Diamagnetic susceptibility and microstructure of diamides

Experimental data for the diamagnetic susceptibility of two series of monodisperse normal diamides R′─HNOC─(CH₂)_n/2─(CH₂)_n/2─CONH─R′] and ketodiamides (R′─HNOC─(CH₂)_n/2─CO─(CH₂)_n/2─CONH─R′] with R′═CH₃─(CH₂)_s─ranging between n=2 and 8 in the solid state are reported. For the normal diamides the molecular susceptibility χ_M increases linearly with n, though the χ_M values are lower than Pascal data derived from the additivity of atomic susceptibilities. For the ketodiamides, χ_M is longer a linear function of n. Thus, while even ketodiamides show χ_M values near to Pascal data the odd members present an abnormally high diamagnetism. The study of the melting points for the two series supports the trend of diamagnetic data, thus stressing the anomalous behavior of the odd ketodiamides which present very low melting temperatures. The x-ray long spacings of the two diamide series have been quantitatively correlated with various chain inclined crystal forms. However, while normal diamides show a zigzag extended chain conformation with formation of hydrogen bonded sheets (two hydrogen bonds per molecule), ketodiamides adopt a twisted conformation which seems to contribute to the enhancement of diamagnetism. In addition, the long periods of the odd ketodiamides reveal tilted twisted molecular structures which are associated with the observed anomalous elevation of χ_M and depression of melting points.