Ultraviolet photodissociation action spectroscopy of the N-pyridinium cation

© 2015 AIP Publishing LLC. The S1←S0 electronic transition of the N-pyridinium ion (C5H5NH+) is investigated using ultraviolet photodissociation (PD) spectroscopy of the bare ion and also the N2-tagged complex. Gas-phase N-pyridinium ions photodissociate by the loss of molecular hydrogen (H2) in the photon energy range 37 000-45 000 cm-1 with structurally diagnostic ion-molecule reactions identifying the 2-pyridinylium ion as the exclusive co-product. The photodissociation action spectra reveal vibronic details that, with the aid of electronic structure calculations, support the proposal that dissociation occurs through an intramolecular rearrangement on the ground electronic state following internal conversion. Quantum chemical calculations are used to analyze the measured spectra. Most of the vibronic features are attributed to progressions of totally symmetric ring deformation modes and out-of-plane modes active in the isomerization of the planar excited state towards the non-planar excited state global minimum

Blue to near-IR energy transfer cascade within a dye-doped polymer matrix, mediated by a photochromic molecular switch

The spectroscopic properties of a poly(methyl methacrylate) matrix doped with a coumarin dye, a cyanine dye, and a photochromic spiropyran dye have been investigated. Before UV irradiation of the matrix, excitation of the coumarin dye results in minimal energy transfer to the cyanine dye. The energy transfer is substantially enhanced following UV irradiation of the matrix, which converts the colourless spiropyran isomer to the coloured merocyanine isomer, which then acts as an intermediate bridge by accepting energy from the coumarin dye and then donating energy to the cyanine dye. This demonstration of a switchable energy transfer cascade should help initiate new research directions in molecular photonics

Infrared Spectra and ab initio Calculations for Fluoride-acetylene Clusters: F--(HCCH)(n), n=3-6

C1 - Journal Articles RefereedInfrared spectra and ab initio calculations are presented for gas phase clusters consisting of a fluoride anion attached to acetylene ligands. Spectra obtained in the C-H stretch region contain a single strong band, consistent with cluster structures in which roughly equivalent acetylene ligands are hydrogen bonded to a central fluoride anion core. Minimum energy structures predicted from ab initio calculations at the MP2 level of theory are highly symmetric with acetylene ligands equally spaced about a central fluoride anion core. The predicted H-bonded C-H stretching frequencies, after scaling to correct for anharmonicity, agree well with the experimental band positions

Electronic absorptions of the benzylium cation

C1 - Journal Articles RefereedThe electronic transitions of the benzylium cation (Bz(+)) are investigated over the 250-550 nm range by monitoring the photodissociation of mass-selected C(7)H(7)(+)-Ar(n) (n = 1, 2) complexes in a tandem mass spectrometer. The Bz(+)-Ar spectrum displays two distinct band systems, the S(1)←S(0) band system extending from 370 to 530 nm with an origin at 19,067 ± 15 cm(-1), and a much stronger S(3)←S(0) band system extending from 270 to 320 nm with an origin at 32,035 ± 15 cm(-1). Whereas the S(1)←S(0) absorption exhibits well resolved vibrational progressions, the S(3)←S(0) absorption is broad and relatively structureless. Vibronic structure of the S(1)←S(0) system, which is interpreted with the aid of time-dependent density functional theory and Franck-Condon simulations, reflects the activity of four totally symmetric ring deformation modes (ν(5), ν(6), ν(9), ν(13)). We find no evidence for the ultraviolet absorption of the tropylium cation, which according to the neon matrix spectrum should occur over the 260 - 275 nm range [A. Nagy, J. Fulara, I. Garkusha, and J. Maier, Angew. Chem., Int. Ed. 50, 3022 (2011)]

Interaction of the Beryllium Cation with Molecular Hydrogen and Deuterium

The structural and spectroscopic properties of the Be(+)-H2 and Be(+)-D2 electrostatic complexes are investigated theoretically. A three-dimensional ground-state potential energy surface is generated ab initio at the CCSD(T) level and used for calculating the lower rovibrational energy levels variationally. The minimum of the potential energy surface corresponds to a well depth of 3168 cm(-1), an intermolecular separation of 1.776 Å, with the bond of the H2 subunit being 0.027 Å longer than for the free molecule. Taking vibrational zero point energy into account, the complexes containing para H2 and ortho D2 are predicted to have dissociation energies of 2678 and 2786 cm(-1), respectively. The νHH band of Be(+)-H2 is predicted to be red-shifted from the free dihydrogen transition by -323 cm(-1), whereas the corresponding shift for Be(+)-D2 is predicted to be -229 cm(-1). The dissociation energy of the Be(+)-D2 complex is calculated to be slightly higher than the energy required to vibrationally excite the D2 subunit, raising the possibility that the onset of dissociation can be observed in the infrared predissociation spectrum at a particular rotational energy level in the νDD manifold

Electronic Spectra of Gas-Phase Polycyclic Aromatic Nitrogen Heterocycle Cations: Isoquinoline(+) and Quinoline(+)

C1 - Journal Articles RefereedElectronic spectra of the gas-phase isoquinoline(+)-Ar and quinoline(+)-Ar complexes are recorded using photodissociation spectroscopy by monitoring the Ar loss channel. The D(3)←D(0) and D(4)←D(0) band origins for isoquinoline(+)-Ar are observed at 15245 ± 15 cm(-1) and 21960 ± 15 cm(-1), respectively, whereas for quinoline(+)-Ar they appear at 16050 ± 15 cm(-1) and 21955 ± 15 cm(-1), respectively. Strong vibronic progressions for the D(3)←D(0) band systems of both isoquinoline(+)-Ar and quinoline(+)-Ar are modeled and assigned in terms of ring deformation and carbon-carbon stretch vibrational modes using time-dependent density functional theory calculations in conjunction with Franck-Condon simulations. The properties of the isoquinoline(+) and quinoline(+) molecules are compared with those of the isoelectronic naphthalene(+) molecule. The existence of strong progressions in the visible spectra of isoquinoline(+)-Ar and quinoline(+)-Ar suggests that the corresponding isoquinoline(+) and quinoline(+) molecular cations are unlikely to be responsible for diffuse interstellar bands

Photoisomerization of beta-Ionone Protonated Schiff Base in the Gas Phase

The photoisomerization of β-ionone protonated Schiff base (BIPSB) is investigated in the gas phase by irradiating mobility-selected ions in a tandem ion mobility spectrometer with tunable radiation. Four distinguishable isomers are produced by electrospray ionization whose structures are deduced from their collision cross sections and photoisomerization behavior along with density functional theory calculations. They include two geometric isomers of BIPSB with trans or cis configurations about the polyene chain's terminal C═N double bond, a bicyclic structure formed through electrocyclization of the polyene chain, and a Z-retro-γ-ionone isomer. Although trans-BIPSB and 9-cis-BIPSB have similar photoisomerization action spectra, with a maximum response at 375 nm, they photoconvert to different isomers. The trans-BIPSB isomer transforms to the bicyclic form upon exposure to light over the 320-400 nm range, whereas the cis-BIPSB isomer is prevented by steric hindrance from forming the bicyclic BIPSB isomer following irradiation and is proposed instead to form the 7,9-di-cis isomer. Neither the bicyclic isomer nor the Z-retro-γ-ionone isomer respond strongly to near-UV light