Norbert Münzel

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.

Uv photoelectron spectrum of cyclobutadiene. free cyclobutadiene stable up to high temperatures

Abstract The Hel photoelectron spectrum of cyclobutadiene (CB) has been obtained under conditions which demonstrate that free CB is stable up to temperatures of several hundred °C. A new experimental argument for the rectangular geometry of CB is presented. Shake-up structures are unimportant for the interpretation of the PE spectrum of CB. LNDO/S PERTCI, MNDO PERTCI and previous experimental vertical ionization energy estimates accord with the experimental data.

Matrix isolation radiation chemistry and photochemistry: electronic absorption spectra of o-xylylene and benzocyclobutene radical cations; localization of koopmans and non-koopmans bands in the photoelectron spectra of o-xylylene, styrene, toluene, o-xylene and benzocyclobutene

Abstract Ar matrix-isolated benzocyclobutene, o -xylylene and styrene have been X-irradiated and the products of the radiation reactions elucidated. From a chemical point of view the easy reaction of benzocyclobutene to neutral o -xylylene is quite remarkable. Besides o -xylylene, the radiolysis of benzocyclobutene yields the radical cations of o -xylylene and of styrene. These cations as well as the benzocyclobutene radical cation are independently produced by resonant two-photon ionization of matrix-isolated benzocyclobutene ( o -xylylene +· and benzocyclobutene +· ) and by radiolysis of matrix-isolated styrene (styrene +· ). Benzocyclobutene +· is easily phototransformed into o -xylylene +· . The UV-visible absorption spectra of the radiolysis products as well as of the photolyzed radiolysis products have been recorded. High-resolution photoelectron spectra of benzene, toluene, o -xylene, benzocyclobutene and styrene have been obtained. The electronic excitation spectra of the radical cations as well as the ionization spectra of the various molecules have been calculated using the semiempirical methods for spectroscopy, LNDO/S PERTCI and MNDO/S PERTCI, and the interrelations between both sorts of spectra analyzed. The agreement between theory and experiment is on the whole very satisfactory. It is shown that the investigation of the UV-visible absorption spectra of the radical cations of the substituted benzenes leads to an une-quivocal localization of a higher π band amid several σ bands in the corresponding photoelectron spectra and that the study of the UV-visible absorption spectrum of o -xylylene radical cation unequivocally confirms a low-energy shake-up band in the photoelectron spectrum of o -xylylene.

Detection of 2,5-dimethylene-2,5-dihydrothiophene and thiophenoradialene

Abstract The reactive species 2,5-dimethylene-2,5-dihydrothiophene and thiophenoradialene have been detected for the first time. The method of HCl elimination has proved to be a generally applicable method for generating heterocyclic xylylenes and radialenes.

UV/VIS absorption spectrum, geometry and electronic structure of transient o-benzyne

Abstract The UV/VIS spectrum of transient o -benzyne is presented along with the LNDO/S PERTCI excitation spectrum. The first absorption band of this compound occurs at the unusually long wavelength of 380 nm. The results indicate that, electronically, o -benzyne can be considered as benzene with an additional σ bond.

Uv photoelectron spectrum of o-xylylene — detection of a low-energy non-koopmans (shake-up) ionization

Abstract The HeI spectrum of the transient title compound has been obtained using the variable-temperature photoelectron- spectroscopy technique. Strong experimental evidence for a low-energy shake-up structure in the ionization pattern of the polyene-like system is presented. Agreement between the semi-empirical PERTCI and experimental ion-state energies is very good.

The electronic spectrum of o-benzyne

Semiempirical valence electron calculations using the CNDO/S SECI, HAM/3 Cl, LNDO/S PERTCI, MNDOC PERTCI, and MNDO/S PERTCI methods have been applied to the interpretation of the recently reported UV/vis absorption spectrum of transiento-benzyne. Detailed calculations were performed for benzene,o-benzyne, cyclopentadienylidenecarbene, cyclopentadienylideneketene, and cyclopentadienylideneketone (6-fulvenone). From these calculations and experimental data for benzene,o-benzyne, and cyclopentadienylideneketone, an interpretation of the electronic absorption spectrum ofo-benzyne is proposed. The interpretation includes the proposal that the first band and a part of the third band originally assigned too-benzyne may be due to cyclopentadienylideneketene, a common byproduct (or intermediate) of the photolyses of benzocyclobutenedione and 3-diazobenzofuranone.

LNDO/S PERTCI Electronic Spectra of Some Radical Cations

A theoretical approach to the interpretation of UV/VIS spectra of radical cations is made. LNDO/S PERTCI calculations including all singles and doubles with respect to the ion ground and excited state main configurations are made. Evaluations of the oscillator strengths are carried out using the full ground and ion states Cl vectors. The method is applied to [trans-butadiene]+, [all-tans-hexatriene]+, [all-trans-octatetraene]+, [cis -butadiene]+, [all-cis -hexatriene]+, [trans-stilbene]+, [biphenyl]+, [diphenylacetylene]+, [benzocyclobutene]+, and [o-xylylene]+. Calculated electronic excitation spectra for these systems are presented. They are compared with the UV/VIS spectra in low temperature matrices. It is shown, that LNDO/S PERTCI results can be of help in the interpetation of UV/VIS spectra of cations and thus in the difficult task of identifying such systems or cationic reaction products of cations.