Aromatic Charge Resonance Interaction Probed by Infrared Spectroscopy

Abstract

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...

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

“…83 The employed dispersion-corrected functional accounts well for the electrostatic, induction, and dispersion forces of the investigated clusters, and reproduces their experimental binding energies and IR spectra to satisfactory accuracy. 45,49,68,[84][85][86][87][88][89][90][91] For example, the binding energy computed for W 2 (D 0 = 1103 cm À1 ) is in excellent agreement with the measured value (D 0 = 1105 AE 10 cm À1 ), 92 indicating that the H-bond interactions in the hydration networks are modeled accurately by this computational approach. All coordinates are allowed to relax during the search for stationary points, and their nature as minima or transition states are verified by harmonic frequency analysis.…”

The S∴π hemibond and its competition with the S∴S hemibond in the simplest model system: infrared spectroscopy of the [benzene-(H2S)n]+ (n = 1–4) radical cation clusters

“…Moreover, competition among multiple intermolecular interactions can be also studied by the choice of the component of the cluster. While for the S∴S hemibond and π∴π hemibond (ordinarily called “charge resonance”), detailed spectroscopic studies on their simple model cluster systems have been reported,13–18 no such a study has been performed for the S∴π hemibond. Then, in the present work, the radical cation clusters of benzene (Bz) and hydrogen sulfide, [Bz-(H 2 S) n ] + , ( n = 1–4), are studied by infrared (IR) spectroscopy in the SH and CH stretch regions.…”

Microhydration of ionized building blocks of DNA/RNA: infrared spectra of pyrimidine$$^{+}$$-$$(\hbox {H}_{2}\hbox {O})_{\text {1-3}}$$ clusters

“…a e-mail: dopfer@physik.tu-berlin.de (corresponding author) infrared (IR) spectroscopy and computational chemistry is one of the powerful approaches to probe such interactions at the microscopic level [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Herein, we apply this combined technique to investigate the microhydration network of the pyrimidine cation (Pym + ).…”