Theoretical and experimental studies on concerted elimination of 1, 2‐bromochloroethane monocation to C₂H₄⁺ and BrCl

Abstract

Ab initio calculations for the concerted elimination of 1, 2-bromochloroethane monocation (1,2-C 2 H 4 BrCl +) to C 2 H 4 + and BrCl are performed using the Minnesota density functional M06-2X and the def2-TZVP basis set. Ab initio calculations are also carried out for the concerted elimination of 1,2-C 2 H 4 BrCl + to C 2 H 4 and BrCl + because the positive charge can be assigned to either moiety in the dissociative ionization process of 1,2-C 2 H 4 BrCl +. Our results demonstrate that the concerted elimination channel of 1,2-C 2 H 4 BrCl + to C 2 H 4 + and BrCl is preferred and that 1,2-C 2 H 4 BrCl + surpasses two energy barriers and then separates into C 2 H 4 + + BrCl through an asynchronous concerted process. Experimentally, we confirm that this elimination channel is from the dissociative ionization process of 1,2-C 2 H 4 BrCl + by using dc-slice imaging technique. The time-of-flight mass spectra of 1,2-bromochloroethane induced by femtosecond laser pulses show that C 2 H 4 + occurs at a laser intensity of 6.0 × 10 13 W/cm 2 , and BrCl + occurs at a higher laser intensity than C 2 H 4 +. This finding is consistent with the theoretical result that the appearance energy of C 2 H 4 + is lower than that of BrCl +. As such, the low-velocity component of BrCl + is absent from our sliced images.