Ab initio study of MF2 (M=Mn, Fe, Co, Ni) rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the ground-state structural, electronic, and magnetic properties of the rutile-type compounds MF2 (M=Mn, Fe, Co, and Ni). All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the [001], [111], and [100] (or [010]) directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed.

I2: uma molécula didática

Iodine vapor is a very suitable substance to learn about molecular energy levels and transitions, and to introduce spectroscopic techniques. As a diatomic molecule its spectra are relatively simple and allow straightforward treatment of the data leading to the potential energy curves and to quantum mechanics concepts. The overtone bands, in the resonance Raman scattering, and the band progressions, in the electronic spectra, play an important role in the calculation of the Morse potential curves for the fundamental and excited electronic state. A weaker chemical bond in the electronic excited state, compared to the fundamental state, is evidenced by the increase in the equilibrium interatomic distance. The resonance Raman scattering of I2 is highlighted due to its importance for obtaining the anharmonicity constant in the fundamental electronic state.

Influence of ni addition to a low-loaded palladium catalyst on the selective hydrogenation of 1-heptyne

Semi-hydrogenation of alkynes has industrial and academic relevance on a large scale. To increase the activity, selectivity and lifetime of monometallic catalysts, the development of bimetallic catalysts has been investigated. 1-Heptyne hydrogenation over low-loaded Pd and Ni monometallic and PdNi bimetallic catalysts was studied in liquid phase at mild conditions. XPS results suggest that nickel addition to Pd modifies the electronic state of palladium as nickel loading is increased. Low-loaded Pd catalysts showed the highest selectivities (> 95%). The most active prepared catalyst, PdNi(1%), was more selective than the Lindlar catalyst.

Fluorescence spectra and ab initio theoretical studies of HCOOH /

A fluorescence excitation spectrum of formic acid monomer (HCOOH) , has been recorded in the 278-246 nm region and has been attributed to an n >7r* electron promotion in the anti conformer. The S^< S^ electronic origins of the HCOOH/HCOOD/DCOOH/DCOOD isotopomers were assigned to weak bands observed at 37431.5/37461.5/37445.5/37479.3 cm'''. From a band contour analysis of the 0°^ band of HCOOH, the rotational constants for the excited state were estimated: A'=1.8619, B'=0.4073, and C'=0.3730 cm'\ Four vibrational modes, 1/3(0=0), j/^(0-C=0) , J/g(C-H^^^) and i/,(0-H^yJ were observed in the spectrum. The activity of the antisymmetric aldehyde wagging and hydroxyl torsional modes in forming progressions is central to the analysis, leading to the conclusion that the two hydrogens are distorted from the molecular plane, 0-C=0, in the upper S. state. Ab initio calculations were performed at the 6-3 IG* SCF level using the Gaussian 86 system of programs to aid in the vibrational assignments. The computations show that the potential surface which describes the low frequency OH torsion (twisting motion) and the CH wagging (molecular inversion) motions is complex in the S^ excited electronic state. The OH and CH bonds were calculated to be twisted with respect to the 0-C=0 molecular frame by 63.66 and 4 5.76 degrees, respectively. The calculations predicted the existence of the second (syn) rotamer which is 338 cm'^ above the equilibrium configuration with OH and CH angles displaced from the plane by 47.91 and 41.32 degrees.

An analysis of the methyl rotation dynamics in the So (x' A) and T (a A) states of thioacetaldehyde from Pyrolysis jet spectra /

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetaldehyde CH3CHS, CH3CDS, CD3CHS and CD3CDS have been observed over the region 15800 - 17300 cm"^ in a continuous pyrolysis jet. The vibronic band structure of the singlet-triplet n -* n* transition were attributed to the strong coupling of the methyl torsion and aldehydic hydrogen wagging modes . The vibronic peaks have been assigned in terms of two upper electronic state (T^) vibrations; the methyl torsion mode v^g, and the aldehydic hydrogen wagging mode v^^. The electronic origin O^a^ is unequivocally assigned as follows: CH3CHS (16294.9 cm"'' ), CH3CDS (16360.9 cm"'' ), CD3CHS (16299.7 cm"^ ), and CD3CDS (16367.2 cm"'' ). To obtain structural and dynamical information about the two electronic states, potential surfaces V(e,a) for the 6 (methyl torsion) and a (hydrogen wagging) motions were generated by ab initio quantum mechanical calculations with a 6-3 IG* basis in which the structural parameters were fully relaxed. The kinetic energy coefficients BQ(a,e) , B^(a,G) , and the cross coupling term B^(a,e) , were accurately represented as functions of the two active coordinates, a and 9. The calculations reveal that the molecule adopts an eclipsed conformation for the lower Sq electronic state (a=0°,e=0"') with a barrier height to internal rotation of 541.5 cm"^ which is to be compared to 549.8 cm"^ obtained from the microwave experiment. The conformation of the upper T^ electronic state was found to be staggered (a=24 . 68° ,e=-45. 66° ) . The saddle point in the path traced out by the aldehyde wagging motion was calculated to be 175 cm"^ above the equilibrium configuration. The corresponding maxima in the path taken by methyl torsion was found to be 322 cm'\ The small amplitude normal vibrational modes were also calculated to aid in the assignment of the spectra. Torsional-wagging energy manifolds for the two states were derived from the Hamiltonian H(a,e) which was solved variationally using an extended two dimensional Fourier expansion as a basis set. A torsionalinversion band spectrum was derived from the calculated energy levels and Franck-Condon factors, and was compared with the experimental supersonic-jet spectra. Most of the anomalies which were associated with the interpretation of the observed spectrum could be accounted for by the band profiles derived from ab initio SCF calculations. A model describing the jet spectra was derived by scaling the ab initio potential functions. The global least squares fitting generates a triplet state potential which has a minimum at (a=22.38° ,e=-41.08°) . The flatter potential in the scaled model yielded excellent agreement between the observed and calculated frequency intervals.

Rotational analysis of the 5 1 0 6 1 0 band of formyl chloride

Please consult the paper edition of this thesis to read. It is available on the 5th Floor of the Library at Call Number: Z 9999 C54 L434 1989

Études spectroscopiques expérimentales et théoriques de complexes de métaux de transition

Cette thèse présente une série d'études qui visent la compréhension de la structure électronique de complexes de métaux de transition en employant diverses méthodes de spectroscopie. L'information sur la structure électronique aide à comprendre et développer des nouveaux matériaux, des nouvelles voies de synthèses, ainsi que des nouveaux modèles théoriques. Habituellement, afin d'explorer la structure électronique d'un système qui comporte en son centre un métal de transition, l'information fournie par les spectres d'un seul composé n'est pas suffisante. On étudie une série de composés similaires, qui ont le même métal de transition à un degré d'oxydation donné, ainsi que des ligands qui forment des liaisons de différentes forces et caractéristiques avec le métal. Cependant, ces changements, bien qu'on les désire de faible impact, créent une grande perturbation de la structure électronique visée par les études. Afin d'étudier en profondeur une seule structure électronique, nous employons une stratégie d'analyse moins perturbante. Nous appliquons une pression hydrostatique sur les complexes de métaux de transition. Cette pression perturbe le système suffisamment pour nous livrer davantage d'informations sur la structure électronique, sans la « dénaturer ». Afin d'étudier précisément ces systèmes perturbés, la technique d'application de pression est conjuguée, dans la littérature, aux diverses techniques de spectroscopie d'absorption UV-visible, de luminescence, ainsi que de diffusion Raman. Pour extraire un maximum d'informations de ces expériences, on emploie des techniques de calculs de structure électronique ainsi que de dynamique des noyaux. Dans cette thèse, on tente de mettre en lumière la structure électronique de composés de molybdène(IV), de platine(II) et palladium(II) à l'aide de la technique de pression couplée aux spectroscopies de luminescence et de diffusion Raman. Dans le chapitre 3, on observe un déplacement de la bande de luminescence de +12 cm-1/kbar entre la pression ambiante et 25 kbar pour le complexe trans-[MoOCl(CN-t-Bu)4]BPh4, dont le centre métallique molybdène(IV)est de configuration électronique 4d2. Il s'agit de la première variation positive observée pour un complexe de type métal-oxo. À des pressions plus élevées, la tendance s'inverse. Le maximum d'énergie de la bande de luminescence se déplace de -8 cm-1/kbar. Ce changement de variation présage d'une compétition interne entre les ligands situés sur les différents axes de l'octaèdre. À l'aide de calculs basés sur la théorie de la fonctionnelle de la densité, on propose un mécanisme pour expliquer ce phénomène. Au cours du chapitre 4, on étudie des complexes de palladium(II) et de platine(II) qui ont les mêmes ligands. Un de ces ligands est le 1,4,7-trithiacyclononane (ttcn). On constate qu'à basse pression le ligand est bidentate. Par contre, lorsque la pression augmente, on constate, par exemple à l'aide du complexe [Pt(ttcn)Cl2], qu'une interaction anti-liante supplémentaire se produit entre le ligand ttcn et le métal, ce qui change la nature de l'orbitale HOMO. On observe un déplacement de la bande de luminescence de -19 cm-1/kbar. Tel que pour le complexe de molybdène(IV), le déplacement de la bande de luminescence dépend de la compétition entre les ligands situés sur les différents axes de l'octaèdre. L'interaction liante entre l'ion platine(II) et l'atome de soufre axial est l'effet le plus plausible qui peut induire un déplacement de la bande de luminescence vers les basses énergies. Ceci nous indique que cette interaction domine. Par contre, pour ce qui est du complexe palladium(II), la compétition est remportée par d'autres effets, car le déplacement de la bande de luminescence est de +6 cm-1/kbar. Encore une fois, des calculs, basés sur la théorie de la fonctionnelle de la densité, aident à explorer les causes de ces observations en suggérant des explications corroborées simultanément par les diverses expériences de spectroscopie. Lors du chapitre 5, une étude plus exacte de la structure électronique ainsi que de la dynamique des noyaux de complexes de métaux de transition est présentée. En effet, les complexes de palladium(II) et de platine(II), de type [M(X)4]2-, ont une structure simple, très symétrique. Le premier état excité de ces molécules subit la distorsion Jahn-Teller. On veut établir un protocole de travail pour les expérimentateurs afin d'analyser des spectres de molécules pour lesquelles l'approximation de Born-Oppenheimer n'est pas valide. On utilise la théorie de la fonctionnelle de la densité dépendante du temps ainsi que le modèle de Heidelberg afin de décrire des effets non adiabatique. On tente d'établir l'influence des effets non adiabatiques sur les spectres de ce type de complexe.

Ab initio study of MF2 (M=Mn, Fe, Co, Ni) rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the ground-state structural, electronic, and magnetic properties of the rutile-type compounds MF2 (M=Mn, Fe, Co, and Ni). All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the [001], [111], and [100] (or [010]) directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed.

“Investigations on Vibrational Overtones in the NIR region and Laser Induced Fluorescence in some selected compounds”

The central theme of this research concerns the study of vibrationally excited molecules. We have used the local mode description of such vibrational states, and this -model has now gained general acceptance. A central feature of the model is the Wloealizafion of vibrational energy. A study of these high—energy localized states provides example, becauseof this localization, overtone spectra, which measure the absorption of T vibrational energy, are extremely sensitive to the properties of X-H bonds. We also use -overtone spectra to study the conformation of molecules, i.e., the relative internal orientation of their bonds. The thesis comprises six chapters

Multiferroic Behavior of Gd Based Manganite

Here we report the multiferroic nature of charge ordered manganite Gd0.5Sr0.5MnO3 for the first time. The temperature variation of dielectric constant shows broad relaxor type ferroelectric transition at around 210K and magnetization measurements shows weak ferromagnetism at 50K. The dielectric peak is very close to charge ordering temperature which is an evidence of the link between electronic state and increase of dielectric response. Butterfly variation of capacitance with voltage confirms ferroelectric nature of the sample at room temperature

Femtosecond time-resolved wave packet motion in molecular multiphoton ionization and fragmentation

The dynamics of molecular multiphoton ionization and fragmentation of a diatomic molecule (Na_2) have been studied in molecular beam experiments. Femtosecond laser pulses from an amplified colliding-pulse mode-locked (CPM) ring dye laser are employed to induce and probe the molecular transitions. The final continuum states are analyzed by photoelectron spectroscopy, by ion mass spectrometry and by measuring the kinetic energy of the formed ionic fragments. Pump-probe spectra employing 70-fs laser pulses have been measured to study the time dependence of molecular multiphoton ionization and fragmentation. The oscillatory structure of the transient spectra showing the dynamics on the femtosecond time scale can best be understood in terms of the motion of wave packets in bound molecular potentials. The transient Na_2^+ ionization and the transient Na^+ fragmentation spectra show that contributions from direct photoionization of a singly excited electronic state and from excitation and autoionization of a bound doubly excited molecular state determine the time evolution of molecular multiphoton ionization.

Femtosecond pump-probe study of the spreading and recurrence of a vibrational wave packet in Na_2

The motion of a vibrational wave packet in the bound A(^1 \summe^+_u) electronic state of the sodium dimer is detected in a femtosecond pump/probe molecular beam experiment. For short times harmonic motion is seen in the total ion yield of Na^+_2 as a function of delay time between the two laser pulses. The spreading of the wave packet results in the loss of the periodic variation of the ion signal. For longer delay times (47 ps) the wave packet regains its initial form which is reflected in the revival structure of the Na^+_2 signal. Time-dependent quantum calculations reproduce the measured effects.

Iron oxide nanoparticles and VO(x)/Hexadecylamine nanotubes composite

In this work, we present the synthesis and characterization of a hybrid nanocomposite constituted by iron oxide nanoparticles and vanadium oxide/Hexadecylamine (VO(x)/Hexa) nanotubes. Transmission Electron Microscopy (TEM) images show small particles (around 20 nm) in contact with the external wall of the multiwall tubes, which consist of alternate layers of VO(x) and Hexa. By Energy Dispersive Spectroscopy (EDS), we detected iron ions within the tube walls and we have also established that the nanoparticles are composed of segregated iron oxide. The samples were studied by Electron Paramagnetic Resonances (EPR) and dc-magnetization as a function of the magnetic field. The analysis of the magnetization and EPR data confirms that a fraction of the V atoms are in the V(4+) electronic state and that the nanoparticles exhibit a superparamagnetic behavior. The percentage of V and Fe present in the nanocomposite was determined using Instrumental Neutron Activation Analysis (INAA). (C) 2008 Elsevier B.V. All rights reserved.

Bichromophoric behavior of nitrophenyl-triazene anions: a resonance Raman spectroscopy investigation

Highly delocalized molecular frameworks with intense charge transfer transitions, known as push-pull systems, are of central interest in many areas of chemistry, as is the case of nitrophenyl-triazene derivatives. The 1,3-bis(2-nitrophenyl)triazene and 1,3-bis(4-nitrophenyl)triazene were investigated by electronic (UV-Vis) and resonance Raman (RR) spectroscopies. The bichromophoric behavior of 1,3-bis(4-nitrophenyl)triazene anion opens the possibility of tuning with visible radiation, two distinct electronic states. The RR profiles of nitrophenyl-triazene derivatives clearly show that the first allowed electronic state can be assigned to a charge transfer from the ring pi system to the NO2 moiety (ca 520 nm), while the second, as a charge transfer from N-3(-) to the aromatic ring (ca 390 nm). In the para-substituted derivative, a more efficient electron transfer and a greater energy separation between the two excited states are observed. Copyright (C) 2008 John Wiley & Sons, Ltd.

Close-coupling calculations of elastic and inelastic positron-helium scattering

Elastic and inelastic positron-helium scattering have been investigated in different partial waves at medium energies using the close-coupling approximation with realistic wavefunctions employing the following states: He(1s1s), He(1s2s), He(1s2p), He(1s3s), He(1s3p), Ps(1s), Ps(2s) and Ps(2p). All excitations of the helium atom are in the spin-singlet electronic state. Calculations are reported of cross sections to He(1s1s), He(1s2s), and He(1s2p) transitions for incident positron energies up to 200 eV. These cross sections are in good agreement with experimental results.