Ces travaux de thèse sont consacrés au développement de nouvelles méthodologies pour l’analyse structurale et quantitative de composés isomères ou isobares en mélange par spectrométrie de masse en tandem (MS/MS) ainsi que par la technique des ions survivants (SY). À l’aide de cette technique, nous avons développé une méthode de « purification collisionnelle en phase gaz » consistant à purifier un composé par fragmentation sélective du contaminant isomère ou isobare afin de permettre l’analyse structurale et quantitative du composé d’intérêt. Nous avons montré que cette approche peut être utilisée avec succès à la fois lors de l’étape d’excitation collisionnelle (CID) d’une expérience MS/MS, mais également lors du processus d’ionisation (in-source CID). Utilisant cette approche MS/MS sur une fenêtre de 15 m/z, nous avons ainsi pu quantifier, par la méthode de l’étalon interne, un peptide trypsique malgré la présence d’un contaminant isobare. L’optimisation des performances de quantification pour la technique SY a ensuite été étudiée sur des peptides isomères topologiques en mélange et comparée à l’analyse par microscopie infrarouge. Parmi les alcalins, alcalino-terreux et métaux de transition testés, nous avons obtenu les meilleurs résultats avec les adduits au césium. Des résultats comparables à la technique infrarouge ont confirmé la pertinence de notre approche avec de surcroît de meilleures performances analytiques, en particulier en terme de rapidité d’exécution, de sensibilité, d’erreur de prédiction et de limite de quantification. / This PhD work focused to the development of new methodologies for the structural and quantitative analysis of isomers or isobars compounds in mixture using tandem mass spectrometry (MS/MS) and the Survival Yield technique (SY).Using this technique, we have developed a method of « gas phase collisional purification » of purifying a compound by selective fragmentation of the isomeric or isobaric contaminant to allow the structural and quantitative analysis of the compound of interest. We have shown that this approach can be used successfully both during the collisional excitation step (CID) of a MS/MS experiment, but also during the ionization process (in-source CID). Using this MS/MS approach on an isolation window of 15 m/z, we were able to quantify, by the internal standard method, a tryptic peptide despite the presence of an isobaric contaminant.Optimization of quantification performances for the SY technique was then studied on topological isomeric peptides in mixture and compared with infrared microscopy analysis. Among the alkali, alkaline earth and transition metals tested, we obtained the best results with cesium adducts.Results comparable to the infrared technique confirmed the relevance of our approach with, moreover, better analytical performances, in particular in terms of speed of execution, sensitivity, prediction error and limit of quantification.

This thesis is focussed on the investigation of matrices for matrix-assisted diffusion-ordered spectroscopy (MAD). Diffusion-ordered spectroscopy (DOSY) is a family of experiments where the resonances in the chemical shift dimension are further dispersed in an extra dimension according to diffusion coefficient. A typical DOSY spectrum shows one single diffusion coefficient for all the resonances coming from one single species. However, If two or more resonances overlap, the diffusion resolution of the DOSY spectrum is compromised and a spurious diffusion coefficient results, intermediate between the species. In case of signal overlap, the use of more advanced processing methods aids to separate two analytes that differ by at least 30% in diffusion coefficient. In practice, many mixtures contain species of similar diffusion coefficients whose resonances overlap in the chemical shift dimension. The addition of co-solutes can modify the chemical environment (matrix), with which different analytes interact to different extents, and enhance the diffusion resolution of DOSY. However, the addition of co-solutes can risk the benefits of DOSY by increasing the probability of signal overlap. Signal overlap in MAD is avoided by using a 1H NMR-invisible surfactant such as sodium perfluorooctanoate (NaPFO), which has replaced each proton by a fluorine atom. PFO micelles are a tunable matrix which allows the separation of analytes via coulombic interactions by adjusting the pH. Differences in diffusion coefficient in NaPFO solution can be analysed using a modified Lindman's law to model the diffusion coefficient as a function of pH. The model rationalises the binding constants of analytes to PFO micelles with good accuracy, subject to the spectral data quality. Another alternative to resolve diffusion coefficients using the invisible MAD approach is by means of a commercially available alkyl surfactant like cetyltrimethylammonium bromide (CTAB). CTAB in high ionic strength solution forms worm-like micelles whose resonances can be filtered out from the final DOSY spectrum. CTAB worm-like micelles have short transverse relaxation times compared to all of the analytes in the mixture. If a transverse relaxation filter is positioned at the beginning of a standard DOSY pulse sequence, as in PROJECT-Oneshot, the strong CTAB signals vanish and leave behind only the analyte resonances and hence avoid signal overlap. Finally, the use of bovine serum albumin (BSA) as a potential invisible matrix, using a similar approach to CTAB worm-like micelles is investigated, using a relaxation-weighted DOSY pulse sequence to suppress most of the BSA background signal (at a cost in analyte signal to noise ratio). An alternative to suppress most of the BSA background and preserve most of the analyte signal is by means of mild transverse relaxation filtration and spectral editing to obtain an edited DOSY spectrum that shows only the analyte signals. Nonetheless, it is a shame that useful MAD results can only be obtained under a narrow set of conditions: i) different mole ratios BSA: analyte to aid diffusion resolution, ii) mild T2 filtration to improve analyte signal to noise ratio and iii) spectral editing to remove residual BSA background.

No description available.

L'objectif de cette thèse était d’évaluer quelques Metal-Organic Frameworks (MOFs), choisis en fonction de leur taille de pores, de leur volume poreux et de leur stabilité thermique, en adsorption et séparation des hydrocarbures. Pour étudier le comportement général des MOFs nous avons choisi des MOFs avec des centres métalliques insaturés, des MOFs à charpente anionique et des ZIFs neutres et avons étudié leur sélectivité en séparation de trois familles d'hydrocarbures, à savoir alcanes, alcènes, aromatiques. Les MOFs à centre métallique insaturé se comportent généralement comme des zéolithes polaires, les ZIFs comme des zéolithes apolaires et/ou comme des tamis moléculaires. Les adsorbants les plus prometteurs sont testés sur des séparations d’intérêt industriel telles que la séparation des isomères de xylène, la séparation des paraffines linéaires, monobranchées et di-branchées et l’adsorption sélective du thiophène en vu de l’évaluation de ces adsorbants en désulfuration des essences. / The aim of this thesis was to evaluate several Metal Organic Frameworks (MOFs), selected based on criteria of pore size, pore volume and thermal stability, in adsorption and separation of hydrocarbons. For studying the general behavior of MOFs in hydrocarbon adsorption, we have chosen MOFs with open metal sites, MOFs with anionic frameworks and neutral ZIFs. The MOFs with open metal sites behave similar to polar zeolites, the ZIFs behave like apolar zeolites and/or like molecular sieves. Finally we selected the most interesting MOFs and tested them in several separations with industrial interest: xylene isomers, paraffin isomers and selective adsorption of thiophene for the purpose of fuel desulfuration.

A cryogenic (325-80 K) ion mobility-mass spectrometer was designed and constructed in order to improve the analytical figures-of-merit for the chemical analysis of small mass analytes using ion mobility-mass spectrometry. The instrument incorporates an electron ionization source, a quadrupole mass spectrometer, a uniform field drift cell spectrometer encased in a cryogenic envelope, and an orthogonal geometry time-of-flight mass spectrometer. The analytical benefits of low temperature ion mobility are discussed in terms of enhanced separation ability, ion selectivity and sensitivity. The distinction between resolving power and resolution for ion mobility is also discussed. Detailed experimental designs and rationales are provided for each instrument component. Tuning and calibration data and methods are also provided for the technique. Proof-of-concept experiments for an array of analytes including rare gases (argon, krypton, xenon), hydrocarbons (acetone, ethylene glycol, methanol), and halides (carbon tetrachloride) are provided in order to demonstrate the advantages and limitations of the instrument for obtaining analytically useful information. Trendline partitioning of small analyte ions based on chemical composition is demonstrated as a novel chemical analysis method. The utility of mobility-mass analysis for mass selected ions is also demonstrated, particularly for probing the ion chemistry which occurs in the drift tube for small mass ions. As a final demonstration of the separation abilities of the instrument, the electronic states of chromium and titanium (ground and excited) are separated with low temperature. The transition metal electronic state separations demonstrated here are at the highest resolution ever obtained for ion mobility methods. The electronic conformational mass isomers of methanol (conventional and distonic) are also partially separated at low temperature. Various drift gases (helium, neon, and argon) are explored for the methanol system in order to probe stronger ion-neutral interaction potentials and effectuate higher resolution separations of the two isomeric ions. Finally, two versatile ion source designs and a method for axially focusing ions at low pressure (1-10 torr) using electrostatic fields is presented along with some preliminary work on the ion sources.

La tesi que porta per títol "Theoretical Study of the Basicity and the Redox Properties of Heteropolyanions" ofereix informació complementària a la comunitat científica relacionada amb la química de coordinació amb metalls de transició en general, i particularment a experts en òxids metàl·lics d'elevada nuclearitat, o polioxometal·lats (POMs). Els POMs poden encapsular selectivament espècies iòniques i catalitzar força reaccions orgàniques, també són agents blanquejadors de la polpa de fusta respectuosos amb el medi ambient, funcionen com a membranes, sensors, etc. En el futur, podrien ser aplicats amb èxit en medecina ja que estan sent testats com a agents antivirals. La llista de possibles aplicacions és interminable. Sigui com sigui, molts grups estan treballant per explotar les propietats d'aquests òxids metàl·lics. Per tant, un coneixement acurat de les propietats fonamentals dels POMs és necessari si els experimentalistes volen desenvolupar noves tecnologies.La pràctica totalitat dels resultats presentats en la memòria han estat obtinguts amb mètodes de càlcul mecano-quàntics d'alt nivell. La tècnica més àmpliament utilitzada ha estat el mètode relacionat amb la teoria del funcional de la densitat. Aquesta eina, d'ús rutinari des de fa uns pocs anys, s'ha convertit en la preferida per al càlcul de sistemes de gran mida com els POMs. Els principals temes abordats en aquest treball estan relacionats amb l'estructura electrònica d'alguns heteropolianions (HPAs). Aquests es caracteritzen per posseir dos tipus estructurals d'àtoms a part dels lligands oxo: el(s) central(s) i els perifèrics. Dintre d'aquest subgrup dels POMs, s'han investigat les propietats de les dues estructures més comuns: l'anió de Keggin (XM12O40n) i el de WellsDawson (X2M18O62m), així com alguns derivats. Han estat estudiades sèries amb diferents metalls perifèrics (anomenats addenda), metalls centrals (heteroàtoms), isòmers i estructures, de manera que s'han traçat conclusions valuoses i extrapolables a altres sistemes no estudiats. Aquesta classe de POMs presenta una seqüència d'orbitals moleculars peculiar. Els seus orbitals frontera han estat analitzats detalladament per un grup ampli de compostos. S'ha posat especial èmfasi en conèixer la relació entre els orbitals metàl·lics (normalment buits) i les propietats redox dels HPAs. Un dels èxits del treball ha estat explicar de forma inequívoca i general les tendències en els potencials de reducció segons la composició química d'una espècie, així com el comportament dels electrons metàl·lics―(des)localització. Val a dir que, en la gran majoria dels casos, s'ha trobat una concordància molt bona amb les dades experimentals.El segon gran tema abordat en la tesi és la basicitat dels HPAs. En un compost tipus XxMmOyn existeixen diverses posicions oxo, les quals no sempre són equivalents químicament. La seva basicitat pot ésser, doncs, diferent. Un estudi realitzat sobre una sèrie de compostos amb metalls mesclats ha donat escales relatives de basicitat, estimades a partir de la funció potencial electrostàtic, així com a partir d'energies de protonació. S'ha mostrat que els lligands oxo bicoordinats a metalls solen ser els més bàsics, mentre que els oxígens terminals són, generalment, els menys bàsics. Els processos de dimerització en anions de Keggin solen tenir lloc en medi àcid. Aquests passen per una protonació dels monòmers i s´han abordat en dos casos. L'objectiu era mostrar possibles mecanismes de reacció del procés.El tractament teòric dels POMs és encara en la seva etapa inicial, probablement per les dificultats intrínseques associades al càlcul d'aquestes espècies. Aquestes són, des del punt de vista computacional, molècules grans. A més, degut a que són òxids de metalls de transició, acostumen a ser anions altament carregats amb, de vegades, estats electrònics de capa oberta. Els resultats recopilats en la tesi són una prova de que el DFT ha estat hàbilment aplicat a l'estudi dels POMs. / The thesis titled "Theoretical Study of the Basicity and the Redox Properties of Heteropolyanions" offers complementary information to the scientific community concerned to coordination chemistry with transition metals in general, and particularly to experts in high-nuclearity metal oxide compounds. They are the so-called polyoxometalates (POMs). POM species can encapsulate selectively some ionic species as well as catalyse many organic reactions. They are also environmentally-friendly wood pulp bleaching agents, they work as membranes, sensors, etc. In the close future, they could be successfully applied to medicine since they were tested as antiviral agents. The full list of possible applications is endless. Be that as it may, many groups are working with the aim of exploiting the properties of these metal oxides. Thus, an accurate knowledge of their fundamental properties is necessary if experimentalists are to develop new POM-related technologies.The great majority of the results presented in the thesis were obtained by means of high-performance quantum-mechanics computational methods. The most amply utilised tool in this study was the method derived from the density function theory. This tool, only recently utilised routinely, has arisen as the preferred one for computing molecular systems of large size as POMs are. The main topics studied in this work are related to the electronic structure of some heteropolyanions (HPAs). They are characterised by possessing two structural types of atoms, a part from oxo ligands: the central and the peripheral atoms. Within this subset of POMs, the two most common structures were studied: the Keggin (XM12O40n) and the WellsDawson (X2M18O62m) anions, and some of their derivatives, as well. Large series of compounds with varying peripheral atoms (also known as addenda), central metals (heteroatoms), isomers and frameworks, so that we traced valuable conclusions, extrapolable to other systems not studied here. This class of POMs features a particular molecular orbitals sequence. Their frontier orbitals were carefully analysed for a wide set of compounds. We especially stressed the connection between the metal orbitals (normally empty) and the redox properties of HPAs. One of the successes of the work was to explain unequivocally and in a general way the trends in the reduction potentials depending on the chemical composition of a given species, as well as the behaviour of metal electrons―(de)localisation. It is worth noting that, in virtually all cases, we found a fair accordance of our results with experimental data.The second great topic studied in the thesis is the basicity of HPAs. In a compound like XxMmOyn there exist different oxo positions which are not always chemically equivalent. Their basicity might be, then, different. The study carried out for a series of mixed-addenda molecules gives relative basicity scales, estimated from the electrostatic potential function and from protonation energies. It has been showed that the oxo ligands coordinated to two metals are more basic than terminal position, which in general are the least basic. The dimerisation processes in Keggin anions take place, normally, in acidic media. They occur after protonation of the monomers and were investigated for two cases. The goal was to show possible reaction mechanisms for the process.The theoretical treatment of POMs is still in its initial steps, probably due to the intrinsic difficulties associated to the calculation of such species. They are, from a computational point of view, large molecules. In addition, because they are transition-metal oxides, they are highly charged anions that, sometimes, display open-shell electronic states. The compilation of results of the thesis is a proof that the DFT have been skilfully applpied to the study of POMs.

Disease-specific compounds (biomarkers) are analyzed in clinical laboratories to assist with diagnosing diseases. This thesis describes development and validation of liquid chromatography tandem mass spectrometry (LC-MS/MS) based tests for diagnosing a diverse group of endocrine and metabolic diseases. The analytical methods used on-line and off-line sample extraction and analytical derivatization as means of enhancing the analytical sensitivity, specificity and clinical utility. All developed methods were extensively validated and reference intervals for the biomarker concentrations were established in blood samples of healthy adults and children. Advantages of the LC-MS/MS as an analytical technique include possibility of simultaneous measurement of multiple analytes and ability of confirming their identity. In this thesis we proposed and evaluated approaches for the assessment of the specificity of analysis in the methods that use tandem mass spectrometry detection. To enhance throughput of the LC-MS/MS tests for the biomarkers that have endogenous or exogenous isomers an approach was developed for quantitation of isomers from unresolved chromatographic peaks. Using methods developed in this thesis we performed a study of the steroidogenesis in ovarian follicles of healthy women and women with polycystic ovary syndrome (PCOS). Obtained data on the steroid concentrations and associations between the steroid metabolites in the pathway would be helpful for better understanding of the ovarian pathophysiology. Potential biomarkers of PCOS were identified in the thesis; further studies will be necessary to confirm their clinical utility.

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2009" Ivana Adamovic. 12/19/2004. Report is also available in paper and microfiche from NTIS.

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2134" Meera Jay Desai. US Department of Energy 12/19/2004. Report is also available in paper and microfiche from NTIS.

Diffusion is a fundamental process which plays a crucial role in many processes occurring in nature. It is governed by the Fickian laws of diffusion. The laws of diffusion explain how diffusive flux is related to the concentration gradient. However, diffusion occurs even when there is no concentration gradient. Chapter 1 introduces diffusion and related concepts such as random walk, Brownian motion, etc. Present understanding with relation to ionic conduction and diffusion in polar solvents and the anomalies observed in the variation of ionic conductivity with ionic radii has also been discussed. Walden’s rule states that the product of limiting ionic conductivity and viscosity is constant for a given ion in different solvents and it is inversely proportional to ionic radius in a given solvent. However, experimental observations indicate that in a given solvent limiting ionic conductivities show an increase followed by a decrease with increase in ionic radii. This is often referred to as the breakdown of Walden’s rule. Several theories have been proposed in the past to explain the breakdown in Waldens rule. Solvent-berg model, continuum based theories and microscopic theories are some of theories that have been proposed. These theories are discussed briefly. The limitations in these theories are also outlined. There are several computer simulation investigations of ions in water and these are discussed. Also described is diffusion of hydrocarbons in zeolites. Various interesting observations such as window effect, nest effect, single file diffusion and the levitation effect are discussed. In Chapter 2, we have analysed the experimental ionic conductivity data as a function of the ionic radius for monovalent cations and anions in aqueous solution. Molecular dynamics simulations on LiCl and CsCl dissolved in water are also reported. The results suggest that the activation energy is responsible for the anomalous dependence of ionic conductivity on ionic radii. It is seen that ions with high conductivity posses low activation energy. The reason for the variation of activation energy with ionic radii are explained in terms of Derouane’s mutual cancellation of forces or levitation effect. This provides an alternative to the existing theories. Experimental limiting ionic conductivity, λ0 of different alkali ions in water shows markedly different dependences on pressure. Existing theories such as that of Hubbard-Onsager are unable to explain this dependence on pressure of the ionic conductivity for all ions. Experimental ionic conductivity data shows that smaller ions such as Li+ exhibit a monotonic increase in λ0 with pressure. Intermediate sized ions such as K+ exhibit an increase in λ0 followed by a decrease at still higher pressures. Larger ions such as Cs+ exhibit a monotonic decrease in λ0 with increase in pressure. In the present thesis, we have explored this intriguing behaviour shown by alkali ions in water in the next few chapters. In Chapter 3, we report molecular dynamics investigation of potassium chloride solution (KCl) at low dilution in water at several pressures between 1 bar and 2 kbar. Two different potential models have been employed. One of the models successfully reproduces the experimentally observed trend in ionic conductivity of K+ ion in water over 0.001-2 kbar range at 298K. We also propose a theoretical explanation, albeit at a qualitative level, to account for the dependence of ionic conductivity on pressure in terms of the previously studied Levitation Effect. A number of properties of the solvent in the hydration shell are also reported. In Chapter 4, residence times of water in the solute and water hydration shell are reported for KCl in water as a function of pressure. Two different approaches – Impey, McDonald and Madden’s approach as well as the recently proposed stable state picture (SSP) of Laage and Hynes yield somewhat different values for the residence times. The latter suggests that the hydration shell is more labile. As pressure is varied, the analysis suggests drastic changes in the hydration shell around water and little or no change in the hydration shell of the ions at higher pressures. The residence times τIMM as well as τSSP show a decrease with increase in pressure upto 1.5 kbar and a small increase beyond this pressure. This correlates with the dependence of the ionic conductivity of potassium ion on pressure. Similar correlation is also seen for chloride ion between ionic conductivity and residence time in hydration shell. However, no such correlation is seen in the case of water. We also report variation of residence time as a function of t∗, the minimum time that a water has to leave the hydration shell to be excluded from it. In Chapter 5, a molecular dynamics study of LiCl dissolved in water is reported at several pressures between 1 bar and 4 kbars at 240K. Structural properties such as radial distribution function, distribution of the angle between ion-oxygen and dipole vector of water in the hydration shell, angle between ion-oxygen and OH vector, oxygen-ion oxygen angle for water in the hydration shell, mean residence times by two different approaches are reported. Self-diffusivity of both Li+ and Cl− exhibit an increase with pressure in agreement with the experimentally observed trend. We also report the velocity autocorrelation function as a function of pressure. We show that the changes in these can be understood in terms of the levitation effect. For the first time we report the self part of the intermediate scattering function, Fs(k, t), at different pressures. These show for Li+ at small wavenumber k, a bi-exponential decay with time at low pressures. At higher pressures when the ionic conductivity is high, Fs(k, t) exhibits a single exponential decay. We also report wavenumber dependence of the ratio of the full width at half maximum to 2Dk2. These changes in these properties can be accounted for in terms of the levitation effect. The changes in the void structure of water with pressure plays a crucial role in the changes in ionic conductivity of both the ions. In Chapter 6, a detailed molecular dynamics study of self-diffusivity of model ions in water is presented as a function of pressure. First, we have obtained the dependence of self-diffusivity on ionic radius for both cations and anions by varying the radius of the ion, rion. Self-diffusivity exhibits an increase with ionic radius when rion is small and reaches a maximum at some intermediate value, before decreasing with increase in rion for rion > . The velocity autocorrelation function for different sizes of cations as well as anions suggest that the ion with maximum self-diffusivity has facile motion with little back scattering. These trends can be understood in terms of the levitation effect which relates the dependence of self-diffusivity on ionic radius to the bottleneck radius of the pore network provided by the solvent or water. The ratio ζ, defined as the full width at half maximum of the self part of the dynamic structure factor at wavenumber k to its value (2Dk2) at k = 0 is seen to increase with k for ions far away from the diffusivity maximum while a decrease with k is observed for ions closer to the diffusivity maximum. Calculations have also been carried out at pressures of 0.001, 2 and 4 kbars to obtain the variation of ionic conductivity with pressure for model ions of several different sizes. It is shown that for small ions (rion < ), self-diffusivity increases with pressure or exhibits an increase followed by a decrease. In contrast, we show that whenever ionic radius is large, (rion > ), a decrease in self-diffusivity with increase in pressure is seen. We suggest that there is a relation between the dependence of self-diffusivity on ionic radius and its dependence on pressure. The nature of this relationship arises through the levitation effect. Increase in pressure leads to decrease in the bottleneck radius, thus increasing the levitation parameter. For small ions (rion < ), this will lead to increase in diffusivity whereas for large ions (rion > ) this will lead to decrease in diffusivity. For small ions (rion < ), the increase in pressure leads to lowered back scattering in the velocity autocorrelation function. In contrast to this, for large ions (rion ≥ ), any increase in pressure leads to increase in back scattering in the velocity autocorrelation function. For the 1.7 °A anion, the ratio ζ is seen to exhibit a minimum at intermediate k and increase with k at large k for 0.001 kbar pressure. This changes to a less pronounced minimum at 2 kbars and by 4 kbars to a nearly monotonically decreasing function of k. These changes suggest, in agreement with the predictions of the levitation effect, the approach of the bottleneck radius to values similar to that of the ionic radius of 1.7 °A on increasing pressure to 4 kbars. Thus, this work offers an unification in our understanding of the dependence of ionic conductivity on ionic radius and pressure. It is seen that when the ionic radius is varied the numerator of the expression for levitation parameter is varied whereas by varying the pressure, the denominator is varied. The variation of diffusivity with density of the host medium and degree of disorder of the host medium is explored in Chapter 7. The system consists of a binary mixture of a relatively smaller sized solute (whose size is varied) and a larger sized solvent interacting via Lennard-Jones potential. Calculations have been performed at three different reduced densities of 0.7, 0.8 and 0.933. These simulations show that diffusivity exhibits a maximum for some intermediate size of the solute when the solute diameter is varied. The maximum is found at the same size of the solute at all densities which is at variance with the prediction of the levitation effect. In order to understand this anomaly, we have carried out additional simulations in which we have varied the degree of disorder at constant density and find that the diffusivity maximum gradually disappears with increase in disorder. We have also carried out simulations in which we have kept the degree of disorder constant but changed only the density. We find that the maximum in diffusivity is now seen to shift to larger distances with decrease in density. In these simulations we have characterized the disorder by constructing the minimal spanning tree. These results are in excellent agreement with the predictions of the levitation effect. They suggest that the effect of disorder is to shift the maximum in diffusivity towards smaller solute radius while that of the decrease in density is to shift it towards larger solute radius. Thus, in real systems where the degree of disorder is lower at higher density and vice versa, the effect due to density and disorder have opposing influences. These are confirmed by the changes seen in the velocity autocorrelation function, self part of the intermediate scattering function and activation energy. In Chapter 8 we report a molecular dynamics study of the dependence of diffusivity of the cation on cation radii in molten superionic salt containing iodine ion. In this study, we have employed modified Parinello-Rahman-Vashistha interionic pair potential proposed by Shimojo et al (F. Shimojo and M. Kobayashi, J. Phys. Soc. Jpn 60, 3725 (1991)). Our results suggest that the diffusivity of the cation exhibits an increase followed by a decrease as the ionic radius is increased. Several other properties like velocity auto correlation function, intermediate scattering function, activation energy are reported. The next two chapters deal with diffusion of hydrocarbon isomers containing aromatic moiety. Chapter 9 reports structure, energetics and dynamic properties of the three isomers of trimethyl benzene in β-zeolite. Monte Carlo and molecular dynamics simulations have been performed at 300K. Of the three isomers, it is observed that 1,2,4-trimethyl benzene(124 TMB) shows fast dynamics inside the channels of β-zeolite. It is seen that both translational and rotational diffusivities are in the order D (124 TMB) > D (123 TMB) > D (135 TMB). 124 TMB seems to perform jumps between perpendicular channels more frequently whereas 123 and 135 isomers experience more hindrance to these jumps. It is also shown that there is a lower energetic barrier for 124 TMB across the window that separates two perpendicular channels in β-zeolite. Reorientational correlation functions suggest that reorientation of C6 axis (axis perpendicular to the plane of the phenyl ring) is highly restricted in case of 135 TMB. Reorientation of C2 axis (axis on the plane of the phenyl ring) seems to be more facile than that of C6 axis in case of both 123 TMB and 135 TMB. And interestingly, C6 and C2 axis reorientations are equally facile in case of 124 TMB. Chapter 10 presents molecular dynamics simulation results carried out on an equimolar binary mixture of cumene (isopropyl benzene) and pseudo-cumene (1,2,4-trimethyl benzene) in zeolite-NaY at four different temperatures. We compare different structural, energetic and dynamic properties of cumene and pseudo-cumene in zeolite-NaY. Our results suggest that both translational and rotational diffusivities are higher for cumene as compared to pseudo-cumene. Potential energy landscapes show that there is an energetic barrier for diffusion past the 12 MR window plane that separates two neighboring super cages. Such an energetic barrier is large for pseudo-cumene (3 kJ/mol) as compared to that of cumene (1.5 kJ/mol). Activation energies corresponding to both translational and rotational diffusion suggest that pseudo-cumene encounters larger energetic barriers for both translation and rotation as compared to cumene. Reorientational correlation functions suggest that reorientation of C2 axis is more facile than that of C6 axis in case of both cumene and pseudo-cumene. Activation energies corresponding to reorientational relaxations suggest that C6 axis encounters larger energetic barriers as compared to C2 axis in case of both cumene and pseudo-cumene. Chapter 11 discusses the main conclusions of the thesis and directions for future work.