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.

The electronic structure of HAs judged from a comparison of the force constants for stretching the XY bonds in the HFor XY=BO the difference between the first ionization potentials of H-XY and H

Our investigations concerning the protonaffmity of aliphatic amines have been confined so far to primary unbranched species. In order to gain further information about the applicability and the limitations of the statements derived from the results of the previous calculations, we extended our investigations to some tertiary, secondary and branched primary amines. Correlations between the experimental protonaffinities and the Mulliken charges of the nitrogen atom and the NHx-group (x = 0, 1, 2) of the amines, as well as those of the NHx + 1 ,-group of the corresponding ammonium cations have been found. However, these correlations only hold within the different groups of amines (primary, secondary and tertiary) and not between them. A somewhat better correlation throughout all groups is found if one compares the charges of the protons in the NHx + 1-groups of the ammonium cations with the experimental protonaffinities of the parent amines but deviations from the expected order are found in this case, too. This shows the limited value of charges as calculated by the methods used here, in predicting the relative order of protonaffinities of amines if they belong to different groups.

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