The 1,2-rearrangement of the title species was studied by the RHF/STO-3G gradient method. The equilibrium and transition structures are fully optimized. For these systems at the STO-3G level, the shift reactivities are in the order Cl > H > Me > F, which is different from that of their electronegativities. The fixed group with lower shift reactivity enhances that of the shift group. The carbenes with two identical substituents are generally more stable. The substituent with a smaller angle formed by its central atom with the C:C bond migrates first.

Key words: REACTION MECHANISM, ACETYLENE, CARBENE, REARRANGEMENT REACTION, AB INITIO CALCULATION, TRANSITION STATE THEORY, POTENTIAL BARRIER, ANALYTIC, GRADIENT ALGORITHM