In nuclear magnetic resonance spectroscopy, the chemical shift is the
resonant frequency of a nucleus relative to a standard in a magnetic field. Often
the position and number of chemical shifts are diagnostic of the structure
of a molecule. Chemical shifts are also used to describe signals in other forms
of spectroscopy such as photoemission spectroscopy.
Some atomic nuclei possess a magnetic moment, which gives rise to different
energy levels and resonance frequencies in a magnetic field. The total magnetic
field experienced by a nucleus includes local magnetic fields induced by
currents of electrons in the molecular orbitals. The electron distribution of
the same type of nucleus usually varies according to the local geometry, and
with it the local magnetic field at each nucleus. This is reflected in the spin
energy levels. The variations of nuclear magnetic resonance frequencies of the
same kind of nucleus, due to variations in the electron distribution, is called
the chemical shift. The size of the chemical shift is given with respect to
a reference frequency or reference sample, usually a molecule with a barely
distorted electron distribution. Operating frequency
The operating frequency of a magnet is calculated from the Larmor equation
where is the actual strength of the magnet in units like teslas or gauss,
and is the gyromagnetic ratio of the nucleus being tested which is in turn
calculated from its magnetic moment and spin number with the nuclear magneton
and the Planck constant h: Thus, the proton operating frequency for
a 1 T magnet is calculated as: Chemical shift referencing
Chemical shift δ is usually expressed in parts per million by frequency, because
it is calculated from where is the absolute resonance
frequency of the sample and is the absolute resonance frequency of a
standard reference compound, measured in the same applied magnetic field. Since
the numerator is usually expressed in hertz, and the denominator in megahertz,
delta is expressed in ppm. The detected frequencies for 1H, 13C,
and 29Si nuclei are usually referenced against TMS or DSS, which by the
definition above have a chemical shift of zero if chosen as the reference.
Other standard materials are used for setting the chemical shift for other
nuclei. Thus, an NMR signal observed at a
frequency 300 Hz higher than the signal from TMS, where the TMS resonance
frequency is 300 MHz, has a chemical shift of:
Although the absolute resonance frequency depends on the applied
magnetic field, the chemical shift is independent of external magnetic field
strength. On the other hand, the resolution of NMR will increase with
applied magnetic field. The induced magnetic field
The electrons around a nucleus will circulate in a magnetic field and create
a secondary induced magnetic field. This field opposes the applied field as
stipulated by Lenz's law and atoms with higher induced fields are therefore
called shielded, relative to those with lower electron density. The chemical
milieu of an atom can influence its electron density through the polar
effect. Electron-donating alkyl groups, for example, lead to increased shielding
while electron-withdrawing substituents such as nitro groups lead to deshielding
of the nucleus. Not only substituents cause local induced fields. Bonding
electrons can also lead to shielding and deshielding effects. A striking example
of this are the pi bonds in benzene. Circular current through the
hyperconjugated system causes a shielding effect at the molecule's
center and a deshielding effect at its edges. Trends in chemical shift are
explained based on the degree of shielding or deshielding.
Nuclei are found to resonate in a wide range to the left of the internal
standard. When a signal is found with a higher chemical shift:
the applied effective magnetic field is lower, if the resonance frequency is
fixed, the frequency is higher, when the
applied magnetic field is static, the nucleus is more deshielded
the signal or shift is downfield or at low field or paramagnetic
Conversely a lower chemical shift is called a diamagnetic shift, and is
upfield and more shielded. Diamagnetic shielding
In real molecules protons are surrounded by a cloud of charge due to adjacent
bonds and atoms. In an applied magnetic field electrons circulate and produce an
induced field which opposes the applied field. The effective field at the
nucleus will be B = B0 − Bi. The nucleus is said to be experiencing a diamagnetic
shielding. Factors causing chemical shifts
Important factors influencing chemical shift are electron density,
electronegativity of neighboring groups and anisotropic induced magnetic field
effects. Electron density shields a nucleus from
the external field. For example, in proton NMR the electron-poor tropylium
ion has its protons downfield at 9.17 ppm, those of the electron-rich
cyclooctatetraenyl anion move upfield to 6.75 ppm and its dianion even more
upfield to 5.56 ppm. A nucleus in the vicinity of an
electronegative atom experiences reduced electron density and the nucleus is
therefore deshielded. In proton NMR of methyl halides the chemical shift of the
methyl protons increase in the order I