Band of absorption

As an example of application of the method we have considered the case of the acrolein molecule in aqueous solution. We have shown how ASEP/MD permits a unified treatment of the absorption, fluorescence, phosphorescence, internal conversion and intersystem crossing processes. Although, in principle, electrostatic, polarization, dispersion and exchange components of the solute-solvent interaction energy are taken into account, only the firsts two terms are included into the molecular Hamiltonian and, hence, affect the solute wavefunction. Dispersion and exchange components are represented through a Lennard-Jones potential that depends only on the nuclear coordinates. The inclusion of the effect of these components on the solute wavefunction is important in order to understand the solvent effect on the red shift of the bands of absorption spectra of non-polar molecules or the disappearance of... 

The UV-VIS radiation gives rise to changes in the energy of electronic states of a molecule. The probability of electronic transitions in a molecule depends on the presence of multiple bonds in the molecule and on the kind, number and positions of the substituent groups. Determination of the kind of transitions corresponding to the observed bands of absorption spectra enables one to determine the structure of the molecule. 

The spectral positions of the 0 O bands of absorption and fluorescence usually do not coincide exactly. The difference is called the Stokes shift. The excited state is generally more polarizable and often more polar than the ground state. Solvent relaxation then stabilizes the excited state following Franck Condon excitation, so that the 0 0 band of absorption is usually at higher frequency than that of emission. When the excited state is much less polar than the ground state, the 0 0 band of fluorescence may appear at higher frequency than that of absorption anti-Stokes shift). Unfortunately, the term Stokes shift is also used to refer to the gap between the band maxima of absorption and emission (see the footnote to fluorescence in Section 2.1.1), which may be much larger than the shift between the 0 0 bands. 

Historically important in the development of modern atomic theory was the recognition that although polyatomic molecules show more or less broad bands of absorption and emission in the visible and ultraviolet regions of the spectrum, the characteristic light absorption or emission by individual atoms occurs at fairly narrow lines of the spectrum, which correspond to sharply defined wavelengths. The line spectrum of each element is so uniquely characteristic of that element that atomic spectroscopy can be used for precise elementary analysis of many types of chemically complex materials. 

Knowing that doesn t vary in the mid-IR, expression 10.2 leads to the energy values corresponding to the bands of absorption (Figure 10.5) ... 

Note that in a broad band, each photon absorbed, or emitted, has an incremental difference of energy from the other photons, depending upon the vibrations, or phonon modes present during the instantaneous moment of electronic change. It is this instantaneous change, as secondarily affected by the slower phonon modes, which cause broad bands of absorption (and/or excitation) and those of emission. It is because phonon energies are quantized that they can add and subtract from the electronic energy process. 

I ltc electronic spectra of organic molecules containing chrt iuoplioies are usually complex, because the superposition of ibraiicmal transitions on the electronic transitions leads to an intricate combination of overlapping lines. The result is a broad band of absorption that often appears to be coniittuons. The complex na... 

In glancing at the infrared spectra, there are several immediately noticeable bands of absorptions. 

Because there are so many possible transitions, each differing from the others by only a slight amount, each electronic transition consists of a vast number of lines spaced so closely that the spectrophotometer cannot resolve them. Rather, the instmment traces an envelope over the entire pattern. What is observed from these types of combined transitions is that the UV spectrum of a molecule usually consists of a broad band of absorption centered near the wavelength of the major transition. 

These bands of absorptions are observed at frequencies equal to the sum or the difference between two frequencies or fundamental frequency of a fundamental and a harmonic one. By cause of bands of combination appear various normal modes of oscillation of the molecule. The high harmonics and the bands of combination in IR absorption spectra cause considerably complications in their interpretation. 

The radicals NO3 have three intensive bands of absorption in visible region of an optical spectrum with A, = 600, 640 h 675 mn, and in UV... 

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