Spectroscopic Identification of Carbenium and Ammonium Isomers of Protonated Aniline (AnH+): IR Spectra of Weakly Bound AnH+−Ln Clusters (L = Ar, N2)

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.

Microhydrated clusters of a pharmaceutical drug: infrared spectra and structures of amantadineH+(H2O)n

Solvation of pharmaceutical drugs has an important effect on their structure and function. Analysis of infrared photodissociation spectra of amantadineH⁺(H₂O)_n=1-4 clusters in the sensitive OH, NH, and CH stretch range by quantum chemical calculations (B3LYP-D3/cc-pVTZ) provides a first impression of the interaction of this pharmaceutically active cation with water at the molecular level. The size-dependent frequency shifts reveal detailed information about the acidity of the protons of the NH₃⁺ group of N-protonated amantadineH⁺ (AmaH⁺) and the strength of the NH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. The preferred cluster growth begins with sequential hydration of the NH₃⁺ group by NH⋯O ionic H-bonds (n = 1-3), followed by the extension of the solvent network through OH⋯O H-bonds. However, smaller populations of cluster isomers with an H-bonded solvent network and free N-H bonds are already observed for n ≥ 2, indicating the subtle competition between noncooperative ion hydration and cooperative H-bonding. Interestingly, cyclic water ring structures are identified for n ≥ 3, each with two NH⋯O and two OH⋯O H-bonds. Despite the increasing destabilization of the N-H proton donor bonds upon gradual hydration, no proton transfer to the (H₂O)_n solvent cluster is observed up to n = 4. In addition to ammonium cluster ions, a small population of microhydrated iminium isomers is also detected, which is substantially lower for the hydrophilic H₂O than for the hydrophobic Ar environment.

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+ Ln clusters, with L=Ar (n≤3) and L=N2 and H2 O (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.

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

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. Recent studies reveal that the adamantyl cage (C₁₀H₁₅) of Ama can open upon ionization leading to distonic bicyclic iminium isomers, in addition to the canonical nascent Ama⁺ isomer. Herein, we study protonation of Ama using infrared photodissociation spectroscopy (IRPD) of Ar-tagged ions and density functional theory calculations to characterize cage and open-cage isomers of AmaH⁺ and the influence of the electron-donating NH₂ group on the cage-opening reaction potential. In addition to the canonical ammonium isomer (AmaH⁺(I)) with an intact adamantyl cage, we identify at least one slightly less stable protonated bicyclic iminium ion (AmaH⁺(II)). While the ammonium ion is generated by protonation of the basic NH₂ group, AmaH⁺(II) is formally formed by H addition to a distonic bicyclic iminium ion produced upon ionization of Ama and subsequent cage opening.

IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions

Protonation at the formyl oxygen atom of benzaldehydes leading to the formation of carboxonium ions yields two distinct isomers, depending on the relative orientation of the proton either cis or trans with respect to the hydrogen atom on the adjacent carbon. In this context, the IR multiple photon dissociation (IRMPD) spectra of protonated ortho, meta, and para-hydroxybenzaldehydes (OH-BZH⁺ ), delivered into the gas phase by electrospray ionization of hydro-alcoholic solutions, are reported in the 3200-3700 cm^-1 spectral range. This range is characteristic of O-H stretching modes and thus able to differentiate cis and trans carboxonium isomers. Comparison between IRMPD spectra and DFT calculations at the B3LYP/6-311++G(2df2p) level suggests that for both p-OH-BZH⁺ and m-OH-BZH⁺ only cis conformers are present in the ion population analyzed. For o-OH-BZH⁺ , IRMPD spectroscopy points to a mixture comprising one trans and more than one cis conformers. The energy barrier for cis-trans isomerization calculated for each OH-BZH⁺ isomer is a measure of the degree of π-electron delocalization. Furthermore, IRMPD spectra of p-OH-BZH⁺ , m-OH-BZH⁺ and protonated phenol (this last used as reference) were recorded also in the fingerprint range. Both the observed C-O and O-H stretching vibrations appear to be a measure of π-electron delocalization in the ions.

Intracluster proton transfer in protonated benzonitrile–(H2O)n≤6 nanoclusters: hydrated hydronium core for n ≥ 2

Infrared spectroscopy and density functional theory calculations of protonated benzonitrile–(H₂O)_n clusters reveal proton transfer to solvent for n ≥ 2 and the drastic effects of the aromatic dopant molecule on the network of H⁺(H₂O)_n+1.

Stepwise microhydration of aromatic amide cations: water solvation networks revealed by the infrared spectra of acetanilide+-(H2O)n clusters (n ≤ 3)

The structure and activity of peptides and proteins strongly rely on their charge state and the interaction with their hydration environment. Here, infrared photodissociation (IRPD) spectra of size-selected microhydrated clusters of cationic acetanilide (AA⁺, N-phenylacetamide), AA⁺-(H₂O)_n with n ≤ 3, are analysed by dispersion-corrected density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the stepwise microhydration process of this aromatic peptide model. The IRPD spectra are recorded in the informative X-H stretch (ν_OH, ν_NH, ν_CH, amide A, 2800-3800 cm^-1) and fingerprint (amide I-II, 1000-1900 cm^-1) ranges to probe the preferred hydration motifs and the cluster growth. In the most stable AA⁺-(H₂O)_n structures, the H₂O ligands solvate the acidic NH proton of the amide by forming a hydrogen-bonded solvent network, which strongly benefits from cooperative effects arising from the excess positive charge. Comparison with neutral AA-H₂O reveals the strong impact of ionization on the acidity of the NH proton and the topology of the interaction potential. Comparison with related hydrated formanilide clusters demonstrates the influence of methylation of the amide group (H → CH₃) on the shape of the intermolecular potential and the structure of the hydration shell.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

Protonation sites can be controlled by the electrospray source as written in the figure.

Gas-phase vibrational spectroscopy of triphenylamine: the effect of charge on structure and spectra

The effect of ionization by oxidation and protonation on the structure and IR spectrum of isolated, gas-phase triphenylamine (TPA) has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy in the fingerprint range from 600 cm⁻¹ to 1800 cm⁻¹ using an infrared free electron laser.

IR spectrum of the protonated neurotransmitter 2-phenylethylamine: dispersion and anharmonicity of the NH3(+)-π interaction

The structure and dynamics of the highly flexible side chain of (protonated) phenylethylamino neurotransmitters are essential for their function. The geometric, vibrational, and energetic properties of the protonated neutrotransmitter 2-phenylethylamine (H(+)PEA) are characterized in the N-H stretch range by infrared photodissociation (IRPD) spectroscopy of cold ions using rare gas tagging (Rg = Ne and Ar) and anharmonic calculations at the B3LYP-D3/(aug-)cc-pVTZ level including dispersion corrections. A single folded gauche conformer (G) protonated at the basic amino group and stabilized by an intramolecular NH(+)-π interaction is observed. The dispersion-corrected density functional theory calculations reveal the important effects of dispersion on the cation-π interaction and the large vibrational anharmonicity of the NH3(+) group involved in the NH(+)-π hydrogen bond. They allow for assigning overtone and combination bands and explain anomalous intensities observed in previous IR multiple-photon dissociation spectra. Comparison with neutral PEA reveals the large effects of protonation on the geometric and electronic structure.

Microsolvation of CH+ in helium: An ab initio study

In the present study, microsolvation and interaction of the CH[Formula: see text] cation with He[Formula: see text] clusters are investigated by means of ab initio calculations at MP2/aug-cc-pVTZ and QCISD/aug-cc-pVTZ levels. Stabilization energies of the studied complexes including basis set superposition error (BSSE) and zero point energy (ZPE) corrections are in the range of [Formula: see text][Formula: see text]kJ/mol and [Formula: see text][Formula: see text]kJ/mol. A good linear correlation is found between the stabilization energy and stretching frequency shift ([Formula: see text]) in the studied complexes. According to energy decomposition analysis, it is found that polarization effects are the major source of the attraction in these complexes.

Electronically excited state of neutral/protonated, indole/5-hydroxyinodole–water clusters: a theoretical study

Simulated IR-spectra of neutral, protonated indole–water clusters calculated at the RI-MP2, RI-CC2/aug-cc-pVDZ level of theory at the ground and S₁ excited states.

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

Hydration has a drastic impact on the structure and function of flexible biomolecules, such as aromatic ethylamino neurotransmitters. The structure of monohydrated protonated phenylethylamine (H(+) PEA-H2 O) is investigated by infrared photodissociation (IRPD) spectroscopy of cold cluster ions by using rare-gas (Rg=Ne and Ar) tagging and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Monohydration of this prototypical neurotransmitter gives an insight into the first step of the formation of its solvation shell, especially regarding the competition between intra- and intermolecular interactions. The spectra of Rg-tagged H(+) PEA-H2 O reveal the presence of a stable insertion structure in which the water molecule is located between the positively charged ammonium group and the phenyl ring of H(+) PEA, acting both as a hydrogen bond acceptor (NH(+) ⋅⋅⋅O) and donor (OH⋅⋅⋅π). Two other nearly equivalent isomers, in which water is externally H bonded to one of the free NH groups, are also identified. The balance between insertion and external hydration strongly depends on temperature.

UV photodissociation spectroscopy of cryogenically cooled gas phase host–guest complex ions of crown ethers

Crown ethers show a dramatic effect on the electronic spectra and fragmentation patterns of guest species.

Diastereo-specific conformational properties of neutral, protonated and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-amino-indanol by gas phase spectroscopy

The effects of ionisation and protonation on the geometric and electronic structure of a prototypical aromatic amino-alcohol with two chiral centres are revealed by IR and UV spectroscopy.

UV spectroscopy of cold ions as a probe of the protonation site

Where does the proton go?

Gas-phase basicities of polyfunctional molecules. Part 2: Saturated basic sites

The present article is the second part of a general overview of the gas-phase protonation thermochemistry of polyfunctional molecules. The first part of the review (Mass Spectrom. Rev., 2007, 26:775-835) was devoted to the description of the physico-chemical concepts and of the methods of determination, both experimental and theoretical, of gas-phase basicity. Several clues concerning the structural and energetic aspects of the protonation of isolated species have been emphasized. In the present article, specific examples are examined. The field of investigation is limited to molecules containing a "saturated" basic site, that is, nitrogen or oxygen atoms engaged in simple σ bonds with their neighboring. Aliphatic, cyclic and aromatic poly-amines, amino alcohols, alcohols, ethers, and hydroxyl-ethers, are successively presented.

IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters

IR spectra of phenol-Arn (PhOH-Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus pi bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, nuOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH-Ar2 as well as cationic PhOH+-Ar have a pi-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+-Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This pi-->H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The pi-bound nuOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound nuOH band. The analysis of the picosecond IR spectra demonstrates that (i) the pi-->H site switching is an elementary reaction with a time constant of approximately 7 ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100 cm(-1)), (iii) both the position and the width of the H-bound nuOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from pi bonding to H bonding in the PhOH+-Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.