Structures and energies of the lowest lying singlet and triplet states of C₃H₂ and C₃F₂. A theoretical study

The theoretically predicted geometries for the propargylene, cyclopropenylidene, vinylidenecarbene, and cyclopropyne forms of C₃H₂ and C₃F₂ in their singlet and triplet states are reported at the MP2/6-31G(d) level of theory, using spin-restricted wave functions for the singlets and spin-unrestricted wave functions for the triplets. The calculated vibrational spectra for the different isomers have been calculated at HF/6-31G(d) and, in some cases, at MP2/6-31G(d). Improved total energies were calculated for C₃H₂ at MP4/6-311G(2df) and for C₃F₂ at MP4/6-31G(d) using the MP2/6-31G(d) optimized geometries. The relative stabilities of the C₃H₂ isomers are predicted at MP4/6-311G(2df)//MP2/6-31G(d) and for C₃F₂ isomers at MP4/6-31G(d)//MP2/6-31G(d) corrected by zero-point energies, using spin-projected wave functions for the triplet states. To give an estimate for the accuracy of the calculated singlet-triplet gap, the energy differences between the lowest lying singlet and triplet states of CH₂ and CF₂ are calculated and compared with experimental values.