Pi-electron Density Research Articles

Sensing of Pb (II) ion by micro and nanocarbon materials- influence of functionality and pi-electron density

In the aspect of pollution due to heavy metal ions, especially Pb (II) ions, there is an urgent need for simple, effective materials and processes for sensing the ions. The present study approaches the sensing of Pb (II) ions by non-metal (O, N and S) doped micro and nanoscale Carbon materials. The unique emission (Fluorescence) properties of the as-prepared doped samples of graphene oxides and Carbon Microspheres were tapped in the selective sensing process of nanomolar to the micromolar concentration of Pb (II) ions. The role-play of Carbon dimensionality, functionality and sp2 Carbon density (pi electron density) in the sensing mechanism was evaluated. The study provided an overview of fundamental aspects of heavy metal ion sensing by Carbon-based nanomaterials (CBNs).

Synthesis of Aryl‐Functionalized Benzo[d,e]Benzo[4,5]İmidazo[2,1‐a]Isoquinolin‐7‐One Derivatives: DNA Intercalation Causing Cytotoxicity

AbstractThis study focuses on the cytotoxic effect properties of 4‐aryl‐functionalized benzo[d,e]benzo[4,5]imidazo[2,1‐a]isoquinolin‐7‐one (4‐X−Ar−BBI). The electron‐withdrawing and electron‐donating substituents located at the para position of the phenyl ring attached to the heterocyclic structure have altered the pi electron density within the structure. This change in electron density has influenced the intercalation ability of the heterocyclic structure and has impacted the intensity of its interaction with DNA. The 4‐aryl‐benzo[d,e]benzo[4,5]imidazo[2,1‐a]isoquinolin‐7‐one derivatives (4‐X−Ar−BBI) were prepared via the Suzuki–Miyaura cross‐coupling reaction. The optimum reaction conditions were determined to be 70 °C for 5 hours using catalyst 1 and K2CO3, resulting in a yield of 96 % of 4‐H−Ar−BBI. Characterization of all compounds (4‐X−Ar−BBI) was performed using Fourier Transform Infrared spectroscopy, mass spectrometry, and 1H‐ and 19F‐NMR spectroscopies. The cytotoxic activities of the compounds (4‐X−Ar−BBI) were investigated against cancer cell lines, specifically HeLa (human cervix adenocarcinoma epithelial), PC3 (human prostate), and MCF‐7 (human breast). The stability of 4‐CN−Ar−BBI and the binding modes between 4‐CN−Ar−BBI and DNA were investigated via 1H‐NMR spectroscopy and UV‐Vis spectroscopy, respectively. 4‐CH3−3‐NO2−Ar−BBI exhibited the most potent cytotoxic activity against MCF‐7, and 4‐F−Ar−BBI showed the highest activity against HeLa, with IC50 values of 3.675 and 7.370 μM, respectively.

Role of Pi-Electron Density at the Interface of Small Molecule-Sensitized Solar Cells

Conventional dye-sensitized solar cells (DSSCs) involving charge-transfer interfaces face charge injection losses and offsets for TiO2-sensitizer band alignment. The direct charge-transfer mechanism in DSSCs with catechol (CT, 1,2-benzenediol)-based compounds minimizes the injection losses and eliminates band alignment issues, although the photovoltaic performance of the corresponding device is very poor due to the ultrafast (picosecond) recombination of photoexcited electrons. Just as in a natural photosystem, structural selectivity toward inhibition of this recombination needs to be defined. The interfacial electron density and back electron transfer kinetics at the molecular sensitizer–TiO2 interface play a significant role in the overall energy conversion efficiency. Herein, we identified, for the first time, that the π-electron cloud at the sensitizer–TiO2 interface facilitates the degree of recombination. Comparative density functional theory analyses confirmed that these electron clouds act as large recombination sites. Luminol (LM) and isoluminol (ILM) were employed as “small molecule” sensitizers without the cloud, having a secondary amine linker, which increased the photoenergy conversion efficiency of the single-step sensitization-based photovoltaic cell (type-II DSSC) by reducing the recombination. The device with LM exhibited a power-conversion efficiency (PCE) of ca. 1.11% (representing 363% improvement when compared to CT), the highest ever reported in this category. This understanding is insightful for the design of novel small molecular sensitizers for future DSSCs.

Is Hammett Correlation of Dipole Moments of <i>mono</i>-Substituted Benzenes Helpful to Distinguish <i>o,p</i>-Directing Groups from <i>m</i>-Directing Groups in an Aromatic Electrophilic Substitution? A Chemical Education Perspective

In chemistry literature it is so far achieved to distinguish o,p-directing groups from m-directing groups purely on the basis of only relative yields of ortho, para and meta electrophilic substituted products in the benzene ring and it is also based on their ability to disturb the pi-electron density at the carbons. It is for the first time in chemistry literature using dipole moment data a linear free energy relationship (LFER), Hammett equation is used to distinguish o,p-directing groups from m-directing groups. This could be achieved by constructing Hammett plots with dipole moments versus Hammett para substituent constants for electron donating groups and Hammett meta substituent constants for electron withdrawing groups. Good straight lines are obtained with correlation coefficients close to 0.9 with certainly an unmistakable trend.

Novel Poly(ionic liquid) Augmented Membranes for Unconventional Aqueous Phase Applications in Fractionation of Dyes and Sugar.

Poly(ionic) liquid (PIL) augmented membranes were fabricated through self-polymerization of 2-vinyl pyridine and 4-vinyl pyridine followed by dopamine triggered polymerization and bridging with inert polyamide support. The resulting membranes acquired a positive surface charge with a high degree of hydrophilicity. Fourier transformed Infra-red (FTIR) and Energy dispersive X-ray (EDX) spectroscopic investigation revealed the successful augmentation of PIL surface layer, whereas surface morphology was investigated through scanning electron microscopy (SEM) imaging. This manuscript demonstrates pi electron-induced separation of dyes with the trend in permeability: Coomassie Brilliant Blue G (CBBHG) > Remazol Brilliant Blue R (RBBR) > Eichrome Black T (EBT) > Congo Red (CR). CBBG exhibited extended conjugation over large aromatic domain. RBBR and EBT were associated withtheelectron-donating -NH2 group and electron-withdrawing -NO2 group, respectively, hence pi electron density on aromatic ring varied. The steric repulsion between two pairs of ortho hydrogens (Hs) in biphenyl moieties of CR resulted in deviation of planarity and hence aromaticity leading to the lowest permeability. The sugar fractionation followed the trend: Galactose > Mannose > Fructose > Glucose > Xylose. More hydroxyl (-OH) groups in sugars and their conformational alignment in the same direction, exhibited more lone pair of electrons leading to more interaction with PIL and hence better permeability. Pentose showed poorer permeation than hexose, whereas aldose showed better permeation than ketose.

Determination of Excited Singlet-State Dipole Moments of Methoxy and Dimethylamino Substituted Benzylidenebenzosuberones Using Solvatochromic Method

The solvent effects on the electronic absorption and emission fluorescence spectra for a series of chalcone cyclic analogues were studied. The singlet-state excited dipole moments and the ground state dipole moments of the cyclic chalcone analogues E-2- benzylidene-1-benzosuberone E-2-(4′-methoxybenzylidene)-1-benzosuberone E-2-(4′-dimethylaminobenzylidene)-1-benzosuberone were calculated by using solvatochromic shift method by means of equations using the variations of Stokes’ shift with the solvent's dielectric constant and refractive index values. It was found that the excited state dipole moments calculated by the solvatochromic shift method were greater than the ground state dipole moments indicating a substantial redistribution of the pi-electron densities in a more polar excited state for each derivative.

Picosecond Protein Dynamics: The Origin of the Time-Dependent Spectral Shift in the Fluorescence of the Single Trp in the Protein GB1

How a biological system responds to a charge shift is a challenging question directly relevant to biological function. Time-resolved fluorescence of a tryptophan residue reflects protein and solvent response to the difference in pi-electron density between the excited and the ground state. In this study we use molecular dynamics to calculate the time-dependent spectral shift (TDSS) in the fluorescence of Trp-43 in GB1 protein. A new computational method for separating solvent, protein, and fluorophore contributions to TDSS is applied to 100 nonequilibrium trajectories for GB1 in TIP3P water. The results support several nontrivial conclusions. Both longitudinal and transverse relaxation modes of bulk solvent contribute to the TDSS in proteins. All relaxation components slower than the transverse relaxation of bulk solvent have significant contributions from both protein and solvent, with a negative correlation between them. Five exponential terms in the TDSS of GB1 are well separated by their relaxation times. A 0.036 ps term is due to both solvent (60%) and protein (40%). Two exponential terms represent longitudinal (tau(L) approximately = 0.4 ps) and transverse (tau(D) approximately = 5.6 ps) relaxation modes of TIP3P water. A 131 ps term is attributable to a small change in the tertiary structure, with the alpha-helix moving 0.2 A away from the beta-strand containing Trp-43. A 2580 ps term is due to the change in the conformation of the Glu-42 side chain that brings its carboxyl group close to the positively charged end of the excited fluorophore. Interestingly, water cancels 60% of the TDSS resulting from this conformational change.

DFT Mechanistic Study on Diene Metathesis Catalyzed by Ru‐Based Grubbs–Hoveyda‐Type Carbenes: The Key Role of π‐Electron Density Delocalization in the Hoveyda Ligand

The catalytic activity and catalyst recovery of two heterogenized ruthenium-based precatalysts (H and NO(2)(4)) in diene ring-closing metathesis have been studied by means of density functional calculations at the B3LYP level of theory. For comparison and rationalization of the key factors that lead to higher activities and higher catalyst recoveries, four other Grubbs-Hoveyda complexes have also been investigated. The full catalytic cycle (catalyst formation, propagation, and precatalyst regeneration) has been considered. DFT calculations suggest that either for the homogeneous and heterogenized systems the activity of the catalysts mainly depends on the ability of the precursor to generate the propagating carbene. This ability does not correlate with the traditionally identified key factor, the Ru...O interaction strength. In contrast, precatalysts with lower alkoxy-dissociation energy barriers and lower stabilities compared with the propagating carbene also present larger C1-C2 bond length (i.e., lower pi character of the C-C bond that exists between the metal-carbene (Ru=C) and the phenyl ring of the Hoveyda ligand). Catalyst recovery, regardless of whether a release-return mechanism occurs or not, is also mainly determined by the pi delocalization. Therefore, future Grubbs-Hoveyda-type catalyst development should be based on fine-tuning the pi-electron density of the phenyl moiety, with the subsequent effect on the metalloaromaticity of the ruthenafurane ring, rather than considering the modification of the Ru...O interaction.

Binary complexes of tertiary amines with phenylacetylene. Dispersion wins over electrostatics

The structures of the binary complexes between phenylacetylene and several tertiary amines viz., triethylamine, 1-ethylpiperidine, 1-ethylpiperazine, 1-azabicyclo[2.2.2]octane, and 1,4-diazabicyclo[2.2.2]octane were inferred using infrared-optical double resonance spectroscopy. The IR spectra in the acetylenic C-H stretching region clearly rule out the formation of electrostatic dominated C-HN hydrogen bonded complexes. The IR spectra also point to the fact that all the five tertiary amines interact with the extended pi electron density of the phenylacetylene moiety, leading to the formation of multidentate C-Hpi hydrogen bonded complexes. Additionally a very weak electrostatic C-HN hydrogen bond enhances the stability of the complex marginally. The multidentate C-Hpi hydrogen bonded complexes are stabilized by a substantial contribution from the dispersion energy.

Effect of CO on the Oxidative Addition of Arene CH Bonds by Cationic Rhodium Complexes

Sequential addition of CO molecules to cationic aryl-hydrido Rh(III) complexes of phosphine-based (PCP) pincer ligands was found to lead first to C-H reductive elimination and then to C-H oxidative addition, thereby demonstrating a dual role of CO. DFT calculations indicate that the oxidative addition reaction is directly promoted by CO, in contrast to the commonly accepted view that CO hinders such reactions. This intriguing effect was traced to repulsive pi interactions along the aryl-Rh-CO axis, which are augmented by the initially added CO ligand (due to antibonding interactions between occupied Rh d(pi) orbitals and occupied pi orbitals of both CO and the arene moiety), but counteracted by the second CO ligand (due to significant pi back-donation). These repulsive interactions were themselves linked to significant weakening of the pi-acceptor character of CO in the positively charged rhodium complexes, which is concurrent with an enhanced sigma-donating capability. Replacement of the phosphine ligands by an analogous phosphinite-based (POCOP) pincer ligand led to significant changes in reactivity, whereby addition of CO did not result in C-H reductive elimination, but yielded relatively stable mono- and dicarbonyl aryl-hydrido POCOP-Rh(III) complexes. DFT calculations showed that the stability of these complexes arises from the higher electrophilicity of the POCOP ligand, relative to PCP, which leads to partial reduction of the excessive pi-electron density along the aryl-Rh-CO axis. Finally, comparison between the effects of CO and acetonitrile on C-H oxidative addition revealed that they exhibit similar reactivity, despite their markedly different electronic properties. However, DFT calculations indicate that the two ligands operate by different mechanisms.

Microwave measurements and ab initio calculations of structural and electronic properties of N-Et-1,2-azaborine

Rotational transitions for N-Et-1,2-azaborine were measured in the 5-13 GHz range using a Flygare-Balle type Fourier transform spectrometer system. Twelve distinct rotational transitions with over 130 resolved hyperfine components, which included a-dipole and b-dipole transitions, were measured and analyzed to obtain rotational constants and (11)B and (14)N nuclear quadrupole coupling constants in the principal rotational axis system. Rotational constants obtained are A=4477.987(4), B=1490.5083(7), and C=1230.6728(6) MHz. The quadrupole coupling constants for (11)B are eQq(aa)=-1.82(1), (eQq(bb)-eQq(cc))=-3.398(4) MHz, and for (14)N, eQq(aa)=1.25(1), (eQq(bb)-eQq(cc))=0.662(4) MHz. Quantum electronic structure calculations predict a ground-state structure with the ethyl group perpendicular to the azaborine plane and rotational constants in very good agreement with the measured structure and rotational constants. The theoretical conformational analysis of the ethyl group rotation around the N[Single Bond]C bond in relation to the heterocyclic ring yielded an asymmetric torsional potential energy surface with barrier heights of about 900 and 1350 cm(-1) for the N-Et-1,2-azaborine. Results of the measurements and calculations indicate that the basic molecular structure of N-Et-1,2-azaborine is similar to ethylbenzene. Electrostatic potential calculations qualitatively show that pi-electron density is somewhat delocalized around the 1,2-azaborine ring.

Vibrational Coherence in Polar Solutions of ZnII tetrakis(N-methylpyridyl)porphyrin with Soret-Band Excitation: Rapidly Damped Intermolecular Modes with Clustered Solvent Molecules and Slowly Damped Intramolecular Modes from the Porphyrin Macrocycle

Ground-state coherent wavepacket motions arising from intermolecular modes with clustered, first-shell solvent molecules were observed using the femtosecond dynamic absorption technique in polar solutions of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) with excitation in the Soret absorption band. As was observed previously in bacteriochlorophyll a solution, the pump-probe transients in ZnTMPyP solutions are weakly modulated by slowly damped (effective damping time gamma > 1 ps) features that are assigned to intramolecular modes, the skeletal normal modes of vibration of the porphyrin. The 40 cm(-1) and 215 cm(-1) modes from the metal-doming and metal-solvent-ligand modes, respectively, are members of this set of modulation components. A slowly damped 2-4 cm(-1) component is assigned to the internal rotation of the N-methylpyridyl rings with respect to the porphyrin macrocycle; this mode obtains strong resonance Raman intensity enhancement from an extensive delocalization of pi-electron density from the porphyrin in the ground state onto the rings in the pi* excited states. The dominant features observed in the pump-probe transients are a pair of rapidly damped (gamma < 250 fs) modulation components arising from intermolecular modes with solvent molecules. This structural assignment is supported by an isotope-dependent shift of the average mode frequencies in methanol and perdeuterated methanol. The solvent dependence of the mean intermolecular mode frequency is consistent with a van der Waals intermolecular potential that has significant contributions only from the London dispersion and induction interactions; ion-dipole or ion-induced-dipole terms do not make large contributions because the pi-electron density is not extensively delocalized onto the N-methylpyridyl rings. The modulation depth associated with the intermolecular modes exhibits a marked dependence on the electronic structure of the solvent that is probably related to the degree of covalency; the strongest modulations are observed in acetonitrile and dimethylsulfoxide. The results strongly support a structural assignment of the low-frequency modes that are coupled to the primary and secondary electron-transfer reactions in photosynthetic reaction centers to intermolecular modes between the redox-active chromophores and first-solvation shell groups from the surrounding protein, and an important additional function of the intermolecular modes in the stabilization of charged intermediates is suggested.

Configurationally Rigid Pentaorganosilicates

The intramolecular substituent interchange in recently reported pentaorganosilicates is investigated by B3LYP calculations, which show excellent agreement with the experimental thermochemical data. Two types of ligand permutation are discerned (A and B), which both lead to racemization of the helical, spirocyclic anions. IRC calculations show that stereomutation A bifurcates into two enantiomeric reaction paths, which are inhibited by ortho substitution of the bidentate ligands. The other pathway (B) proceeds through a trigonal bipyramidal transition state with one bisequatorial bidentate ligand and is disfavored by increasing the pi-electron density of the ligand. A more electronegative fifth, monodentate substituent increases the barrier of pathway A and lowers that of pathway B, as in bis(biphenyl-2,2'-diyl)fluorosilicate, which is the first tetraorganofluorosilicate to be isolated and fully characterized. These concepts enabled us to design and synthesize methyl- and ethylbis([2]naphthylpyrrol-2,1'-diyl)silicate as Si-chiral pentaorganosilicates that are configurationally rigid at room temperature.

Wormholes in chemical space connecting torus knot and torus link π-electron density topologies

Möbius aromaticities can be considered as deriving from cyclic delocalized pi-electron densities rho(r)(pi) which have the topological form of either a two-component torus link or a single-component torus knot. These two topological forms are distinguished by their (non-zero) linking number L(k), which describes how many times the two components of a torus link cross each other or the single component of a torus knot crosses with itself. The special case of Hückel or benzenoid aromaticity is associated with a pi-electron density that takes the form of a two-component torus link for which the linking number is zero. A class of molecule has been identified which here is termed a Janus aromatic, and which bears the characteristics of both a two-component torus link and a single-component torus knot in the topology of the pi-electron density. This is achieved by the formation of one (or more) wormholes or throats in the pi-electron density connecting the two torus forms, which can impart a Janus-like dual personality to the aromaticity of the system. The impact of such wormholes on the overall pi-delocalized aromaticity of such molecules is approximately estimated using a NICS(rcp) index, and subdivides into two types; those where the forms of aromaticity associated with a torus link and a torus knot cooperate and those where they oppose.

Infrared–optical double resonance spectroscopic measurements and high level ab initio calculations on a binary complex between phenylacetylene and borane-trimethylamine. Understanding the role of C–H⋯π interactions

The structure of the binary complex between phenylacetylene and borane-trimethylamine has been elucidated using IR-UV double resonance spectroscopy in combination with high level ab initio calculations at the CCSD(T) level. Borane-trimethylamine interacts primarily through multiple C-H...pi interactions with the pi electron density of the benzene ring in phenylacetylene. CCSD(T) level calculations provide reliable estimates for the interaction energy and free energy, which are in accord with the experimental observations. The DFT-SAPT calculations point out that the dispersion interaction plays a major role in the formation of the experimentally observed complex, along with a sizable contribution from electrostatics.

A π-stacked phenylacetylene and 1,3,5-triazine heterodimer: a combined spectroscopic and ab initio investigation

The IR-UV double resonance spectroscopy of a complex between phenylacetylene and 1,3,5-triazine reveals that the acetylene C-H group of phenylacetylene is minimally perturbed due to its interaction with 1,3,5-triazine. Further, the IR spectrum clearly indicates that 1,3,5-triazine primarily interacts with pi-electron density of the benzene ring in phenylacetylene. Geometries obtained at the DFT/MO6-2X and MP2/aug-cc-pVDZ levels, combined with highly accurate energy calculations at the complete basis set (CBS) limit of CCSD(T), establish formation of the displaced pi-stacked heterodimer between phenylacetylene and 1,3,5-triazine.

A crossed molecular beam study on the synthesis of the interstellar 2,4-pentadiynylidyne radical (HCCCCC)

Crossed molecular beam experiments are performed to elucidate the synthesis of the 2,4-penta-diynylidyne [HCCCCC(X (2)Pi)] radical under single collision conditions--a crucial reaction intermediate to form polycyclic aromatic hydrocarbons and carbonaceous nanostructures in the interstellar medium and in combustion flames. The experiments demonstrate that the chemical dynamics of ground state carbon reacting with diacetylene [HCCCCH(X (1)Sigma(g)(+))] are indirect and proceed via addition of the electrophilic carbon atom to the pi electron density of the diacetylene molecule yielding ultimately the carbenelike HCCCCCH(X (3)Sigma(g)(-)) molecule. This intermediate fragments via hydrogen atom emission to yield the 2,4-pentadiynylidyne [HCCCCC(X (2)Pi)] radical. The chemical dynamics elucidated also allows us to predict that reaction of carbon atoms with polyynes of the generic formula H(C[triple bond]C)(n)H leads to the formation of hydrogen-terminated carbon clusters of the generic form HC(2n+1) in extreme environments. The acetylene-related reactivity and electronic structure of the diacetylene molecule also allow us to project that reactions of the diacetylene molecule with cyano and ethynyl radicals result in a stepwise extension of the carbon skeleton forming cyanodiacetylene (HCCCCCN) and triacetylene (HCCCCCCH) plus atomic hydrogen. These predictions open the door to extensive laboratory studies involving hitherto poorly understood reactions of the diacetylene molecule under single collision conditions.

Water induced weakly bound electrons in DNA

We have studied the effect of humidity on the electronic properties of DNA base pairs. We found that the hydrogen links of the nucleobases with water molecules lead to a shift of the pi electron density from carbon atoms to nitrogen atoms and can change the symmetry of the wave function for some nucleobases. As a result, the orbital energies are shifted which leads to a decrease in the potential barrier for the hole transfer between the G-C and A-T pairs from 0.7 eV for the dehydrated case to 0.123 eV for the hydrated. More importantly, the pi electron density redistribution activated by hydration is enhanced by the intrastrand interactions. This leads to a modification of the nucleobase chemical structures from the covalent type to a resonance structure with separated charges, where some pi electrons are not locked up into the covalent bonds. Within the (G-C)(2) sequences, there is overlapping of the electronic clouds of such unlocked electrons belonging to the stacked guanines, that significantly increases the electron coupling between them to V(DA)=0.095 eV against the V(DA)=0.025 eV for the dehydrated case. Consequently, the charge transfer between two guanines within the (G-C)(2) sequences is increased by 250 times due to hydration. The presence of nonbonded electrons suppress the band gap up to approximately 3.0 eV, that allows us to consider DNA as a narrow band gap semiconductor.

Intrinsic Conformational Preferences of Cα,α-Dibenzylglycine

The intrinsic conformational preferences of C (alpha,alpha)-dibenzylglycine, a symmetric alpha,alpha-dialkylated amino acid bearing two benzyl substituents on the alpha-carbon atom, have been determined using quantum chemical calculations at the B3LYP/6-31+G(d,p) level. A total of 46 minimum energy conformations were found for the N-acetyl- N'-methylamide derivative, even though only nine of them showed a relative energy lower than 5.0 kcal/mol. The latter involves C 7, C 5, and alpha' backbone conformations stabilized by intramolecular hydrogen bonds and/or N-H...pi interactions. Calculation of the conformational free energies in different environments (gas-phase, carbon tetrachloride, chloroform, methanol, and water solutions) indicates that four different minima (two C 5 and two C 7) are energetically accessible at room temperature in the gas phase, while in methanol and aqueous solutions one such minimum (C 5) becomes the only significant conformation. Comparison with results recently reported for C (alpha,alpha)-diphenylglycine indicates that substitution of phenyl side groups by benzyl enhances the conformational flexibility leading to (i) a reduction of the strain of the peptide backbone and (ii) alleviating the repulsive interactions between the pi electron density of the phenyl groups and the lone pairs of the carbonyl oxygen atoms.

Infrared Spectroscopy of Gas Phase Benzenium Ions: Protonated Benzene and Protonated Toluene, from 750 to 3400 cm–1

Gas phase C 6H 7 (+) and C 7H 9 (+) ions are studied with infrared photodissociation spectroscopy (IRPD) and the method of rare gas tagging. The ions are produced in a pulsed electric discharge supersonic expansion source from benzene or toluene precursors. We observe exclusively the formation of either the C 2 v benzenium ion (protonated benzene) or the para isomer of the toluenium ion (protonated toluene). The infrared spectral signatures associated with each ion are established between 750 and 3400 cm (-1). Comparing the gas phase spectrum of the benzenium ion to the spectrum obtained in a superacid matrix [ Perkampus, H. H.; Baumgarten, E. Angew. Chem. Int. Ed. 1964, 3, 776 ], we find that the C 2 v structure of the gas phase species is minimally affected by the matrix environment. An intense band near 1610 cm (-1) is observed for both ions and is indicative of the allylic pi-electron density associated with the six membered ring in these systems. This spectral signature, also observed for alkyl substituted benzenium ions and protonated naphthalene, compares favorably with the interstellar, unidentified infrared emission band near 6.2 microm (1613 cm (-1)).