  1H nuclear magnetic resonance (NMR) measurements in [YPc2]0, an organic
compound formed by radicals stacking along chains, are presented. The
temperature dependence of the macroscopic susceptibility, of the NMR shift and
of the spin-lattice relaxation rate 1/T1 indicate that the unpaired electron
spins are not delocalized but rather form a S=1/2 antiferromagnetic chain. The
exchange couplings estimated from those measurements are all in quantitative
agreement. The low-energy spin dynamics can be described in terms of diffusive
processes and the temperature dependence of the corresponding diffusion
constant suggests that a spin-gap around 1 K might be present in this compound.
