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Molecular Dipole (molecular + dipole)
Terms modified by Molecular Dipole Selected AbstractsAdvancing beyond charge analysis using the electronic localization function: Chemically intuitive distribution of electrostatic momentsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2008Julien Pilmé Abstract We propose here an evaluation of chemically intuitive distributed electrostatic moments using the topological analysis of the electron localization function (ELF). As this partition of the total charge density provides an accurate representation of the molecular dipole, the distributed electrostatic moments based on the ELF partition (DEMEP) allows computing of local moments located at non atomic centers such as lone pairs, , bonds and , systems. As the local dipole contribution can be decomposed in polarization and charge transfer components, our results indicate that local dipolar polarization of the lone pairs and chemical reactivity are closely related whereas the charge transfer contribution is the key factor driving the local bond dipole. Results on relevant molecules show that local dipole contributions can be used to rationalize inductive polarization effects in alcohols derivatives and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a , character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. That way, the nature of the CO bond has been revisited for several typical systems by means of the DEMEP analysis which appears also helpful to discuss aromaticity. Special attention has been given to the carbon monoxide molecule, to the CuCO complex and to a weak intramolecular N|---CO interaction involved in several biological systems. In this latter case, it is confirmed that the bond formation is mainly linked to the CO bond polarization. Transferability tests show that the approach is suitable for the design of advanced force fields. © 2008 Wiley Periodicals, Inc. J Comput Chem 2008 [source] Molecular polarizability of fullerenes and endohedral metallofullerenesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2002Francisco Torrens Abstract The interacting induced dipoles polarization model implemented in our program POLAR is used for the calculation of the molecular dipole µ and tensor quadrupole moments and also the dipole,dipole polarizability . The method is tested with Scn, Cn (fullerene and graphite) and endohedral Scn@Cm clusters. The polarizability is an important quantity for the identification of clusters with different numbers of atoms and even for the separation of isomers. The results for the polarizability are of the same order of magnitude as from reference calculations performed with our version of the program PAPID. The bulk limit for the polarizability is estimated from the Clausius,Mossotti relationship. The polarizability trend for these clusters as a function of size is different from what one might have expected. The clusters are more polarizable than what one might have inferred from the bulk polarizability. Previous theoretical work yielded the same trend for Sin, Gen and GanAsm small clusters. However, previous experimental work yielded the opposite trend for Sin, GanAsm and GenTem larger clusters. At present, the origin of this difference is problematic. One might argue that smaller clusters need not behave like those of intermediate size. The high polarizability of small clusters is attributed to dangling bonds at the surface of the cluster. In this respect, semiconductor clusters resemble metallic clusters. Copyright © 2002 John Wiley & Sons, Ltd. [source] Oriented Electric Fields Accelerate Diels,Alder Reactions and Control the endo/exo SelectivityCHEMPHYSCHEM, Issue 1 2010Rinat Meir Abstract Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels,Alder (DA) reactions. When the EEF is oriented along the "reaction axis" (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4,6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the "reaction axis" in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis-ŕ-vis the individual molecular dipole. At some critical value of the EEF along the "reaction axis", there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions. [source] Anomalous charge relaxation in channels of pentacene-based organic field-effect transistors: a charge transient spectroscopy studyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2006I. Thurzo Abstract Two types of Si/SiO2/pentacene organic field-effect transistors (OFET) with bottom Au-source (S) and , drain (D) electrodes were examined by charge transient spectroscopy (QTS), applying pulsed bias ,UDS to the channel of an OFET with floating gate electrode. The transient charge Q (t ), flowing through the channel after the removal of the bias pulse, was processed at a constant temperature by a three-channel correlator yielding the signal ,Q = Q (t1) , 3/2Q (2t1) + 1/2Q (4t1), the scanned delay t1 being related to the trailing edge of the bias pulse. Most of the QTS spectra were characterized by peaks of ,Q (t1) with FWHM corresponding to discrete time constants ,m , t1m, while scanning t1 from 2 ,s to 0.1 s. The common feature of the QTS spectra was a linear dependence of the peak height ,Qm on ,UDS for both polarities of the latter, thereby resembling what is expected for dielectric relaxation (polarization). Some devices showed anomalous (reversed) sign of the signal with respect to the polarity of ,UDS, or even features like transitions from the correct sign to the reversed one. In order to customize the anomalies, a model is presented which ignores injection of excess charge carriers and takes into account two contributions to the total transient charge: a/space charge of intrinsic charge carriers piled up at the blocking Au-electrodes during the pulse, relaxing with the dielectric relaxation time ,D = ,0,r/, (, being conductivity of the organics); b/orientation of molecular dipoles (,dip) in the relaxing electric field of the space charge. It is the dipolar component that is responsible for the anomalous charge flow direction manifested by the signal reversal. The origin of the permanent dipole moment of the otherwise non-polar pentacene molecules may be either attached excess or missing atoms (vacancies) of the defect molecules [J. E. Northrup and M. L. Chabinyc, Phys. Rev. B 68, 041202 (2003)]. In cases of non-blocking contacts the dipolar relaxation would lead to QTS peaks of correct sign, to be distinguished from possibly non-negligible contribution of the dielectric relaxation in the semiconductor. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||