Polycyclic aromatic hydrocarbons (PAHs) are suggested to occur in interstellar media and ice grains. It is important to characterize hydrated PAHs and their cations to explore their stability in interstellar and biological media. Herein, the infrared photodissociation (IRPD) spectrum of the naphthalene-HO radical cation (Np-HO) recorded in the O-H and C-H stretch range is analysed by dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level to determine its structure and intermolecular bonding. Monohydration of Np in its A ground electronic state leads to the formation of a bifurcated CHO ionic hydrogen bond (H-bond), in which the lone pairs of HO bind to two adjacent CH proton donors of the two aromatic rings. The frequency-dependent branching ratios observed for IRPD of cold Np-HO-Ar clusters allows the estimation of the dissociation energy of Np-HO as D ∼ 2800 ± 300 cm. The monohydration motif of Np differs qualitatively from that of the benzene cation in both structure and binding energy, indicating the strong influence of the multiple aromatic rings on the hydration of PAH cations. This difference is rationalized by natural bond orbital analysis of the ionic H-bond motif. Comparison with neutral Np-HO reveals the large change in structure and bond strength of the hydrated PAHs upon ionization. While neutral Np-HO is stabilized by weak π H-bonds (OHπ, π-stacking), strong cation-dipole forces favour a planar bifurcated CHO ionic H-bond in Np-HO.
The evolution of the microhydration network around a prototypical PAH+ cation is determined by infrared spectroscopy of size-selected clusters and density functional theory calculations.
Charge resonance is as trong attractive intermolecular force in aromatic dimer radical ions.D espite its importance,this fundamental interaction has not been characterized at high resolution by spectroscopyo fi solated dimers.W e employv ibrational infrared spectroscopyo fc old aromatic pyrrole dimer cations to precisely probe the charge distribution by measuring the frequency of the isolated N À Hstretch mode (n NH ). We observe al inear correlation between n NH and the partial charge qo nt he pyrrole molecule in different environments.S ubtle effects of symmetry reduction, such as substitution of functional groups (here pyrrole replaced by Nmethylpyrrole) or asymmetric solvation (here by an inert N 2 ligand), shift the charge distribution towardt he moiety with lower ionization energy.T his general approach provides ap recise experimental probe of the asymmetry of the charge distribution in such aromatic homo-and heterodimer cations.The intermolecular interactions of aromatic p electrons are important for chemical and biological recognition. [1] Radical ions of arenes are involved in chemical reaction mechanisms, charge transport of modern (bio-)organic materials (organic electronics), and radiation damage in DNAand proteins. [2] In addition to p H-bonding,c ation/anion-p,a nd p-p stacking interactions, [3] the charge-resonance (CR) interaction is af undamental and very strong force in charged arene dimers. [4] Forc ations,t his stabilization arises from sharing the positive charge between two identical or different molecules with the same or comparable ionization energies in a p-stacked dimer.I nahomodimer cation (A 2 + ), the positive charge is equally shared between both molecules, while in ah eterodimer (AB + )t he partner with the lower ionization energy (IE) carries more positive charge.I nt he simplest description, the CR interaction in A 2 + causes as plitting between the two electronic states described by Y AE = Y(A + )Y(A) AE Y(A)Y(A + ); the symmetric Y + ground state is stable and the destabilized antisymmetric Y À state is repulsive.The splitting between both electronic states is twice the stabilization energy of the ground state (ca. 50-100 kJ mol À1 ). Thei nteraction is strongest for A 2 + homodim-ers,a nd decreases in AB + as the difference in the IEs of the two molecules increases (DIE = IE A ÀIE B ). This description of the CR interaction in arene dimer ions is similar to the one employed for odd electron chemical bonds in radical ions (hemibonds) [5] and exciton splitting in delocalized electronic excitation in neutral dimers. [6] In the condensed phase, p-stacked dimer cations of aromatic and polycyclic aromatic hydrocarbons were first detected by electron spin resonance [7] and optical absorption spectroscopy of the intense and broad CR transition connecting the two Y AE states. [7d, 8] Thelatter electronic transition occurs in the low-energy part of the optical spectrum (in the red to near-infrared near 1 mm) and provides adirect measure for the CR splitting.Inthe gas phase,the binding e...
Aromatic hydrocarbons and their protonated ions are important constituents of the interstellar medium (ISM). The recent discovery of benzonitrile (BN; cyanobenzene, C6H5CN) in the ISM suggests that its protonated ion (H+BN) is also present. Herein, we present vibrational signatures of H+BN obtained via infrared photodissociation (IRPD) spectra of its clusters with up to four nonpolar ligands (L = Ar/N2) recorded in the NH (ν NH) and CH (ν CH) stretch range. Protonation of BN occurs at the N atom of the nitrile group. Systematic complexation shifts (Δν NH) observed in the IRPD spectra of H+BN-L n are assigned to cluster structures by comparison to quantum chemical calculations. In the most stable H+BN-L n structures, the first ligand (n = 1) forms a NH+… L ionic hydrogen bond (H-bond), while additional ligands (n = 2–4) are attached to the aromatic ring via π stacking. For L = Ar, a less stable π-bonded H+BN-Ar isomer is also detected, and its IR spectrum provides an accurate experimental estimate of ν NH = 3555 ± 3 cm−1 for bare H+BN, an intense characteristic fingerprint of this ion in the 3 μm range. Comparison of C6H5CNH+ with HCNH+ and CH3CNH+ reveals that the acidity of the NH proton in RCNH+ ions increases in the order R = C6H5 < CH3 < H.
Solvation-dependent intracluster proton transfer (ICPT) within bare and Ar-tagged protonated naphthalene− (water) n clusters, H + (Np−W n ) with n ≤ 3, is characterized by infrared photodissociation (IRPD) spectroscopy in a supersonic plasma expansion. IRPD spectra of size-selected clusters recorded in the CH and OH stretch range (2750−3800 cm −1 ) are analyzed with dispersion-corrected density functional theory (DFT) calculations (B3LYP-D3/aug-cc-pVTZ) to determine both the protonation site and the structure of the hydration network. Ar tagging of H + (Np−W n ) leads to colder spectra with higher spectral resolution. The position of the excess proton is controlled by a subtle balance between the difference in proton affinity (PA) of Np and W n and the involved solvation energies. For n = 1, the excess proton is localized on the Np ring, leading to a H + Np−W structure with a bifurcated CH•••O ionic H-bond, because of the large difference in PA of Np and W. For n = 2, ICPT occurs, and the cluster has a structure in which a symmetric Zundel ion is connected to Np via two strong OH•••π ionic H-bonds. Because of the similar PA values of W 2 and Np, the energetics of the ICPT is largely decided by the higher solvation energy in favor of Np−H + W 2 as compared to H + Np−W 2 . For n ≥ 3, the PA of W n substantially exceeds the one of Np, leading to ICPT. Attachment of the bulky planar Np ring to H + W n causes an increasing perturbation of the bare H + W n cluster with size by symmetry reduction and the strong OH•••π H-bonds. Comparison of H + (Np−W n ) with the related H + (Bz−W n ) clusters (Bz = benzene) indicates the implications of extending the aromatic π-electron system on both the critical threshold size for ICPT (n c = 1 for Bz and n c = 2 for Np) and the structure of the hydration network.
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