Spin dynamics in mesoscopic size magnetic systems:AHtNMR1study in rings of iron (III) ions

Two magnetic molecular clusters containing almost coplanar rings of iron (III) ions with spin S=5/2 have been investigated by ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR and relaxation measurements. The first system, which will be referred to as Fe6, is a molecule of general formula [${\mathrm{NaFe}}_6$(${\mathrm{OCH}}_3$ ${)}_{12}$(${\mathrm{C}}_{17}$ ${\mathrm{O}}_4$ ${\mathrm{H}}_{15}$ ${)}_6$ ${]}^{+}$ ${\mathrm{ClO}}_4^{\mathrm{-}}$ or [${\mathrm{NaFe}}_6$(${\mathrm{OCH}}_3$ ${)}_{12}$(${\mathrm{C}}_{15}$ ${\mathrm{H}}_{11}$ ${\mathrm{O}}_2$ ${)}_6$ ${]}^{+}$ ${\mathrm{ClO}}_4^{\mathrm{-}}$ or [${\mathrm{LiFe}}_6$(${\mathrm{OCH}}_3$ ${)}_{12}$(${\mathrm{C}}_{15}$ ${\mathrm{H}}_{11}$ ${\mathrm{O}}_2$ ${)}_6$ ${]}^{+}$ ${\mathrm{ClO}}_4^{\mathrm{-}}$ while the second type of ring, denoted Fe10, corresponds to the molecule [${\mathrm{Fe}}_{10}$(${\mathrm{OCH}}_3$ ${)}_{20}$(${\mathrm{C}}_2$ ${\mathrm{H}}_2$