  A systematic inelastic neutron scattering study of the superexchange
interaction in three different undoped monolayer cuprates (La_2CuO_4, Nd_2CuO_4
and Pr_2CuO_4) has been performed using conventional triple axis technique. We
deduce the in-plane antiferromagnetic (AF) superexchange coupling $J$ which
actually presents no simple relation versus crystallographic parameters. The
absolute spectral weight of the spin susceptibility has been obtained and it is
found to be smaller than expected even when quantum corrections of the AF
ground state are taken into account.
