Petra Swiderek

Theoretical prediction of the vibrational spectrum of naphthalene in the first excited singlet state

Complete harmonic force fields have been calculated for the ground state (S0) and the first excited singlet state (S1) of naphthalene using the multiconfiguration self‐consistent field (MCSCF) approach. Identical calculations were performed for benzene to test the methodology with already available theoretical and empirical force fields. Two different basis sets were applied (STO‐3G and near double‐zeta) and all π‐orbitals included in the active space. The geometries of ground and excited states were separately optimized. Following the ideas of Pulay, the force constants were scaled before calculating frequencies and normal modes. For the ground states the influence of correlation is discussed by comparison with Pulay’s results. Except for special vibrations where correlation effects turn out to be important, the use of Pulay’s scaling factors leads to a satisfactory description of the in‐plane‐vibrations. In the case of benzene the calculated frequency shifts between S0 and S1 are in complete qualitative...

Low-energy-electron-induced hydroamination of an alkene.

The smallest catalyst: A new strategy to control chemical synthesis by exposure to low-energy electrons relies on the electrostatic attraction caused by the soft ionization of one of the reaction partners. This approach was used to induce a reaction between C(2)H(4) and NH(3) yielding aminoethane. The reaction resembles a hydroamination except that the electron beam replaces the catalyst used in the organic synthesis.

Damage induced by low-energy electrons in solid films of tetrahydrofuran

We report on the low-energy electron-induced production of aldehydes within thin solid films of tetrahydrofuran (THF) condensed on a solid Kr substrate. The aldehyde fragments, which remain trapped within the bulk of the THF film, are detected in situ via their 3,1(n-->pi*) and 3(pi-->pi*) electronic transitions and vibrational excitations in the ground state using high-resolution electron-energy-loss spectroscopy. The production of aldehyde is studied as a function of the electron exposure, film thickness, and incident electron energy between 1 and 18.5 eV. The aldehyde production is calibrated in terms of an electron scattering cross section, which is found to be typically 6-7 x 10(-17) cm(2) between 11 and 19 eV. Its energy dependence is characterized by a small feature around 3 eV, a strong rise from 6 eV up to a maximum at 12.5 eV, followed by two structures centered around 15 and 18 eV. The aldehyde production is discussed in terms of the formation of electron resonances or transient anion states, which may lead to the fragmentation of the molecule and explain the structures seen in the energy dependence of the measured cross section.

Electron-energy-loss spectroscopy of solid phenanthrene and biphenylene: Search for the low-lying triplet states

Abstract High-resolution electron energy-loss spectra are reported for fluorene and biphenyl deposited on a thin film of solid argon at a temperature of 20 K. New triplet states are detected at 3.86 eV in fluorene and 3.93 eV in biphenyl. The assignment of the newly found triplet states is discussed in the framework of the composite molecule approach.

Spin-forbidden transitions in amorphous and crystalline thin films of benzene

Abstract Electron-energy-loss spectra of the lowest singlet–triplet transition in thin films of amorphous and crystalline solid benzene have been recorded at a temperature of 32 K. The structure of the solid films depends on the substrate temperature at which the molecules are deposited. At 32 K films of amorphous character are formed whereas crystalline films are produced at a deposition temperature of 100 K and a thickness of more than 5 layers. This is shown by the angular dependence of the vibrational spectra. While amorphous films display an essentially isotropic scattering behavior, the crystalline films are characterized by a distinct enhancement of dipole-allowed vibrations under specular conditions. The 0–0 transition to the lowest triplet state is found at 3.676 eV in crystalline benzene and at 3.665 eV in the amorphous film. This is the first time that the shift of a spin-forbidden band can be related to changes in the bulk structure of the molecular solid. The reason for this shift is explored by discussing the available experimental and theoretical information about effects of the surrounding medium on the transition energies of aromatic hydrocarbons.

Electron energy loss spectroscopy of solid naphthalene and acenaphthene: search for the low-lying triplet states

Abstract Low-energy electron loss spectra of naphthalene and acenaphthene deposited on a thin film of solid argon have been measured at a temperature of 20 K. The second triplet of acenaphthene (1 3B2) could be detected at 3.74 eV, and a further triplet is tentatively assigned at 4.23 eV. The band observed by Allan in the gas-phase spectrum of naphthalene at 5.20 eV is reassigned.

Electron-induced reactions in condensed films of acetonitrile and ethane

Reactions in pure and mixed films of C(2)H(6) and CD(3)CN deposited on a Au surface at 35 K have been induced by low-energy electrons and investigated by Thermal Desorption Spectrometry (TDS). The incident electron energy (E(0)) was varied between 5 and 16 eV and a number of different products were identified. Beside the main products, CD(4), CD(3)H, and C(2)D(6), molecules resulting from atom scrambling during radical chain reactions (C(2)H(5)D) and recombination products (CD(3)CD(2)CN and C(2)H(5)CD(3)) were identified while others were characteristically absent. The quantity of the different products varied with E(0). The observed electron-driven processes are in accord with previous findings from gas phase experiments on dissociative electron attachment and electron impact ionization. On this basis, reaction mechanisms leading to the formation of the observed products are suggested for different ranges of E(0).

Electron‐energy‐loss spectroscopy of the low‐lying triplet states of styrene

Low‐energy electron‐energy‐loss spectra of styrene deposited on a thin film of solid argon are measured at a temperature of 15 K. The spectra show vibrationally resolved bands in the region of the lowest valence transitions thus allowing to locate the 0–0 transition to the lowest triplet state at 2.69 eV. The second triplet state of styrene is detected for the first time with a 0–0 transition at 3.98 eV. Semiempirical calculations are performed to characterize the bands observed in the spectrum considering the nomenclature of Platt. They suggest that the lowest triplet state has the same spacial wave function as the second singlet state and is closely related to 3La benzene. The second triplet state which has most likely Ba character cannot directly be related to a specific singlet state because the Ba and Bb states are found to mix strongly in the singlet manifold whereas among the triplets they do not.

Vibronic structure in the low‐lying singlet–triplet transitions of benzene and toluene

Low‐energy electron‐energy‐loss spectra of solid benzene and toluene in the range of the three low‐lying triplet states were recorded at a temperature of 15 K. Vibronic structure within the low‐lying triplet bands of toluene is observed for the first time. In the case of benzene the high resolution spectra reveal more details in the vibronic structure than known from previous electron‐ energy‐loss spectra. With this information a modified interpretation of the vibronic structure in the first triplet band of benzene is proposed. The difference between the spectra of toluene and benzene is explained by the influence of vibronic coupling on the lowest triplet state. In addition, the systematic broadening of the vibronic levels within the first and second triplet band of toluene is interpreted as an effect of the side group internal rotation.

Triplet states in oligomeric materials: Electron energy loss spectroscopy of thiophene and bithiophene and extrapolation to the polymer

High-resolution electron-energy-loss spectra of thiophene and bithiophene have been measured in the range of the low-lying singlet–triplet excitations. In combination with ab-initio calculations the observed vibrational structure within the S0→T1 and S0→T2 bands of thiophene is assigned and adiabatic transition energies are determined. The study of bithiophene aimed at the search for the S0→T2 band. This transition has not been unambiguously located. The adiabatic S0→T1 energy of thiophene, together with previous results from literature, yields a consistent set of solid phase data that can be used to model the chain length dependence of S0→T1 excitation energies in oligothiophenes. Based on this data set and others, currently used extrapolation procedures aiming at a prediction of polymer excitation energies are evaluated. In addition, it is shown that recent semiempirical calculations do not correctly describe the convergence of the S0→T1 energies towards infinite chain length. It is therefore advisable to apply suitable modern ab-initio methods to this problem.