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. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
Most frequently, NMR spectroscopy is used by chemists and biochemists to investigate the properties of organic molecules, though it is applicable to any kind of sample that contains nuclei possessing spin. Suitable samples range from small compounds analyzed with 1-dimensional proton or carbon-13 NMR spectroscopy to large proteins or nucleic acids using 3 or 4-dimensional techniques. The impact of NMR spectroscopy on the sciences has been substantial because of the range of information and the diversity of samples, including solutions and solids.
When placed in a magnetic field, NMR active nuclei (such as 1H or 13C) absorb electromagnetic radiation at a frequency characteristic of the isotope. The resonant frequency, energy of the absorption, and the intensity of the signal are proportional to the strength of the magnetic field. For example, in a 21 Tesla magnetic field, protons resonate at 900 MHz. It is common to refer to a 21 T magnet as a 900 MHz magnet, although different nuclei resonate at a different frequency at this field strength in proportion to their nuclear magnetic moments.
Most frequently, NMR spectroscopy is used by chemists and biochemists to investigate the properties of organic molecules, though it is applicable to any kind of sample that contains nuclei possessing spin. Suitable samples range from small compounds analyzed with 1-dimensional proton or carbon-13 NMR spectroscopy to large proteins or nucleic acids using 3 or 4-dimensional techniques. The impact of NMR spectroscopy on the sciences has been substantial because of the range of information and the diversity of samples, including solutions and solids.
When placed in a magnetic field, NMR active nuclei (such as 1H or 13C) absorb electromagnetic radiation at a frequency characteristic of the isotope. The resonant frequency, energy of the absorption, and the intensity of the signal are proportional to the strength of the magnetic field. For example, in a 21 Tesla magnetic field, protons resonate at 900 MHz. It is common to refer to a 21 T magnet as a 900 MHz magnet, although different nuclei resonate at a different frequency at this field strength in proportion to their nuclear magnetic moments.
Nuclear Magnetic Resonance (NMR) of Alkenes
In 1H NMR spectrum, hydrogen atoms bound to a carbon consisting of a double bond (these hydrogens are called alkenyl hydrogens) are typically found in low field of the NMR spectrum, which is the left side, and the hydrogens are said to be deshielded. The cause for this is due to the movement of the electrons in the pi bond of the carbon-carbon double bond.
Alkenyl hydrogens create an external
magnetic field that is perpendicular to the double bond axis and causes
the electrons in the pi bond to enter a circular motion (shown in red).
The circular motion actually reinforces the external field at the edge
of the double bond on both sides of the pi bond but creates a local
field (shown in purple and green) that opposes the external field in the
center of the double bond. Because of this pulling force within the pi
bond across the double bond which reinforces the regions occupied by
alkenyl hydrogens, the alkenyl hydrogens are strongly deshielded.
Additionally, alkenyl hydrogens do not
have to be all chemical-shift equivalent, and when they aren't, coupling
will be observed which is the different peaks in an MNR spectrum.
When alkynel hydrogen atoms are are not symmetrically substituted
on a double bonded carbon, the hydrogens of a cis and trans isomer will
yield a different shift on the NMR spectrum. Because the coupling
constant is smaller in a cis isomer than in a trans isomer, the NMR
spectrums of the two isomers are different conveying the hydrogens in a cis isomer to be slightly more upfield to-- the right of the spectrum-- and trans hydrogens to be more downfield to the left.
Sometimes coupling will lead to very
complicated patterns as a result of the J values that vary widely due to
the relationship between the hydrogens involved. When this occurs,
information can still be derived to determine the structure of a
molecule by looking at the number of signals, the chemical shift of each
one, integration, and splitting patterns similarly to identifying
alkane NMR.
