considerable increase of the hydrogen bond strength in F− · H2O and H2O · HOH and to a small decrease of the binding energy in Li+ · H2 O. The observed strengthening of the hydrogen bonding interaction due to correlation is shown to be partly compensated by the change in the vibrational zero-point energy of the composed systems compared to the non-interacting subsystems. Approximate force constants corresponding to the intersystem vibrations in Li+ · H2O, F− · H2 O, and H2O · HOH are deduced from the calculated potential curve data on the SCF and the CI level of accuracy.

Correlation effects in the ionization of hydrocarbons

The spectral intensity for ionization as a function of binding energy for the valence electrons of ethylene, allene, butatriene, trans‐butadiene, acetylene, benzene, methane, ethane, and cyclopropane is computed by a many‐body Green’s function method. The results are used to interpret unidentified structures in experimental ionization spectra. For the ionization out of the inner valence orbitals of the unsaturated molecules the spectral intensity is found to be distributed over several lines, in sharp contrast to the ionization out of the inner valence orbitals of the saturated molecules where the greater part of the intensity appears in one main line. The reasons for this behavior are discussed. It is also found that there is a correspondence between the behavior of the spectral intensity in the inner valence region and the satellite structure in the outer valence region. For C6H6, C4H4, and C4H6 interesting satellite lines of considerable intensity are predicted to be situated in the outer valence regio...

The potential surface of X̃ 3B1 methylene (CH2) and the singlet–triplet splitting

The data in the two immediately preceding papers, when combined with the extant microwave, infrared, and photodetachment spectroscopic data, provide 152 rotation and rotation‐bending energy level separations in X 3B1 methylene (involving 12CH2, 13CH2, and CD2). In the present paper we fit all this data using the two nonrigid bender Hamiltonians NRB1 and NRB2. The more refined model (NRB2) leads to the following results for triplet methylene: re=1.0766±0.0014 A, αe=134.037°±0.045°, and the barrier height to linearity=1931±30 cm−1 (the uncertainties are three times the standard errors). Rotation‐bending energy levels for CH2, CD2, and CHD are calculated for v2≤4 and N≤6. The determination of the rotation‐bending energy levels in CH2 leads to an improved determination of the singlet–triplet splitting T0(a1A1) in methylene as 3156±5 cm−1 (9.023±0.014 kcal/mol, 0.3913±0.0006 eV). Although the rotation‐bending energy levels are accurately predicted it is not possible to predict the stretching frequencies of C...

Rotational excitation of CO by He impact

Abstract To study rotational excitations of CO by He impact, configuration-interaction potential energy surfaces have been computed with two different basis sets. The surfaces are compared to one another, to an electron-gas surface, and to an experimentally determined surface. In addition, converged close-coupling calculations of the collision cross sections have been done on these surfaces for energies up to 100 cm −1 and compared. On the most accurate CI surface, cross sections have been computed using the infinite-order sudden (IOS) and quasi-classical methods as well.

The electronic structure of molecules by a many‐body approach. I. Ionization potentials and one‐electron properties of benzene

The ionization potentials of benzene are studied by an ab initio many‐body approach which includes the effects of electron correlation and reorganization beyond the one‐particle approximation. The calculations confirm the assignment of the photoelectron spectrum experimentally proposed by Jonsson and Lindholm: 1e1g(π), 2e2g, 1a2u(π), 2e1u, 1b2u, 1b1u, 2a1g, 1e2g in order of increasing binding energy. To definitely establish the ordering of the ionization potentials in the second band, which has been very controversial, the corresponding vibrational structure has been calculated. A number of one‐electron properties are calculated in the one‐particle approximation and compared to experimental work and other theoretical calculations.

Theoretical studies of photoexcitation and ionization in H2O

Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H_2O employing Franck–Condon and static‐exchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground‐state equilibrium geometry, and previously devised moment‐theory techniques are employed in constructing the continuum oscillator‐strength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact–excitation cross sections, and dipole (e, 2e)/(e, e+ion) and synchrotron‐radiation studies of partial‐channel photoionization cross sections. The various calculated excitation series in the outer‐valence (1b(^−1)_1, 3a(^−1)_1, 1b(^−1)_2) region are found to include contributions from valence‐like 2b_2 (σ*) and 4a_1(γ*) virtual orbitals, as well as appropriate nsa_1, npa_1, nda_1, npb_1, npb_2, ndb_1, ndb_2, and nda_2 Rydberg states. Transition energies and intensities in the ∼7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b(^−1)_1)X(^ 2)B_1, (3a_1(^−1))^2A_1, and (1b_2(^−1))(^2)B_2 partial‐channel cross sections are found to be largely atomic‐like and dominated by 2p→kd components, although the 2b_2(σ*) orbital gives rise to resonance‐like contributions just above threshold in the 3a_1→kb_2 and 1b_2→kb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1→kb_1 channel, accounting for a prominent feature in the recent high‐resolution synchrotron‐radiation measurements. When this feature is taken into account, the calculations of the three outer‐valence channels are in excellent accord with recent synchrotron‐radiation and dipole (e, 2e) photoionization cross‐sectional measurements. The calculated inner‐valence (2a_1(^−1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final‐state configuration‐mixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a_1 oxygen K‐edge electron energy‐loss profile in H_2O are seen to be clarified fully by the present calculations.

Anharmonic potential function and effective geometries for the NH3 molecule

Abstract An accurate analytic anharmonic potential function for NH3 is determined by fitting to experimental rovibrational transition data of the 14NH3, 15NH3, 14ND3, 15ND3, and 14NT3 isotopic species using the nonrigid invertor Hamiltonian method [V. Spirko, J. Mol. Spectrosc.101, 30–47 (1983).]. Information from ab initio potential energy calculations is used to obtain a numerically stable and physically meaningful solution to the fitting procedure. From the potential function determined in this study effective geometries are evaluated for the low lying roinversional states of 14NH3 and 14ND3. To allow for a quantitative analysis of the inversional dependence of other molecular constants, the roinversional matrix elements 〈ψv2,J,k(ϱ)|sin2nϑ|ψ′v2,J,k(ϱ)〉 ( ϑ = ϱ − π 2 , ϱ being the inversion angle, and n an integer number) are evaluated for spectroscopically important values of the inversion (v2) and rotation (J, k) quantum numbers.

The electronic structure of molecules by a many-body approach III. Ionization potentials and one-electron properties of furan and thiophene

Abstract The valence ionization potentials (IPs) of furan and thiophene are studied by an ab initio many-body approach which includes the effects of electron correlation and reorganization beyond the Hartree—Fock approximation. For both molecules it is found that the ordering of the IPs as obtained in the Hartree—Fock approximation is correct. The assignment made for furan agrees with the ab initio calculation of Siegbahn, but it does not agree with the ordering proposed by Derrick et al. from their experimental investigations. For thiophene both the ordering of Derrick et al. and the one of Gelius et al. is shown to be incorrect concerning the position of the 1b 1 (π) IP. For both molecules the first two IPs are due to the 1a 2 (π) and the 2b 1 (π) molecular orbitals. For furan four orbitals of σ-type symmetry are placed between the 2b 1 and the 1b 1 π-orbitals, for thiophene there is only one. Several one-electron properties are calculated in the one-particle approximation and compared with experimental and other theoretical data. The localized molecular orbitals are also discussed.

SCF MO LCGO studies on hydrogen bonding the system (FHOH

The energy hypersurface of the system NH3 · H2O is investigated for a number of different internuclear geometries. In the minimum energy structure involving a linear hydrogen bond, NH3 acts as proton acceptor. The binding energy of the system is calculated to be 6.28 kcal/mole and the bond distance d(NO) to be 3.07 A. The potential energy curve of the inversion of the hydrogenbonded NH3 is computed and discussed.

SCF MO LCGO studies of hydrogen bonding: The hydrogen fluoride dimer

Abstract Different geometrical configurations of the hydrogen fluoride dimer have been studied by SCF LCGO MO calculations expanding the molecular wavefuntions into an extended basis set of gaussian type functions. For the minimum energy geometry a structure with a single linear hydrogen bond between the fluorine nuclei was obtained with an equilibrium FF distance of d (FF) = 2.85A (experimental FF distance in the cyclic hexamer 2.53A) and an HFH bond angle of β = 140° (experimental 140° ± 5°). The binding energy was computed to be B = 4.50 kcal/mole (experimental 6.0 ± 1.5 kcal/mole).