1,720,979 research outputs found
Vertical electron affinities in substituted trimethylsilanes
No full textThe gas-phase negative electron affinities obtained by means of electron transmission spectroscopy (ETS) in the series (CH3)3Si-X (with X = I, Br, Cl, Si(CH3)3, SCH3, OCH3, N(CH3)2, CH3) are compared. The electron-acceptor properies of these compounds are enhanced when the substituent contains third-row (or heavier) elements. Geometries and charge distributions in the neutral molecules are calculated at the 3-21G* and 6-31G** ab initio levels (except for X = I, Br). The LUMO energies predicted by 3-21G*, 6-31G**, *SCF and semiempirical MNDO calculations are compared with the resonance energy trend observed in the ET spectra
Ab initio determination of the geometric structure of oligo-2-thienyl ketones
No full textConformations, structural parameters and charge distributions of 2-acetylthiofene (THCO), di-2-thienyl ketone (2THCO) and higher oligomers (3THCO and 4THCO) have been determined by ab initio calculations at the HF/6-31G** level of theory. Electron correlation effects have been investigated at MP2/6-31G** level for THCO and at the MP2/6-311G**//HF/6-31G** level for 2THCO. The most probable conformation of oligo-2-thienylene ketones is rod-like, with a ring deviation from the main molecular plane of about 20° and with prevailing S,O-cis orientation
Spectroscopic and theoretical determination of the electronic structure of anisole, thioanisole, and methoxy- and methylthiobenzonitriles: A contribution to the study of organic conducting polymers
No full textGas-phase ionization and attachment energy (IE and AE) values of some para- and ortho-cyano derivatives of anisole and thioanisole (p-NCPhXCH3, X = O, S and o-NCPhXR, X = O, R = CH3; X = S, R = H, CH3, and C(CH3)3) have been determined experimentally. The assignments of the spectra, based on those of the parent compounds PhXMe, NCPh, and XMe2 (Me = CH3), agree with the results of theoretical HF/6-31G** calculations. The calculations correctly reproduce the prevalence of the planar rotamer of the oxy-derivatives with respect to the gauche one, while overestimating the relative stability of the gauche conformer of the thio-derivatives.
The two rotamers of the thio-derivatives have similar energy and their valence energy levels do not sizeably differ. In addition, PhSMe and p-NCPhSMe have HOMO-LUMO energy gaps (9.5 eV) in the oxy-derivatives and there are indications that the methylthio group has larger polarizability than the methoxy group. These data suggest that poly(p-phenylene sulfide) is more suitable than poly(p-phenylene oxide) to carry electricity under mild doping
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Determination of the electronic structure of oligo(2-thienyl ketones) and extrapolation to poly(thienylene ketone)
No full textGas phase ionization and attachment energy (IE and AE) values of the first few oligomers of 2-thienylketone have been determined experimentally. The assignment of the ultraviolet photoelectron spectra agree with the results of theoretical HF/6-31G**calculations. The constant value of the HOMO energy allows extrapolation to the corresponding polymer. A combined analysis of the AE values, the electrochemical reduction data and the results of ab initio 6-31G** and semi-empirical calculations allow one to obtain the LUMO energy for the oligomers and for an ideal gas-phase polythienyleneketone.
The narrow valence band and the high IE value of the polymer are not propitious to p-doping. On the other hand, the high electron affinity of the oligomers and the polymer, the reversibility of the oligomer electrochemical reduction and the large conduction band of the polymer indicate that the oligomers, and likely the polymer, could give rise to n-doped materials having high electrical conductivity
Ultraviolet Photoelectron, Electron Transmission and ab-initio Study of the Factors Determining the Stability of imines
No full textThe electronic structure of several alkyl-, N_bis(trimethylsilyl)methyl- (BTMSM), and C_phenyl-substituted imines 1 - 11 have been determined by ab-initio 3-21G* calculations. The calculations reproduce with good accuracy the available geometry of methyleneimine, 1, obtained by microwaves spectroscopy1 and ab-initio 6-31G** calculations2 as well as the outermost energy levels experimentally determined by UPS for several derivatives. The analysis of the results of the calculations indicate that alkyl substitution of 1 at the nitrogen atom reduces the charge separation at the double bond increasing stabilisation, while substitution at the carbon atom has the opposite effect. Steric hindrance of the substituent(s) and the energy gaps between filled (nitrogen lone pair, HOMO, and πCN) and virtual (π*CN) orbitals play a secondary role on stability.
All the BTMSM and phenyl derivatives studied are stable in air at room temperature irrespective of other substituents present. Our analysis indicates that these substituents change the energy, the ordering and the localisation properties of the valence MOs. In particular, the nitrogen lone pair orbital is no more the HOMO
Comparative spectroscopic and theoretical studies on the conformation of some alpha-diethoxyphosphoryl carbonyl compounds and their alpha-ethylsulfonyl analogues
No full textThe comparative νCO IR analysis between β-carbonylphosphonates [XC(O)CH2 P(O)(OR)2: X=Me 1, Ph 2, OEt 3 , NEt2 4 and SEt 5; R=Et] (series I) and β-carbonylsulfones [XC(O)CH2SO2R: X=Me 6, Ph 7, OEt 8 , NEt2 9 and SEt 10; R=Et] (series II) along with ab initio 6-31G** calculations of 1a and 6a (R=Me) suggest the existence of only a single gauche conformer for I. The negative carbonyl frequency shifts for both series follow approximately the electron-affinities of the π*CO orbital of the parent compounds MeC(O)X 11-15. The less positive asymmetric sulfonyl frequency shifts (ΔνSO2) for II in relation to the phosphoryl frequency shifts (ΔνPO) for I and the larger negative carbonyl frequency shifts for II with respect to the corresponding values for I are in line with the upfield 13C NMR chemical shifts of the carbonyl carbon for II compared to I. These trends agree with the shorter O(SO2)...C(CO)¬ contact in comparison with the O(PO)...C(CO)¬ one and are discussed in terms of Olp * π *CO charge transfer and electrostatic interactions which are stronger for series II than for I, indicating that the sulfonyl oxygen atom is a better electron donor than the phosphoryl oxygen atom. Intrinsic geometrical parameters of O=S-CH2 and O=P-CH2¬ moieties seems to be responsible for this behaviour indicated by X-ray and ab initio calculations of dialkyl methylsulfonylmethanephosphonate MeSO2CH2P(O)(OR)2 (R=Et 18, Me 18a)
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