Rotational spectroscopy

The microwave spectroscopy or (pure ) rotation spectroscopy is one belonging to the group of the molecule and high-frequency spectroscopy study method. It is used preferably the investigation of gases, liquids and solids. Basis of the method is the absorption of electromagnetic waves in the frequency range of about 0.5 to 100 GHz ( microwaves) by the excitation of molecular rotations, or transitions of atoms between hyperfine structure. Microwave spectroscopy can be applied to both time-or frequency- resolved.

Theoretically, the Rotatationsspektrum is described by the rigid rotator. In the description of the rigid rotor, the centrifugal distortion ( distortion due to the centrifugal force) can be added in the form of Zentrifugaldehnungsterms with the Zentrifugaldehnungskonstante. In view of the description of molecules in linear gyro, spherical top, symmetric top and asymmetric top are divided. Rotational spectra are usually observed as absorption spectra.

Rigid rotator

In the simplest case the rotation of a molecule can be described by a rigid rotator with its rotational energy, the moment of inertia about the axis of rotation and. This results in the accompanying picture. In the case of symmetric top moments of inertia agree to all three principal axes of inertia.

Experimental conditions

Generally suitable only molecules for microwave spectroscopy, which form dipoles or execute oscillations, which changed the dipole moment. As a measuring method, on the one hand, the absorption at various frequencies to be measured, or use is made of the Fourier transform and analyzes a time-dependent absorption by the frequencies contained therein.

To obtain precise and as unambiguous as possible attributable absorption spectra, the interaction of the molecules must be minimized with each other. Mostly is therefore working with small amounts of gaseous species in large evacuated measuring containers.

Sample shape

Microwave spectra of gases are characterized by sharp absorption lines, the cause is usually the rotation of dipolbehafteten molecules. The sharp lines arising from the difference in energy quantum mechanically -determined energy levels of molecular vibrations or rotations.

The microwave spectroscopy can also be used for the elucidation of structure and dynamics of liquids. The spectra of liquids are distinguished from others by very broad absorption bands, which go through several frequency ranges. In the microwave spectrum of molecules provide a contribution that have a dipole moment. The strength of the dipole moment is preferably in the amount of absorption, whereas the rate of molecular motion ( wobble rotation) determines the position of the absorption band on the frequency scale. Is generally found a correlation between the viscosity of a liquid and the Bewegungsgsgeschwindigkeit the dipoles.

Usually can be microwave spectra with a superposition of Debye functions (named after Peter Debye ) describe each individual Debye- function motion is assigned.

Application

With the aid of microwave spectroscopy gases information can be obtained, such as:

• Bond lengths in simply structured molecules
• Conformations of certain chemical compounds, known as rotary hyperfine structure in the absorption spectrum comprise
• Structures of short-lived, non- isolable species also produce rotation hyperfine structures, using the molecular-beam method,
• Have electronic environment and electron density distribution around certain atomic nuclei around which so-called Quadrupolhyperfeinstrukturen in the absorption spectrum.

The microwave spectroscopy is mainly used in physical chemistry to the study of molecular properties that are impossible or difficult to obtain through other methods.