NMR lab PCHEM.docx - Lucas Paquin CHEM 166A NMR Determination of the Rotational Barrier in N N\u2019Dimethylacetamide Introduction Nuclear magnetic

Lucas Paquin CHEM 166A March 19, 2018 NMR Determination of the Rotational Barrier in N, N ’ - Dimethylacetamide Introduction: Nuclear magnetic resonance (NMR) spectroscopy is one of the most commonly used spectroscopic tools for studying dynamic processes. These dynamic processes can include conformational changes and chemical changes. Examples of conformational studies using dynamic NMR are the boat-chair interconversions in cyclohexanes and cis-trans isomerization. Chemical studies include proton exchange reactions, such as the protonation of amines, keto-enol tautomerization, and complexation. There are two requirements for the use of dynamic NMR. The first requirement is that the magnetically active nucleus, protons in this case, must change environments so that the chemical shift of the nucleus differs from one environment to another. This process is called chemical exchange. Chemical exchange represents the transfer of a nucleus from one molecular environment to another (2) . The second is that the time scale of the exchange must be either slow enough or fast enough to cause the NMR lines to be broadened. The time scale where broadening occurs is called the NMR time scale. In this experiment, the barrier to rotation in N, N'- dimethylacetamide (NNDMA) will be determined. The great part about this method is that dynamic aspects of systems that are at chemical equilibrium can be studied. For example, rate information can be obtained for virtual reactions, such as the cis-trans isomerization of NNDMA, in which reactants and products are chemically identical: Second, the NMR time scale includes a range of reaction rates that are often encountered in the laboratory, 10 -1 -10 -5 s -1 . In addition, rotational barriers in the range 12-80 kJ/mole can be studied by this method (3) . For NNDMA, if the -N(CH 3 ) 2 substituent rotates freely, it would be expected for the NMR spectrum to consist of two singlets, with an intensity ratio of 2:1 (2 methyl’s on nitrogen, 1 methyl on the carbonyl carbon). The NMR spectrum is not consistent with free rotation about the peptide bond. Instead of one peak at 3 ppm and one at 2 ppm, there are two peaks at ~3 ppm in addition to the one at 2 ppm. This difference results from the two N-CH3 groups being in magnetically inequivalent environments. One methyl group is cis to the carbonyl bond and the other is cis to the acetyl methyl group. The upfield resonance at ~3 ppm is assigned to the -NCH3 group cis to the acetyl methyl; the downfield resonance at ~3 ppm is assigned to the -NCH3 group cis to the carbonyl bond. Meanwhile, the 2 ppm resonance is assigned to the acetyl methyl protons. Amides are the simplest model compounds for the peptide bond in proteins. The conformation of the peptide bond plays a significant role in determining the backbone structure 1

of proteins (4) . X-Ray crystallographic results indicate that the trans configuration (5) is the dominant form in amides, polypeptides, and proteins. However, the cis form does occur in some proteins and polypeptides (6) .