THE GAS-PHASE REACTIVITY AND THEORETICAL STRUCTURES OF GERMANIUM IONSGENERATED BY DIRECT LASER VAPORIZATION

The ion series Ge-x(+/-),x=1-7, have been generated using direct laser vaporisation and detected using Fourier transform ion cyclotron reson ance mass spectrometry (FTICR-MS). Optimised non-local spin density ap proximation-density functional theory (NLSD) structures for these ions and their corresponding neutrals are presented. These results are use d to interpret the relative intensity variation in positive-ion and ne gative-ion mass spectra and deduce the most prevalent neutral species in the laser plume. Ion-molecule reaction studies confirm the transiti ons to closed cyclic structures, as predicted by the NLSD calculations , at x = 3 for the anions and x = 4 for the cations. The ions most rea ctive towards CH3OH, CH3CH2OH, H2S and C-C3H6, were found to be the sm all cations with terminal, electron-rich (radical) Ge centres, while a ll the anions, with the exception of Ge-2(-), were essentially unreact ive. The NLSD results reveal that there is little structural change up on either electron attachment or ionisation of the neutral structures. Small energy separations are predicted between the a(3) Pi(u) and X-3 Sigma(g)(-) states of Ge-2 (6.2 kcal mol(-1)), the a(3)B(1) and X(1)A (1) spin states of Ge-3 (4.6 kcal mol(-1)) and the singlet and tripler spin stares of Ge-5 (about 4 kcal mol(-1) vertical separation). A Ryd berg-like ''square'' quartet state of Ge-4(-) was also discovered, 30 kcal mol- higher in energy than the lowest energy D-2h doublet structu re. NLSD did not locate minima corresponding to the anticipated (2) Pi (u) ground states of the mixed valence dimers Ge-2(+) and Ge-2(-), ins tead locating the states (4) Sigma(g)(+)Ge(2)(+) and 2 Sigma(g)(+)Ge(2 )(-) both of which have electron vacancies below the HOMO. Collision i nduced dissociation (CID) results from FTICR-MS studies, and the recen t literature laser photoelectron-zero electron kinetic energy (LPES-ZE KE) results for the anions Ge-2(-) through to Ge-2(-) were interpreted using the theoretical results from this study. (C) 1997 Elsevier Scie nce B.V.