Structures and energies of the chlorine-substituted analogues of C 3H 2: an ab initio and density functional theory comparative study

Abstract

A theoretical study of the HC 3Cl and HC 3Cl + isomers has been carried out. The geometries and vibrational frequencies have been obtained at the MP2 and B3LYP levels, whereas relative energies have been computed at the MP4, QCISD(T), G2 and CCSD(T) levels. All levels of theory predict that the global minimum of HC 3Cl is singlet chlorocyclopropenylidene, although singlet chlorovinylidenecarbene and singlet and triplet chloropropargylene are relatively close in energy (about 50, 61 and 66 kJ/mol, respectively at the CCSD(T)/cc-pVTZ level). All low-lying HC 3Cl states have relatively high dipole moments. In particular the global minimum is estimated to have a dipole moment around 3.046 D. In the case of HC 3Cl + there are two species which lie very close in energy, a cyclic isomer derived from chlorocyclopropenylidene, and an open-chain structure obtained from ionization of chloropropargylene. G2 and CCSD(T) calculations predict that the latter would be the global minimum, with the former lying just about 4–6 kJ/mol higher in energy. From our calculations it can be concluded that the B3LYP method leads to rather good relative energy predictions, since in most cases the results are quite close to the much more expensive CCSD(T) method.