Electromagnetic radiation, absorption

Spectroscopy The science of analyzing the spectra of atoms and molecules. Emission spectroscopy deals with exciting atoms or molecules and measuring the wavelength of the emitted electromagnetic radiation. Absorption spectroscopy measures the wavelengths of absorbed radiation. 

Electromagnetic Interference Shielding, Electromagnetic Radiation Absorption, and Joining of Plastics... 

Every molecular species is capable of absorbing its own characteristic frequencies of electromagnetic radiation, as described in Figure 24-5. This process transfers energy to the molecule and results in a decrease in the intensity of the incident electromagnetic radiation. Absorption of the radiation thus attenuates the beam in accordance with the absorption law described later. 

From a chemical kinetic point of view, the electromagnetic radiation absorption process of a photon having an energy hv. can be seen as the following pseudoreaction scheme ... 

Gaseous H CI has a strong absorption band centered at about X = 3.40 X 10 m in the infrared portion of the electromagnetic radiation spec-tmm. On the assumption that D bonds to Cl with the same str ength that H does, predict the frequency of vibration in Hz and rad of D CI. 

As diverse as these techniques are all of them are based on the absorption of energy by a molecule and all measure how a molecule responds to that absorption In describing these techniques our emphasis will be on then application to structure determination We 11 start with a brief discussion of electromagnetic radiation which is the source of the energy that a molecule absorbs m NMR IR and UV VIS spectroscopy... 

Structure determination m modern day organic chemistry relies heavily on instrumental methods Several of the most widely used ones depend on the absorption of electromagnetic radiation... 

Absorption of electromagnetic radiation causes a molecule to be excited from Its most stable state (the ground state) to a higher energy state (an excited state)... 

Section 13 20 IR spectroscopy probes molecular structure by examining transitions between vibrational energy levels using electromagnetic radiation m the 625-4000 cm range The presence or absence of a peak at a charac tenstic frequency tells us whether a certain functional group is present Table 13 4 lists IR absorption frequencies for common structural units... 

Section 13 21 Transitions between electronic energy levels involving electromagnetic radiation m the 200-800 nm range form the basis of UV VIS spec troscopy The absorption peaks tend to be broad but are often useful m indicating the presence of particular tt electron systems within a mole cule... 

Colorimetry, in which a sample absorbs visible light, is one example of a spectroscopic method of analysis. At the end of the nineteenth century, spectroscopy was limited to the absorption, emission, and scattering of visible, ultraviolet, and infrared electromagnetic radiation. During the twentieth century, spectroscopy has been extended to include other forms of electromagnetic radiation (photon spectroscopy), such as X-rays, microwaves, and radio waves, as well as energetic particles (particle spectroscopy), such as electrons and ions. ... 

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

Absorption of a photon is accompanied by the excitation of an electron from a lower-energy atomic orbital to an orbital of higher energy. Not all possible transitions between atomic orbitals are allowed. For sodium the only allowed transitions are those in which there is a change of +1 in the orbital quantum number ) thus transitions from s—orbitals are allowed, but transitions from s d orbitals are forbidden. The wavelengths of electromagnetic radiation that must be absorbed to cause several allowed transitions are shown in Figure 10.18. 

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation. 

Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1. As described, equation 10.1 does not distinguish between the different ways in which the attenuation of radiation occurs. Besides absorption by the analyte, several additional phenomena contribute to the net attenuation of radiation, including reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss of the electromagnetic radiation s power, we use a method blank (Figure 10.20b). The radiation s power exiting from the method blank is taken to be Pq. 

The absorptivity and molar absorptivity give, in effect, the probability that the analyte will absorb a photon of given energy. As a result, values for both a and 8 depend on the wavelength of electromagnetic radiation. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

Spectroscopy is basically an experimental subject and is concerned with the absorption, emission or scattering of electromagnetic radiation by atoms or molecules. As we shall see in Chapter 3, electromagnetic radiation covers a wide wavelength range, from radio waves to y-rays, and the atoms or molecules may be in the gas, liquid or solid phase or, of great importance in surface chemistry, adsorbed on a solid surface. 

In the process of absorption or emission of infrared radiation involving transitions between two vibrational states the interaction is usually between the molecule and the electric, rather than the magnetic, component of the electromagnetic radiation (see Section 2.1). For this... 

Atomic and Molecular Energy Levels. Absorption and emission of electromagnetic radiation can occur by any of several mechanisms. Those important in spectroscopy are resonant interactions in which the photon energy matches the energy difference between discrete stationary energy states (eigenstates) of an atomic or molecular system = hv. This is known as the Bohr frequency condition. Transitions between... 

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