Nuclear magnetic relaxation studies of semiconductor nanocrystals and solids

Semiconductor nanocrystals, small biomolecules, and {dollar}sp{lcub}13{rcub}{dollar}C enriched solids have been studied by analyzing the relaxation properties of their nuclear magnetic resonance (NMR) spectra. More specifically, for the first time, details of the structure of the surface of semiconductor nanocrystals was obtained. The surface structure was deduced from high resolution {dollar}sp 1{dollar}H and {dollar}sp{lcub}13{rcub}{dollar}C liquid state spectra of the thiophenol ligands on the surface of the nanocrystals, which were assigned using standard 2-dimensional liquid state techniques. Intensity calibrated {dollar}sp1{dollar}H NMR spectra were recorded as a function of nanocrystal radius, and the surface coverage by thiophenol was found to be low, varying from 5.6% and 26% as the nanocrystal radius changed from 11.8 to 19.2 A. The longitudinal and transverse relaxation times of the {dollar}sp1{dollar}H and {dollar}sp{lcub}13{rcub}{dollar}C resonances of the thiophenol ligands show that the spectra are homogeneously broadened and that the broadening increases as the nanocrystal radius becomes smaller. This suggests that the thiophenol ligands are rotating with respect to the nanocrystal surface and that the correlation time of this motion increases with decreasing radius.; We present a method for measuring {dollar}sp{lcub}14{rcub}{dollar}N-{dollar}sp1{dollar}H J-couplings in small biomolecules by measuring the rate of scalar relaxation of the second kind. These couplings are related to molecular conformation. By measuring the {dollar}sp{lcub}14{rcub}{dollar}N longitudinal relaxation time and the difference in the {dollar}sp1H{dollar} transverse and longitudinal relaxation rates, the {dollar}sp{lcub}14{rcub}{dollar}N-{dollar}sp1{dollar}H J-couplings can be determined. The method is demonstrated on pyridine and the small peptide oxytocin. To measure the relaxation times in crowded spectra with overlapping peaks in 1D, we present new selective 2D T{dollar}sb1{dollar} and T{dollar}sb2{dollar} experiments. The results show that the technique is viable; however, relaxation effects due to chemical shift anisotropy and modulation by strong coupling interfere with the technique.; Finally, we demonstrate the possibility of carbon-carbon cross relaxation in {dollar}sp{lcub}13{rcub}{dollar}C enriched solids. {dollar}sp{lcub}13{rcub}{dollar}C magic angle spinning exchange experiments performed on polycrystalline samples of {dollar}sp{lcub}13{rcub}{dollar}C{dollar}sb2{dollar} zinc acetate and {dollar}sp{lcub}13{rcub}{dollar}C{dollar}sb3{dollar} L-alanine show correlations between the resolved carbon sites in the molecule which grow at a rate proportional to the distance between the carbons. Additionally, the quenching of normal H cross relaxation leaves open the possibility to observe higher order effects and we have observed the presence of two spin dipolar order between the {dollar}sp{lcub}13{rcub}{dollar}C nuclei, which is explained by a third order perturbation theory. (Abstract shortened by UMI.)...