Molecular Anion Spectroscopy and Stability

A detailed and thorough computational investigation of the SF ₆ anion is undertaken, with additional calculations of other stable SF _n anions. The calculations are used to in- terpret recent infrared multi-photon dissociation (IRMPD) experiments of SF ₆ and SF ₅ anions done with the FELIX free electron tunable infrared laser in the spectral range of 450-850 cm^-1. The vibrational spectroscopy of negative molecular ions is crucial towards understanding the dynamics of electron transfer processes, as well as providing experimental validation for computational models of molecular bonding in anions. Many molecular negative ions have been shown to exhibit long (i.e. > 1¹s) lifetimes with respect to electron autodetachment, and a recent reformulation of QET, or quasi- equilibrium theory, has shown success in reproducing these experimental lifetimes for a variety of molecules. In particular, the SF₆ anion has an interesting history of disparate results for autodetachment lifetime measurements, largely dependent upon the type of experimental method employed. Analysis of the experimental data in reference to the theoretical predictions of QET suggests this disparity may be related to the degree of IVR, or inter-vibrational coupling occurring in the anion. Calculation of the electron autodetachment lifetime according to QET requires accurate values for the electron affinity as well as the vibrational frequencies for both the neutral and the anion. Ab-initio and DFT computations are extremely useful for providing reasonable estimates of the anion vibrational frequencies, however, experimental constraints upon these values are essential. An additional benefit of anion vibrational spectroscopy is the elucidation of molecular structure and symmetry. This is particularly relevant for the case of SF₆ , as certain computational methods show interesting results in regard to the distortion to lowered C_4v symmetry. This is in contradiction to the expected O_h symmetry of the SF^- ₆ anion, and the potential energy surface along this multi-dimensional vector of S-F bond dissociation is further computationally explored, as are the theoretical implications of a multiple-well potential surface. v