Construction of a Quantum-Spin System ofS=1/2Antiferromagnetic Chain with the Next-Nearest-Neighbor Interactions

We have newly synthesized organic radicals F 2 PIMNH and Cl 2 PIMNH, where F 2 PIMNH = 2-[4 ′ - N - t e r t -butylamino-2 ′ ,6 ′ -difluorophenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1 H -imidazol-1-oxyl and Cl 2 PIMNH = 2-[4 ′ - N - t e r t -butylamino-2 ′ ,6 ′ -dichlorophenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1 H -imidazol-1-oxyl. Both crystals include zigzag chains made of hydrogen bondings. The magnetism of Cl 2 PIMNH is explained by the Heisenberg antiferromagnetic uniform chain model with 2 J / k B =-3.6 K and gapless ground state is suggested. On the other hand, static magnetic measurements of F 2 PIMNH 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.