PROTONATED FLUORINE, HF2- AB-INITIO POTENTIAL-ENERGY SURFACE AND VIBRATIONAL SPLITTINGS FOR PROTON-TRANSFER( )

We calculate 1542 ab initio points for the HF2+ ground state at the QC ISD(T)/6-311 ++G(2df,2pd) level for a wide range of geometries. We fit the ab initio points to a multiparameter analytic function to obtain a multidimensional potential energy surface (PES) valid far large ampl itude hydrogen motion. We then calculate and assign vibrational levels for this PES,There is intramolecular proton transfer when the hydroge n atom tunnels through a triangular transition state separating the tw o equivalent equilibrium geometries. The barrier to proton transfer is 9547 cm(-1) (8340 cm(-1) with zero-point correction). Below the barri er to proton transfer, the energy levels are split and measurable spli ttings are predicted for relatively low-lying vibrational levels that may be experimentally accessible. The first three levels with splittin g greater than 0.01 cm(-1) are, in order of increasing energy, (0,3,1) , (0,4,0), and (0,3,2), while the first three levels with splitting gr eater than 0.1 cm(-1) are, in: order of increasing energy, (0,4,1), (0 ,5,0), and (0,4,2), where nu(2) is the H-F-F bend quantum number and n u(3) is the F-F stretch quantum number. We conclude that H-F-F bend ex citation is essential far proton transfer, and that F-F stretch excita tion facilitates proton transfer. Ln addition, there is a 3:1 Fermi re sonance between the HF stretch (harmonic frequency 3334 cm(-1)) and th e H-F-F bend (harmonic frequency 1141 cm(-1)), and levels with HF stre tch excitation can have significant splittings. For example, the split ting is greater than 0.01 cm-l for the (1,1,1) level, which is Fermi r esonant with (0,4,1) and greater than 0.1 cm(-1) for the (1,2,1) level , which is Fermi resonant with (0,5,1). This is relevant for the exper imental observation of the vibrational splittings since the IR intensi ty of the HF stretch is four times that of the H-F-F bend. (C) 1998 Am erican Institute of physics. [S0021-9606(98)30944-7].