Hydration of biomolecules and pharmaceutical compounds has a strong impact on their structure, reactivity, and function. Herein, we explore the microhydration structure around the radical cation of the widespread pharmaceutical...

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C−H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C 10 H 16 + –H 2 O, Ad + –W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion‐corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn–Teller distorted Ad + via a strong CH⋅⋅⋅O ionic H‐bond supported by charge–dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad + in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν 3 band of W, resulting from weak angular anisotropy of the Ad + –W potential.

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.

The infrared spectrum of the disilane cation, Si 2

The protonated form of amantadine (1-C10H15NH2, Ama), the amino derivative of adamantane (C10H16, Ada), is a wide-spread antiviral and anti-Parkinsonian drug and plays a key role in many pharmaceutical processes....

The IR spectrum of Si3H8(+) ions produced in a supersonic plasma molecular beam expansion of SiH4, He, and Ar is inferred from photodissociation of cold Si3H8(+)-Ar complexes. Vibrational analysis of the spectrum is consistent with a Si3H8(+) structure (2(+)) obtained by a barrierless addition reaction of SiH4 to the disilene ion (H2Si=SiH2(+)) in the silane plasma. In this structure, one of the electronegative H atoms of SiH4 donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric Si-H-Si bridge of the 2(+) isomer with a bond energy of approximately 60 kJ mol(-1) is characteristic for a weak three-center two-electron bond, which is identified by its strongly IR active asymmetric Si-H-Si stretching fundamental at about 1765 cm(-1). The observed 2(+) isomer is calculated to be only a few kJ mol(-1) less stable than the global minimum structure of Si3H8(+) (1(+)), which is derived from vertical ionization of trisilane. Although more stable, 1(+) is not detected in the measured IR spectrum of Si3H8(+)-Ar, and its lower abundance in the supersonic plasma is rationalized by the production mechanism of Si3H8(+) in the silane plasma, in which a high barrier between 2(+) and 1(+) prevents the efficient formation of 1(+). The potential energy surface of Si3H8(+) is characterized in some detail by quantum chemical calculations. The structural, vibrational, electronic and energetic properties as well as the chemical bonding mechanism are investigated for a variety of low-energy Si3H8(+) isomers and their fragments. The weak intermolecular bonds of the Ar ligands in the Si3H8(+)-Ar isomers arise from dispersion and induction forces and induce only a minor perturbation of the bare Si3H8(+) ions. Comparison with the potential energy surface of C3H8(+) reveals the differences between the silicon and carbon species.

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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.