Accurate calculation of the ionization energies of the chlorine lone pairs in 1,1,1-trifluoro-2-chloroethane (HCFC-133a).

The vertical ionization energies of the chlorine lone pairs in HCFC-133a have been calculated at the SCF (via Koopmans' theorem and including orbital relaxation) and correlated (ROMP2, OVGF, and ROCCSD(T)) levels. Dunning aug-cc-pVXZ (X = D, T, and Q) basis sets were employed, and the ROMP2 and ROCCSD(T) results were extrapolated to the complete basis set (CBS) limit. Our highest-level results (obtained at the ROCCSD(T)/CBS level) were 11.99 and 12.08 eV for the Cl lone pairs of A″ and A' symmetry, respectively. The values obtained at the computationally much less demanding ROMP2/CBS level were just 0.10 and 0.13 eV higher than the highest-level ones. Using the Cl lone-pair band of the photoelectron spectrum of the HCF(2)Cl and CF(3)Cl molecules as a guide, it is considered very unlikely that these two lone pairs can be discriminated in the photoelectron spectrum of the title molecule. The use of the calculated IPs to estimate the energies of the Rydberg states of HCFC-133a is also discussed.