Jan Szczepanski

Electronic and vibrational spectra of matrix isolated anthracene radical cations: Experimental and theoretical aspects

Radical cations of anthracene have been formed by vapor phase electron impact followed by trapping in an argon matrix at 12 K. Visible/ultraviolet and infrared absorption spectra of the anthracene cations in an argon matrix have been run. Significant differences in the infrared band intensities between neutral and cationic anthracene have been observed. The effects of photolysis and added CCl4 have been studied and their influence on the infrared band intensities correlated with known visible bands attributable to the anthracene cation. Theoretical calculations using Pariser–Parr–Pople and intermediate neglect of differential overlap methodologies with high level multireference perturbation configuration interaction, specifically modified for spectroscopic applications, have been performed. Both approaches predict the previously observed photoelectron spectrum well. For the optical absorption, the match with the experimental spectrum is also good, but there are notable differences between the two predicti...

Vibrational and electronic spectra of matrix-isolated pentacene cations and anions

Abstract Pentacene cations and anions have been formed by electron bombardment of a vapor phase mixture of pentacene, Ar and CCl 4 and trapped in an argon matrix. The electronic and infrared spectra of both ions have been recorded. Increasing concentrations of CCl 4 resulted in the eventual disappearance of the anion spectrum. The intensity distributions of the IR spectra of the ions are similar but differ markedly from the distribution of neutral pentacene. The ion intensities resemble the distribution of the so-called interstellar ‘unidentified infrared’ bands. The visible bands of neutral, cationic and anionic pentacene are compared with the diffuse interstellar bands.

Glycine and its hydrated complexes: a matrix isolation infrared study.

The hydration of glycine is investigated by comparing the structures of bare glycine to its hydrated complexes, glycine.H(2)O and glycine.(H(2)O)(2). The Fourier transform infrared spectra of glycine and glycine.water complexes, embedded in Ar matrices at 12 K, have been recorded and the results were compared to density functional theory (DFT) calculations. An initial comparison of the experimental spectra was made to the harmonic infrared spectra of putative structures calculated at the MPW1PW91/6-311++G(d,p) level of theory. The results suggest that bare glycine adopts a C(s) symmetry structure (G-1), where the hydrogens of the amino NH(2) hydrogen-bond intramolecularly with the carboxylic acid C horizontal lineO oxygen. Also observed as minor constituents are the next two lowest-energy structures, one in which the carboxylic acid (O-)H group hydrogen-bonds to the amino NH(2) group (G-2), and the other where intramolecular hydrogen bonding occurs between the NH(2) and the carboxylic acid O(-H) groups (G-3). The abundances of these structures are estimated at 84%, 9% and 8%, respectively. The least favored structure, G-3, can be eliminated by annealing the matrix to 35 K. Addition of the first water molecule to G-1 takes place at the carboxylic acid group, with simultaneous hydrogen bonding of the water molecule to the carboxylic acid (C=)O and (O-)H. The results are consistent with the predominance of this structure, although there is evidence for a small amount of a hydrated G-2 structure. Addition of the second water molecule is less definitive, as only a small number of intense infrared modes can be unambiguously assigned to glycine.(H(2)O)(2). Anharmonic frequency calculations based on second-order vibrational perturbation theory have also been carried out. It is shown that such calculations can generate improved estimates (i.e., approximately 2%) of the experimental frequencies for glycine and glycine.H(2)O, provided that the potential energy surfaces are modeled with high-level ab initio approaches (MP2/aug-cc-pVDZ).

Visible and infrared spectra of matrix-isolated perylene cations

Abstract Perylene radical cations have been produced via low-energy electron impact of a vapor phase mixture of perylene, argon and carbon tetrachloride. The cations have been trapped and stabilized in an argon matrix at 12K in sufficient quantities to observe their infrared (and visible) absorption spectra. A strong green (534 nm) electronic absorption band, previously produced by γ-irradiation of perylene in an s -butylchloride matrix (77 K), has been ascribed to the perylene cation. A positive correlation has been found between this band and the newly observed infrared bands at 1128, 1318, 1346, 1349, and 1551 cm −1 . The integrated intensities of these and other bands, assigned tothe perylene cation, have been measured and are discussed in light of the recent suggestion that polycyclic aromatic hydrocarbons are the carriers of the unidentified infrared emission bands observed from interstellar space.

Infrared spectroscopy of gas-phase complexes of Fe+ and polycyclic aromatic hydrocarbon molecules

The observed depletion of iron in the interstellar medium has been suggested to result from its efficient complexation with, among others, polycyclic aromatic hydrocarbon molecules (PAHs). We present here the first experimental vibrational spectra of cationic iron-PAH complexes with benzene, naphthalene, and fluorene. The spectra were obtained by infrared multiple-photon dissociation (IRMPD) spectroscopy of the complex ions trapped in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. A continuously tunable free electron laser provided intense radiation in the astrophysically interesting wavelength range of 6-16 mu m. Supporting calculations of the geometries, relative stabilities, and harmonic vibrational frequencies of the complexes were carried out using density functional theory with the MPW1PW91/6-31+G(d,p) functional/basis set. In all cases, the experimental spectra indicate that Fe+ is bound to the six-membered carbon ring of the aromatic ligand in a high-spin (quartet) electronic ground state for the mono-complexes, Fe(benzene)(+) and Fe(naphthalene)+, and in a low-spin (doublet) electronic ground state for the bis-complexes,Fe(benzene)(+)2, Fe(naphthalene)(+)2, and Fe(Cuorene)(+)2 complexes. Comparison of the complex spectra to the bare (neutral and cationic) PAH spectra reveals their spectroscopic fingerprint, e. g., in the splitting of the out-of-plane CH-bending modes, which could aid in their interstellar detection.

Infrared spectroscopy of matrix-isolated carbon clusters, with emphasis on C8 and C9

Abstract Infrared spectra of neutral linear all-carbon clusters, produced by laser ablation of graphite, have been investigated in Ne, Ar and Kr matrices. The effects of thermal annealing on the infrared intensities have been used together with results from recent high level ab initio and density functional theory (DFT) calculations to locate several heretofore unknown infrared bands of linear C 8 and C 9 clusters. Specifically, two infrared bands located at 2071.5 cm −1 and 1710.5 cm −1 (Ar matrix), are shown to be well correlated and to match closely the density functional theory (DFT) predictions of 2109 cm −1 (1366.3 km/mol) and 1703 cm −1 (984.8 km/mol) by Hutter, Luthi and Diederich (HLD) for the linear C 8 cluster. A moderately intense band has also been located at 2078 cm −1 (Ar matrix) and its shown to correlate well with the two known C 9 bands at 1601 cm −1 and 1998 cm −1 . This band is also shown to match well with the DFT predictions of HLD and is assigned to the v 5 mode of linear C 9 . Combination bands involving symmetric and asymmetric stretching modes have been observed for C 5 , C 6 , C 7 and C 9 and are used to estimate the totally symmetric frequencies for these species. Good agreement with theory ios found. Finally, anharmonicity constants for the prominent asymmetric stretching frequencies are determined from the overtone bands of C 3 , C 5 , C 6 , C 7 and C 9 .

Vibrational and electronic spectra of matrix-isolated tetracene cations

Abstract The electronic and vibrational spectra of tetracene cations, generated by vapor-phase electron bombardment and deposited in an argon matrix, have been recorded. Integrated intensities of the infrared bands of neutral and cationic tetracene compare well with a recent density functional calculation by Langhoff. As observed in previous studies of PAH ions, the intense neutral out-of-plane CH bends decrease drastically in the cation, while the very weak neutral in-plane CC stretches and CH bends gain dramatically upon ionization. The possible contribution of tetracene cations and neutrals to the ‘unidentified interstellar infrared’ bands is discussed.

Photon-induced Complete Dehydrogenation of Putative Interstellar Polycyclic Aromatic Hydrocarbon Cations: Coronene and Naphtho[2,3-a]pyrene

Fourier transform ion cyclotron resonance mass spectra of coronene and naphtho[2,3-a]pyrene radical cations exposed to broadband ultraviolet/visible radiation are presented. Parent ions are formed and confined in a Penning trap at 3.0 T and photolyzed with an Xe arc lamp. Each of the two radical cations is found to dehydrogenate completely to leave the C+24 bare carbon cluster cation. The extent of dehydrogenation depends directly on irradiation power. We infer that polycyclic aromatic hydrocarbon cations are inherently unstable to photolysis. We discuss some implications of hydrogen loss to yield bare carbon cluster cations. The relevance of dehydrogenation to unidentified interstellar infrared emission bands is discussed briefly.

H2 ejection from polycyclic aromatic hydrocarbons: infrared multiphoton dissociation study of protonated acenaphthene and 9,10-dihydrophenanthrene

The infrared multiple-photon dissociation (IRMPD) spectra of protonated acenaphthene ([ACN+H]+) and 9,10-dihydrophenanthrene ([DHP+H]+) have been recorded using an infrared free electron laser after the compounds were protonated by electrospray ionization and trapped in a Fourier transform ion cyclotron mass spectrometer. In both compounds, the loss of two mass units is predominant. Density functional calculations (B3LYP/6-311++G(d,p)) of the infrared spectra of all possible protonated isomers of each species showed that the observed IRMPD spectra are best fit to the isomer with the largest proton affinity and lowest relative electronic energy. Potential energy surfaces of the most stable isomers of [ACN+H]+ and [DHP+H]+ have been calculated for H and H2 loss. The lowest energy barriers are for loss of H2, with predicted energies 4.28 and 4.15 eV, respectively. After H2 ejection, the adjacent aliphatic hydrogens migrate to the bare ejection site and stabilize the remaining fragment. Single H loss may occur from [ACN+H]+ but the energy required is higher. No single H loss is predicted from [DHP+H]+, only H migration around the carbon skeleton. The vibrational bands in the parent closed-shell protonated polycyclic aromatic hydrocarbons are compared to bands observed from the interstellar medium.

H2 Ejection from Polycyclic Aromatic Hydrocarbons: Infrared Multiphoton Dissociation Study of Protonated 1,2-Dihydronaphthalene

1,2-Dihydronaphthalene (DHN) has been studied by matrix isolation infrared absorption spectroscopy, multiphoton infrared photodissociation (IRMPD) action spectroscopy, and density functional theory calculations. Formed by electrospray ionization, protonated 1,2-dihydronapthalene was injected into a Fourier transform ion cyclotron resonance mass spectrometer coupled to an infrared-tunable free electron laser and its IRMPD spectrum recorded. Multiphoton infrared irradiation of the protonated parent (m/z 131) yields two dissociation products, one with m/z 129 and the other with m/z 91. Results from density functional theory calculations (B3LYP/6-31++G(d,p)) were compared to the low-temperature matrix isolation infrared spectrum of neutral DHN, with excellent results. Calculations reveal that the most probable site of protonation is the 3-position, producing the trihydronaphthalene (THN) cation, 1,2,3-THN(+). The observed IRMPD spectrum of vapor-phase protonated parent matches well with that computed for 1,2,3-THN(+). Extensive B3LYP/6-31G(d,p) calculations of the potential energy surface of 1,2,3-THN(+) have been performed and provide insight into the mechanism of the two-channel photodissociation. These results provide support for a new model of the formation of H(2) in the interstellar medium. This model involves hydrogenation of a PAH cation to produce one or more aliphatic hydrogen-bearing carbons on the PAH framework, followed by photolytic formation and ejection of H(2).