Long-lived states in multi-spin systems

Abstract

Long-lived states are nuclear spin configurations that, in suitable circumstances, decay very slowly towards thermal equilibrium. The first paper on the subject reported a long-lived order in 2,3-dibromothiophene between a pair of inequivalent proton nuclei of about 100 s at about 20 mT. The lifetime exceeded the relaxation time of longitudinal magnetization T1 by more than one order of magnitude. Currently many systems cansurvive T1 even at magnetic fields of several Tesla. Long lifetimes may benefit different methodologies used to investigate for example nuclear spin diffusion, chemical reactivity and metabolic processes. In addition hyperpolarization methods may profit from long-lived states in order to enhance both sensitivity and temporal resolution. Although this research field is relatively young, the first publication being 11 years old, about 150 investigations so far have been published on peer-reviewed scientific journals on this subject. In this work the main focus is to extend the analysis to multiple spin systems. The structure of the thesis is composed of a theoretical and an experimental part. We propose a model based on nuclear spin permutations that uses the formalism of discrete group theory. This approach allows the classification of nuclear wave functions and internal Hamiltonian operators, according to nuclear spin permutation symmetry, in order to predict the number of long-lived orders and their analytical expression. The mathematical structure can also be applied to investigate fundamental bounds on spin conversion in the presence of symmetry. The theoretical model is grounded on a set of approximations used to define the symmetry operations and the corresponding permutation symmetry groups. The experimental section includes examples of long-lived orders occurring under different magnetic, geometric and dynamic conditions. This large variety of regimes shows on one hand the ubiquitous character of long-lived species. On the other hand a common trait is identified in the formal characterisation that uses permutation and rotational symmetry concepts. For this reason a permutation symmetry characterisation is presented alongside the experimental description. The role of local geometry in a rigid spin system is highlighted by comparing two isomers with a different local arrangement of spin nuclei, and showing how a long-lived order is predicted and detected only in one case. Non rigid molecules can also display long-lived character. This is demonstrated by considering the methyl group 13CH3 in ?-picoline. Interestingly, as proton nuclei are magnetically equivalent, the long-lived order accessibility cannot employ coherent mechanisms. Finally a derivative of naphthalene is shown to possess an exceptionally long lifetime in solution and at room temperature. The accessibility of a very long lifetime opens up the possibility to store (hyper)polarization into singlet order and retrieve it later in time. A set of preliminary dissolution dynamic nuclear polarization experiments are also presented as a first attempt in this sense.