In this work, liquid phase Carbon-thirteen DNP experimental data were collected in a flow transfer system for different mono-substituted aromatic compounds, such as, anisole, nitrobenzene and halobenzenes. These substitute groups covered a wide range of electronic effects from strong electron donating groups to strong electron withdrawing groups.
Hammett linear free energy relationship was used to quantitatively study the electronic effect of substituent on the carbon-thirteen DNP enhancement. It was found that the carbon-thirteen DNP enhancements at meta and para positions exhibits a reasonable correlation. A better correlation of carbon-thirteen DNP enhancements with the Hammett inductive factor was observed.
A large scalar dominated enhancement at ipso (C-1) position was observed for iodobenzene. This suggests that facile transfer of spin polarization to the C-1 carbon via the highly polarizable iodine atom is possible and the dominate electron transfer mechanism for this system.
A model of polarization transfer via spin diffusion of abundant proton spins was examined. The results indicate that it is not important.
