Construction of a quantum-spin system of S=1/2 antiferromagnetic chain with the next-nearest-neighbor interactions

We have newly synthesized organic radicals F2PIMNH and Cl2PIMNH, where F2PIMNH = 2-[4'-N-tert-butylamino-2',6'-difluoropheny;]-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl and Cl2PIMNH = 2-[4'-N-tert-butylamino-2',6'-dichlorophenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl. Both crystals include zigzag chains made of hydrogen bondings. The magnetism of Cl2PIMNH is explained by the Heisenberg antiferromagnetic uniform chain model with 2J/k(B) = -3.6 K and gapless ground state is suggested. On the other hand, static magnetic measurements of F2PIMNH suggest the nonmagnetic ground state and the existence of a finite gap in its excitation spectrum. In this crystal, the next-nearest-neighbor contacts are seen which are believed to be the origin of the energy gap.