Organic radicals as potential magnetic materials and bioprobes

Nitronyl nitroxides bearing substituents with hydrogen bonding potential were synthesized and characterized by magnetism and crystallography. Most of the radicals showed antiferromagnetic interactions at low temperatures. The magnetism of the radicals were rationalized with their crystal packing. In two of the systems, mAmPN and pAcPN, strong antiferromagnetic interactions were observed which was attributed to spin-pairing resulting from antiparallel stack of the nitroxide N-Os. In pAmPN radical, there appeared to be two opposing interactions going on at low temperatures with the ferromagnetic interactions dominating the antiferromagnetic interactions at temperatures lower than 2 K. Two amine salt radicals were made from acidification of para -aminophenyl nitronyl nitroxide. These two systems showed extremely strong antiferromagnetic interactions throughout the temperature range of 1.8 K to 300 K. UV-vis absorption experiments at variable concentrations on the salt radicals displayed aggregation of the molecules at about 0.2 mM concentrations and above. The huge antiferromagnetic interaction was attributed to aggregation. Mixed crystal or alloy of two nitronyl nitroxides H4BImNN and F4BImNN was studied by crystallography and magnetism. The two components individually showed ferromagnetic interactions The alloy also exhibited ferromagnetic interactions that was rationalized with its hydrogen-bonded packing. Biradicals with different interspin distances were synthesized and analyzed by four pulse double electron electron spectroscopy (DEER) using Jeschke\u27s DEER analysis program. The DEER interspin distances for two of the systems 26TAnt and 14TTer were in excellent agreement with their theoretically predicted values derived from molecular modeling. Thus the two systems are good candidates as standards for DEER experiments on spin-labeled biomolecules