DO ALL SOLID AMIDES HYDROGEN-BOND - RAMAN EVIDENCE TO THE CONTRARY

We report Raman and preresonant Raman spectra of three cyclic amides: epsilon-caprolactam, 2-azacyclononanone, and 2-azacyclotridecanone, in the solid phase. Raman spectra of the amides, particularly preresonant Raman spectra, provide direct information on hydrogen bonding through the relative intensities of the C=O stretch and C-N stretch bands, the frequency width of the C=O band, the frequency of the N-H stretch band, and the frequency width of the N-H band. In our Raman spectra of epsilon-caprolactam all these features indicate the presence of hydrogen bonding in the solid, as expected for the amides. However, for 2-azacyclononanone and 2-azacyclotridecanone all the Raman spectral indicators of hydrogen bonding are negative. If characteristic changes in the molecular electronic structure and vibrational spectra of the amides are accepted as defining a hydrogen bond interaction, then we are forced to conclude that some amides are not hydrogen bonded in the solid.