Electronic spectroscopy of jet-cooled half-sandwich organometallic free radicals: laser-induced fluorescence study of the monomethylcyclopentadienyl complexes of zinc and cadmium

Abstract

Laser excitation and single vibronic level fluorescence spectra have been obtained for the open-faced sandwich complexes methylcyclopentadienylzinc and methylcyclopentadienylcadmium, ZnC₅H₄CH₃ and CdC₅H₄CH₃, respectively. These molecules are produced in a supersonic jet expansion using a laser vaporization/photolysis technique. Two nearby excited electronic states were observed in the excitation spectra. These two states arise from splitting of the òE₁ states of ZnC₅H₅ and CdC₅H₅ owing to the lower symmetry of the metal methylcyclopentadienyl complexes. In the case of CdC₅H₄CH₃, a third electronic state was also identified. Both laser excitation and dispersed fluorescence spectra of these molecules show extensive vibrational structure originating from both skeletal and intra-ring vibrations. From these features, we were able to determine where the metal atom is most likely located, i.e. above the ring in a ring-bonding (η⁵) fashion. In addition, we also observe low-frequency modes which we attribute to hindered rotation of the methyl group. Barriers to methyl free rotation and changes in conformation of the methyl group were determined from this structure. This report constitutes the first spectroscopic study of these free radicals and the first detailed investigation of methyl torsional barriers in open-shell organometallic molecules.