The NMR chemical shift has been a very sensitive and extremely useful tool for chemists for discriminating between electronic environments. This intrinsic sensitivity to environment at nuclear sites becomes even more obvious as ever higher magnetic fields spread out the unique environments in the frequency scale. Jameson has contributed significantly to a fundamental understanding of the NMR chemical shift as a molecular electronic property. One characteristic of Jameson's work is her interest in general trends and over-arching relationships, not just specific systems. Her contributions have usually involved providing the basis for a fundamental understanding of a limited body of data, and on this basis, predicting a systematic global pattern which emerges much later. [ Periodic Table-wide trends in NMR parameters] In her early work, while searching for an explanation for the huge 129Xe chemical shifts in the xenon fluorides, at a time when NMR shifts had been measured for only a very few nuclei in a limited number of compounds, she predicted that the range of chemical shifts should vary from one nucleus to another in the Periodic Table in the same way that r-3 average value for the p electron (or d for transition series) in the free atom. Accumulated data on nuclei in all parts of the Periodic Table now demonstrate this periodic behavior in a dramatic way. That seminal study forms the basis for the now commonly known scaling of chemical shifts of one type of nucleus to that of another type in analogous compounds and for the comparative sensitivities of different NMR nuclei to their electronic environment.

The NMR chemical shift is a difference between two values of the nuclear magnetic shielding. Part of the motivation of Jameson's work in NMR in the gas phase has been to understand the way in which intermolecular interactions affect the nuclear magnetic shielding, in other words, the exploration of the nuclear magnetic shielding surface, analogous to the intermolecular potential energy surface. The observed resonance signal of a 129Xe nucleus in xenon gas, for example, is a weighted average over the shielding surface corresponding to various Xe-Xe distances. She predicted the general shape of the intermolecular shielding surface, based on insight gained from the temperature dependence of the density coefficients of the chemical shifts measured in gases in her laboratory. The shape is not unlike the shape of a potential energy surface, deshielding relative to the infinitely separated situation, reaching a minimum at shorter distances, then becoming more shielded as the united atom (zero separation) limit is approached. At that time, no ab initio calculations had provided such a picture, and the calculations that did appear in the literature showed an intermolecular shielding function that was the wrong sign in the interesting separations, and monotonically deshielding as the distance between the interacting pair decreased, going to unphysically large negative values at short distances. Only later (1993) have Jameson's ab initio calculations shown that in the rare gas pair the shape of the purely theoretical shielding surface is indeed of a shape much like that predicted by Jameson nearly twenty years earlier. Almost all the reported studies of the effects of molecular collisions on the NMR chemical shift in the gas phase have emerged from Jameson's laboratory. [ Gas phase intermolecular chemical shifts Her experimental and theoretical work on Xe in mixtures of gases [Xe NMR in gases] forms the basis for the wide application of Xe NMR shifts for probing various polycrystalline surfaces, zeolites, other porous solids, and polymers.

The other nuclear magnetic shielding surface is the intramolecular one. This concept has emerged out of two separate but related phenomena: the observed temperature dependence of the NMR chemical shift in the gas in the limit of zero density [Temperature dependence of chemical shifts in isolated molecules] and the small but ubiquitous NMR isotope shifts. All but one of the reported experiments observing phenomena of the first type were carried out in the Jameson laboratory. A global view of the large volume of isotope shift data, expressed in the same theoretical framework as used for the temperature dependence of the chemical shift in the isolated molecule was provided by Jameson in two papers in 1977. The nuclear magnetic shielding surface is the collection of values of shielding corresponding to the various geometries of the molecule. Weighted averaging over this surface occurs as the molecule undergoes rotation and vibration, according to the probabilities of various geometries, as expressed by the anharmonic vibrational wavefunctions. [Rovibrational averaging of molecular electronic properties] This interpretation provides a connection between the very easily measured isotope shifts and the product of two factors: a dynamic factor which depends on masses and force constants and an electronic factor which is a measure of the sensitivity of the shielding at the resonant nucleus to a small perturbation at the location of the isotopic substitution. The full theory involves many such terms containing these two factors; the leading term provides an interpretation of a very large collection of isotope shift data in a simple yet global way. [Isotope shifts in NMR] Moreover, it leads to the interesting idea that isotope shifts provide important electronic information that depends on the electronic pathway between the two points, in the same way as does chemical shifts arising from functional group substitution, but are so benign as to not disturb the electronic distribution in the way that chemical group substitution does. With this model, Jameson predicted a large number of trends in the derivatives of the shielding surfaces at the equilibrium geometry of the molecule, all of which have recently been verified by ab initio calculations of the derivatives. She has also carried out ab initio and density functional theory calculations of nuclear shielding surfaces for molecules and for rare gases interacting with larger molecular systems.[Nuclear shielding surfaces]

Jameson has determined absolute values of nuclear shielding for 19F, 13C, 15N, 29Si, 31P, 77Se, and 125Te in the gas in the zero-density limit (virtually in the isolated molecule) for many compounds, thereby providing quantities which are much closer to what is calculated theoretically than the usual differences (chemical shifts) measured in condensed phases. [Absolute shielding scales] These absolute shielding scales place all measured chemical shifts on an absolute basis, providing more stringent tests of high level quantum mechanical calculations. Other areas of interest have included NMR spin-spin coupling constants. [Spin spin coupling] Very early on she provided a simple model which predicted absolute signs of one-bond couplings and the changes in sign for pairs of nuclei as one goes across the Periodic Table when only a handful of such signs were known. The same model provides the basis for the shapes of spin spin coupling surfaces and isotope effects on J couplings. In summary, using NMR chemical shifts and coupling constants as examples, Jameson has drawn attention to the role of two important contributions to molecular electronic properties in general: intermolecular effects and rovibrational averaging, which should be considered in any comparisons of the experimental quantities with ab initio calculations, and she has made the general concept of molecular electronic property surfaces an important tool in the understanding of macroscopic observables.

Her work on spin relaxation studies in the gas phase provides a measure of the rate constants associated with either the reorientation of a molecular frame or the changes in the rotational angular momentum vector of a molecule upon collision with another. Each is an independent sensitive measure of the anisotropy of the intermolecular interaction potential of the collision pair. The relation between these two rates had been predicted by many theoretical models for molecular reorientation in liquids. Jameson's gas phase experiments test these models in the gas limit where the model predictions differ substantially from each other. From her lab has emerged the largest body of data on such rate constants (or the related cross sections) for over a hundred different collision pairs as a function of temperature; the observed molecules are linear or spherical tops such as N2, CO, CO2, NNO, CH4, CF4, SiH4, SiF4, SF6, SeF6, and TeF6. [Gas phase spin relaxation By using two different nuclei as probes, Jameson and her group have obtained both types of cross sections for the same collision pair and with a simple model she provided explanations for the general patterns observed in both cross sections. Her classical trajectory calculations are beginning to reproduce these quantities, thereby fulfilling the original promise of spin relaxation in the gas phase as a very sensitive test of the anisotropy of the intermolecular potential.[Classical trajectory calculations]

Her recent work in zeolites, the crystalline materials widely used for separations, oil recovery, and catalysis, capitalizes on Jameson's acknowledged leadership in the field of interpretation of gas phase NMR chemical shifts and is something sorely needed for advancing the understanding of how xenon interacts with the zeolite past the empirical state it had been in. Her studies of distribution and dynamic behavior of adsorbed species in what had been traditionally called "microporous" solids addresses a topical problem of both basic and technological importance. Actually the pores are in the nano-scale. What makes this research unusual is the combination of a detailed molecular level understanding of the fundamental processes with an awareness of the relevance of the results to potential technological applications. Prof. Jameson has established an international reputation in the fields of 129Xe NMR and zeolites and is the leading expert in theoretical understanding of chemical shifts.

The unique and powerful aspect of this research program is the combination of NMR experiments with theoretical calculations and simulations. Jameson's group at the University of Illinois at Chicago integrates Monte Carlo simulations (GCMC) [Monte Carlo simulations of sorbates in porous solids], ab initio quantum calculations [Ab initio calculations] and NMR experiments. Such a combination is essential if any fundamental advances in the understanding of distributions and dynamics in microporous solids are to occur. [Adsorption and diffusion in zeolites] Her early work focused on zeolite NaA, a well-characterized crystalline system Jameson's lab has determined directly the distribution of Xe atoms in the cavities of a microporous solid by observing individually the trapped clusters Xe, Xe2, Xe3, ..., Xe8 in NMR. The chemical shifts of the clusters vary with temperature as they undergo changes in configurations within the cavities. For the first time Jameson has reproduced the observed distributions, the cluster chemical shifts, and the temperature dependence by a computer simulation using only potential energy and shielding surfaces, with no adjustable parameters. [Monte Carlo simulations of sorbates in porous solids] These are the most detailed tests yet of any computer simulation, the observed quantities depend on the equilibrium distribution of the xenon between the bulk gas and inside the zeolite (the adsorption isotherm), the distribution of the adsorbed xenon among the cages (the cluster sizes and their fractions) and the distribution of the atoms of a given cluster within a cage (the chemical shift of the cluster depends on this). Her simulations provide these quantities and are tested by the detailed experiments in the same lab. This judicious choice of system where individual clusters Xen are observable in the 129Xe NMR spectrum avoids having to interpret exchange-averaged spectra, where all the contributions are convoluted into a single chemical shift value. Thus, these investigations provide a basic understanding and the results highlights the subtleties and intricacies of the contributions of various intermolecular interactions to the Xe shielding function. The investigation of pore sizes, cation substitutions and distributions, alumina-silica ratio are all important issues relevant to applications in catalysis and separation science. The systematic approach used in this laboratory is the most sensible way to develop a quantitative understanding of the behavior of Xe in microporous solids in general. For example, the complete substitution of K+ for Na+ ion in the zeolite, while leaving the zeolite framework unchanged, verified the magnitude of the alkali ion contributions to the 129Xe chemical shift in experimental and theoretical studies as detailed as that in NaA. In another experimental breakthrough, the divalent cation contributions were determined by magic angle spinning NMR experiments in the same A zeolite framework where the Na+ ions were incrementally substituted by Ca2+ ions. In the same spectrum the individual progressions of Xen peaks were observed for cages containing no Ca2+ ion, one, two, or three Ca2+ ions, thereby observing directly the distributions of Xe atoms among cages of one type, as well as the competition between cages of different types for the sorbate atoms. Simulations successfully reproduced all observed trends and verified the importance of the role of electric polarization of the Xe atom by cations in any zeolite.

In another set of experiments and simulations, Jameson's lab has determined the rate of cage-to-cage migration of xenon atoms in a zeolite. By attaching a magnetic label to the Xe atoms in Xe6 for example, it is possible to follow in time the appearance and disappearance of this label in all the other clusters, as the Xe atom leaves cavities of Xe6 and go elsewhere. The time evolution of all peaks following a perturbation of one peak is simulated successfully when the rate constants are correctly chosen, One of the most exciting and significant results of this work is that the microscopic cage-to-cage migration rates kmn vary with cluster sizes, m and n. Simulations using these rate constants also lead to the correct equilibrium distribution of xenon atoms among the cavities, as they should, verifying the consistency of the detailed dynamic information with the detailed equilibrium information. This work provides the most detailed picture yet of the process of diffusion within a microporous solid. These studies demonstrate that simulations can provide strict constraints on the quantitative interpretation of the experimental results. This represents an important contribution because Xe NMR is used empirically to characterize solids and polymers (usually in the fast exchange limit). Competitive adsorption is a very important issue in the technological applications of zeolites. Jameson's experiments and simulations of binary mixtures in zeolites has provided a very detailed picture of competitive adsorption and diffusion as never before possible, right down to directly observing for the first time mixed clusters such as XeKr, Xe2Kr, ... Xe6Kr. In other cases where the co-adsorbed molecules (such as Ar, CO, CH4, CO2) are in fast exchange, she has directly determined the average number of co-adsorbed molecules in the same cage as exactly n Xe atoms for a given loading of the two sorbates. This level of detail is unprecedented in the entire history of adsorption studies. Although a large amount of 129Xe NMR data in zeolites and other porous media have been reported over the past dozen years, Jameson's studies are the first to provide a quantitative theoretical understanding by reproducing detailed experimental results entirely from first principles.

A fundamental understanding of the processes of adsorption and diffusion within microporous solids is very important to their technological applications. Although a large number of scientists and engineers all over the world have been involved in such studies, only now with Jameson's recent papers have the most detailed experiments and quantitative interpretation of adsorption and diffusion processes been put forth. Given her style of doing science, it comes as no surprise that the concept of the intermolecular shielding surfaces developed by her from ab initio and gas phase studies make such detailed interpretations possible in heterogeneous systems which are completely consistent with her body of previous work in NMR of gas phase binary mixtures.

Jameson has developed a well-conceived systematic program to address fundamental aspects of adsorption at the molecular level in porous media. The broad scale of the attack on a well-defined problem in a well-characterized system has been particularly fruitful, combining as it does theoretical chemical shift calculations as a function of interatomic distances with GCMC calculations and comparing both with detailed experimental results under a variety of conditions. Her work on kinetics and on Monte Carlo analysis of population distributions are completely original. The research has already led to significant new knowledge. The kind and variety of information obtained are unique. Jameson's work has convincingly demonstrated that information, unprecedented in its detail and fundamental significance can be extracted from such NMR experiments. This is the first major entree into the structure and dynamics of inclusion compounds in general, which are obviously relevant to the understanding and development of catalytic systems for industry. Jameson's thoughtful approach to the fundamental problems addressed will continue to have substantial impact upon progress in the field.

Jameson has established that the NMR lineshape of a confined molecule reflects the average NMR tensor properties which in turn reflect the geometry and the internal structure of the confining pore or channel and the intermolecular interaction of the molecule with the wall atoms of the confining structure. [Xe line shapes in nanochannels] This was accomplished by making theoretical predictions of line shapes as a function of shape and size of the cage or channel, electronic nature of the wall atoms, the extent of hydrogen bonding of cage atoms, the point group symmetry of the cage, as a function of temperature and of occupancy. In a series of papers, she and her group have used a combination of quantum mechanical shielding surfaces and grand canonical Monte Carlo simulations to predict the line shapes and the systematic changes which are observed with temperature, with occupancy, in a wide range of zeolitic materials (Linde type A zeolites, silicalite, SSZ-24, ALPO4-11, etc.), self-assembled nanochanneled molecular crystals such as tris(o-phenylenedioxy)cyclotriphosphazene (TPP) and dipeptides (prior to observations in this case), and also in channels decorated with paramagnetic centers. Using the same approach, she has explored the Xe shielding tensors for Xe in cages, such as a large number of clathrate hydrate cages of four different ice structures, organic cages of four types of cryptophanes and C60. [Simulations of Xe chemical shifts in cages]

A series of fundamental studies on model systems (carried out in collaboration with R. A. Harris at Berkeley) elucidates the relation between the nuclear magnetic shielding (which gives rise to the NMR chemical shift) and the intrinsic chirality of a molecule or environment, establishes the relation between odd and even parts of the potential and the various symmetric and antisymmetric components of the shielding tensor, and proves that a chiral potential alone is sufficient to provide discrimination of various diastereomers via their chemical shifts. These fundamental studies in simple model systems leads to reliable assignements of diastereomeric chemical shifts observed for Xe atom in derivatized cryptophane cages, including the Xe biosensor diastereomers. [Chirality and shielding]

Predictions of the chemical shifts of Xe in liquid solutions including water, alkanes, perfluoroalkanes and perfluorooctylbromide using molecular dynamics provide the first ever calculations of such shifts which for decades had been interpreted entirely in terms of the refractive index of the solvent.[Xe in aqueous solution] The ability to reproduce the chemical shift of Xe in aqueous solution is particularly gratifying, since the shielding surfaces have been obtained from ab initio calculations for the Xe shielding in various ice cages in the clathrate hydrates, taking into account not only the cage water molecules, but the complete set of waters in the next shell of hydrogen bonding partners, as well as the electrostatic contributions from the remaining water molecules in the entire crystal. Using a similar simulation box system introduced by S. Murad, and atomistic classical molecular dynamics methods,[Molecular dynamics simulations] not only solubilities and other average properties such as NMR chemical shifts in solutions can be obtained, but also rates of dynamic processes such as exchange, reorientation, permeation of channels, ion transport studies in carbon nanotube models for ion channels and molecular transport across lipid membranes.Coarse-grained molecular dynamics simulations of model lipid-bilayer membranes which are self-assembled from an isotropic mixture permit the studies of permeation processes across these membranes, providing important insight into mechanisms of transport of Xe and other small molecules such as O2 and CO2 across these membranes, with or without a transmembrane protein channel such as OmpA; penetration of gold nanocrystals and ligand-coated gold nanoparticles cause changes in the membrane physical and structural properties, and cause molecular-level events such as creation of pores (water columns), ion transport along the water-filled pores, lipid flip-flops, and lipid displacement from the membrane, revealing the molecular-level mechanisms for these events under various conditions.[Ion and Molecular transport]

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