the branch of physical chemistry that studies the relationship between the magnetic and the chemical properties of matter and investigates the effect of magnetic fields on chemical processes.

Magnetochemistry is based on the modern physics of magnetic phenomena and crystal chemistry. Studying the relation between magnetic and chemical properties makes it possible to elucidate the features of the chemical structure of a substance. Both permanent and variable magnetic fields are used for this purpose. In the case of variable fields, it is necessary to distinguish between magnetic phenomena resulting from an absence of resonance effects and phenomena directly related to resonance. The study of magnetic phenomena of the first type is in principle no different from the investigation of such phenomena in permanent fields. However, independent methods of investigation are necessary to study the specific effects of the resonance absorption by a substance of electromagnetic energy. These effects are observed in variable (primarily high-frequency) fields under fixed conditions (electron paramagnetic resonance, nuclear magnetic resonance, ferromagnetic resonance, and chemical polarization of nuclei).

Upon formation of a chemical bond, the spins of valence electrons assume an antiparallel orientation, resulting in the mutual compensation of their magnetic moments. Consequently, the majority of chemical compounds exhibit diamagnetic proper-ties. The diamagnetic substances include ionic compounds, such as NaCl and KC1, the electron structure of whose ions imitates the electron structure of the atoms of the noble gases, and cova-lent saturated inorganic (and especially organic) compounds, such as CO₂ and CH₄.

Given no mutual deformation of electron shells, the diamagnetic susceptibility of a compound equals the sum of the susceptibilities of its component atoms or ions. Comparison of the experimentally measured diamagnetic susceptibility of a compound and its value calculated additively makes it possible to detect electron shell deformation, which is associated with features of chemical structure. For example, a noticeable reduction in the total diamagnetism of an organic compound is brought about by the presence of a double bond in the molecule. Conversely, an aromatic bond, which is characterized by the movement of delocalized electrons along the aromatic ring, leads to a marked increase in diamagnetism and anisotropy. (The magnetic susceptibility X₁ measured perpendicular to the plane of the aromatic ring considerably exceeds the susceptibility X_ǀǀ measured parallel to the plane.) The regular patterns indicated make it possible to use the measurements of the magnetic susceptibility of diamagnetic compounds to identify the compounds and obtain approximate information on the nature of their chemical bonds.

Substances with unsaturated chemical bonds characteristically exhibit uncompensated magnetic moments and usually contain atoms of transition elements (for example, elements of the iron group or the rare-earth elements). Ionic compounds of this type generally have paramagnetic properties. Investigation of the temperature curve for the magnetic susceptibility of these substances makes it possible to determine the magnitude of the ionic magnetic moment and to infer the valence of the component atoms and their electron structure. Most frequently encountered, however, are substances containing atoms of transition elements with a covalent bond. These chemical compounds may be paramagnetic, ferromagnetic, or antiferromagnetic. In the first two cases, if the magnetic susceptibility and its corresponding temperature curve are known, it is possible to estimate the effective magnetic moment and to make certain assumptions about the nature of the chemical bond. In a number of cases involving ferromagnetic and ferrimagnetic compounds, it is also possible to determine the effective magnetic moment for an ion or atom of a transition element and the number of unpaired electrons (that is, to determine its electronic configuration), according to the relationship between the magnetic properties of the compounds and the field intensity and temperature. These data supplement the results of other physicochemical investigations.

Constant magnetic fields have no direct effect on the nature of the chemical bond or on the chemical equilibrium. In a number of cases, however, they can influence the kinetics of certain chemical processes.

External magnetic fields, acting on the coagulation of minute particles of iron scale (significant quantities of which are frequently found in air and water), can have a considerable effect on certain physico-chemical processes in the gaseous and liquid phases. Magnetochemical measurements are widely used to detect these dispersed impurities and control the purity of chemical experiments.