Infrared spectrum of the 1-cyanoadamantane cation: evidence of hydrogen transfer and cage-opening upon ionization

The radical cations of diamondoids are important intermediates in their functionalization reactions and are also candidates as carriers for astronomical absorption and emission features. Although neutral diamondoids have been studied extensively, information regarding their radical cations is largely lacking, particularly for functionalized diamondoid derivatives. Herein, we characterize the structure of the 1-cyanoadamantane radical cation (C₁₀H₁₅CN⁺, AdCN⁺) using infrared photodissociation (IRPD) spectroscopy of mass selected AdCN⁺N₂ clusters in the XH stretch range (2400-3500 cm^-1) and dispersion-corrected density functional theory calculations (B3LYP-D3BJ/cc-pVTZ). A group of three distinct CH stretch bands are observed in the 2800-3000 cm^-1 range, in addition to a highly redshifted absorption at 2580 cm^-1 attributed to the acidic CH proton predicted by calculations. An unexpected broad absorption peaking at 3320 cm^-1 is also detected and assigned to an NH stretch mode based on its width and frequency. Calculations indicate that hydrogen atom transfer (HAT) from the adamantyl cage (C₁₀H₁₅, Ady) to the N atom of the CN group yields lower energy structures, with an open-cage isomer exhibiting such hydrogen transfer being the global minimum on the potential energy surface. The energy barriers involved in the formation of this open-cage isomer are also lower than those calculated for generation of the analogous open-cage 1-amantadine cation isomer which has previously been identified by IRPD. The combined consideration of IRPD spectra and calculations indicates a major population of the nascent canonical closed-cage isomer and a smaller population of the global minimum isomer featuring both cage-opening and hydrogen transfer.