Electrospray ionization mass spectrometric investigation of ammonium ion complexes with anomeric 2,3-O-isopropylidene-1α- and -1β-D-ribofuranosyl azides: anomeric and kinetic isotope effects in ammonium affinities

[Methanol + ammonium acetate] solutions of anomeric 2,3‐O‐isopropylidene‐1α‐ and 1β‐ribofuranosyl azides were investigated by electrospray ionization mass spectrometry (ESI‐MS). The compounds included d6‐labeled and/or unlabeled isopropylidene groups that enable the identification of peaks characteristic of the ammonium‐attached monomeric (MNH4+), ammonium‐bound homodimeric ([M]2NH4+) and heterodimeric ([MNH4M1]+) complex ions in ESI mass spectra of solutions of a pair of compounds. The intensities of the product ion peaks obtained by the collisionally activated ammonium‐bound dimeric ions are related to the secondary isotope effect kα/kαd6 = 0.88 and kβ/kβd6 = 1.25 or to isotope plus anomeric effects kα/kβd6 = 1.43 and kβ/kαd6 = 0.59 in the ammonium affinities of these compounds. The calculations of solely anomeric effects in the ammonium affinities of α and β anomeric compounds obtained from the data presented previously give two series of values: kα/kβ = (kα/kαd6)(kαd6/kβ) = 1.49 and kαd6/kβd6 = (kαd6/kβ)(kβ/kβd6) = 2.12 or kα/kβ = (kα/kβd6)(kβd6/kβ) = 1.14 and kαd6/kβd6 = (kαd6/kα)(kα/kβd6) = 1.63. The disparities of these results indicate the different structures of hydrogen bonding in ammonium‐bound dimeric complexes which decompose to monomeric ions with different rate constants. Comparison of experimental results obtained by the qualitative approach of the kinetic method and ammonium affinities of these compounds calculated by the semi‐empirical molecular orbital method (AM1) show that the [MNH4M1]+ dimeric complex ions dissociate to the most stable MNH4+ and M1NH4+ monomeric ions. The obtained relative order of ammonium affinities of these compounds is: αd6 > α > β > βd6. Copyright © 2001 John Wiley & Sons, Ltd.