J-coupling
Encyclopedia
J-coupling is the coupling between two nuclear spin
s due to the influence of bonding electron
s on the magnetic field
running between the two nuclei. J-coupling contains information about dihedral angle
s, which can be estimated using the Karplus equation
. It is an important observable effect in 1D nuclear magnetic resonance spectroscopy
.
and D. E. Maxwell reported a spin echo experiment which indicates the existence of an interaction between two protons in dichloroacetaldehyde
. In the echo experiment, two short, intense pulses of radiofrequency magnetic field
are applied to spin ensemble at the nuclear resonance condition and are separated by time interval of τ. The echo appears with a given maximum amplitude at time 2τ. For each setting of τ, the maximum of the echo signal is measured and plotted as a function of τ. If the spin ensemble consists of magnetic moment
, a monotonic decay in the echo envelope is obtained. In the Hahn-Maxwell experiment, the decay was modulated by two frequencies: one frequency corresponded with the difference in chemical shift
between the two non equivalent spins and a second frequency, J, that was smaller and independent of magnetic field strength. (J/2π = 0.7 cycle per second)
Such interaction came as a great surprise. The direct interaction between two magnetic dipole depends on the relative position of two nuclei in such a way that when averaged on all various orientation of the molecule it equals to zero.
In November 1951, N. F. Ramsey
and E. M. Purcell
proposed a mechanism that explained the observation and gave rise to an interaction of the form I1.I2. The mechanism is the magnetic interaction between each nucleus and the electron spin of its own atom together with the exchange coupling of the electron spins with each other.
In the 1990s, direct evidence was found for the presence of J-couplings between magnetically active nuclei on both sides of the hydrogen bond
. Initially, it was surprising to observe such couplings across hydrogen bonds since J-couplings are usually associated with the presence of purely covalent bond
s. However, it is now well established that the H-bond J-couplings follow the same electron-mediated polarization mechanism as their covalent counterparts.
of a molecular system may be taken as:
H = D1 +D2 +D3.
D1 = electron orbital-orbital, spin-orbital, spin-spin and electron spin-external field interactions
D2 = magnetic interactions between nuclear spin and electron spin
D3 = direct interaction of nuclei with each other
for a singlet
molecular state and frequent molecular collisions, D1 and D3 are almost zero. The full form of J-coupling interaction between spins Ij and Ik on the same molecule is:
H = 2π Ij. Jjk. Ik
where Jjk is the j-coupling tensor, a 3x3 real matrix. It depends on molecular orientation. In isotropic liquid it reduces to a number, so called scalar coupling. In 1D NMR, scalar coupling leads to oscillations in FID as well as splitting of lines in the spectrum.
et al. in 1994 is commonly the method of choice for accurate measurements of J couplings.
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
s due to the influence of bonding electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s on the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
running between the two nuclei. J-coupling contains information about dihedral angle
Dihedral angle
In geometry, a dihedral or torsion angle is the angle between two planes.The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection...
s, which can be estimated using the Karplus equation
Karplus equation
The Karplus equation, named after Martin Karplus, describes the correlation between 3J-coupling constants and dihedral torsion angles in nuclear magnetic resonance spectroscopy:J = A \cos^2 \phi + B \cos\,\phi + C...
. It is an important observable effect in 1D nuclear magnetic resonance spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
.
Discovery
In October 1951, E. L. HahnErwin Hahn
Erwin L. Hahn is a U.S. physicist, best known for his work on nuclear magnetic resonance . In 1950 he discovered the spin echo....
and D. E. Maxwell reported a spin echo experiment which indicates the existence of an interaction between two protons in dichloroacetaldehyde
Chloroacetaldehyde
Chloroacetaldehyde is the organic compound with the formula ClCH2CHO. Like some related compounds, it is highly electrophilic reagent and a potentially dangerous alkylating agent. The compound is not normally encountered in the anhydrous form, but rather as the hydrate , ClCH2CH2. ...
. In the echo experiment, two short, intense pulses of radiofrequency magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
are applied to spin ensemble at the nuclear resonance condition and are separated by time interval of τ. The echo appears with a given maximum amplitude at time 2τ. For each setting of τ, the maximum of the echo signal is measured and plotted as a function of τ. If the spin ensemble consists of magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
, a monotonic decay in the echo envelope is obtained. In the Hahn-Maxwell experiment, the decay was modulated by two frequencies: one frequency corresponded with the difference in chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
between the two non equivalent spins and a second frequency, J, that was smaller and independent of magnetic field strength. (J/2π = 0.7 cycle per second)
Such interaction came as a great surprise. The direct interaction between two magnetic dipole depends on the relative position of two nuclei in such a way that when averaged on all various orientation of the molecule it equals to zero.
In November 1951, N. F. Ramsey
Norman Foster Ramsey, Jr.
Norman Foster Ramsey, Jr. was an American physicist. A physics professor at Harvard University since 1947, Ramsey also held several posts with such government and international agencies as NATO and the United States Atomic Energy Commission...
and E. M. Purcell
Edward Mills Purcell
Edward Mills Purcell was an American physicist who shared the 1952 Nobel Prize for Physics for his independent discovery of nuclear magnetic resonance in liquids and in solids. Nuclear magnetic resonance has become widely used to study the molecular structure of pure materials and the...
proposed a mechanism that explained the observation and gave rise to an interaction of the form I1.I2. The mechanism is the magnetic interaction between each nucleus and the electron spin of its own atom together with the exchange coupling of the electron spins with each other.
In the 1990s, direct evidence was found for the presence of J-couplings between magnetically active nuclei on both sides of the hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
. Initially, it was surprising to observe such couplings across hydrogen bonds since J-couplings are usually associated with the presence of purely covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s. However, it is now well established that the H-bond J-couplings follow the same electron-mediated polarization mechanism as their covalent counterparts.
J-coupling Hamiltonian
The HamiltonianHamiltonian (quantum mechanics)
In quantum mechanics, the Hamiltonian H, also Ȟ or Ĥ, is the operator corresponding to the total energy of the system. Its spectrum is the set of possible outcomes when one measures the total energy of a system...
of a molecular system may be taken as:
H = D1 +D2 +D3.
D1 = electron orbital-orbital, spin-orbital, spin-spin and electron spin-external field interactions
D2 = magnetic interactions between nuclear spin and electron spin
D3 = direct interaction of nuclei with each other
for a singlet
Singlet
A pair of spin-1/2 particles can be combined to form one of three states of total spin 1 called the triplet, or a state of spin 0 which is called the singlet. In theoretical physics, a singlet usually refers to a one-dimensional representation...
molecular state and frequent molecular collisions, D1 and D3 are almost zero. The full form of J-coupling interaction between spins Ij and Ik on the same molecule is:
H = 2π Ij. Jjk. Ik
where Jjk is the j-coupling tensor, a 3x3 real matrix. It depends on molecular orientation. In isotropic liquid it reduces to a number, so called scalar coupling. In 1D NMR, scalar coupling leads to oscillations in FID as well as splitting of lines in the spectrum.
Measurement of J-coupling
The Quantitative J correlation developed by Ad BaxAd Bax
Adriaan "Ad" Bax is a molecular Biophysicist. He was born in Holland and is currently a US Citizen. He is the Chief of the Section on Biophysical NMR Spectroscopy at the National Institutes of Health....
et al. in 1994 is commonly the method of choice for accurate measurements of J couplings.
See also
- Earth's field NMREarth's field NMRNuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . EFNMR is a special case of low field NMR....
- Exclusive correlation spectroscopyExclusive correlation spectroscopyExclusive correlation spectroscopy is an NMR correlation experiment introduced by O. W. Sørensen, Christian Griesinger, Richard R. Ernst and coworkers for the accurate measurement of small J-couplings....
(ECOSY) - Magnetic dipole-dipole interactionMagnetic dipole-dipole interactionMagnetic dipole–dipole interaction, also called dipolar coupling, refers to the direct interaction between two magnetic dipoles. The potential energy of the interaction is as follows:...
(dipolar coupling) - Proton NMRProton NMRProton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
- Residual dipolar couplingResidual dipolar couplingThe residual dipolar coupling between two spins in a molecule occurs if the molecules in solution exhibit a partial alignment leading to an incomplete averaging of spatially anisotropic dipolar couplings....