FT-IR spectrometric study of N-t-butoxycarbonylglycine N′,N′-dimethylamide and its interaction with proton donors

Abstract

FT-IR spectra of N-t-butoxycarbonylglycine-N′,N′-dimethylamide and its deuteriated ND counterpart, in solution and in the solid state, have been analysed between 3800 and 400 cm^–1. An assignment is proposed for some important vibrations, especially for the so-called amide vibrations, by comparison with literature data on similar molecules. The frequency shifts and intensity changes of the vibrations sensitive to the physical state of this model dipeptide strongly suggest that intermolecular hydrogen bonds involving mainly the NH and CO (amide) groups are formed in the solid state. Thermodynamic parameters (equilibrium constants and enthalpies) of the hydrogen-bond formation with phenol derivatives have been determined in carbon tetrachloride and in 1,2-dichloroethane. The complexes investigated are of medium strength. The FT-IR spectra in the ν_CO region and the double perturbation of the ν_NH band assigned to the five-membered intramolecular NH ⋯ OC hydrogen-bonded ring clearly show that complex formation occurs at both the amide and urethane carbonyl groups. About 45% of the complexes are formed on the CO (urethane) function, almost independent of the acidity of the phenols. The unusual broadness of the OH ⋯ OC complex band originates from the superposition of two complex bands. N-t-Butoxycarbonylglycine-N′,N′-dimethylamide can be considered as being built up from two model molecules, methyl-N-methylcarbamate and N,N-dimethylacetamide.