Potential energy curves for the HF − and CH 3F − anions: a coupled cluster study

Abstract

Very accurate ab initio calculations using the single-reference (SR) coupled cluster (CC) theory were performed for HF, CH 3F and their negative ions. The CC theory with singly and doubly excited clusters (CCSD approach) and its non-iterative counterpart accounting for connected triexcited components (CCSD(T) approach) were used as primary theoretical methods, and the HF and HF − systems were also investigated using the complete active space self-consistent field (CASSCF) approach. Emphasis was placed on the stability of the negatively charged ions and the potential energy curves for dissociative electron attachment in HF and CH 3F. In agreement with the majority of earlier calculations, the HF − and CH 3F − anions are unstable (i.e. have negative electron affinities) and can only capture an electron when the HF and CF bonds are significantly stretched. It is demonstrated that electron capture by HF and CH 3F correlates quite well with the maxima in the corresponding dipole moment functions. The potential energy curve for HF − is attactive, i.e. without a barrier for electron attachment, representing the HF molecule plus free electron near the equilibrium geometry of HF and crossing the neutral HF curve at the HF internuclear separation of 2.2 bohr in order to reach the H(1 s 2S) + F − (2 6 p 1S) limit (the equilibrium HF bond length is 1.733 bohr). The stretching of the HF bond and the energy required to capture an electron, resulting from CC calculations, are lower than in earlier calculations. The potential energy curve for CF bond breaking in CH 3F − has barriers for the dissociative electron attachment ( ΔE ‡ a ) and associative electron detachment ( ΔE ‡ d ) at the CF distance of approximately 3.7 bohr (the equilibrium CF separation is 2.612 bohr). Both barriers resulting from CC calculations, which are lower than in earlier calculations (CCSD method gives ΔE ‡ a = 1.67 eV , ΔE ‡ d = 0.28 eV ; CCSD(T) results are ΔE ‡ a = 1.56 eV , ΔE ‡ d = 0.20 eV ), are sensitive to many-electron correlation effects. The dissociative electron attachment in methyl fluoride is accompanied by a strong variation of the angle describing the umbrella motion of the methyl group and the appearance of a shallow (approximately 2 kcal mol −1) van der Waals' minimum in the corresponding CH 3F − curve representing the CH 3·F − complex. The implications of our findings for electron transfer phenomena, in particular for the photoinitiated charge transfer processes of the harpooning type, are discussed. The role of diffuse functions in the calculations for the temporary anion states and the role of the basis set and the theoretical approach in the modelling of potentials for electron attachment to a molecule are also briefly discussed.