UNIMOLECULAR DECOMPOSITIONS OF IONIZED ISOPROPYL METHYL-ETHER - AN AB-INITIO AND RRKM STUDY

In connection with the unimolecular decompositions of ionized isopropy l methyl ether, namely, methane elimination and methyl radical loss, a b initio molecular orbital calculations at the UMP2, QCISD, and QCISD( T) levels of theory with the 6-31G(d) and 6-31G(d,p) basis sets have b een used to investigate the relevant parts of the C4H10O.+ ground-stat e potential energy surface. The calculations demonstrate that at inter nal energies below the threshold for loss of a methyl radical, bond di ssociation to an ion-neutral complex, [CH3O+=CHCH3 CH3.], and methane elimination therefrom both occur. Sets of reactant and transition stat e frequencies calculated at the UMP2/ 6-31G(d) level, as well as energ etics based on QCISD(T)/6-31G(d,p)//UMP2/6-31G(d) + ZPVE calculations, were employed in RRKM calculations. By invoking quantum-mechanical ba rrier tunneling in the methane elimination channel, we were able to re produce several experimental observations: (a) the methane elimination reaction dominates at internal energies near the threshold for dissoc iation; (b) the onsets for the methane and methyl losses differ by onl y a few tenths of a kilocalorie per mole; and (c) there is a very stro ng primary isotope effect of about 60 on the CH2D2 and CHD3 losses fro m CH3OCH(CHD2)(2)(.+).