Table of Contents
NMR Spectroscopy Definition
NMR stands for Nuclear Magnetic Resonance Spectroscopy is a technique that investigates the magnetic properties of a substance. It records the magnetic spectral patterns emitted by the nucleus of that compound. This process also helps determine the magnetic structure of the compound or sample.
What is NMR Spectroscopy?
NMR Spectrometer, employs electromagnetic radiation, as the radio waves to analyze the magnetic properties of the substance. This Spectrometer emits a radio wave that is directed at the sample. The atoms of the sample absorbing this radiation get exited. The atoms will resonate at specific frequencies to return the signal.
The signals are altered by the atoms depending on their structures and bonds. The frequencies of returning signals are detected by machines and are specific to each molecule. This gives us detail about the shape and the size of the molecules. Each atom of that compound contributes to the resonance shift detected by the NMR machinery.
The resonance shift is affected by the magnetic field at the nucleus. This resonance is affected by the bonds of atoms. Using this information the structure of the molecule can be inferred and the functional groups present in them.
Advantages of NMR Spectroscopy
There is a large spectral volume library that has been collected by immense effort through the years, now this library can be utilized as a reference to identify the functional groups. Since the spectral patterns of the functional groups are known, then based on the NMR reading the chemical and structural properties of the compound can be determined.
NMR spectroscopy is more advantageous than mass spectroscopy. As it is not required to purify the compound before analysis. All atoms of the compound can be observed from NMR analysis and it is a quantitative measurement, unlike mass spectroscopy. But in some cases, NMR may not be the way to go.
NMR Spectroscopy and Unknown Compounds
A resonance reading is obtained by NMR spectroscopy that helps us to identify the purity of the substance and its molecular structure. Purity is determined by whether the substance comprises only one kind of molecule. Generally, chemicals or solvents are found that decrease the purity of the substance.
The NMR reading depicts several spikes, these spikes are compared to the spectral libraries and the chemical structure of the compound is analysed.
Measuring Each Atom
One of the features of the spikes that is important is its length. The relative proportion of a given atom can be inferred by this property. This can help us to construct the molecular structure of the sample, since we know for instance, how many carbon or hydrogen atoms are present in the compound.
NMR Spectroscopy and Understanding the Position of Atoms
NMR Spectroscopy technique also gives us insight about the relative position of the atoms present. The neighbouring groups or atoms can affect the resonance signals of an atom. For instance, if a polar group is located next to the hydrogen atom then it that case the NMR reading will be higher as compared to when a non-polar group is located proximal to the Hydrogen atom.
also gives us information about the relative position of our atoms. A hydrogen atom can give off several different resonance signals depending on its neighboring atoms or groups. For example, a hydrogen atom located next to a polar group, such as an oxygen-containing carboxyl group, will give off a higher NMR reading than a hydrogen atom neighbored by non-polar methane groups.
When a polar atom or group like carboxy group is bonded to the hydrogen atom, then it will show an incline in resonance shift leading to higher NMR reading. The general pattern observed is that non-polar atoms show lower NMR reading as opposed to the polar atoms or groups.
NMR can be utilized to identify functional groups easily. Groups like amine group, hydroxyl group, carboxy group all are characterised by specific NMR resonance shfts. The identification of functional groups helps us to decide the basic structure of the sample.
The Principles of NMR Spectroscopy
The working principle of NMR is based on the charge and spin of the nucleus. These 2 properties of nucleus help to respond to a magnetic field. In the presence of an external magnetic field or running an electric current causes energy transfer to a higher state.
At a particular radio frequency and wavelength this energy transfer can be detected. Once the spin of the nucleus return to the baseline it emits the energy that is detected and read by the NMR machinery.
When an external magnetic field is applied, the nucleus can spin in either conditions, either in direction of magnetic field or opposed to it. If the nucleus spins in the opposite direction then it possess more energy as compared to the nucleus spinning in the same direction. The phenomenon of electron shielding affects the resonant frequency.
As a general rule the higher electronegativity of the nucleus resuts in higher resonance frequency. The presence of more electronegative groups lowers the chemical shift. Whereas if more electron donating groups are present it will lead to higher chemical shifts. This increase in shift can be due to presence of aromatic groups that delocalize current.
Interpreting NMR Spectroscopy Signals
The number and intensity of signals tells the number of equivalent protons and the type of proton present respectively. The position of peak and the position of signal indicates amount of de-shielding and the chemical shift, respectively. The proton signal based on its proton neighbors is split by number of peaks referred to as Signal Splitting.
Signal Splitting in NMR Spectroscopy
The signal splitting process gives us vital information about the type and position of protons in the molecule. The proton signal s affected by their hydrogen neighbors such that it splits into smaller peaks. In case there are no neighboring hydrogen atoms a single peak can be observed. But if 2 hydrogen atoms occur proximaly the resonance will be seen as doublet.
Whereas if both hydrogen atoms occur on adjacent atoms then we observe triple signal.Similarly, if there occurs 3 hydrogen on adjacent groups it will result in 4 peaks. Based on the number of hydrogens on adjacent functional groups represented as N, the NMR resonance will be N+1 peaks.