NUCLEAR MAGNETIC RESONANCE
Nuclear magnetic resonance spectroscopy , most commonly known as NMR spectroscopy or magnetic resonance spectroscopy ( MRS ), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. Besides identification, NMR spectroscopy provides detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. The most common types of NMR are proton and carbon-13 NMR spectroscopy, but it is applicable to any kind of sample that contains nuclei possessing spin.
NMR spectra are unique, well-resolved, analytically tractable and often highly predictable for small molecules. Different functional groups are obviously distinguishable, and identical functional groups with differing neighboring substituents still give distinguishable signals.
Correlation spectroscopy is a development of ordinary NMR. In two-dimensional NMR, the emission is centered around a single frequency, and correlated resonances are observed.
There are also more complex 3D and 4D methods and a variety of methods designed to suppress or amplify particular types of resonances.
Nuclear spin and the splitting of energy levels in a magnetic field
Subatomic particles (electrons, protons and neutrons) can be imagined as spinning on their axes. 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. 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;

1.
If the number of neutrons and the number of protons are both even, then the nucleus has NO spin.
2.
If the number of neutrons plus the number of protons is odd, then the nucleus has a halfinteger spin (i.e. 1/2, 3/2, 5/2)
3.
If the number of neutrons and the number of protons are both odd, then the nucleus has an integer spin (i.e. 1, 2, 3)
The overall spin, I , is important. Quantum mechanics tells us that a nucleus of spin I will have 2 I + 1 possible orientation. A nucleus with spin 1/2 will have 2 possible orientations.
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. 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. 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.
Detection of NMR Signal
Image illustrating the production of NMR spectra.
Deuterated solvents
The vast majority of molecules in a solution are solvent molecules, and most regular solvents are hydrocarbons and so contain NMR-active protons. In order to avoid detecting only signals from solvent hydrogen atoms, deuterated solvents are used where 99+ % of the protons are replaced with deuterium (hydrogen-2). The most widely used deuterated solvent is deuterochloroform (CDCl
3
), although other solvents may be used depending on the solubility of a sample. Deuterium oxide (D
2
O) and deuterated DMSO (DMSO-d
6
) are often used when CDCl
3
doesn't work, and other deuterated solvents can be used as needed.

Chemical shift
Chemical shift is defined as nuclear shielding / applied magnetic field . Chemical shift is a function of the nucleus and its environment. It is measured relative to a reference compound. For
1
H NMR, the reference is usually tetramethylsilane, Si (CH
3
)
4
.
The magnetic field at the nucleus is not equal to the applied magnetic field; electrons around the nucleus shield it from the applied field. The difference between the applied magnetic field and the field at the nucleus is termed the nuclear shielding .
The chemical shift provides information about the structure of the molecule. The conversion of the raw data to this information is called assigning the spectrum.
Interpreting NMR spectra – chemical shifts
Chemical shifts for proton and carbon-13 NMR.

Correlation spectroscopy
Correlation spectroscopy is one of several types of two-dimensional nuclear magnetic resonance (NMR) spectroscopy or 2D-NMR. This type of NMR experiment is best known by its acronym, COSY. Other types of two-dimensional NMR include J-spectroscopy, exchange spectroscopy (EXSY), Nuclear Overhauser effect spectroscopy (NOESY), total correlation spectroscopy (TOCSY) and heteronuclear correlation experiments, such as
HSQC, HMQC, and HMBC. In correlation spectroscopy, emission is centered on the peak of an individual nucleus; if its magnetic field is correlated with another nucleus by throughbond (COSY, HSQC, etc.) or through-space (NOE) coupling, a response can also be detected on the frequency of the correlated nucleus. Two-dimensional NMR spectra provide more information about a molecule than one-dimensional NMR spectra and are especially useful in determining the structure of a molecule, particularly for molecules that are too complicated to work with using one-dimensional NMR.
1H NMR

13C NMR
1-dimensional NMR spectra of carbon-13 and hydrogen.
COSY
Homonuclear correlated spectroscopy. Correlation between protons that are coupled to each other. (Bax et al., 1981). There are many modified versions of the basic COSY experiment: DQF-COSY (double-quantum filtered), COSY45, LRCOSY, ECOSY.
TOCSY: Total Correlation Spectroscopy (also referred to as the HOHAHA experiment).
Uses a spin-lock for coherence transfer. During the spin-lock all protons of a coupled system become "strongly coupled," leading to cross peaks between all resonances of a coupled system. Has some advantages over COSY in that small couplings can be detected
(in COSY small couplings give very weak cross-peaks because the peaks are antiphase).

Homonuclear correlation spectroscopy (COSY)
C-H-COSY OR HETCOR
Heteronuclear correlation, usually between 1H and 13 C resonances mediated by JC-H. The experiment can be run using either 1JC-H or longer range couplings. It has poor sensitivity because the observed nucleus is
13
C and has been largely replaced by the inverse detection experiments HMBC and HMQC .
HMQC
Proton detected heteronuclear multiquantum coherence. Similar to CH-COSY or
HETCOR experiment, except that the inverse detection using a DEPT sequence provides

much better sensitivity (Bax et al., 1986). Used to correlate proton and carbon signals using either one bond or longer-range couplings.
HSQC
Heteronuclear Single Quantum Coherence. This is a CH correlation experiment which uses proton detection of the 13C signals using an INEPT sequence. It shows higher resolution in the C-dimension than does the related HMQC experiment.
HMBC
Heteronuclear Multi-Bond Connectivity. This experiment is a lot like HMQC, but it is optimized to detect proton-carbon correlation over 2 and/or 3 bonds (Bax et al., 1986).
Although the sensitivity is better than direct detection methods, it is still not very good, since the magnetization has to be transferred twice, first from proton to carbon and then back to proton via weak long-range interactions, which require relatively long delay times and thus much opportunity for loss of signal intensity by T1 and T2 relaxation. For a modification to distinguish 2 from 3-bond coupling see: Griesinger, Tetrahedron Lett.
1996, 37, 363.

Heteronuclear multiple-bond correlation spectroscopy (HMBC)
Examples of 2-dimensional NMR spectra.
EXSY
Exchange Spectroscopy - the 2D equivalent of the Forsen saturation transfer experiment.
The transfer of magnetization, however, is due to chemical exchange between A and B, rather than an NOE . EXSY uses an identical pulse sequence to the NOESY experiment,

and is thus subject to interference by NOE . The magnetization transfer will be the equivalent of a negative NOE (Perrin, C. L.; Dwyer, T. Chem. Rev. 1990, 90, 935).
NOESY , ROESY
Proton-proton correlation mediated by dipolar coupling ( NOE effect). Correlation between protons that are close in space. This is the single most powerful NMR technique for determining the 3-dimensional structure of molecules - from conformations of small molecules to the 3-dimensional structure of small proteins.
In 2- (or multi-) dimensional NMR spectroscopy multipulse sequences are employed to provide additional information not obtainable (or not easily obtainable) from onedimensional spectra. The most useful and commonly used forms of 2D NMR spectroscopy provide correlations between proton (or other NMR-active nuclei) signals based on some interaction between them, most frequently J-coupling (H-H. H-C or even C-C), but it could also be dipolar coupling (the NOE interaction), rates of chemical exchange (DNMR swapping of spin environments) or relative diffusion rates ( DOSY and related experiments).
HOESY
Heteronuclear Overhauser Effect Spectroscopy. Correlation between protons and heteronuclei that are close in space. Requires that the heteronucleus be relaxed significantly by dipole-dipole interactions with nearby protons or other magnetic nuclei, and so fails with nuclei relaxed exclusively by chemical shift anisotropy or quadrupolar mechanisms. Used extensively in the spectroscopy of organolithium reagents, since 6Li works well in this experiment. INADEQUATE: Incredible Natural Abundance Double

Quantum Transfer Experiment. 2-Dimensional JC-C This double quantum coherence experiment produces a C-C correlation using 1JCC. It is perhaps the most powerful structure-determination tool of all, but suffers from very low sensitivity (Bax et al., 1981)
HMBC Spectra
This is a proton-detected experiment that shows carbons that are 2-3 bonds away from protons. It is absolute value.
1.
A common artifact is the appearance of one-bond correlations. HMBC is "tuned" to detect the small couplings arising from long-range interactions, but this is not perfect. These one-bond artifacts appear as doublets in f2, with J=1JCH.
Occasionally, this splitting is itself useful information. However, most of the time, one should be wary of these artifacts.
2.
HMBC incorporates a delay which under ideal circumstances is 1/2JCH. Since long-range couplings occupy a relatively wide range of 5-25 Hz, this delay is often set at a compromise value of 60 ms (8 Hz). This means that not all of the correlations will appear, and certainly not with equal intensity. Additionally, note that three-bond couplings are often larger than two-bond ones.
3.
BENEFITS OF NMR

Provides both qualitative and quantitative information.

Samples can be recovered.

A small quantity of material is required for analysis.

Preparation of sample is easy.

SHORTCOMMINGS OF NMR

Poor sensitivity.

Requires special infrastructure and training.

Spectra takes a long time to interpret.

Potential fire hazard.

Very expensive.
VARIOUS APPLICATIONS OF NMR

Compound identification and confirmation.

Chemical composition determination.

Molecular conformation determination.

Raw materials fingerprinting.

Quality assurance and control.