Synthesis of open shell organic molecules for magnetic materials applications and investigations
Abstract (Summary)A variety of molecules were investigated to help determine structure-property relationships governing the limits of unpaired electron delocalization and interaction. Nitreno-radicals were synthesized and examined by ESR. Connectivites of nitrene groups were correlated to ZFS |D/hc | and |E/ hc | values. Meta -connected quartet state systems showed larger |D/hc | compared to para -connectivity. Calculations did not show more delocalization in either connectivity. Prepared quartets were studied by ESR Curie analysis over 4-70 K. Nitrosonium perchlorate complexes of azidophenyl nitronyl nitroxides were studied by X-ray crystallography and found to be more planar than their radical precursors. Magnetic susceptibility measurements showed little response, confirming oxidation to cation. Prepared para -connected rigid dinitrenes were studied by ESR and Curie analysis. 1,4-Bis-(nitrenophenyl)-1,3-butadiyne (29 ) showed an excited-state triplet biradical spectrum, and is presumably a ground-state singlet. This proves that the unpaired Ã?-electrons in 29 are strongly exchange coupled and strongly delocalized. 1,4-Bis-(4-nitrenophenylethynyl)-benzene (30 ) showed an excited-state quintet spectrum but is also presumed to be a ground-state singlet. The quintet spectrum indicates that the unpaired Ã?-electrons in 30 are not strongly exchange coupled, and do not delocalize to the central phenyl ring. A rigid dinitrene, 1,5-dinitrenonaphthalene ( 113 ), was found to rearrange to a cumulated alkene with terminal nitrile groups. No biradical state was observed. Apparently 113 exhibits strong exchange delocalization of its unpaired Ã?-electrons to favor a singlet state over the triplet, since the triplet is not observed: computations support a singlet ground-state. Septet trinitrene 1,3-dicyano-5-methyl-2,4,6-trinitrenobenzene ( 115 ) was prepared and studied by ESR and Curie analysis. A high-spin ground state was observed with estimated ZFS of |D/hc | = 0.090 cm-1 and |E/hc | [approximate] 0 cm -1 . The azide groups photolyze in step-wise fashion giving triplet mononitrene, quintet, and septet. There is sufficient delocalization of the nitrene Ã?-electrons to favor the septet state in the overall system. An iminoyl nitroxide with a hydrogen-bonding 6-uradinyl group ( 119 ) was prepared. X-ray crystallography showed planarity dominated by two-point hydrogenbonding. Room temperature solution ESR shows seven lines (aN = 8 G, 3 G) compared to five lines (typical aN = 7 G) that result from a nitronyl nitroxide.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2003