Nuclear magnetic resonance NMR spectroscopy is a powerful analytical technique used to determine the structure of unknown organic compounds. It is based on the interaction of atomic nuclei with an external magnetic field, which causes the nuclei to absorb and re-emit electromagnetic radiation. The resulting NMR spectra provide valuable information about the functional groups, connectivity, and stereochemistry of the compound. Here are the steps involved in using NMR spectroscopy data to identify the structure of an unknown organic compound:
1. Obtain NMR spectra: The first step is to obtain the NMR spectra of the unknown compound. Typically, two types of NMR spectra are recorded: proton NMR 1H NMR and carbon NMR 13C NMR . These spectra provide information about the number and types of hydrogen and carbon atoms in the compound, respectively.
2. Analyze the 1H NMR spectrum: The 1H NMR spectrum displays peaks corresponding to different hydrogen atoms in the compound. The chemical shift position of the peak provides information about the electronic environment of the hydrogen atom, which can help identify the functional groups present in the compound. The integration area under the peak indicates the number of hydrogen atoms represented by the peak. The splitting pattern multiplicity of the peaks provides information about the connectivity of the hydrogen atoms to neighboring carbon atoms.
3. Analyze the 13C NMR spectrum: The 13C NMR spectrum displays peaks corresponding to different carbon atoms in the compound. The chemical shift of these peaks provides information about the electronic environment of the carbon atoms, which can help identify the functional groups present in the compound. The splitting pattern of the peaks can provide information about the connectivity of the carbon atoms to neighboring atoms.
4. Identify functional groups: Based on the chemical shifts and splitting patterns observed in the 1H and 13C NMR spectra, identify the functional groups present in the compound. Common functional groups include alcohols, amines, aldehydes, ketones, carboxylic acids, esters, and aromatic rings.
5. Determine connectivity: Use the splitting patterns and coupling constants observed in the 1H NMR spectrum to determine the connectivity of the hydrogen atoms to neighboring carbon atoms. This information, combined with the connectivity information obtained from the 13C NMR spectrum, can help establish the overall connectivity of the compound.
6. Determine stereochemistry: If the compound contains stereocenters, the stereochemistry can be determined by analyzing the coupling constants and splitting patterns observed in the 1H NMR spectrum. Additionally, two-dimensional NMR techniques, such as COSY correlation spectroscopy and NOESY nuclear Overhauser effect spectroscopy , can provide further information about the spatial arrangement of atoms in the compound.
Practical example: Let's say we have an unknown compound with the molecular formula C4H8O2. The 1H NMR spectrum shows a singlet peak at 3.6 ppm integration = 6H and a triplet peak at 1.2 ppm integration = 2H . The 13C NMR spectrum shows three peaks at 170 ppm, 60 ppm, and 14 ppm.
Based on the chemical shifts and integration values in the 1H NMR spectrum, we can identify the presence of a methyl group CH3 and an ethoxy group OCH2CH3 . The 13C NMR spectrum supports this interpretation, with peaks corresponding to a carbonyl carbon 170 ppm , an ether carbon 60 ppm , and a methyl carbon 14 ppm .
Combining this information, we can propose the structure of the unknown compound as ethyl acetate CH3COOCH2CH3 . The connectivity and stereochemistry of the compound can be confirmed using two-dimensional NMR techniques, such as COSY and NOESY, if necessary.