ORIGINS OF ABNORMALLY HIGH IR C=O FREQUENCIES OF SEMICARBAZONES IN THE SOLID-STATE .3. RAMAN-SPECTRA OF SEMICARBAZONES

Raman spectra of 28 semicarbazones (R1R2C=N-NH-CO-NH2; R1 = aryl, alkyl; R2 = H, CH3, alkyl) were obtained in the solid state. Several semicarbazones were deuterated at the NH and NH2 positions to help with the band assignments. These Raman spectra were compared with the corresponding solid-state IR spectra which, in many instances, have been found to exhibit abnormally high C=O frequencies. The observed differences in the infrared (IR) and Raman C=O frequencies were up to 83 cm-1. In general, these differences were higher for the compounds which exhibit abnormally high IR C=O (> 1700 cm-1) than for those in the normal range (ca. 1690 cm-1). It is concluded that the abnormally high IR C=O frequencies are caused, at least in part, by the vibrational coupling of the C=O in the solid-state H-bonded structure.