The utilization of simple photochemical reactions for the storage of solar energy in the form of chemical energy in energy-rich products has often been considered in the further development and improvement of e. g. simple thermosolar techniques. The hitherto proposed criteria for the qualification of an abiotic photochemical system are, however, mostly of a qualitative nature, so a mutal comparison of the systems is not precise enough. In this article it is shown how a useful correlation on the basis of time-independent experimental data can be achieved and how, from the viewpoint of photochemistry, a comparative classification of known reactions is possible. The following reactions are compared: the [2 + 21-photocycloadditions of norbornadiene, dimethyl 2,3-norbornadienedicarboxylate, and dicyclopentadienone, the photoisomerization of trans-to cis-diacetylindigo, the photodissociation of nitrosyl chloride as well as a photocatalytic redox reaction. The quantity of material required and storage efficiency are by far the most favorable in the case of trans-diacetylindigo. The main disadvantage of the latter however, is that the energy-rich cis-form rapidly reverts to the stable frans-form at elevated temperatures

Die 4,5-Dimethylen-1,3-dioxolane 1 -5 zeigen, obwohl sie s-cis-fixierte Diensysteme darstellen, im Vergleich zu den 1,2-Dimethylencycloalkanen 8 -10 und 2,3-Dimethylen-1,4-dioxan (6) sehr geringe Dienreaktivitaten, wie durch die Geschwindigkeitskonstanten und die Aktivierungsparameter belegt wird. Der EinfluB der durch Photoelektronenspektroskopie festgelegten Ionisationspotentiale und der nach dem MIND0/3-Verfahren berechneien Molekulgeometrien auf die Reaktivitlt wird untersucht. Es zeigt sich, daR fur die Reaktivitat der s-cis-fixierten Diene nicht der Interplanarwinkel, sondern die Scherung (1,4-Abstand) des Dien-Systems von entscheidender Bedeutung ist. On the Diels-Alder Reactivity of s-cis-Fixed 1,3-DienesContrary to the 1,2-dimethylenecycloalkanes 8 -10 and 2,3-dimethylene-1,4-dioxane (6), the 4,5-dimethylene-1,3-dioxolanes 1 -5 show very poor diene reactivities in the Diels-Alder reaction though possessing an s-cis-fixed diene system. This is documented by bimolecular rate constants as well as activation parameters. The influence of the ionisation potentials measured by P E spectroscopy and of the molecular geometries calculated by the MIND0/3 method on the reactivity is studied. It is shown that not the dehedral angle but the scissoring (2,4-distance) of the diene system is of decisive importance for the reactivity of the s-cis-fixed dienes.Fur die Abschatzung der Diels-Alder-Reaktivitaten ist das sog. ,,Grenzorbitalmodell" ein haufig benutztes Verfahren -41. Die damit gewonnenen qualitativen Ergebnisse hangen nicht zuletzt davon ab, inwieweit sterische Faktoren sowie Veranderungen in der Geometrie des DienGerustes verstanden und hinreichend gut berucksichtigt werden konnen.Fur die ,,normale" Diels-Alder-Reaktion hat im allgemeinen eine Verringerung der Energiedifferenz zwischen dem H O M O des Diens und dem LUMO des Dienophils eine Reaktivitatssteigerung zur Folge 'I. Sustmunn und Schubert haben die HOMO-LUMO-Energiedifferenzen gegen den Logarithmus der bimolekularen Geschwindigkeitskonstanten aufgetragen und fanden eine grobe Korrelation ' I. Sie untersuchten offenkettige Diene, fur die die relativen Konzentrationen der s-cis-Isomeren nicht bekannt sind; groI3e Abweichungen traten bei cyclischen 1,3-Dienen auf. Eine lineare Beziehung wurde bei der Reaktion von Cyanethylenen mit Cyclopentadien und 9,lO-Dimethylanthracen gefunden ' I.

No abstract

Recently we reported the results of some semiempirical and ab initio studies in which we compared the electronic structure of the hitherto unknown borinine with those of benzene and pyridine. The results of our calculations led us to the conclusion that the elusive nature of borabenzene is caused by its high reactivity, which might at least in part be due to the pronounced σ acceptor properties of a low-lying σ* molecular orbital. We now present the results of further ab initio and semiempirical (MNDO) investigations in which we performed full geometry optimizations for the molecule using two different basis sets (STO-3G, 4-31G) and also calculated the vibrational spectra of the 10B and 11B isotopomeric borabenzene molecules at the 4-31 G level of ab initio theory and with the semiempirical MNDO method. The calculated vibrational spectrum might be helpful to the experimentalist in identifying the molecule, for example trapped in a rare gas matrix among the side products. The calculated orbital energies can be useful in identifying the molecule by means of its photoelectron spectrum.

Quantumchemical Calculations on Borine (Borabenzene)Although many Borinato-transition metal-complexes have been isolated after the first synthesis and characterisation of such a molecule in 1970, nobody succeeded so far in isolating the unsubstituted neutral ligand Borine (Borabenzene) C5H5B.It is the aim of this theoretical investigation to find at least a partial answer to the question why the free borine should be a highly reactive molecule.The results of semiempirical and ab-initio-calculations presented here show that the LUMO of the free and unsubstituted borine in contrast to benzene and pyridine should be a er*-molecular orbital which causes pronounced a-acceptor properties.