(Remember that no coupling is observed between equivalent nuclei: the 2 H atoms are equivalent.) There are 0 neighbours so n = 0 and n + 1 = 1. (c) The 1H nuclei in set (c) do not have any H on neighbouring atoms. The signal due to set (b) at 4.49 ppm is split into a 1 : 3 : 3 : 1 quartet. There are 3 neighbours so n = 3 and n + 1 = 4. (b) The 1H nuclei in set (b) are next to the 3 H in set (a). The signal due to set (a) at 1.21 ppm is split into a 1 : 2 : 1 triplet. There are 2 neighbours so n = 2 and n + 1 = 3. (a) The 1H nuclei in set (a) are next to the 2 H in set (b).There are 3 chemical environments, (a), (b) and (c), and so 3 signals in the spectrum. There is a mirror plane that cuts through the C=C bond.(c) H bonded to C=C: chemical shift 4.9 - 5.9 ppm (observed at 5.26 ppm). (b) C H 2 bonded to O: chemical shift 3.2 - 5.4 ppm (observed at 4.49 ppm). (a) C H 3 bonded to C: chemical shift 0.8 - 1.2 ppm (observed at 1.21 ppm). Using the 1H NMR chemical shift table, the 1H chemical shifts are predicted to be: It can be rationalized using the steps outlined above: The 1H NMR of the molecule oppsite is shown below. The process of predicting the spectra from a known structure is best illustrated with an example. If 1H and 13C NMR spectra are both available, make sure that your analysis is consistent.Įxample 1: Predicting the Spectrum from a Structure In the 1H NMR spectrum, use the number of lines in to which each signal is split through coupling and the ' n + 1' rule to determine the number of neighbouring atoms and hence work out which groups are connected (and which are not). In 1H NMR spectrum, use the integration to determine the number of 1H nuclei in each chemical environment. For example, if there are 4 C atoms in the formula but only 2 chemical shifts in the 13C NMR spectrum then the molecule must possess symmetry which makes some of the nuclei equivalent. Use the number of chemical shifts and the chemical formula (if available) to work out the number of equivalent nuclei and hence the presence or absence of symmetry in the molecule. Use the chemical shifts and charateristic chemical shift tables ( 1H NMR, 13C NMR) to determine the functional groups present. Whether you are predicting what the NMR spectrum of a compound will look like or trying to determine the structure of a compound from its NMR spectrum, a number of steps must be followed: A NMR spectrum contains lots of information about the functional groups and connectivity in a molecule.
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