A combined vacuum ultraviolet laser and synchrotron pulsed field ionization study of BCl3

Abstract

The pulsed field ionization-photoelectron (PFI-PE) spectrum of boron trichloride (BCl₃) in the region of 93 590–95 640 cm⁻¹ has been measured using vacuum ultraviolet (VUV) laser. At energies 0–1100 cm⁻¹ above the adiabatic ionization energy (IE) of BCl₃, the bending vibration progression of BCl₃⁺ is clearly resolved in the PFI-PE spectrum, whereas the spectrum at energies 1200–1900 cm⁻¹ above the IE(BCl₃) is found to exhibit dense vibrational structure. This observation unambiguously shows that BCl₃⁺ in its ground state has C_2V symmetry. Ab initio calculations performed at the CCSD(T)/CBS level with high-level corrections are consistent with this observation, indicating that the BCl₃⁺( ²B₂) ground state has two long and one short B–Cl bonds. Furthermore, the CCSD(T)/CBS calculations predict the existence of two BCl₃⁺ transitional structures with D_3h and C_2V symmetries lying ≈800 and 1300 cm⁻¹, respectively, above the BCl₃⁺( ²B₂) ground state. This prediction is also consistent with the dense features observed in the PFI-PE spectrum in the region of 1200–1900 cm⁻¹ above the IE(BCl₃). The assignment of the PFI-PE vibrational bands gives the IE(BCl₃) = 93 891 ± 2 cm⁻¹ (11.6410 ± 0.0003 eV) and the bending frequencies for BCl₃⁺( ²B₂), ν₁⁺(b₂) = 194 cm⁻¹ and ν₁⁺(a₁) = 209 cm⁻¹. We have also examined the dissociative photoionization process BCl₃ + hν → BCl₂⁺ + Cl + e⁻ using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy (AE) for this process to be 12.495 ± 0.002 eV. Combining this 0 K AE value and the IE(BCl₃), we have determined the 0 K bond dissociation energy (D₀) for Cl₂B⁺–Cl as 0.854 ± 0.002 eV. This experimental and theoretical study indicates that the CCSD(T, Full)/CBS calculations with high-level corrections are highly reliable for the predictions of IE(BCl₃), AE(BCl₂⁺) and D₀(Cl₂B⁺–Cl) with error limits of less than 35 meV. However, the CCSD(T, Full)/CBS predictions for ΔH_f0°(BCl₃), ΔH_f0°(BCl₂⁺), and ΔH_f0°(BCl₃⁺) are less reliable with discrepancies up to 0.1 eV as compared to the experimental determinations.