HINDERED INTERNAL ROTATION AND DIMERIZATION OF N,N-DIMETHYLFORMAMIDE IN CARBON TETRACHLORIDE

The well-known hindered internal rotation of N,N-dimethylformamide (DMF) is reinvestigated by a nmr total line-shape (TLS) treatment taking into account both the variance of δν (nonexchanging N-methyl doublet chemical shift) and the coupling between the formyl and N-methyl protons. The agreement between experimental and calculated spectra is excellent and the thermodynamic parameters derived are Ea - 20.5 ± 0.2 kcal/mole, νo = (5 ± 3)1012 sec-1, δG128˚* = 21.0 kcal/mole, δH* = 20.2 ± 0.2 kcal/mole, and δS* = –1.7eu, showing that DMF is not unexpectedly different from other amides. DMF in CCL4 is examined and the rate of internal rotation (from TLS) at different concentrations is shown to correlate qualitatively with the extent of dimerization of DMF. The nature of the dimer is discussed briefly, and it is shown that there may be a contribution to the chemical shift behavior of DMF in solution by anisotropic shielding effects of the carbonyl groups in an oriented cyclic dimer. © 1969, American Chemical Society. All rights reserved.