(The) transition states studies of $C_2H_5X$ and $CH_2CHNO$ with semiempirical and ab initio MO methods

Abstract

The transition states of ethyl halides and nitrosoethene are studied with semiempirical (MINDO/3, MNDO, AM1, PM3) and ab initio (Gaussian $86,631G^{**}$) MO methods, respectively. When we calculate the transition states of ethyl halides with semiempirical MO methods, the optimized geometrical structures are good agreement with ab initio results, but the activation energies do not agree with experimental and ab initio results. The activation energies and optimized geometrical structure of ethyl fluoride with semiempirical MO methods show large deviation from experimental and 4-31G values. $6-31G^{**}$ calculations for nitrosoethene and nitrosoethyne show them unstable. Nitrosoethene is only 20.2 kcal/mol more stable than it``s radicals ($C_2H_3$ + NO) and nitrosoethyne is only 27.6 kcal/mol more stable than it``s radicals(C$_2$H + NO) with $6-13G^{**}$ calculation. Thus these molecules nitrosoethene and nitrosoethyne) will react even at low temperature. In our studies, nitrosoethene decompose to it``s radicals and then reacts via cyclic intermediate proposed by Sherwood.