Infrared Spectrum of the Amantadine Cation: Opening of the Diamondoid Cage upon Ionization

Radical cations of diamondoids, a fundamental class of highly stable cycloalkanes, are intermediates in functionalization reactions and possibly present in the interstellar medium. Herein, we characterize the structure of the radical cation of 1-amantadine (1-C 10 H 15 NH 2 +, Ama + ), the amino derivative of the parent adamantane (C 10 H 16 +, Ada + ), by infrared spectroscopy and density functional theory calculations. The structural isomers of Ama + produced by electron ionization are probed by infrared photodissociation of cold Ar-tagged ions. In addition to the canonical nascent Ama + isomer with an intact C 10 H 15 cage, we identify two distonic bicyclic iminium isomers in which the adamantyl cage opens upon ionization, one of which is lower in energy than the cage isomer. The reaction profile with barriers and intermediates for this cage-opening reaction are determined. Comparison with Ada + suggests that this type of ionization-induced cage-opening may be a common feature for diamondoids and important for their reactivity.