Dynamic nuclear polarization detection sensitivity

An extremely sensitive technique able to detect the nature of radical pairs in a photochemical reaction is called chemically induced dynamic nuclear polarization (CIDNP), which depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample, irradiated in situ, in the cavity of a NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (28). 

In another class of experiments, hyperpolarized states are generated by spin-sensitive chemical reactions. These include para-hydrogen-induced polarization (PHIP) [3-5] and chemically induced dynamic nuclear polarization (CIDNP) [6-8]. The latter involves non-equilibrium nuclear spin state populations that are produced in chemical reactions that proceed through radical pair intermediates. CIDNP s applicability has been focused towards the study of chemical reactions and the detection of surface exposed residues in proteins [9], but has so far remained limited to specialized chemical systems. 

However, despite their exquisite spectral sensitivity to stmcture, dynamics, and morphology, conventional NMR methods suffer from a common drawback that in many circumstances, can limit their power and applicability - a notoriously low detection sensitivity (especially compared to optical methods). This fundamental insensitivity originates from the miniscule size of nuclear magnetic moments, which results in an exceedingly small (Boltzmann) equilibrium nuclear spin polarization, P, generally given by... 

The enormous improvement in the nuclear spin polarization achievable by these optical approaches has great potential to impact conventional NMR smdies in a variety of ways. Besides simply increasing the detection sensitivity of NMR, the enhanced nuclear spin polarization may be useful for obtaining enhanced spectral selectivity (e.g., for some spatial, structural, or dynamical feature of a sample) enhanced contrast and resolution in MRI and improved density matrix purity (e.g., for NMR quantum computation). Finally, it is worth noting that the optical fields used in some of these methods can be gated, thereby permitting time-resolved studies that would not be possible with conventional NMR approaches. 

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