Studies of liquid phase intermolecular interactions utilizing ¹H and ¹³C dynamic nuclear polarization and nuclear magnetic resonance techniques
Liquid phase ¹³C DNP experimental data were collected in a flow transfer system for different organic molecules, such as acetone, acetaldehyde, diethyl malonate, ethyl acetoacetate, diphenylmethane, and triphenylmethane. These molecules represent a wide range of functional groups with different acidities of the respective carbon-hydrogen bonds. The ¹³C DNP results demonstrated that the scalar dominated enhancement is sensitive to the acidity of carbon-hydrogen bonds as well as to the correlation times of the sample molecules. A hydrogen bonding spin polarization model is used, for the first time, to interpret the scalar components induced by the nitroxide free radical at the carbon sites of the acidic carbon-hydrogen bonds.
Three aromatic molecules: nitrobenzene, 1, 2-dichlorobenzene, and toluene, are studied by the solution ¹³C DNP technique. The scalar components for the ring carbons are sensitive to the electronic environment of these carbon sites. A spin delocaliztion model is used, for the first time, to explain the scalar contribitions for the ¹³C DNP enhancements of the ring carbons.
Both ¹H and ¹³C DNP experiments are performed for the Taxol/TEMPO (2, 2, 6, 6-tetramethyl-1-piperidinyloxy) system. The different ¹H enhancements for the hydrogens in the two acetyl groups indicate the different accessibility of these groups to the free radical. The ¹³C DNP results for the skeleton carbons of Taxol show the different accessibility of these carbon sites to the free radical.
The solution ¹³C DNP result of adamantane indicates that the DNP enhancements and thus the correlation times for the two different carbon sites are very close under the high free radical concentration.
The ¹³C DNP study of C₇₀ empty cage fullerene suggests that the endcap carbons are more accessible than those at the center of the cage, and that the scalar coupling between the cage carbons and the free radical is very weak.