Insights into the Photoelectron Spectroscopy of Chlorofluoroethenes Studied by Density-Functional and Coupled-Cluster Theories.

The first two ionic states of chlorofluoroethenes were studied by using both time-independent and time-dependent density-functional theories. We calculated the equilibrium geometries and harmonic vibrational frequencies of 1,1-, cis-, and trans-C2H2FCl and their cations by using the B3LYP and B3PW91 functionals together with the cc-pVTZ and aug-cc-pVTZ basis sets. Franck-Condon factors were computed by the method developed in our group, in which the Duschinsky effect was treated explicitly. A new technique, named alignment transformation, followed by Euler transformations was developed to achieve the Eckart conditions. The adiabatic ionization energies were calculated by the CCSD(T) method extrapolated to the complete basis set limit. Insights into the simulated photoelectron spectra of C2H2FCl indicate that the resolutions of recent threshold photoelectron experiments are not high enough to detect individual transitions. The high-resolution photoelectron spectra of C2H2FCl are predicted for future reference. The computed adiabatic ionization energies of the three isomers of C2H2FCl are in accord with the experiments with the absolute deviations ranging from 0.004 to 0.021 eV. We suggest that the agreement between experimental and theoretical spectra should be a key criterion to judge whether a spectral assignment is reasonable.