NMR Spectroscopy

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The electrons that surround the nucleus are in motion so they created their own electromagnetic field. This field opposes the the applied magnetic field and so reduces the field experienced by the nucleus. Thus the electrons are said to shield the nucleus. Since the magnetic field experienced at the nucleus defines the energy difference between spin states it also defines what the chemical shift will be for that nucleus.

Electron with-drawing groups can decrease the electron density at the nucleus, deshielding the nucleus and result in a larger chemical shift. Compare the data in the table below. This is an effect of the halide atom pulling the electron density away from the methyl group.

Spectral Parameters:

This exposes the nuclei of both the C and H atoms, "deshielding" the nuclei and shifting the peak downfield. The effects are cumulative so the presence of more electron withdrawing groups will produce a greater deshielding and therefore a larger chemical shift, i. These inductive effects are not only felt by the immediately adjacent atoms, but the deshielding can occur further down the chain, i. The new magnetic field will have an effect on the shielding of atoms within the field.

The best example of this is benzene see the figure below. Protons that are involved in hydrogen bonding i. This is due to the deshielding that occurs in the hydrogen bond.

Theoretical principles

Since hydrogen bonds are dynamic, constantly forming, breaking and forming again, there will be a wide range of hydrogen bonds strengths and consequently a wide range of deshielding. This as well as solvation effects, acidity, concentration and temperature make it very difficult to predict the chemical shifts for these atoms. Experimentally -OH and -NH can be identified by carrying out a simple D 2 O exchange experiment since these protons are exchangeable. What do we mean by "type" of hydrogen atom? Since the amount of shielding is dependent on the local chemical environment, the exact chemical shift for H atoms can vary widely.

There are three basic methods you can use to determine if H atoms are identical. This is the simplest but slowest method.

The idea is to replace every H atom, one at a time with another atom i. Each different product indicates a different type of H atom. This requires to you describe each H atom verbally. If you have a different description then the H atoms are different. The symmetry method is the most sophisticated but requires a knowledge of molecular symmetry.

H atoms that are related by mirror planes, axis of rotation or a center of inversion are equivalent to one another. The first method is the easiet but slowest, the last is the fatest but requires a good knowledge of molecular symmetry. See the thre examples below. There are several important pieces of information that you can obtain from an 1 H-NMR. The first is the chemical shift of the peak.

Basic Introduction to NMR Spectroscopy

This will aid in identifying the type of H atom that produced the signal. The second is the integration ratios of the peaks. The area under a peak of a 1 H-NMR is directly proportional to the number of H atoms that produced the peak. The area is calculated by integrating the area, done automatically for you by the software. On older spectra the integration curve was drawn on the spectra, modern software will produce this as a table attached to the spectra.

Nuclear magnetic resonance spectroscopy - Wikipedia

The example below is for methyl t-butyl ether. The integrals shown as the red curves are in a ratio of or This implies that the peak on the left corresponds to the methyl group attached to the oxygen as expected since the O will deshield the H atoms. The peak on the right is the three methyl groups of the t-butyl group, less deshielded as it is further away from the O atom. Note that we get the simplest ratio of H atom types , methyl : t-butyl not the true ratio of The NMR above has absorptions which are called singlets , a single sharp peak.

However, in most cases, this is not the norm. Absorptions are split into groups of peaks due to coupling between adjacent protons in the molecule. Browser does not support script.

High Resolution Nuclear Magnetic Resonance (NMR) Spectroscopy

Winterbourne House and Garden University Music. Research and Cultural collections. See all schools, departments, research and professional services Liberal Arts and Natural Sciences. Conferences and hospitality Facilities search Birmingham Day Nurseries. Libraries Guild of students. Online Shop Freedom of Speech. In many atoms such as 12 C these spins are paired against each other, such that the nucleus of the atom has no overall spin.

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  7. However, in some atoms such as 1 H and 13 C the nucleus does possess an overall spin. The rules for determining the net spin of a nucleus are as follows;. The overall spin, I , is important. In the absence of an external magnetic field, these orientations are of equal energy. If a magnetic field is applied, then the energy levels split.

    Nuclear Magnetic Resonance (NMR) Spectroscopy

    Each level is given a magnetic quantum number, m. When the nucleus is in a magnetic field, the initial populations of the energy levels are determined by thermodynamics, as described by the Boltzmann distribution.

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    This is very important, and it means that the lower energy level will contain slightly more nuclei than the higher leve l. It is possible to excite these nuclei into the higher level with electromagnetic radiation. The frequency of radiation needed is determined by the difference in energy between the energy levels. The nucleus has a positive charge and is spinning. This generates a small magnetic field. The nucleus therefore possesses a magnetic moment, m , which is proportional to its spin, I. The constant, g , is called the magnetogyric ratio and is a fundamental nuclear constant which has a different value for every nucleus.

    This means that if the magnetic field, B , is increased, so is D E. It also means that if a nucleus has a relatively large magnetogyric ratio, then D E is correspondingly large. If you had trouble understanding this section, try reading the next bit The absorption of radiation by a nucleus in a magnetic field and then come back.

    In this discussion, we will be taking a "classical" view of the behaviour of the nucleus - that is, the behaviour of a charged particle in a magnetic field. This nucleus is in the lower energy level i. The nucleus is spinning on its axis. In the presence of a magnetic field, this axis of rotation will precess around the magnetic field;. The frequency of precession is termed the Larmor frequency , which is identical to the transition frequency.

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