He(I) Photoelectron Spectroscopy and Electronic Structure of Alkyllithium Clusters

Abstract

The electronic structure of some alkyllithium clusters has been investigated by He(I) photoelectron spectroscopy. Two distinct types of spectra have been observed. This is explained by the different degree of association of the compounds in the gas phase, as determined by the steric demand of the alkyl substituent. From the analysis of the spectra it can be concluded that isopropyl- (i-PrLi), sec-butyl- (s-BuLi), and tert-butyllithium (t-BuLi) form only tetramers, while ethyl- (EtLi), n-propyl- (n-PrLi), n-butyl- (n-BuLi), and isobutyllithium (i-BuLi) are mixtures of tetrameric and hexameric clusters in the gas phase. This observation is in accord with former mass spectrometric investigations [Plavšić, D.; Srzić, D.; Klasinc, L. J. Phys. Chem. 1986, 90, 2075; Berkowitz, J.; Bafus, D. A.; Brown, T. L. J. Phys. Chem. 1961, 65, 1380]. The band shapes and intensities are interpreted by large geometry changes taking place during the ionization as a consequence of Jahn−Teller distortion of the ionic clusters and the change of electrostatic interactions. The nature and extent of the distortion is examined by ab initio quantum chemical calculations. The effect of the substituents and the degree of association on the electronic structure is also clarified.