Theoretical investigation of the transition states leading to HCl elimination in 2-chloropropene

This paper describes ab initio electronic structure calculations on the planar transition states of 2-chloropropene leading to HCl elimination in the ground electronic state to form either propyne or allene as the cofragment. The calculations provide optimized geometries of the transition states for these two reaction channels, together with vibrational frequencies, barrier heights, and reaction endothermicities. The calculated barrier heights for the two distinct four-centre HCl elimination transition states, one leading to HCl and propyne and the other leading to HCl and allene, are 72.5 kcal mol¡1 (77.8 kcalmol¡1 without zero-point correction) and 73.2 kcalmol¡1 (78.7 kcalmol¡1) at the MP2/6-311G(d,p) level, 71.0 kcal mol¡1 (76.3 kcal mol¡1) and 70.5 kcalmol¡1 (76.0 kcalmol¡1) at the QCISD(T)/6-311‡G(d,p)// MP2/6-311G(d,p) level, and 66.9 kcalmol¡1 (71.7 kcal mol¡1) and 67.3 kcalmol¡1 (72.1 kcalmol¡1) at the G3//B3LYP level of theory. Calculated harmonic vibrational frequencies at the B3LYP/6-31G(d) level along with transition state barrier heights from the G3//B3LYP level of theory are used to obtain RRKM reaction rate constants for each transition state, which determine the branching ratio between the two HCl elimination channels. Even at internal energies well above both HCl elimination barriers, the HCl elimination leading to propyne is strongly favoured. The smaller rate constant for the HCl elimination leading to allene can be attributed to the strong hindrance of the methyl rotor in the corresponding transition state.