Infrared+Spectroscopy

The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The higher-energy near-IR, approximately 14000–4000 cm−1 (0.8–2.5 μm wavelength) can excite [|overtone] or [|harmonic] vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5–25 μm) may be used to study the fundamental vibrations and associated [|rotational-vibrational] structure. The far-infrared, approximately 400–10 [|cm−1] (25–1000 μm), lying adjacent to the [|microwave] region, has low energy and may be used for [|rotational spectroscopy]. The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties. Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are [|resonant frequencies], i.e. the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates. The energies are determined by the shape of the molecular [|potential energy surfaces], the masses of the atoms, and the associated [|vibronic coupling]. In particular, in the [|Born–Oppenheimer] and harmonic approximations, i.e. 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. 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. Thus, the frequency of the vibrations can be associated with a particular bond type. In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes in the dipole. A permanent dipole is not necessary, as the rule requires only a change in dipole moment.[|[][|1][|]] A molecule can vibrate in many ways, and each way is called a //vibrational mode.// For molecules with N atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes. Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum, but only in the Raman spectrum. Asymmetrical diatomic molecules, e.g. CO, absorb in the IR spectrum. More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e. big molecules have many peaks in their IR spectra. The atoms in a CH2 group, commonly found in [|organic compounds], can vibrate in six different ways: **symmetric and antisymmetric stretching**, **scissoring**, **rocking**, **wagging** and **twisting**: stretching ||~ Antisymmetrical stretching ||~ Scissoring || (These figures do not represent the "[|recoil]" of the C atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter H atoms).
 * Infrared spectroscopy** (IR spectroscopy) is the [|spectroscopy] that deals with the [|infrared] region of the [|electromagnetic spectrum], that is light with a longer [|wavelength] and lower [|frequency] than [|visible light]. It covers a range of techniques, mostly based on [|absorption spectroscopy]. As with all spectroscopic techniques, it can be used to identify and study [|chemicals]. A common laboratory instrument that uses this technique is a [|Fourier transform infrared] (FTIR) [|spectrometer].
 * ~ Symmetrical
 * [[image:http://upload.wikimedia.org/wikipedia/commons/0/0e/Symmetrical_stretching.gif width="210" height="150" caption="Symmetrical stretching.gif" link="http://en.wikipedia.org/wiki/File:Symmetrical_stretching.gif"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/0/0c/Asymmetrical_stretching.gif width="210" height="150" caption="Asymmetrical stretching.gif" link="http://en.wikipedia.org/wiki/File:Asymmetrical_stretching.gif"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/6/60/Scissoring.gif width="210" height="150" caption="Scissoring.gif" link="http://en.wikipedia.org/wiki/File:Scissoring.gif"]] ||
 * ~ Rocking ||~ Wagging ||~ Twisting ||
 * [[image:http://upload.wikimedia.org/wikipedia/commons/1/14/Modo_rotacao.gif width="210" height="150" caption="Modo rotacao.gif" link="http://en.wikipedia.org/wiki/File:Modo_rotacao.gif"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/8/84/Wagging.gif width="210" height="150" caption="Wagging.gif" link="http://en.wikipedia.org/wiki/File:Wagging.gif"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/4/40/Twisting.gif width="210" height="150" caption="Twisting.gif" link="http://en.wikipedia.org/wiki/File:Twisting.gif"]] ||