AB-INITIO THEORETICAL-STUDY OF DIPOLE-BOUND ANIONS OF MOLECULAR-COMPLEXES - (HF)(3)(-) AND (HF)(4)(-) ANIONS

Ab initio calculations have been performed to determine structures and vertical electron detachment energy (VDE) of the hydrogen fluoride tr imer and tetramer anions, (HF)(3)(-) and (HF)(4)(-). In these systems the excess electron is bound by the dipole field of the complex. It wa s determined that, unlike the neutral complexes which prefer the cycli c structures, the equilibrium geometries of the anions have ''zig-zag' ' shapes. For both complexes the predicted VDEs are positive [210 meV and 363 meV for (HF)(3)(-) and (HF)(4)(-), respectively], indicating t hat the anions are stable systems with respect to the vertical electro n detachment. These results were obtained at the coupled-cluster level of theory with single, double and triple excitations [CCSD(T) method; the triple-excitation contribution in this method is calculated appro ximately using the perturbation approach] with the anion geometries ob tained using the second-order Moller-Plesset perturbation theory (MP2) method. The same approach was also used to determine the adiabatic el ectron affinities (AEA) of (HF)(3) and (HF)(4). In addition to the ele ctronic contribution, we also calculated the contributions (using the harmonic approximation) resulting from different zero-point Vibration energies of the neutral and anionic clusters. The calculations predict ed that while the AEA of.(HF)3 is positive (44 meV), the AEA for (HF)( 4) is marginally negative (-16 meV). This suggests that the (HF)(3)(-) anion should be a stable system, while the (HF)(4)(-) is probably met astable. (C) 1997 American Institute of Physics.