W. von Niessen

Study of the Electronic Structure of Molecules. XII. Hydrogen Bridges in the Guanine–Cytosine Pair and in the Dimeric Form of Formic Acid

In this work we analyze molecular complexes with two hydrogen bonds (A=H2=B) and with three hydrogen bonds (A≡H3≡B). In an introductory discussion of molecular complexes with a single hydrogen bond (A–H1–B), it is argued that the presence or absence of a double‐well potential depends primarily on (a) the repulsive or attractive nature of the A–H1 and H1–B potentials, (b) on the relative position of the molecular fragments A and B, (c) on the attractive or repulsive interaction of the fragments A and B. Since the A–H1–B complex can be stable if one of the potentials for A–H1 or H1–B is repulsive, provided that the attraction of A and B compensates the above repulsions, it is suggested to use the designation “hydrogen bridge” as a more general designation for what is commonly referred to as “hydrogen bond.” The guanine–cytosine (G–C) base pair is taken as an example of three hydrogen bridges (h–bs). A number of computations were performed to assess the shape of the potential curve for the h–b in the G–C pa...

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...

On the accuracy of ionization potentials calculated by Green’s functions

A many‐body Green’s function method is used to calculate vertical valence ionization potentials to high accuracy for the atoms and molecules Ne, N2, F2, CO2, P2, H2O, and H2S. Large basis sets including several sets of polarization functions are used in the calculations to reach the limit of the presently achievable accuracy for molecular systems. The maximum errors in the computed ionization potentials are 0.1 to 0.25 eV depending on the molecule and the basis set. The results are extremely stable, when large basis sets are used. Comparison with other methods is made.

Density Localization of Atomic and Molecular Orbitals. I

A new intrinsic method for the localization of atomic and molecular orbitals is presented, which will be called (charge) density localization method. This new procedure follows an earlier suggestion of Edmiston and Ruedenberg. It is based on the minimization of the sum of the charge density overlap integrals of the orbitals: Σi<j ∫ |〈 r|i〉|2|〈 r|j〉|2d3r, |〈 r|i〉|2 being the charge density of orbitals |i〉. Because of the physical relation between the exchange interaction of wave packets representing fermions and their mutual overlap it is expected that this method is related to the energy localization procedure of Edmiston and Ruedenberg. The density localization method is applied to the atoms Be and Ne, where the four outer shell localized orbitals point to the corners of a tetrahedron. The results are compared with those of other localization procedures. Good agreement is obtained. It is shown in the present and in forthcoming publications that the same physical situation can be described by different bu...

Strong Correlation Effects in inner Valence Ionization of N2 AND CO

Abstract The results of many-body calculations of the valence and inner-valence ionization potentials and their intensities are reported for N2 and CO. For N2 we find that the 2σg line is smashed to several pieces of roughly equal intensity. It is not possible to identify any of these lines as the ǒmainǒ line representing the ionization of a 2Ug electron and the remaining ones as satellite lines. For CO there survives a line which carries about half of the 3e intensity and which can be interpreted to represent the ionization of an electron out of the 3σ orbital. The results explain the peculiar shape of the broad innervalence peaks of N2 and CO. For both N2 and CO rich satellite structure is found in qualitative agreement with experimental X-ray photoelectron spectra.

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.

The electronic structure of molecules by a many-body approach: II. Ionization potentials one-electron properties of pyridine and phosphoridine

Abstract The valence ionization potential (IPs) of pyridine and phosphoridine are studied by an ab initio many-body approach which includes the effects of electron correlation and reorganization beyond the Hartree-Fock approximation. For pyridine the order of the first three IPs is a 2 (π), a 1 (n), b 1 (π), but the IPs of the a 2 and a 1 orbitals are so close together that they have to be regarded as identical in binding energy, which is also concluded from experiment. Whereas for pyridine the ordering of the IPs calculated in the HF approximation is incorrect, it is correct for phosphoridine. For this latter molecule the first three ionization potentials are due to ionization from the b 1 (π), a 2 (π), and a 1 (n) orbitals. Several one-electron properties are calculated and compared with experimental and other theoretical data. The localized molecular orbitals are discussed as well.

Theoretical studies of inner-valence-shell photoionization cross sections in N2 and CO

Abstract Theoretical studies in the intensity-borrowing sudden approximation are reported of inner-valence-shell photoionization cross sections in N2 and CO. The required ionic-state energies and spectroscopic amplitudes are obtained from appropriate Greens-function and configuration-interaction calculations, and previously devised Stieltjes-Tchebycheff moment-theory techniques are employed in determinations of corresponding continuum dipole transition moments in the static-exchange approximation. Comparisons are made of the Greens-function calculations in the two-particle-hole Tamm-Dancoff approximation with wavefunction results obtained from single-excitation and polarization configuration-interaction calculations. Detailed descriptions are given of the calculated spectroscopic intensity distributions and of the hole-particle configurational compositions of the corresponding inner-valence-shell ionic states. and comparisons are made with previously reported wavefunction studies in N2+ and CO+. Spectroscopic assignments are suggested on basis of the present calculations for the strong features observed recently in higher-resolution inner-valence-shell photoelectron spectra. The corresponding calculated partial-channel photoionization cross sections for the designated C2Σg+, F2Σg+, G2Σg+, and (2σg−1)2Σg+ bands in N2 and C2Σ+, D2Π, F2Σ+, G2Σ+, and (3σ−1)2Σ+ bands in CO are found to be in good quantitative accord with dipole (e, 2e), (e, e + ion), and synchrotron-radiation studies.

Strong vibronic coupling effects in ionization spectra: The “mystery band” of butatriene

Abstract A theory of vibronic coupling in molecules is presented and applied to butatriene. The energies and coupling constants which enter the calculation are computed using ab initio Hartree—Fock and many-body methods. The influence of the energy splitting and the coupling constants on the calculated spectrum is discussed. It is definitely shown that the “mystery band” in the photoelectron spectrum of butatriene arises from the vibronic coupling between the electronic states 2 B 3g and 2 B 3u . To reproduce the experimental observations it is essential to include in the calculation both totally and non-totally symmetric vibrational modes.

Experimental and theoretical investigation of the complete valence shell ionization spectra of CO2and N2O

Abstract The valence electron ionization spectra of CO 2 , and N 2 O are studied by dipole (e—2e) spectroscopy and 2ph-TDA many-body Green function calculations. Intense satellite structure in the (e—2e) spectra between ≈ 20 eV and ≈ 30 eV binding energy is assigned with the help of the calculations. While for CO 2 , only satellite lines of 2 Π u symmetry appear with significant intensity, intense satellite lines of both 2 Π and 2 γ symmetry are found for N 2 O in the 20–30 eV energy range. The theory predicts a complete breakdown of the molecular orbital picture of ionization to occur for the two innermost valence electrons of CO 2 and N 2 O. The inner valence part of the ionization spectra of CO 2 and N 2 0 is found to be considerably more complex than has hitherto been assumed. The experimental spectra confirm the main features of the theoretical results.