Halogenated isomers of the interstellar C3H2:: an ab initio comparative study

Stereoisomeric carbenic structures of singlet as well as triplet interstellar C3H2 and its halogenated analogues, HC3F, HC3Cl and HC3Br, are compared and contrasted using exactly the same ab initio methods. For each species three structures are considered: cyclopropenylidene (1), vinylidenecarbene (2), and propargylene (3). The effects of the halogens on the relative stabilities of these isomers are reported, using six levels of theory: HF/6-31G*, HF/6-311++G**, B3LYP/6-311++G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G** and QCISD (T)/6-311++G**. The energy, geometrical parameters and harmonic vibrational frequencies are compared and contrasted. Among the 24 structures studied, singlets of 1 appear as global minima. (C) 2003 Elsevier B.V. All rights reserved.