NMR relaxation studies of hydrogen bonding in simple amide compounds

CHED 1011

Tiffany E. Muns, TMuns716@hawks.rwu.edu and Nancy E. Breen, nbreen@rwu.edu. Department of Chemistry, Roger Williams University, 1 Old Ferry Rd, Box 5376, Bristol, RI 02809
Results of NMR relaxation studies of simple amide compounds are presented. The 13C spin lattice NMR relaxation times of N-methyl acetamide, N, N-dimethyl acetatmide, ε-caprolactam and 2-azacyclotridecanone were measured in various solvents on a JEOL ECX 300 MHz NMR spectrometer. The relaxation times were found to be sensitive to the hydrogen bonded properties of the solvent. For example, the carbon atom in the amide bond of ε-caprolactam had a T1 relaxation time that varied from 10 seconds in CDCl3 to 38 seconds in CD3CN. The shorter T1 relaxation time observed in CDCl3 can be explained by the nature of the H-bonding present in the sample. In ε-caprolactam, the amide bond adopts the cis-configuration because of ring strain. When it forms hydrogen bonds with itself, as it does in CDCl3, it forms cyclic dimers. In this geometry, the C=O-H-N is more rigid and therefore undergoes less random molecular motion than when it is not hydrogen bonded to itself. This makes for a longer correlation time and a shorter T1 relaxation time. In general it was observed that when inter-amide hydrogen bonding is possible, shorter T1 relaxation times are found. Whereas, when no inter-amide hydrogen bonding is possible or when the dominant interaction is solute-solvent, much longer T1 relaxation times are observed.