The infrared spectra of a sample is collected by passing a beam of infrared light through the sample. For example, the atoms in a CH 2 group, commonly found in organic compounds can vibrate in six different ways: symmetrical and antisymmetrical stretching, scissoring, rocking, wagging and twisting: More complex molecules have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. Simple diatomic molecules have only one bond, which may stretch. Thus, the frequency of the vibrations can be associated with a particular bond type. Nevertheless, the resonant frequencies can be in a first approach related to the strength of the bond, and the mass of the atoms at either end of it. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are determined by the normal modes corresponding to the molecular electronic ground state potential energy surface. In particular, in the Born-Oppenheimer and harmonic approximations, i.e. In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. These resonant frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, by the associated vibronic coupling. Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels. They are neither strict divisions nor based on exact molecular or electromagnetic properties. The names and classifications of these subregions are merely conventions. The higher energy near-IR, approximately 14000-4000 cm -1 (1.4–0.8 μm) can excite overtone or harmonic vibrations. The mid-infrared, approximately 4000-400 cm -1 (30–1.4 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared, approximately 400-10 cm -1 (1000–30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The infrared portion of the electromagnetic spectrum is divided into three regions the near-, mid- and far- infrared, named for their relation to the visible spectrum.
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