Ensemble-averaged ab initio magnetic resonance and double resonance spectra and line shapes in heterogenous condensed phase systems.

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The characterization of microscopic structure and molecular motion in solids and liquids that lack long-range order is still a challenging topic, especially for heterogenous media with many components. One of the most powerful techniques to study such systems is magnetic resonance (MR) spectroscopy, both on nuclei (NMR) and electrons (EPR).In order to understand the information in a magnetic resonance spectrum, one needs a theory to connect the spectrum to the underlying molecular structure and dynamics. However, the spectra are usually very complicated, and even with a theory in hand, its exact application for such complicated scenarios is a formidable task that is well beyond present day computational capabilities. In such scenarios computer simulations are very valuable. In this project, I am proposing several developments along these lines, that is the application of molecular simulation methods to understand magnetic resonance and magnetic double resonance spectra. In the latter case the experiment involves simultaneously the magnetic signature of atomic nuclei and of electrons.Specifically, building on our previous work, what we aim to achieve is to combine several theoretical techniques together, developing carefully-tested methods that can aid in interpreting the results of some of the most powerful magnetic resonance techniques.The first technique that we would like to focus on is "ENDOR", a magnetic double resonance technique that has the capacity to provide detailed knowledge about the structure of heterogenous media around a molecular probe. First we will address some fundamental questions regarding the precise relation between the structure of the probe, the condensed phase, and the spectrum. Our long-term aim here is to develop a carefully tested toolbox that calculates the ENDOR spectrum from a virtual molecular system, and iteratively modifies the molecular system until the best match with experiment is achieved.The second technique is "dynamic nuclear polarization" or "DNP", again a double resonance technique that is closely related to ENDOR. In a surprising recent finding, one particular flavor of DNP was discovered to be operative in insulating solids, which disagreed with more traditional wisdom. We have recently addressed this perplexing finding and successfully explained it. In this project we would like to further develop our model, and push further towards new predictions that can possibly guide the experimental developments.The final aspect of magnetic resonance that we would like to investigate is a more basic one. We would like to establish a quantitative link between the indirect (electron-mediated) magnetic coupling between atomic nuclei in condensed phases, and the characteristics of the hydrogen bond, its geometry, and its covalency.