Ion-molecule reactions in the gas phase. Collision energy influence on the competitive S(N)i/S(N)2 orientation from adduct ions of epimeric 1,2-indanediols prepared by ammonia chemical ionization

Under chemical ionization (CI) conditions, cis and trans 1,2-indanediols mainly react with the NH3/NH4+ system via a nucleophilic substitution process. In the CI source several mechanisms can occur yielding the substituted [M+NH4-H2O]+ ions at m/z 150. Collisionally activated reaction (CAR)spectra provide additional information concerning the structure of selected low kinetic energy species. These ion-molecule reactions with methylamine are performed for [M + NH4-H2O]+ ions in the collision cell of a triple quadrupole mass spectrometer. In this way, the nucleophilic substitution process is demonstrated to depend on the stereochemistry of the indanediol precursor. Stereospecific S(N)2 and S(N)i pathways operate for the cis and trans derivatives, respectively. Such competitive orientations are explained by considering hydrogen bonds and the steric hindrance due to the homobenzylic group.