Quantum-chemical investigations of small molecular anions

Dedicated to Dr Gerhard Herzberg on the occasion of his 90th birthday Recent large-scale ab initio calculations for small negative molecular ions are reviewed. Accurate equilibrium geometries are established for several species like NH2−, HCC−, NO2 −, CH2N− C5 − and C6 −. Predictions are made for various spectroscopic properties like vibrational frequencies, rotational constants and infrared intensities. The effects of a shallow energy minimum in the T-shaped configuration on the rovibrational term energies of HCC − are investigated. The calculated vibrational structures of the photoelectron spectra of SiH3 − and CH2N − are in very good agreement with the experiment. The present calculations support the assignment of an absorption observed at 608 nm in a neon matrix to the [Ctilde]2Πg ← [Xtilde]2Πu transition of C6 −. Electron affinities are obtained with an accuracy of 0-05 eV or better. Spectroscopic properties of intermediates in simple SN2 reactions are calculated.