In
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
,
magic angle spinning (MAS) is a technique often used to perform experiments in solid-state NMR
spectroscopySpectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise...
.
By spinning the sample (usually at a frequency of 1 to 70 kHz) at the
magic angleThe magic angle is a precisely defined angle, the value of which is approximately 54.7°. The magic angle is a root of a second-order Legendre polynomial, , and so any interaction which depends on this second-order Legendre polynomial vanishes at the magic angle...
θ
m (ca. 54.74°, where cos
2θ
m=1/3) with respect to the direction of the
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
, the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum.
In any condensed phase, a nuclear spin experiences a great number of interactions.
In
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
,
magic angle spinning (MAS) is a technique often used to perform experiments in solid-state NMR
spectroscopySpectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise...
.
By spinning the sample (usually at a frequency of 1 to 70 kHz) at the
magic angleThe magic angle is a precisely defined angle, the value of which is approximately 54.7°. The magic angle is a root of a second-order Legendre polynomial, , and so any interaction which depends on this second-order Legendre polynomial vanishes at the magic angle...
θ
m (ca. 54.74°, where cos
2θ
m=1/3) with respect to the direction of the
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
, the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum.
In any condensed phase, a nuclear spin experiences a great number of interactions. The main three interactions (dipolar, chemical shift anisotropy, quadrupolar) often lead to very broad and featureless lines. However, these three interactions in solids are time-dependent and can be averaged by MAS. The nuclear dipole-dipole interaction, between
magnetic momentThe magnetic moment of a system is a measure of the strength and the direction of its magnetism. More technically , the term magnetic moment of a system usually refers to its magnetic dipole moment, and quantifies the contribution...
s of nuclei averages to zero only at the magic angle, θ
m . The chemical shift anisotropy, a nuclear-electron interaction, averages to a non-zero value. The quadrupolar interaction is only partially averaged by MAS leaving a residual secondary quadrupolar interaction. In liquids, e.g. a solution of an
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
, most of these interactions will average out because of the rapid time-averaged molecular motion that occurs. This orientation averaging in solution is mimicked by MAS of a solid. This causes the signal to become much narrower, giving rise to the isotropic value (which is of interest for structural determination of solid materials and compounds) and spinning sidebands which occur at multiples of the spinning speed and can be used to determine the chemical shift anisotropy of the nuclei.
The physical spinning of the sample is achieved via an air turbine mechanism. These turbines (or rotors) come in a variety of diameters (outside diameter), from 2.0-15.0 mm, and are usually spun on air or nitrogen gas. The rotors are made from a number of different materials such as ceramics e.g. zirconia,
silicon nitrideSilicon nitride is a chemical compound of silicon and nitrogen. It is a hard ceramic having high strength over a broad temperature range, moderate thermal conductivity, low coefficient of thermal expansion, moderately high elastic modulus, and unusually high fracture toughness for a ceramic...
or polymers such as poly-methyl-methacrylate (PMMA),
polyoxymethylenePolyoxymethylene is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability...
(POM). The cylindrical rotors are axially symmetric about the axis of rotation. Samples are packed into the rotors and these are then sealed with a single or double end cap. These caps are made from number of different materials e.g. Kel-F, Vespel, zirconia or boron nitride depending on the application required.