Opening of the Diamondoid Cage upon Ionization Probed by Infrared Spectra of the Amantadine Cation Solvated by Ar, N2 , and H2 O

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 mass‐selected Ama⁺L_n clusters, with L=Ar (n≤3) and L=N₂ and H₂O (n=1), and dispersion‐corrected density functional theory calculations (B3LYP−D3/cc‐pVTZ). Three isomers of Ama⁺ generated by electron ionization are identified by the vibrational properties of their rather different NH₂ groups. The ligands bind preferentially to the acidic NH₂ 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 NH₂ group substantially lowers the barriers for the isomerization reaction.

Real-time observation of photoionization-induced water migration dynamics in 4-methylformanilide-water by picosecond time-resolved infrared spectroscopy and ab initio molecular dynamics simulations

A novel time-resolved pump-probe spectroscopic approach that enables to keep high resolution in both the time and energy domain, nanosecond excitation-picosecond ionization-picosecond infrared probe (ns-ps-ps TRIR) spectroscopy, has been applied to the trans-4-methylformanilide-water (4MetFA-W) cluster. Water migration dynamics from the CO to the NH binding site in a peptide linkage triggered by photoionization of 4MetFA-W is directly monitored by the ps time evolution of IR spectra, and the presence of an intermediate state is revealed. The time evolution is analyzed by rate equations based on a four-state model of the migration dynamics. Time constants for the initial to the intermediate and hot product and to the final product are obtained. The acceleration of the dynamics by methyl substitution and the strong contribution of intracluster vibrational energy redistribution in the termination of the solvation dynamics is suggested. This picture is well confirmed by the ab initio on-the-fly molecular dynamics simulations. Vibrational assignments of 4MetFA and 4MetFA-W in the neutral (S₀ and S₁) and ionic (D₀) electronic states measured by ns IR dip and electron-impact IR photodissociation spectroscopy are also discussed prior to the results of time-resolved spectroscopy.

Ionization-Induced π → H Site Switching in Resorcinol-Arn (n = 1 and 2) Clusters Probed by Infrared Spectroscopy

Infrared (IR) spectra of resorcinol (Rs)-Ar_n clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S₀ and D₀) by IR dip and resonance-enhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S₀ keep their frequency regardless of the number of Ar atoms and the conformation of the OH groups in Rs (rotamers RsI and RsII), demonstrating that the Ar atoms are attached to the aromatic π-ring (π-bound structure) in S₀. In the D₀ state, the IR spectra of Rs⁺-Ar_n reflect the difference in the Rs conformations (RsI⁺ and RsII⁺). For n = 1, the IR spectra of both rotamers are almost the same as those of the corresponding monomer cations, indicating that Ar ligands essentially remain π-bonded after ionization. In contrast, the IR spectra of Rs⁺-Ar₂ show hydrogen-bonded and free OH stretching vibrations, demonstrating that for a significant fraction of the clusters, the Ar atoms migrate from the π-bound site to the OH groups. The ionization-induced π → H migration yields are not unity for both rotamers RsI⁺-Ar₂ and RsII⁺-Ar₂. This result is in sharp contrast to phenol⁺-Ar₂, in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs⁺-Ar₂ is discussed in terms of the number of OH binding sites and Franck-Condon factors. The ionization excess energy dependence of the IR spectra of Rs⁺-Ar₂ and its Rs⁺-Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.

Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde: Theoretical Insights into Photoswitchability of Protonated Systems

The photoswitchability of a protonated aromatic compound (2-aminobenzaldehyde, 2ABZH⁺) in its individual and microhydrated states has been addressed based on the RI-MP2/RI-CC2 theoretical methods. Our calculated results give insight into the ultrafast nonradiative deactivation mechanism of the 2ABZH⁺, driven by a conical intersection between the S₁/ S₀ potential energy surfaces. Also, it has been predicted that protonation accompanies a significant blue shift effect on the first ¹ππ* excited state of 2ABZ by 0.87 eV (at least 50 nm).

Microhydration of PAH+ cations: evolution of hydration network in naphthalene+-(H2O) n clusters (n ≤ 5)

The interaction of polycyclic aromatic hydrocarbon molecules with water (H2O = W) is of fundamental importance in chemistry and biology. Herein, size-selected microhydrated naphthalene cation nanoclusters, Np+-W n (n ≤ 5), are characterized by infrared photodissociation (IRPD) spectroscopy in the C-H and O-H stretch range to follow the stepwise evolution of the hydration network around this prototypical PAH+ cation. The IRPD spectra are highly sensitive to the hydration structure and are analyzed by dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) to determine the predominant structural isomers. For n = 1, W forms a bifurcated CH···O ionic hydrogen bond (H-bond) to two acidic CH protons of the bicyclic ring. For n ≥ 2, the formation of H-bonded solvent networks dominates over interior ion solvation, because of strong cooperativity in the former case. For n ≥ 3, cyclic W n solvent structures are attached to the CH protons of Np+. However, while for n = 3 the W3 ring binds in the CH···O plane to Np+, for n ≥ 4 the cyclic W n clusters are additionally stabilized by stacking interactions, leading to sandwich-type configurations. No intracluster proton transfer from Np+ to the W n solvent is observed in the studied size range (n ≤ 5), because of the high proton affinity of the naphthyl radical compared to W n . This is different from microhydrated benzene+ clusters, (Bz-W n )+, for which proton transfer is energetically favorable for n ≥ 4 due to the much lower proton affinity of the phenyl radical. Hence, because of the presence of polycyclic rings, the interaction of PAH+ cations with W is qualitatively different from that of monocyclic Bz+ with respect to interaction strength, structure of the hydration shell, and chemical reactivity. These differences are rationalized and quantified by quantum chemical analysis using the natural bond orbital (NBO) and noncovalent interaction (NCI) approaches.

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

Sequential microhydration of cationic 5-hydroxyindole (5HI+): infrared photodissociation spectra of 5HI+–Wn clusters (W = H2O, n ≤ 4)

The hydrogen-bonding properties of the acidic OH and NH groups of the 5-hydroxyindole cation are probed by infrared spectroscopy and DFT calculations of its microhydrated clusters.

Infrared spectroscopy of hydrated polycyclic aromatic hydrocarbon cations: naphthalene+–water

The combination of infrared spectroscopy and quantum chemical calculations unravels the salient properties of the bifurcated CH⋯O ionic hydrogen bond typical for the PAH⁺–H₂O interaction.