Absorption of an electromagnetic wave

The Hamiltonian that governs rotations of isolated H-bonded molecules in the gas phase is particularly simple when, in a first approximation, this H-bonded molecule is considered as rigid, as no potential energy is present. It therefore consists of only a kinetic energy term that is written as 

It clearly indicates that trimethylammonium N(CH3)3 is a stronger base than ammonia NH3. This ionic character becomes more pronounced in Br-H - N(CH3)3 and complete when the halogen atom is I (iodine), so that the H-bond estabhshed between hydrogen iodide and trimethylammonium is I N(CH3)3 instead of the above written initial neutral species I H - N(CH3)3. 

Rotational features of almost aU H-bonded complexes in the gaseous phase appear in the microwave region, with wavenumbers less than 10 cm They correspond to transitions between pure rotational levels, pure meaning that vibrations remain unchanged, or no vibrational transition accompanies such rotational transitions. Rotational features, however, also appear in the IR spectra of these H-bonded complexes. IR bands correspond to transitions between various vibrational levels of a molecule. When this molecule is isolated, as in the gas phase, these transitions are always accompanied by transitions between rotational levels that obey the same selection rules as pure rotational transitions detected in microwave spectroscopy. The information conveyed by these rotational features in IR spectra are therefore most similar to those conveyed by microwave spectra, even if the mechanism at the origin of their appearance is different. Their interests lie in the use of an IR spectrometer, a common instrument in many laboratories, instead of a microwave spectrometer, which is a much more specialized instrament. However, the resolution of usual IR spectrometers are lower than that of microwave spectrometers that use Fabry-Perot cavities. This IR technique has been used in the case of simple H-bonded dimers with relatively small moments of inertia, such as, for instance, F-H- -N C-H (3). Such complexes are far from simple to manipulate, but provide particularly simple IR spectra with a limited number of bands that do not show any overlap. 

In NMR spectroscopy a radiowave induces transitions between spin levels of the nuclei of the molecules that compose the sample that is immersed in a static magnetic field. The frequencies of these radio waves fall below 1 GHz. They consequently induce transitions between spin levels that are separated by energies always smaller than kT, even at such low temperatures as 1K, as can be seen from values of constants h and k defined in the appendix 

Larmor frequencies are typically of 500 MHz for a proton immersed in a static magnetic field of amplitude 5 = 12 T, a very strong, but nowadays current magnetic field. In the same magnetic field, the Larmor frequency of a fluorine nucleus F is a bit smaller, that of a carbon atom that represents more than 99% of all C-atoms in natural conditions has no spin and is consequently not directly seen in NMR) is four times smaller and that of a N about 0.07 that of the proton. This first approximation consequently allows us to define various ranges of radiofrequencies to be used to induce transitions between spin levels of a definite nucleus, H, F, N, N, etc. 

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