Condensed-Phase Effects on the Structural Properties of FCH₂CN–BF₃ and ClCH₂CN–BF₃: A Matrix-Isolation and Computational Study

Abstract

We have measured several IR bands of FCH₂CN–BF₃ and ClCH₂CN–BF₃ in solid nitrogen, argon, and neon. These bands include the B–F asymmetric stretch (ν_BF^a), the B–F symmetric stretch (ν_BF^s), the BF₃ symmetric deformation or “umbrella” mode (δ_BF^s), and the CN stretch (ν_CN). For both complexes, the frequencies of these modes shift across the various media, particularly the B–F asymmetric stretching band, and thus they indicate that the inert gas matrix environments significantly alter the structural properties of FCH₂CN–BF₃ and ClCH₂CN–BF₃. Furthermore, the frequencies shift in a manner that parallels the dielectric constant of these media, which suggests a progressive contraction of the B–N distances in these systems and also that it parallels the ability of the medium to stabilize the increase in polarity that accompanies the bond contraction. We have also mapped the B–N distance potentials for FCH₂CN–BF₃ and ClCH₂CN–BF₃ using several density functional and post-Hartree–Fock methods, all of which reveal a flat, shelflike region that extends from the gas-phase minimum (near 2.4 Å) toward the inner wall (to about 1.7 Å). Furthermore, we were able to rationalize the medium effects on the structure by constructing hybrid bond potentials composed of the electrostatic component of the solvation free energy and the gas-phase electronic energy. These curves indicate that the solvation energies are greatest at short B–N distances (at which the complex is more polar), and ultimately, the potential minima shift inward as the dielectric constant of the medium increases