Structures and energies of the chlorine-substituted analogues of C3H2: an ab initio and density functional theory comparative study

A theoretical study of the HC3Cl and HC3Cl+ isomers has been carried out. T he geometries and vibrational frequencies have been obtained at the MP2 and B3LYP levels, whereas relative energies have been computed at the MP4, QCI SD(T), G2 and CCSD(T) levels. All levels of theory predict that the global minimum of HC3Cl is singlet chlorocyclopropenylidene, although singlet chlo rovinylidenecarbene and singlet and triplet chloropropargylene are relative ly close in energy (about 50, 61 and 66 kJ/mol, respectively at the CCSD(T) /cc-pVTZ level). All low-lying HC3Cl states have relatively high dipole mom ents. In particular the global minimum is estimated to have a dipole moment around 3.046 D. In the case of HC3Cl+ there are two species which lie very close in energy, a cyclic isomer derived from chlorocyclopropenylidene, an d an open-chain structure obtained from ionization of chloropropargylene. G 2 and CCSD(T) calculations predict that the latter would be the global mini mum, 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 goo d relative energy predictions, since in most cases the results are quite cl ose to the much more expensive CCSD(T) method. (C) 2000 Elsevier Science B. V. All rights reserved.