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Title: The amide planarity as studied by NMR of oriented molecules - The spectrum of N-methylacetamide
Authors: Ramaprasad, S.
Kellerhals, H.P.
Kunwar, A.C.
Khetrapal, C.L.
Issue Date: 1976
Publisher: Gordon and Breach Science Publishers.
Citation: Molecular Crystals and Liquid Crystals, 1976, Vol.34, p19-23
Abstract: The use of the technique of NMR spectroscopy of molecules oriented in the nematic phase of liquid crystals for the study of the 'planarity' of the three bonds meeting at the nitrogen atom in a peptide unit has been suggested1 and applied to the simplest member, namely, N-methyl formamide. Such studies are of fundamental importance since in most of the theoretical calculations, a peptide unit is assumed to be completely planar whereas the necessity of considering non-planar distortions in such a unit has been pointed out.2. For N-methyl formamide, using reasonable values of the bond lengths and the bond angles, it was found that the results are consistent with the molecule having a plane of symmetry which may arise, either from the H.CO.NH.C moiety of the molecule being rigidly planar or from a 'rapid inversion' through the nitrogen atom. The results support the fromer if there is a 'free' rotation of the CH3 group about the N-CH3 bond. However, if the methyl group rotation is hindered in a 3-fold potential, the results are in better agreement with the latter. In this case the conformations for which a C-H bond of the methyl group is 'staggered' with respect to the N-H bond are more favoured; the dihedral angles b.Deltaohgr and b.nuH defined according to the standard convention2 give the nonplanar distortions about the nitrogen atom. ohgr is the dihedral angle (Cagr1CNCagr2) between the planes Cagr1CN and CNCagr2 (fig. 1). It is 180(d)grave for a trans planar unit. b.Deltaohgr is the variation in (positive in the clockise direction). b.nuH is the dihedral angle (Cagr1) between the planes Cagr1CN and CNH. It ia 0(d)grave for a trans planar unit. For N-methyl formamide, a value of nearly 10(d)grave was estimated for the dihedral angle b.Deltaohgr and b.nuH under the assumption that they are equal in magnitude but opposite in sign.2. Furthermore, separate spectra due to the cis and the trans species could be observed and interpreted in N-methyl formamide3 whereas in N-methyl acetamide, the spectrum due to trans species alone was observed.
Description: Restricted Access.
ISSN: 1058-725X
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Copyright: 1976 OPA (Overseas Publishers Association) N.V.
Appears in Collections:Research Papers (SCM)

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