Molecular beam method
The molecular beam method is a method of chemistry with which kinetic and structural information about chemical reactions can be obtained.
principle
At least two beams ( molecular beams or atomic beams ) are generated from one type of particle each. The molecules are made to react by crossing or combining these rays in a vacuum .
The speed and collision of the molecular beams lead to a certain scatter of:
The vacuum prevents the influence of "foreign molecules",
- which could deflect the molecular beams and products,
- which could undesirably participate in the reaction.
In reactions under normal laboratory conditions, these "foreign molecules" are molecules of the solvent , air, and possibly also a protective gas.
The detection can be done with mass spectrometry for kinetic information, or with rotation spectroscopy , infrared spectroscopy for structural information.
Kinetic studies
The molecular beam method is used in kinetics to research the collision processes of the atoms and molecules involved in a reaction without interfering intermolecular interactions . The interpretation of the collision processes provides statistical information, in particular on the minimum energy required for the reaction. Because not every collision of apparently reactive particles ends in a reaction of these particles.
The molecular beams are crossed, the molecules partially react with each other in the crossing area. The molecules scatter through collision and partial reaction . The scattered molecules are detected, their mass is determined and from this the composition of the scattered particles is determined.
The following phenomena occur:
- Molecules that do not collide in the intersection area retain the direction of travel of the original molecular beam.
- Molecules that collide and react mainly take a flight direction, which results from the flight directions of the original molecular beams.
- Molecules that collide but do not react are strongly scattered in all directions.
The deviation from the angle of the original flight direction due to collision or reaction, the scattering angle of the reaction products, size of the intersection area of the molecular beams, as well as the flight time of the particles can be evaluated using the simple impact theory and refined impact theory.
Structural studies
The collisions are less interesting in structural studies. Here structures of intermediate products and end products of reactions are examined.
Only particles are measured that react, i.e. that more or less take the direction of flight that results from the direction of flight of the original molecular beams.
With microwave spectroscopy and infrared spectroscopy, sometimes with the help of other spectroscopic methods, information about bond lengths, rotations, conformations, vibrations, etc. that occur in the intermediate and end products can be obtained and from this information can be used to interpret the structure and the bond state in otherwise poorly accessible species.
application
The molecular beam method is often used for a wide variety of kinetic studies in physical chemistry . The structural analysis with the molecular beam method is increasingly used nowadays in the research of "exotic" complexes , e.g. B. of mercury - noble gas complexes.
Both types of application are more used for basic research and are therefore rarely used industrially.
literature
Gerd Wedler; Physical chemistry textbook ; Wiley-VCH; 1997