Electronic excitations of fluoroethylenes

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Understanding the electronic structure of fluorinated ethylenes is crucial for predicting their chemical behavior.
• Previous studies have faced challenges in accurately calculating excitation energies and interpreting spectral features.

Purpose of the Study:

• To investigate the lowest-lying singlet and triplet electronic states of various fluoroethylenes.
• To accurately calculate excitation energies and compare them with experimental data.
• To elucidate the nature of electronic transitions and state mixing in these molecules.

Main Methods:

• Utilized symmetry-adapted cluster configuration interaction (SAC-CI) theory for electronic state calculations.
• Employed extensive basis sets, including Dunning's aug-cc-pVTZ with Rydberg functions.
• Analyzed electronic state mixing and potential reasons for spectral shifts.

Main Results:

• Calculated excitation energies show good agreement with experimental values, including challenging pi-->pi(*) transitions.
• Observed significant mixing between electronic states, particularly pi-pi(*) and pi-3ppi states in trifluoroethylene.
• No pure pi-sigma(*) excited states were identified in most fluoroethylenes.
• The study refutes the proposed interaction between pi-pi(*) and sigma-sigma(*) states as the cause for the tetrafluoroethylene spectral blueshift.

Conclusions:

• The computational approach provides reliable excitation energies and aids in spectral assignment for fluoroethylenes.
• Structural changes are suggested as a more plausible explanation for the observed spectral shifts in tetrafluoroethylene.
• The findings offer valuable insights into the electronic properties and photophysics of fluorinated ethylenes.