He(I) photoelectron spectroscopy and electronic structure of alkyllithium clusters

The electronic structure of some alkyllithium clusters has been investigate d by He(I) photoelectron spectroscopy. Two distinct types of spectra have b een observed. This is explained by the different degree of association of t he compounds in the gas phase, as determined by the steric demand of the al kyl substituent. From the analysis of the spectra it can be concluded that isopropyl- (i-PrLi), sec-butyl- (s-BuLi), and tert-butyllithium (t-BuLi) fo rm only tetramers, while ethyl- (EtLi), n-propyl- (n-PrLi), n-butyl- (n-BuL i), and isobutyllithium (i-BuLi) are mixtures of tetrameric and hexameric c lusters in the gas phase. This observation is in accord with former mass sp ectrometric investigations [Plavsic, D.; Srzic, D.; Klasinc, L. J. Phys. Ch em. 1988, 90, 2075; Berkowitz, J.; Bafus, D.A; Brown, T. L. J. Phys. Chem. 1961, 65, 1980]. The band shapes and intensities are interpreted by large g eometry changes taking place during the ionization as a consequence of Jahn -Teller distortion of the ionic clusters and the change of electrostatic in teractions. The nature and extent of the distortion is examined by ab initi o quantum chemical calculations. The effect of the substituents and the deg ree of association on the electronic structure is also clarified.