Spectroscopic Evidence for Hydrogen Bonds: Effects of Chelation on the Carbonyl Frequency

The vibrational band characteristic of the carbonyl linkage was measured for o-hydroxyacetophenone, o-methylaminobenzoate, o-methylsalicylate, o-hydroxybenzaldehyde and for compounds similar to these but lacking an adjacent proton-donor group. Chelation through hydrogen bonding was found to shift the C=O band to longer wave-lengths by about 0.15μ to 0.20μ, or 40 cm—1 to 53 cm—1, and to increase its intensity. Similar effects were observed for the carbonyl band of the enol forms of acetyl-acetone, ethyl malonate, ethyl acetoacetate. The magnitude of the effects of hydrogen bonding on the carbonyl band shows that energy changes in the proton-acceptor groups cannot be neglected in the computation of the energy of the hydrogen bond from spectroscopic data. The results also show that measurements on the proton-acceptor group can often be used as criteria for the presence of hydrogen bonding when experimental difficulties prevent accurate measurements of the effects on the vibrational band of the proton-donor group. Smaller effects were noted for ethylenic linkages of some of these compounds.