THE ADDITION OF WATER TO ETHYLENE AND TRANS-BUTENE RADICAL-CATION - MODEL SYSTEMS FOR THE REACTION OF ALKENE RADICAL CATIONS WITH NUCLEOPHILES

The reactions of one, two, or four water molecules with ethylene radic al cation and the reaction of one water molecule with trans-2-butene r adical cation have been investigated by ab initio calculations at the UMP2/631G level. in part, PMP4/6-311+G** single point energies have b een added. The reaction of one water molecule with ethylene radical ca tion occurs without barrier in the gas phase to form a distonic radica l cation. This intermediate species reacts readily with a second water molecule to interchange water molecules through two S(N)2-type reacti on pathways. The influence of solvation on the substitution barrier ha s been studied in various ways and has been shown to increase the gas phase substitution barrier approximately twofold in water. The thermod ynamic acidity of the intermediate distonic radical cation has been es timated using a thermodynamic cycle. From combined ab initio and solut ion simulation results, the acidity of the distonic ion is predicted t o be somewhat smaller as compared to that of protonated ethanol. The r eaction of trans-2-butene with water in the gas phase does not lead to the formation of a distonic radical cation. Instead, an ion-dipole co mplex is formed. This result is linked to the stabilization of alkene radical cations by alkyl substituents.