Magnetochemistry

The magnetochemistry is a branch of physical chemistry that deals with the magnetic properties of materials employed or substances. It was founded around 1905 by the French physicist Paul Langevin and expanded by the German chemist Wilhelm Klemm .

Determination of the effects

The measurement of the magnetic susceptibility or the magnetic moment enables conclusions to be drawn about the electron configurations of metal ions or non-metallic molecules. Magnetochemical effects can be determined in a simple manner via the magnetic susceptibility with a Gouy's balance . Other similar methods of determining susceptibility according to Quincke or according to Faraday-Curie are also based on the change in weight and magnetic field.

Due to the influence of a magnetic field, many chemical substances show an orientation that can be measured directly as a change in weight. One can visualize the individual atoms as small elementary magnets that - like many small iron filings - align themselves in a magnetic field. Depending on the substance class, there is a small measurable interaction with the magnetic field (diamagnetism), a stronger interaction (paramagnetism) or a very strong interaction (ferromagnetism). A small circular electrical movement of an electron around its own axis, which is induced by a magnet, can be assumed as an idea. The influence of a magnet on a circulating current, on a current-carrying coil, also generates a magnetic dipole moment, the vector of which is perpendicular to the plane of the circulating current (right-hand rule). Analogously, with every magnetic field on a magnetically active substance, a rotary movement is generated on every elementary electron. If the vector of the magnetic dipole moment does not coincide with the directional vector of the magnetic field, gyratory movements of the elementary magnets occur.

Mathematically, the torque of elementary magnets is described by the vector product of the magnetic dipole moment and the magnetic field. In mathematics, the vector product indicates the perpendicular to a plane of two vectors and corresponds to the length of its vector to the projected area of ​​both vectors.

The strength of the influence of a magnetic field on substances can be determined by weighing. Magnetic susceptibility is a very important magnetochemical parameter. Using a neodymium magnet and a fine balance, the magnetic susceptibility can be roughly determined using the Cortel method.

The change in weight is directly proportional to the change in force. The change in force caused by individual magnetochemical substances is specified with the substance-specific susceptibility ( ). This is a proportionality factor with no unit. Often, however, the susceptibility is related to the consistency of the substance (number of moles of substance per cubic meter, molar susceptibility ); substances can only be compared by referring to the density. ${\displaystyle \chi _{\ }}$ ${\displaystyle \chi _{\text{m}}}$

Magnetic flux density and magnetic field strength are linked via the following relationship:

${\displaystyle B=\mu _{\text{0}}\cdot H\cdot (1+\chi _{\text{m}})}$

${\displaystyle B}$= magnetic flux density (kg ⋅ s ⁻²  ⋅ A ⁻¹ )

${\displaystyle H}$= magnetic field strength (A ⋅ m ⁻¹ )

${\displaystyle \mu _{\text{0}}}$= Permeability constant of the vacuum (1.256 ⋅ 10 ⁻⁸ m ⋅ kg ⋅ C ⁻² )

If negative, it is a diamagnetic substance; is positive, it is a paramagnetic substance. Very high values ​​for are ferromagnetic substances. These substances even have magnetization in the absence of a magnetic field. ${\displaystyle \chi _{\text{m}}}$ ${\displaystyle \chi _{\text{m}}}$ ${\displaystyle \chi _{\text{m}}}$

According to Gouy's principle, a paramagnetic sample is inserted between the poles of two strong permanent magnets. A magnetic field opposite to the applied magnetic field is induced by the elementary magnets of the sample. Since the force is perpendicular to the applied magnetic field, the sample is pushed slightly upwards. The force effect can be measured using a scale. If the field strength at the sample surface at the entrance and exit is correctly determined, the susceptibility of the substance can be determined by the weight with the equation:

${\displaystyle F=0{,}5\cdot (\chi _{\text{m}}-\chi _{\text{a}})\cdot A\cdot \mu _{\text{0}}(H_{\text{E}}^{2}-H_{\text{A}}^{2})}$

${\displaystyle H_{\text{E}}}$: Field strength at sample entry

${\displaystyle H_{\text{A}}}$: Field strength at sample exit

${\displaystyle A}$: Area of ​​a uniformly thick test specimen

be determined.

The same equation is also valid for the investigation of substances using a neodymium magnet.

Para- and diamagnetism can also be detected in the optical area. A watch glass (see also: Newton's rings) is filled with a solution of a paramagnetic substance and the watch glass is placed between the acute-angled pole pieces of two attractive permanent magnets. If a light beam is radiated onto the sample parallel to the pole pieces, the beams striking the substance are separated into two beams and are visible on the side of the wall opposite to the incident light. With diamagnetic substances, the bundle of rays is compressed.

The magnetizability can be determined on the basis of the magnetic field strength of the magnet, the specific density of the filled substance and the change in weight. Paramagnetic substances tend to migrate into the area of ​​high field strengths (e.g. in the case of two round magnets in the center of the circle), diamagnetic substances migrate into the area of ​​low field strengths (e.g. in the case of two round magnets to the edges of the circle). Paramagnetism is temperature dependent, diamagnetism is not.

causes

The cause of the susceptibility lies in the individual electrons around the atomic nucleus. In order to understand the effect, one can assume, as a model, that unpaired electrons rotate on a circular path around the atomic nucleus, similar to a current-carrying coil, and thereby generate a magnetic field. If the single spins cancel , d. H. If the electron orbitals of the molecule or atom are each filled with two electrons in opposite directions (e.g. in the case of noble gases or noble gas-like ions Na ⁺ , Ca ²⁺ , Cl ^- ), the substance has no electron spin and is diamagnetic. The diamagnetic susceptibility is always negative and significantly smaller (factor: 0.01-0.1) than the paramagnetic susceptibility. In paramagnetic substances there are unpaired electrons. Using tables on the electron configurations of elements or the orbital theory of molecules, the number of unpaired electrons can be determined (e.g. one for the hydrogen atom, four for Fe ^II and two for Cu ^II ) and the magnetic moment ( e.g. for the iron metal group or in the case of the lanthanides ) via the spin relationship

${\displaystyle m={\frac {1}{\sqrt {n\cdot (n+2)}}}}$

to calculate.

Measurements

Structural issues such as the oxidation number or the type of bond can be clarified through magnetochemical measurements . FeSO ₄ and [Fe (H ₂ O) ₄ ] Cl _{2 have} a magnetic moment of approx. 5.2  µ _B , whereas K ₄ [Fe (CN) ₆ ] and Fe (CO) _{5 have} a magnetic moment of zero. While the former has an ionic structure between the cation and ligand, the latter have a strongly polarized bond.

Results

• In inorganic chemistry, magnetochemistry has made important contributions to the development of ligand field theory and to the understanding of the metallic state.
• In organic chemistry, magnetochemical measurements are used to demonstrate polymerisation processes (the gradual disappearance of double bonds can be demonstrated magnetochemically), to measure aroma and organic radicals.
• In addition to the dia- and paramagnetic substances, there are also substances that show ferromagnetic , antiferromagnetic or ferrimagnetic behavior. When exposed to a magnetic field, the magnetization increases sharply or they become permanent magnets themselves.
• By means of magnetochemistry, important theoretical foundations for NMR spectroscopy could be laid. However, the core susceptibility is lower by a factor of 10 ⁴ than the diamagnetic susceptibility, so that the core susceptibility cannot be proven by weighing.

literature

• Magnetochemistry. In: Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 2: Cm-G. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1981, ISBN 3-440-04512-9 .
• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 , pp. 983-995.
• Heiko Lueken : Magnetochemistry . BG Teubner, Stuttgart / Leipzig 1999, ISBN 3-519-03530-8 .
• Alarich Weiss, Helmut Witte: Magnetochemistry . Verlag Chemie, Weinheim 1973, ISBN 3-527-25398-X .
• Adolf Cortel: Demonstration on Paramagnetism with an Electronic Balance. In: Journal of Chemical Education. Vol 75, January 1998, p. 61.

Individual evidence

1. ↑ Adolf Cortel: Demonstration on paramagnetism with at Electronic balance. In: Journal of Chemical Education. Vol 75, Jan. 1998, p. 61.
2. ↑ Kevin C. de Berg, Kenneth J. Chapman: Determination of the Magnetic Moments of Transition Metal Complexes Using Rare Earth Magnets. In: Journal of Chemical Education. Vol. 78, May 2001, p. 670 ff.
3. ↑ Charles Rich times, Patricia K. Ruff: Demonstrating and Measuring Relative Molar Magnetic Susceptibility Using a neodymium magnet. In: Journal of Chemical Education. Vol. 81, August 2004, p. 1155.
4. ^ Fritz Voit: Magnetism. In: Artur Friedrich (Ed.): Handbook of experimental school physics. Part 6: Electricity theory I. Aulis Verlag Deubner & Co., Cologne 1964, DNB 456881778 , p. 210.