Primer on NMR analysis This is a synopsis of the elements for analyzing NMR. This is not meant to replace learning the underlying basis for why these items are true ? learning the NMR theory will help you remember the concepts summarized here. Unique atoms have unique signals Atoms in unique magnetic environments will have different signals. Atoms in the same magnetic environments will have a single signal. It is important to be able to identify the number of unique atoms in a molecule to know how many signals you will see in the NMR. Remember to look for planes of symmetry to identify sets of the same protons. Below each molecule has sets of unique protons circled: OH CH 3 Br O CH 3 CH 3 CH 3 CH 3 CH 3 H H Note: Remember, signals that happen to overlap in chemical shift will appear to be unresolved or messy signals. It is usually easy to spot this based on the integrations of the signals. Three parameters for analysis The three parameters used for analyzing NMRs are chemical shift (ppm, or ?), integrations and splitting. Chemical shift: Chemical shift is based on the amount of electron density around the atoms being probed. This can be estimated based on electronegativity differences between atoms and resonance forms. Less electron density causes the signal to resonate at lower magnetic field strength (downfield), which is to the left on the NMR spectrum. More electron density causes the signal to resonate at higher magnetic field strength (upfield), which is to the right on the NMR spectrum. Tables in the lab and lecture text can give general ppm ranges for many protons. Tetramethylsilane (TMS), the internal standard for most 1 H NMR, is at 0 ppm on the furthest right of the spectrum because Si is less electronegative than carbon, which leads the methyl groups to resonate at a high field relative to almost all organic molecules. Note: Protons on hetero atoms (N, O) are often hydrogen bonded, which leads to many protons of the sample in slightly different environments. This means that there can be a wide range of values at which those protons can appear. It will also broaden the signal, even to the point where the signal is not seen. Integrations: The area under the curve of each individual signal will be proportional to the number of atoms that give that signal. The actual numbers of the integrations can vary (even between different NMRs of the same molecule) but the ratio of the integrations will always represent the number of atoms under each signal. Important caveats: In general, integrations can be +/- 10% If signals overlap, the total integration of the overlapping signals will be proportional to the total number of atoms. Protons on hetero atoms (N, O) are often hydrogen bonded, which leads to many protons of the sample in slightly different environments. This can alter the integration due to broadening of the peaks. Splitting: The signals of any set of equivalent protons are affected by any other protons that it can ?see?, which means any protons within 3 bonds (benzene rings, being a single electronic structure, can be affected by protons further away). A signal will be split into n+1 peaks by n equivalent protons nearby. If there is more than one set of equivalent protons, it will be split multiple times. The heights of the peaks will follow Pascal?s triangle (see http://en.wikipedia.org/wiki/Pascal's_triangle). Analysis In general, you should identify fragments of structure by using the integrations and splitting together to see what signals are interacting with each other. Once you have identified fragments you can use the molecular formula and chemical shifts to help piece them together. Even without the molecular formula the chemical shifts should provide information on the functional groups that are next to each signal. Common solvent peaks The most common solvent/standard peaks seen in proton NMR are shown below: Solvent chemical shift (ppm) Appears because CHCl 3 7.23 Part of the NMR solvent Acetone (CH 3 ) 2 CO 2.05 NMR tube not completely dry from cleaning Water (H 2 O) 1.5 Left over from reaction/ workup Tetramethylsilane (TMS) (CH 3 ) 4 Si 0.0 Internal standard lynseyt Microsoft Word - Primer on NMR analysis.doc
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