The Visible and the Ultraviolet Spectroscopy
In the Electromagnetic (EM) spectra , the far UV is in the region of the wavelength (λ) 4nm – 200nm, the near UV is in the region of the λ of 200nm – 400nm and the visible light exist in the range of the λ 400nm – 800nm. When the EM radiation in the UV or visible region is passed through an organic compound, a portion of the radiation is usually absorbed by compound. The energy of the absorbed radiation from these regions are used to electron transitions from lower electronic energy levels to higher electronic energy levels. The magnitude of the absorption of this radiation depends on the wavelength of the radiation and the structure of the molecules.
The UV – Visible spectrometers are the instruments those used to measure the absorbance of the radiation at each wavelength of the visible and UV regions. In general the absorbance of the wavelength of 200nm – 800nm can be measured by most of the visible-UV spectrometers, so the far UV is less useful in the spectroscopic methods as its wavelength range is from 4nm to 200nm. In these instruments, a beam of visible/UV radiation (200nm-400nm and 400nm-800nm) is directed through a solution of the compound being analyzed. Also, these instruments are design to make a comparison of the intensities of Incident light (I0) and the transmitted light (I), to produce a graph plot of the Wavelength of the entire region versus the Absorbance (A) of light at each wavelength. Such a graph is called an Absorption Spectra.
The absorption at a particular wavelength is defined by the equation;
A = log(I0/I)
It is also can be derived from the Beer Lambert's law , which says that,
I = I0 x -ЄCl
log(I/I0) = -ЄCl
log(I0/I) = ЄCl
So that,
A = log(I0/I) = ЄCl
Where,
A- observed absorbance
Є- molar absorptivity or molar extinction coefficient
C- molar concentration of the sample
l- length of the sample tube
I0- intensity of incident light
I- intensity of transmitted light
The wavelength of maximum absorption (λmax) and the maximum molar absorptivity (Єmax) are differ from one compound to another. Most of the absorption bands (or peaks) in the absorption spectra are broad because, each electron energy level has associated with its many vibrational and rotational energy levels. Thus, electronic transitions may occu from any of several vibrational and rotational energy levels of one electronic level to any of several vibrational and rotational states of a high electronic levels.
Alkenes and non-conjugated dienes usually have λmax below 200nm which is out of the range of visible-UV spectrometers. But the compounds with conjugated multiple bonds have λmax longer than 200nm. For example, Ethene gives a λmax at 171nm, 1,4-pentadiene gives a λmax at 178nm and 1,3-butadiene gives a λmax at 217nm.
In the Electromagnetic (EM) spectra , the far UV is in the region of the wavelength (λ) 4nm – 200nm, the near UV is in the region of the λ of 200nm – 400nm and the visible light exist in the range of the λ 400nm – 800nm. When the EM radiation in the UV or visible region is passed through an organic compound, a portion of the radiation is usually absorbed by compound. The energy of the absorbed radiation from these regions are used to electron transitions from lower electronic energy levels to higher electronic energy levels. The magnitude of the absorption of this radiation depends on the wavelength of the radiation and the structure of the molecules.
The UV – Visible spectrometers are the instruments those used to measure the absorbance of the radiation at each wavelength of the visible and UV regions. In general the absorbance of the wavelength of 200nm – 800nm can be measured by most of the visible-UV spectrometers, so the far UV is less useful in the spectroscopic methods as its wavelength range is from 4nm to 200nm. In these instruments, a beam of visible/UV radiation (200nm-400nm and 400nm-800nm) is directed through a solution of the compound being analyzed. Also, these instruments are design to make a comparison of the intensities of Incident light (I0) and the transmitted light (I), to produce a graph plot of the Wavelength of the entire region versus the Absorbance (A) of light at each wavelength. Such a graph is called an Absorption Spectra.
The absorption at a particular wavelength is defined by the equation;
A = log(I0/I)
It is also can be derived from the Beer Lambert's law , which says that,
I = I0 x -ЄCl
log(I/I0) = -ЄCl
log(I0/I) = ЄCl
So that,
A = log(I0/I) = ЄCl
Where,
A- observed absorbance
Є- molar absorptivity or molar extinction coefficient
C- molar concentration of the sample
l- length of the sample tube
I0- intensity of incident light
I- intensity of transmitted light
The wavelength of maximum absorption (λmax) and the maximum molar absorptivity (Єmax) are differ from one compound to another. Most of the absorption bands (or peaks) in the absorption spectra are broad because, each electron energy level has associated with its many vibrational and rotational energy levels. Thus, electronic transitions may occu from any of several vibrational and rotational energy levels of one electronic level to any of several vibrational and rotational states of a high electronic levels.
Alkenes and non-conjugated dienes usually have λmax below 200nm which is out of the range of visible-UV spectrometers. But the compounds with conjugated multiple bonds have λmax longer than 200nm. For example, Ethene gives a λmax at 171nm, 1,4-pentadiene gives a λmax at 178nm and 1,3-butadiene gives a λmax at 217nm.
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