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ُElectronic transitions spectroscopy

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الكلية كلية العلوم     القسم قسم الكيمياء     المرحلة 4
أستاذ المادة عباس عبد علي دريع الصالحي       12/03/2018 18:35:11
University of Babylon Undergraduate Studies
College of Science
Department of Chemistry
Course No. Chsc 424 Physical chemistry
Fourth year - Semester 2
Credit Hour: 3 hrs.
Scholar units: three units
Lectures of Quantum mechanics
Second Semester, Scholar year 2017-2018
Prof. Dr. Abbas A-Ali Draea
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Lecture No. Five: Electronic Transitions Spectroscopy
1-Introudction.
2-Advantages of Electronics spectroscopy.
3- UV-Visible spectra.
4- The Importance of Conjugation.
5-Shifting of electronic transitions.

1-Introudction:
Electronic changes have been occurs in this region, since the energy limits equal to 8000Cm-1 frequency (ranged from 750nm into less 200nm). Its classified into three different region (A, B, and C), since the visible is C-region, that s ranged from 750nm into400nm and the near UV-region (B) is ranged from 400nm into 200nm. Last region-A is the vacuumed UV-region that’s ranged from 200nm and less than (oxygen absorbed). The energy value limits reach the outer shell electrons of chemical species and promotion to higher energy levels.
They found some assumption of the electronic transitions in molecule since nuclei remain frozen (R unchanged):-
1- Vertical transitions between potential energy curves.
2- There is no selection rule governing the allowed vibrational changes accompanying an electronic transition.
3- The probability of undertaking a v” ? v’ transition is governed by Franck?Condon factors.

2-Advantages of Electronics spectroscopy:
Electronics spectroscopy studies give up information about electronics structures of molecules, electronics energies at different molecular levels, and the know lodgment of emission and absorption spectra for colored chemical compounds. A large change in energy are coming from electronic transitions also rearrangements of electrons that’s produced change in electrostatic power on nuclei. At the same time of electronic transition there are vibrational and rotational; so can be analysis the vibrational structure from electronic transition spectra. If sample spectra at gas phase, but at solid phase, liquid phase there are confused in spectral lines as one band width broad peaks and cannot be analysis. Therefore the spectra in gas phase are very complexes than other phases. In some time absorption energy is so enough to dissociation the molecules. E total =E electronic + E vibration + E rotation
All chemical species have electronic spectra because the change in electronic distribution produced the change in dipole moment. Alternative in rotational spectra the molecules must have electrical dipole moment but in vibration spectra the molecules must have change in the electrical dipole moment through the movements. All semi diatomic molecules can be have electronic spectra also within rotation &vibration like N2, H2, Br2.
According to the theory of molecular orbital, there are different types of transitions occurs due electron promotion from lowering energy orbital levels into higher molecular orbital energy levels, or transitions can be represented by transition from atomic character orbital into higher energy molecular orbital.
3- UV-Visible spectra:
Chemical compounds, this is mainly a study of molecules and their electronic transitions. Molar Absorptivity (?) ranges from 0 to 105 for use in absorbance measurements. Transitions with ? < 103 are considered to be of low intensity. In organic molecules, most bonding electrons are excited by ? < 185 nm (Vacuumed-UV). Recall that E(eV) = 1239 / ? (nm).
Most functional groups have lone pairs whose energies place them in the near UV and visible range. These groups are called “chromophores”, although this is a bit odd, since all molecules are “chromophores” under the right conditions. The visible region of the spectrum comprises photon energies of 36 to 72 kCal/mole, and the near ultraviolet region, out to 200 nm, extends this energy range to 143 kCal/mole. Ultraviolet radiation having wavelengths less than 200 nm is difficult to handle, and is seldom used as a routine tool for structural analysis. The energies noted above are sufficient to promote or excite a molecular electron to a higher energy orbital. Consequently, absorption spectroscopy carried out in this region is sometimes called "electronic spectroscopy". A diagram showing the various kinds of electronic excitation that may occur in organic molecules is shown on the left. Of the six transitions outlined, only the two lowest energy ones (left-most, colored blue) are achieved by the energies available in the 200 to 800 nm spectrum. As a rule, energetically favored electron promotion will be from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and the resulting species is called an excited state. .
4- The Importance of Conjugation:
A comparison of the absorption spectrum of 1-pentene, ?max = 178 nm, with that of isoprene clearly demonstrates the importance of chromophore conjugation. The spectrum on conjugation of double and triple bonds also shifts the absorption maximum to longer wavelengths. From the polyene spectra displayed, it is clear that each additional double bond in the conjugated pi-electron system shifts the absorption maximum about 30 nm in the same direction. Also, the molar absorptivity (?) roughly doubles with each new conjugated double bond. Spectroscopists use the terms defined in the table on the right when describing shifts in absorption. Extending conjugation generally results in bathochromic and hypochromic shifts in absorption.
The appearance of several absorption peaks or shoulders for a given chromophore is common for highly conjugated systems, and is often solvent dependent. This fine structure reflects not only the different conformations such systems may assume, but also electronic transitions between the different vibrational energy levels possible for each electronic state. Vibrational fine structure of this kind is most pronounced in vapor phase spectra, and is increasingly broadened and obscured in solution as the solvent is changed from hexane to methanol.

Terminology for Absorption Shifts
Nature of Shift Descriptive Term
To Longer Wavelength Bathochromic
To Shorter Wavelength Hypsochromic
To Greater Absorbance Hyperchromic
To Lower Absorbance Hypochromic

5-Shifting of electronic transitions:
They found different reasons for the remove the transition into up normal positions:-
A- Effect of substituted groups on the original spectrum.
Alternative the functional groups (provided with hetero atoms) with unsaturated center will remove or shifted the absorption into red region (longer wave length- red shift).

Alternative of two unsaturated centers (double bonds) causes the delocalization of electron alone the axis of molecule, were the absorption of ???* will remove into red shift (low frequency) with low energy value.


B- Effect of solvent
In some of polar solvent, transitions are shifted into red region due interaction of polar excited groups with polar solvents.

But in some interaction between the molecules and solvents, they give up more stability (localization) of electrons, therefore absorption position alter to shorter wave length (blue shift) with highly energy value (hydrogen bonding). The transition didn t occur unless additional energy value to break down the hydrogen bond and transition in blue region.
The calculation of the hydrogen bonding strength in compounds can be doing thereby measuring the transition into two different solvent polarities. Taking the difference between them in energy value as in following example: n??* of carbonyl group in acetone.
In Hexane 279nm (506.3 kJ mol-1)
In Water 264.5nm (527.2 kJ mol-1)
Energy difference equal to20.9 kJ mol-1

C- Effect of complementary groups
(Charge transfer spectrum)
When they found more than one substituted group into aromatic ring with different effect (one of them is electron donating and the other is electron drawing) in Para position for each other s, these groups can be called complementary groups.
The transition is called Charge transfer, and the band is Charge transfer band, this phenomenon is due the resonance exchange effect. If the two substituted groups into the aromatic ring have the same electronic effect (both of them drawing or donating) the resultant spectrum as individual spectrum of one group and there aren’t complementary groups. The same thing occurs if they are not in Para position from each other s.
D- Effect of hydrogen bonded (inter &intra).
This reason is come out from hydrogen bond formation in two different case, the first is inter hydrogen bonding formation in the same chemical species moiety like in salicylic acid ,the second case is intra hydrogen bonding formation between two chemical species moiety in bulk solution like ethanol and acetone. Both types will alter the transition into blue shift (shorter wave length).


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