Using NMR to observe the restricted rotation in amide bonds

Nuclear Magnetic Resonance (NMR) spectroscopy can be utilized to explore different domains in the natural sciences. NMR is a great tool for the analysis of molecular properties such as the amide bond, which has a restricted rotation around the C–N bond. In Biochemistry, the amide bond is referred to as the peptide bond. This bond is formed by the union of a carboxyl group of one amino acid with the amino group of another amino acid. In other words, the two amino acids form a peptide bond through a dehydration reaction (Scheme 1).

Scheme 1. Dehydration reaction between two residues to form a longer amino chain and water as a by-product.

In the amide, N,N–dimethylacetamide (DMA), we can see in the ¹H NMR spectrum that the two methyl groups bonded to the nitrogen produce two different signals (Figure 1). To comprehend this, we need to look at resonance stabilization. Resonance stabilization is when electrons are delocalized amid multiple atoms (Scheme 2). In the case of amides, this creates a hybrid structure that results in the C–N bond having some double-bond character (therefore the rotation is restricted). At room temperature, DMA due to the contribution of the double character results in the methyl groups bonded to the nitrogen to be chemically different. At higher temperatures the energy barrier to rotation is overcome and there is rapid rotation of the C–N bond, which creates an environment that is equivalent for both N–methyl groups.

Figure 1. ¹H NMR Spectrum of DMA, demonstrating that the two N–methyl groups have different chemical shifts.

Scheme 2. Resonance structures of DMA, the hybridization results in the delocalization of electrons (indicated by the dotted line) between the O–C–N bonds.

In sum, NMR can be used to study the molecular properties of compounds. This can aid in interpreting compounds and understanding more advanced chemical concepts. For example, dynamic processes that include rotational barriers and conformational changes of cis-trans isomers.^[1] If you have any questions regarding these concepts or how our instruments can be of aid, please contact us.

References

^[1] Huggins, M.T.; Kesharwani, T.; Buttrick, J.; Nicholson C. J. Chem. Educ. 2020, 97, 1425 – 1429
^[2] https://researchguides.library.vanderbilt.edu/c.php?g=69346&p=449918 (accessed February 2023) - Vanderbilt University