139,588 research outputs found
Time-dependent density functional study of the electronic spectra of oligoacenes in the charge states -1, 0, +1, and +2
We present a systematic theoretical study of the five smallest oligoacenes
(naphthalene, anthracene, tetracene, pentacene, and hexacene) in their
anionic,neutral, cationic, and dicationic charge states. We used density
functional theory (DFT) to obtain the ground-state optimised geometries, and
time-dependent DFT (TD-DFT) to evaluate the electronic absorption spectra.
Total-energy differences enabled us to evaluate the electron affinities and
first and second ionisation energies, the quasiparticle correction to the
HOMO-LUMO energy gap and an estimate of the excitonic effects in the neutral
molecules. Electronic absorption spectra have been computed by combining two
different implementations of TD-DFT: the frequency-space method to study
general trends as a function of charge-state and molecular size for the
lowest-lying in-plane long-polarised and short-polarised
electronic transitions, and the real-time propagation scheme to obtain the
whole photo-absorption cross-section up to the far-UV. Doubly-ionised PAHs are
found to display strong electronic transitions of character
in the near-IR, visible, and near-UV spectral ranges, like their singly-charged
counterparts. While, as expected, the broad plasmon-like structure with its
maximum at about 17-18 eV is relatively insensitive to the charge-state of the
molecule, a systematic decrease with increasing positive charge of the
absorption cross-section between about 6 and about 12 eV is observed for each
member of the class.Comment: 38 pages, 11 figures, 7 tables, accepted for publication in Chemical
Physic
Academic Language in Book Clubs
Book clubs are increasingly used by teachers to help raise ELL literacy levels. Yet, there has been little research on whether this tool actually contributes to improved oral language proficiency. The research question is, after being taught the academic language needed to ask and answer higher order questions, how well are students able to ask and answer questions? This study examines to what extent students are able to engage in higher-order questioning after receiving direct instructional support through book clubs. The methods employ anecdotal notes, transcripts of student conversations, and student written work to assess how students can discuss text before and after being taught scaffolds. Using Bloom’s Taxonomy of the Cognitive Domain to measure results, the author concludes that: 1) engagement in authentic discourse through book clubs can increase long-term language learning; and 2) and book clubs are an effective forum to teach higher order questioning
On the treatment of LUMO energies for their use as descriptors
Calculated energies of lowest unoccupied molecular orbitals (LUMOs) are frequently employed as descriptors in studies of quantitative structure–activity relationships and linear free energy relationships involving electron transfer. However, the quantum chemical programs with which these are carried out, whether Hartree–Fock or density functional theory, do not treat orbitals of different character, for example, C=C π* and C-Cl σ*, consistently, nor is there consistency among different families of compounds. These problems can be ameliorated with the use of the experimental equivalent of the LUMO energy, the vertical attachment energy (VAE), or by shifting and scaling LUMO energies to a training set of available VAEs in similar compounds. Examples from the literature are used to illustrate these points
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