Single quantum, line shape

Empedocles, S. A. and Bawendi, M. G. (1999) Influence of spectral diffusion on the line shapes of single CdSe nanocrystallite quantum dots. J. Phys. Chem. B, 103, 1826-1830. 

When the H- H dipole-dipole interaction can be measured for a specific pair of H nuclei, studies of the temperature dependence of both the H NMR line-shape and the H NMR relaxation provide a powerful way of probing the molecular dynamics, even in very low temperature regimes at which the dynamics often exhibit quantum tunnelling behaviour. In such cases, H NMR can be superior to quasielastic neutron scattering experiments in terms of both practicality and resolution. The experimental analysis can be made even more informative by carrying out H NMR measurements on single crystal samples. In principle, studies of both the H NMR lineshape and relaxation properties can be used to derive correlation times (rc) for the motion in practice, however, spin-lattice relaxation time (T measurements are more often used to measure rc as they are sensitive to the effects of motion over considerably wider temperature ranges. 

Figure 4.12 (A) Comparison of PLE spectra of quantum dots near a single Ag nanoprism (NP) (diamonds) and quantum dots alone (blue circles). Intensities have been normalized to compare the line shape of the two spectra. (B) Series of images of quantum dot photolxuninescence near the Ag nanoprism in (A). Adapted from reference 43.

In solution-state NMR, many important experiments incorporate the creation and evolution of MQ coherence (MQC).5,6,84-86 Since MQC cannot be directly detected, experiments that follow the evolution of a MQC are inherently at least two-dimensional. This is the case with H- H DQ MAS spectroscopy. Figure 7 shows a corresponding pulse sequence and coherence transfer pathway diagram first, a DQC is excited, which subsequently evolves during an incremented time period q the DQC is then converted into observable single-quantum (SQ) coherence (SQC), which is detected in the acquisition period, q. To select the desired coherence transfer pathways, e.g., only DQC during q, a phase cycling scheme is employed.79,80 Pure absorption-mode two-dimensional line shapes are ensured by the selection of symmetric pathways such that the time-domain... 

A new approach to combining CP with MQ MAS in a 2D NMR experiment has been demonstrated, involving CP from H to the single-quantum coherences of a quadrupolar nucleus.In two separate methods, pure-absorption line shapes have been obtained using a z-filter and a reversed split-ti method. 

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