Role of Hydrogen Bonding on the Reactivity of Thiyl Radicals: A Mass Spectrometric and Computational Study Using the Distonic Radical Ion Approach.

Experimental and computational quantum chemistry investigations of the gas-phase ion-molecule reactions between the distonic ions (+)H3N(CH2)nS(•) (n = 2-4) and the reagents dimethyl disulfide, allyl bromide, and allyl iodide demonstrate that intramolecular hydrogen bonding can modulate the reactivity of thiyl radicals. Thus, the 3-ammonium-1-propanethiyl radical (n = 3) exhibits the lowest reactivity of these distonic ions toward all substrates. Theoretical calculations on this distonic ion highlight that its most stable conformation involves a six-membered ring configuration, and that it has the strongest intramolecular hydrogen bond. In addition, the calculations indicate that the barrier heights for radical abstraction by this hydrogen-bond-stabilized 3-ammonium-1-propanethiyl radical are the highest among the systems examined, consistent with the experimental observations.