ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
High-level coupled-cluster computations of the two lightest trihalogen cations (F3+ and Cl3+) predict the ground electronic state to be X˜ 1A1. As expected, the trifluorine cation is even less stable than the trichlorine cation, which has been detected and studied experimentally. The Brueckner-reference coupled-cluster doubles and perturbatively connected triples method with a basis set of beyond triple-ζ quality predicts the classical X˜ 1A1 F3+→2P F+2Π F2+ dissociation energy to be 15 kcal/mol. We expect that more complete basis sets and higher levels of theoretical treatment will not qualitatively change this dissociation barrier, and thus the trifluorine cation should be a viable species. The lowest linear triplet states of both F3+ and Cl3+ at the correlated levels of theory are bound by only 2–3 kcal/mol. The electronic wave function for the X˜ 1A1 state of F3+ exhibits substantial multireference character and, similar to X˜ 1A1 O3, proves to be a difficult case for single-reference ab initio methods based on a spin-restricted Hartree–Fock (RHF) determinant. More specifically, RHF-based coupled-cluster singles and doubles method and its extension with connected triple excitations predict different orderings of the X˜ 1A1 F3+ stretching frequencies (ω1 and ω3). Reliable predictions for the harmonic vibrational frequencies of this system are obtained through the use of two Brueckner-reference coupled-cluster methods and a large basis set of beyond triple-ζ quality [our best predictions are ω1(A1)=825 cm−1, ω2(A1)=376 cm−1, ω3(B2) =752 cm−1]. Comparison with the previous ab initio analyses of F3+ stresses the need for a very high level of treatment of dynamic electron correlation to obtain chemically accurate results. The issue of inversion symmetry breaking in a possible dissociation product of the trifluorine molecular cation, F2+, is also addressed and it is shown that a "symmetry dilemma" in the region near the equilibrium F–F distance (∼1.3 Å) can be resolved through the use of coupled-cluster methods based on a Brueckner-reference determinant, which has a reference instability shifted away from its position in spin-restricted open-shell and spin-unrestricted Hartree–Fock determinants. © 1998 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.476752
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