Electronic and optical properties of families of polycyclic aromatic hydrocarbons: a systematic (time-dependent) density functional theory study

Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their crystalline state are among the most promising materials for organic opto-electronics. Following previous works on oligoacenes we present a systematic comparative study of the electronic, optical, and transport properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using density functional theory (DFT) and time-dependent DFT we computed: (i) electron affinities and first ionization energies; (ii) quasiparticle correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv) electronic absorption spectra of neutral and $\pm1$ charged systems. The excitonic effects are estimated by comparing the optical gap and the quasiparticle corrected HOMO-LUMO energy gap. For each molecular property computed, general trends as a function of molecular size and charge state are discussed. Overall, we find that circumacenes have the best transport properties, displaying a steeper decrease of the molecular reorganization energy at increasing sizes, while oligorylenes are much more efficient in absorbing low-energy photons in comparison to the other classes.Comment: 26 pages, 9 figures, 4 tables, accepted for pubblication in Chemical Physics (14/04/2011

Communication: In search of four-atom chiral metal clusters

A combined study utilizing anion photoelectron spectroscopy and density functional theory was conducted to search for four-atom, chiral, metal, and mostly metal clusters. The clusters considered were AuCoMnBi−/0, AlAuMnO−/0, AgMnOAl−/0, and AuAlPtAg−/0, where the superscripts, −/0, refer to anionic and neutral cluster species, respectively. Based on the agreement of experimentally and theoretically determined values of both electron affinities and vertical detachment energies, the calculated cluster geometries were validated and examined for chirality. Among both anionic and neutral clusters, five structures were identified as beingchiral

Activation of CH4_4, NH3_3, and N2_2 by Tantalum Ions, Clusters and Their Oxides: What Can Be Learnt from Studies of Ions in the Gas Phase

The emission control of harmful compounds and greenhouse gases and the development of alternative, sustainable fuel sources is a major focus in current research. A solution for this problem lies in the development of efficient catalytic materials. Here, gas phase model systems represent prominent examples for obtaining fundamental insights on reaction properties of prospective catalytic systems. In this work, we review results from studies of tantalum clusters and their oxides in the gas phase and discuss insights with a potential relevance for applied systems. We focus on reactions that are essential for sustainable chemistry in the future. In detail, we address the activation of methane, which may enable the transformation of a greenhouse gas to a chemical feedstock, and we discuss the activation of NH$_3$, which may function as an alternative energy carrier whose unwanted emission needs to be curbed in future applications. Finally, we consider the activation of N$_2$ as a third reaction, since reducing the high energy demand of ammonia synthesis still bears significant challenges. While tantalum may be an interesting catalytic material, the discussed studies may also serve as benchmark for investigations of other materials

Electron transfer and hydride transfer pathways in the Stoltz-Grubbs reducing system (KOtBu-Et3SiH)

Recent studies by Stoltz, Grubbs et al. have shown that triethylsilane and potassium tert-butoxide react to form a highly attractive and versatile system that shows (reversible) silylation of arenes and heteroarenes as well as reductive cleavage of C-O bonds in aryl ethers and C-S bonds in aryl thioethers. Their extensive mechanistic studies indicate a complex network of reactions with a number of possible intermediates and mechanisms, but their reactions likely feature silyl radicals undergoing addition reactions and SH2 reactions. This paper focuses on the same system, but through computational and experimental studies, reports complementary facets of its chemistry based on (a) single electron transfer (SET), and (b) hydride delivery reactions to arenes

Tuning SMSI Kinetics on Pt-loaded TiO2_2(110) by Choosing the Pressure: A Combined UHV / Near-Ambient Pressure XPS Study

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O$_2$ and H$_2$ on the SMSI-state of well-defined Pt/TiO$_2$(110) catalysts at pressures of up to 0.1 Torr. By tuning the O$_2$ pressure, we can either selectively oxidize the TiO$_2$ support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1x10$^{-5}$ Torr O$_2$, but orders of magnitudes less effective at higher O$_2$ pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred

From C-H Bond Insertion to Hydrogen Atom Transfer: Tuning the Reaction Mechanisms of Methane Activation by the Oxidation of Ta₂⁺

The activation of methane under mild conditions is a challenging but rewarding goal; the underlying key parameters, however, remain elusive. In this study on isolated tantalum Ta2+ compounds exposed to methane in a ring-electrode ion trap, strong changes in the reactivity are observed depending on the compound\u27s degree of oxidation. While the general reaction behavior is presented for species ranging from Ta2+ to Ta2O6+ based on experimental kinetic studies, we focus in more detail on the dehydrogenation reactions occurring on Ta2O2+ and the hydrogen atom transfer (HAT) on Ta2O5+, for which density functional theory calculations were performed. In the first part, we elucidate the role of Ta–C–Ta bridging motifs in product structures as driving forces for the dehydrogenation of methane on Ta2O2+; in the second part, we investigate the origins of the HAT – a hitherto unknown reaction scheme for binary tantalum oxides. For the latter, we show that the reactivity originates from the spin density on oxygen atoms, which is a typical characteristic of the reaction on other metal oxides. This reflects a change in the reactivity from oxidized metallic systems to metal oxides and demonstrates that chemical modifications of tantalum compounds can achieve different methane activation schemes

Wpływ treningu i biegu półmaratońskiego na prawą komorę u biegaczy amatorów

Introduction. Endurance running may lead to heart remodelling. There are little data on the right ventricular (RV) changes in amateur athletes running shorter than marathon distances. The study aimed to investigate whether training and running a half marathon affect the anatomy and function of the RV in amateur runners and whether these changes affect the athlete's competitive performance. Material and methods. The study included 45 recreational runners with a mean age of 32.96 (5.12) years, 27 men. Echocardiography was performed before the ten-weeks training period and before and after the half marathon run. The morphological and functional parameters of the RV were analysed, including two-dimensional, Doppler and speckle-tracking echocardiography. Results. In training period, the RV outflow tract (27.98 [5.46] vs. 30.07 [4.90]; p = 0.003) and the RV index of myocardial performance (0.36 [0.29; 0.45] vs. 0.39 [0.33; 0.52]; p = 0.017) increased significantly and no changes were found for E/e’. After the half marathon run, the absolute value of the RV free wall global longitudinal strain increased significantly (–25.89 [3.08] vs. –27.20 [3.42]; p = 0.008). Athletes who trained more intensively during the training period achieved significantly better half marathon results (r = –0.4; p ≤ 0.05). Conclusions. More enhanced physiological RV remodelling under exercise in amateur athletes results in better half marathon finishing times. The preparation period and 21.0975 kilometres run do not affect the diastolic function of the RV in recreational runners. The RV systolic function improves immediately after the half marathon performance.Wstęp. Biegi długodystansowe mogą prowadzić do przebudowy serca. Niewiele jest danych na temat zmian w prawej komorze u sportowców amatorów biegających na dystansach krótszych niż maratoński. Celem pracy było wykazanie, czy trening i ukończenie półmaratonu wpływają na anatomię i funkcję prawej komory u biegaczy amatorów oraz czy zmiany te wpływają na osiągany przez sportowca wynik. Materiał i metody. Badaniem objęto 45 biegaczy amatorów w średnim wieku 32,96 (5,12) lat, w tym 27 mężczyzn. Echokardiografię wykonano przed 10-tygodniowym okresem treningowym oraz przed i po biegu półmaratońskim. Analizie poddano parametry morfologiczne i czynnościowe prawej komory. Wykonano echokardiografię dwuwymiarową, dopplerowską i stosowano technikę śledzenia plamki akustycznej. Wyniki. W okresie treningowym droga odpływu prawej komory (27,98 [5,46] vs. 30,07 [4,90]; p = 0,003) oraz wskaźnik sprawności prawej komory (0,36 [0,29; 0,45] vs. 0,39 [0,33; 0,52]; p = 0,017) wzrosły znacząco i nie stwierdzono zmian dla E/e’. Po półmaratonie wartość bezwzględna globalnego podłużnego odkształcenia wolnej ściany prawej komory istotnie wzrosła (–25,89 [3,08] vs. –27,20 [3,42]; p = 0,008). Sportowcy, którzy w okresie treningowym trenowali intensywniej, osiągnęli istotnie lepsze wyniki w półmaratonie (r = –0,4; p ≤ 0,05). Wnioski. Silniejsza fizjologiczna przebudowa prawej komory serca pod wpływem ćwiczeń u sportowców amatorów skutkuje lepszym czasem ukończenia półmaratonu. Okres przygotowań i przebiegnięcie 21,0975 km nie wpływa na funkcję rozkurczową prawej komory u biegaczy rekreacyjnych. Funkcja skurczowa prawej komory poprawia się natychmiast po ukończeniu półmaratonu

IR-photodissociation and photodetachment spectroscopy of Cl− · (NH3)x (IR:

Complexes of ammonia mols. and one chloride ion have been studied by photodetachment and IR-photodissocn. spectroscopy. For the smallest anionic complex, the stabilization energy with respect to the bare chloride ion and vibrational frequencies have been detd. Two bands showed a splitting due to rotational branches, which could be represented by simulation. Rotational consts. obtained by former ab initio calcns. are confirmed and rotational consts. of a vibrationally excited state are supplied. IR-photodissocn. spectra of clusters with up to four ammonia mols. per chloride ion were recorded. [on SciFinder(R)

Photodetachment photoelectron spectroscopy of the weakly bound OClO-center dot H2O complex

The weakly bound complex OClO-·H2O and its neutral counterpart are of interest for atm. chem. as well as for studies of microsolvation. Electron affinity and binding energies can be deduced from photodetachment photoelectron spectra of the bare mol. and its H2O complex. Such spectra are presented here. An unusual large binding energy of the anionic cluster of 0.75 eV was found. [on SciFinder(R)

The electron affinity of phenanthrene

Phenanthrene is studied by photodetachment-photoelectron spectroscopy. Due to the absence of a parent ion peak in the anion mass spectrum the electron affinity could not be detd. directly. However, this absence is the first indication that this mol. has a neg. electron affinity. The first three water complexes of phenanthrene were studied, supplying insights into its microsolvation property. Moreover, the electron affinity of the bare mol. could be detd. to be -0.01±0.04 eV by an extrapolation method using the water cluster data. The exptl. work is supported by ab initio calcns. for detg. the structure of the water complexes. Finally a correlation between the electron affinity and the redn. potential of polycyclic arom. hydrocarbons is investigated. [on SciFinder(R)