Armin Schweig

Measurement of relative conformational stabilities by variable temperature photoelectron spectroscopy. A study of rotational isomerism in thioanisole

Abstract The photoelectron spectra of thioanisole have been recorded over the range of temperature 20-500°C. The observed spectral changes are quantitatively explained in terms of recently proposed rotational isomerism in this molecule. Thereby estimates of the ratio of partition functions of the two dominant isomers as well as of their energy difference are obtained. This analysis constitutes the first quantitative application of photoelectron spectroscopy to internal rotation. It is further shown that semi-empirical quantum chemical results are in harmony with the rotational isomerism concept for thioanisole.

A photoelectron spectroscopic study of keto-enol tautomerism in acetylacetones - a new application of photoelectron spectroscopy☆

Abstract The photoelectron spectra of acetylacetone, 3-methylacetylacetone, and 3,3-dimethylacetylacetone have been recorded over the range of temperature 25–250°C. From the changes in the spectra with temperature the equilibrium constants for the keto ⇋ enol reaction and their temperature dependence have been obtained. The reaction enthalpies derived by means of photoelectron spectroscopy are in good agreement with gas-phase values previously obtained by other methods.

Photoionization cross sections in the valence electron approximation. III. Nonlinear molecules (theory)

A theoretical method involving the plane‐wave approximation and the use of semiempirical LCAO‐SCF‐MOs of the CNDO and MINDO type is developed for the calculation of photoionization cross sections of arbitrary molecules consisting of atoms from the first three rows of the Periodic Table. Three qualitative concepts for the interpretation of the formalism are discussed, and the dependence of the cross sections on the photon energy, on the electron density distribution in the initial orbital, and on the different behaviour of s, p, and d electrons is treated explicitly. Examples for the application to HeI and HeII photoelectron spectroscopy are given, too.

Photoionization cross sections in the valence electron approximation. Relative intensities in the HeI photoelectron spectra of linear molecules

Abstract Theoretical and experimental values for the relative band intensities in the Hel-photoelectron spectra of eighteen linear molecules are compared. Qualitative rules concerning band intensities are discussed.

Photoionization cross sections in the valence electron approximation. I. Linear molecules

Abstract Expressions for the photoionization cross sections of valence molecular orbitals are given for linear molecules consisting of atoms from the first three rows of the Periodic Table. Preliminary results for HF, F2, HCl and Cl2 are compared with experimental values.

Photoionization cross sections: Interpretation of band intensities in He I and He II photoelectron spectra☆

Abstract Relative photoionization cross sections of molecules consisting of atoms from the first three rows of the Periodic Table are computed by a theoretical method developed previously which involves the plane-wave approximation for the photoelectron and the use of semiempirical LCAO-SCF-MOs of the CNDO- or MINDO- type for the initial orbital. The calculated values are compared with experimental photoelectron band intensities obtained by integrating the band areas in He I and He II photoelectron spectra. The observed relative intensity changes of bands in the spectrum in going from He I to He II excitation are attributed to variations in the one- and/or two-centre contributions to the cross sections from the electron density at the atoms and in the bonds, respectively. The analysis of the relative intensities of bands in the He I and He II spectra thus leads to conclusions about the electron density distribution in the initial orbital and about the assignment of the bands. In a qualitative sense, the experimental He I/He II intensity changes are usually correctly predicted by our theoretical method, but there are often considerable quantitative discrepancies between the measured and calculated values which probably arise from the inherent simplifications of our approach.

CNDO/2 (complete neglect of differential overlap)-method for third-row molecules

An extension of the CNDO/2 method to compounds containing third-row elements (Germanium, Arsenic, Selenium and Bromine) is presented. Bond lengths, bond angles, dipole moments, and ionization potentials are considered.

Matrix isolation photochemistry : photoequilibrium between transient o-benzyne, carbon monoxide and transient cyclopentadienylideneketene in an argon matrix

Abstract A photochemical study of benzocyclobutenedione is presented using narrow-band excitation and time-dependent spectroscopic measurements. Various photointermediates and the title photoequilibrium are firmly established. The UV/VIS absorption spectra of pure o-benzyne (1) and cyclopentadienylideneketene (4) are given. The results prove that the IR band at 2087 cm−1 previously ascribed to 1 and assigned to the vibration is instead the ketene band of 4.

Photoionization cross sections: He I and He II photoelectron spectra of saturated three-membered rings☆

Abstract The relative band intensities in the He I and He II photoelectron spectra of cyclopropane, oxirane, and thirane are interpreted by means of a recent theoretical approach. For each compound, the assignment of the two bands in the 15–18 eV range derived from our intensity arguments is opposite to the sequence of the respective ab initio eigenvalues.

SHAPE RESONANCES IN PHOTOIONIZATION: CORRELATION WITH STO-3G MO RESULTS

Abstract Shape resonances in nine linear molecules are investigated by multiple-scattering and molecular-orbital calculations. Each resonance correlates with a bound-state virtual orbital. It is shown that minimal-basis-set molecular-orbital calculations may be used to guess the number of resonances expected and to estimate their energies via regression analysis.