Opening of the Diamondoid Cage upon Ionization Probed by Infrared Spectra of the Amantadine Cation Solvated by Ar, N₂, and H₂O

Abstract

Radical cations of diamondoids, a fundamental class of very stable cyclic hydrocarbon molecules, play an important role in their functionalization reactions and the chemistry of the interstellar medium. Herein, we characterize the structure, energy, and intermolecular interaction of clusters of the amantadine radical cation (Ama + , 1-aminoadamantane) with solvent molecules of different interaction strength by infrared photodissociation (IRPD) spectroscopy of massselected Ama + L n clusters, with L=Ar (n � 3) and L=N 2 and H 2 O (n = 1), and dispersion-corrected density functional theory calculations (B3LYPÀ D3/cc-pVTZ). Three isomers of Ama + generated by electron ionization are identified by the vibrational properties of their rather different NH 2 groups. The ligands bind preferentially to the acidic NH 2 protons, and the strength of the NH…L ionic H-bonds are probed by the solvation-induced red-shifts in the NH stretch modes. The three Ama + isomers include the most abundant canonical cage isomer (I) produced by vertical ionization, which is separated by appreciable barriers from two bicyclic distonic iminium ions obtained from cage-opening (primary radical II) and subsequent 1,2 H-shift (tertiary radical III), the latter of which is the global minimum on the Ama + potential energy surface. The effect of solvation on the energetics of the potential energy profile revealed by the calculations is consistent with the observed relative abundance of the three isomers. Comparison to the adamantane cation indicates that substitution of H by the electron-donating NH 2 group substantially lowers the barriers for the isomerization reaction.[a] M.

Infrared spectra and structures of protonated amantadine isomers: detection of ammonium and open-cage iminium ions

“…In addition to the AmaH + Ar dimer ( n = 1), we have also recorded the IRPD spectra of AmaH + Ar n with n = 2 and 3 in the XH stretch range. Similar to the case of Ama + Ar n , 48 the incremental spectral shifts and splittings upon tagging with further Ar ligands are very small and do not provide any additional information about the vibrational and isomer assignment. Hence, we do not discuss them in detail further here.…”

“…The IR spectra calculated for all AmaH + Ar isomers for each monomer ( I–IV ) show only minimal shifts of less than 11 ( I ), 4 ( II ), 7 ( III ), and 8 cm −1 ( IV ) in ν NH 2/3 , β NH 2/3 , γ NH 3 , and ν CN upon Ar-tagging, respectively. Similar to Ama + Ar, 47,48 the Ar ligands form in the most stable AmaH + Ar isomers weak and strongly nonlinear NH⋯Ar H-bonds ( R NH⋯Ar = 2.58–2.86 Å, ϕ ArHN = 119.8–165.4°), accompanied by additional van der Waals contacts to adjacent CH 2 /CH 3 groups ( R CH⋯Ar = 2.96–3.15 Å), yielding D 0 = 8.7–10.3 kJ mol −1 . As a result, the involved N–H bonds are slightly stretched (by 1 mÅ), while the involved C–H bonds are shortened (by 1 mÅ) and free C–H bonds can be elongated.…”

“…As shown recently, ionization of Ama can trigger a rearrangement reaction, producing via cage opening and subsequent 1,2 H-shift the bicyclic distonic Ama + ( III ) iminium radical cation (involving a barrier of V b = 123 kJ mol −1 ), whose C9 radical center is a good acceptor for a H radical. 47,48 Consequently, AmaH + ( II ) has also an open-cage structure with one CH 3 group (C3H 3 ) and a C9H group with its C–H bond oriented toward the nearly planar CNH 2 + group ( r C9H⋯C1 = 2.450 Å). As a result, the cage opens much more than for Ama + ( III ) ( θ C1C9C3 = 155.3° vs. 106.1°, r C1⋯C3 = 4.47 vs. 3.35 Å), 47 and the C–C bonds at the former C9 radical center drastically elongate ( r C3C9 = 1.482 vs. 1.529 Å, r C9C8 = 1.492 vs. 1.536 Å).…”

“…Experimentally, we apply IR photodissociation (IRPD) spectroscopy to mass-selected Ar-tagged AmaH + ions, whereby the weakly-bonded Ar ligand has essentially no impact on the structural, energetic, vibrational, and chemical properties of AmaH + , as recently demonstrated for the Ama + radical cation. 47,48 Despite its high pharmaceutical importance, no spectroscopic studies are available for isolated AmaH + . Mass spectrometry experiments provide information about its fragmentation and proton affinity.…”

“…46 In addition to its pharmaceutical use, the diamondoid derivative Ama is also of interest for studying ionizationinduced rearrangement processes of the adamantyl cage. 47,48 By replacing H with the electron-donating NH 2 group at the apical 1-position, 42,49 the chemical properties of diamond-like structures are mostly retained but the barrier to cage opening is strongly reduced. 47,48 The geometric, vibrational, and electronic properties of Ama are well-known from infrared (IR), Raman, and electron momentum spectroscopy.…”