Abnormally High IR Frequencies for the Carbonyl Group of Semicarbazones of the Benzaldehyde and Acetophenone Series

We have synthesized 86 semicarbazones of variously substituted benzaldehyde and acetophenone and of several other aldehydes and ketones for comparison. The IR data in Nujol mull revealed numerous abnormally high frequencies for the carbonyl of semicarbazones of up to ca. 1760 cm^-1 (cf. 1690 cm^-1 as a normal value for semicarbazones carbonyls and 1640–1650 cm^-1 for the amide and urea carbonyls). These high C=0 frequencies shift to normal values in DMSO solution. The origin of the high C=0 frequencies could not be ascribed to the stereoelectronic effects of the substituents on the aromatic ring. It appeared that the high C=0 frequencies are caused by the solid-state structure. The X-ray structural determination of two semicarbazones indicated a network of H-bonds in which each C=0 is involved in a bifurcated H-bond with NH₂ and NH from two different molecules. The solution and solid-state ¹³C NMR studies of semicarbazones are also described.