Microwave radiation rotational spectroscopy

Because a chemical bond is only about 10 10 m long, special techniques have to be used to measure its length. There are two principal techniques one for solids and the other for gases. The technique used for solids, x-ray diffraction, is described in Major Technique 3, billowing Chapter 5. Microwave spectroscopy, discussed here, is used to determine bond lengths in gas-phase molecules. This branch of spectroscopy makes use of the ability of rotating molecules to absorb microwave radiation, which has a wavelength close to 1 cm. 

Microwave spectroscopy The study of the interaction of microwave radiation with the rotational motion of a molecule. Microwave transitions are not restricted to molecules and can occur in atoms whenever the separation between the energy levels is in the microwave region of the spectrum. 

Rotational quanta are much smaller than vibrational quanta, and correspond to electromagnetic radiation in the microwave region, typically in the range 103-105MHz (lcm-1 = 3.3 x 104 MHz). Rotational transitions can be excited directly, in microwave absorption spectroscopy (pure rotational spectroscopy), but can also be observed in the rotational fine structure in high-resolution vibrational or electronic spectra. 

Absorption of microwave radiation to excite molecular rotation is allowed only if the molecule has a permanent dipole moment. This restriction is less severe than it may sound, however, because centrifugal distortion can disturb the molecular symmetry enough to allow weak absorption, especially in transitions between the higher rotational states which may appear in the far IR (c. 100cm-1). Microwave spectroscopy can provide a wealth of other molecular data, mostly of interest to physical chemists rather than inorganic chemists. Because of the ways in which molecular rotation is affected by vibration, it is possible to obtain vibrational frequencies from pure rotational spectra, often more accurately than is possible by direct vibrational spectroscopy. 

Polyatomic molecules have more complex microwave spectra, but the basic principle is the same any molecule with a dipole moment can absorb microwave radiation. This means, for example, that the only important absorber of microwaves in the air is water (as scientists discovered while developing radar systems during World War II). In fact, microwave spectroscopy became a major field of research after that war, because military requirements had dramatically improved the available technology for microwave generation and detection. A more prosaic use of microwave absorption of water is the microwave oven it works by exciting water rotations, and the tumbling then heats all other components of food. 

An important development in microwave/optical double resonance, called microwave/optical polarisation spectroscopy, was described by Ernst and Torring [42], The principles of this technique are illustrated in figure 11.22. A linearly polarised probe beam from a tunable laser is sent through the gas sample and a nearly crossed linear polariser, before its final detection. Polarised microwave radiation resonant with a rotational transition in the gas sample is introduced via a microwave horn as shown, and resonant absorption results in a partial change in polarisation of... 

For characterizing a dipolar molecule in its electronic ground state, few methods are more instructive than pulsed-nozzle Fourier-trans-form microwave spectroscopy (32). As illustrated schematically in Fig. 5, a short pulse of microwave radiation directed at the gas pulse excites a rotational transition in the species of interest subsequently the rotationally excited molecules reemit radiation, which is detected. This technique provides a remarkably sensitive probe for transients, the properties of which can be specified with all the precision and detail peculiar to rotational spectroscopy only microseconds after their production. In relation to a weakly bound adduct A --B formed by two molecular reagents A and B, for example, we may draw on the rotational spectrum to determine such salient molecular properties as symmetry, radial and angular geometry, the intermolecular stretching force constant and internal dynamics, the electric charge distribution, and the electric dipole and quadrupole moments of A -B (see Table I). 

Microwave spectroscopy Molecules rotate in the gaseous state. Upon absorbing microwave radiation, the molecules rotate faster. The frequencies of radiation absorbed by molecules and the relative intensities of such absorptions depend upon the geometry of the molecules. A mathematical analysis of the microwave spectrum of a molecule can provide chemists with bond angles and bond lengths. This information is much more accurate than that obtained by infrared spectroscopy. Study of the effect of a magnetic field on a microwave spectrum allows the dipole moment of the molecule to be calculated. 

The vibrational-rotational states of diatomic molecules are probed in spectroscopic experiments using radiofrequency or microwave radiation (low energy, pure rotational spectroscopy) or infrared radiahon (vibrahonal spectroscopy). The former requires a permanent dipole moment for a transition to take place and a change in the rotational quantum number of A/ = 1 (A/ = -1 in emission). The latter requires that the dipole moment change in the course of the vibrational motion and that An = 1 (An = -1 in emission) and A/ = 1 (except for diatomic radicals where A/ can also be 0). These selechon rules lead to a pattern of lines in the high-resolution vibrational spectrum, and the lines make up a vibrational band. 

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