Satish Pradhan Dnyanasadhana College, Thane-400604 Satish Pradhan Dnyanasadhana College, Thane-400604. Department of Chemistry M.Sc. Analytical Chemistry Sem-II 1.2 Molecular ultra violet and visible spectroscopy

1.2 Molecular ultra violet and visible spectroscopy Types of transitions, Factors affecting molecular absorption: pH, temperature, solvent, effect of substituent's Derivative and dual wavelength spectroscopy applications including simultaneous determinations.

ABSORPTION SPECTROSCOPY The transition may take place from lower energy level to higher energy level by absorbing energy is as called absorption spectroscopy ABSORPTION SPECTRUM The result obtained as a result of a number of such transitions is called absorption spectrum.

EMISSION SPECTROSCOPY The transition may take place from higher energy level to a lower energy level thereby emitting the excess energy as a photon. It is then called emission spectroscopy EMISSION SPECTRUM. The result obtained as a result of no. Of such transitions is called emission spectrum.

Energy for Atom Atom

Energy of a Molecule Vibrational Energy TRANSLATIONAL ENERGY Electronic Energy Rotational Energy TRANSLATIONAL ENERGY

Electronic Energy of Molecule * Antibonding * Antibonding ∏ n *  * n *  * n Nonbonding  Bonding Energy  Bonding

CH3----------------- CH3 Rotational Energy CH3----------------- CH3 Motion of molecule from the centre of joining the nuclei

Bond length 154 pm, Stretching Vibrational Energy Bond length 154 pm, Stretching 10 pm. For a C-C bond with a bond length of 154 pm, the variation is about 10 pm.

C C C 4o Bending 10 pm Vibrational Energy For C-C-C bond angle a change of 4o is typical. This moves a carbon atom about 10 pm.

ABSORBING SPECIES The absorption of ultraviolet or visible radiation by a molecular species M can be considered to be a two step process, excitation M + h M* The lifetime of the excited species is brief (10-8 to 10-9 s). Relaxation involves conversion of the excitation energy to heat. M* M + heat The absorption of ultraviolet or visible radiation generally results from excitation of bonding electrons.

Orbitals containing electrons are called as Bonding Orbitals Types of Orbitals Bonding Orbitals : Orbitals containing electrons are called as Bonding Orbitals Antibonding Orbitals : vacant or unounoccupied Orbitals ( Not containing electrons) are called as Bonding Orbital's .

Closed shell electrons Covalent single bonded electrons Types of Electrons in a molecule Closed shell electrons Not absorbing / Not in bonding Covalent single bonded electrons σ Sigma Electrons in π orbitals Paired nonbonding outershell electrons n electrons C---C Sigma bond σ Sigma electrons C==C C C H—O---H

ELECTRONIC TRANSITIONS charge transfer electrons , , and n electrons d and f electrons charge transfer electrons Organic Molecule Inorganic molecule Organic + Inorganic

Types of Absorbing Electrons E-in bond formation between atoms; Nonbonding or unshared outer electrons Electrons from The molecular orbitals

The molecular orbital's pi () molecular orbital Electrons from The molecular orbital's  Molecular orbital Called as  electrons pi () molecular orbital Called as  electrons Nonbonding electrons

Types of Absorbing Electrons The electrons that contribute to absorption by a molecule are: Those that participate directly in bond formation between atoms; Nonbonding or unshared outer electrons that are largely localized about such atoms as oxygen, the halogens, sulfur, and nitrogen. The molecular orbitals associated with single bonds are designated as sigma () orbitals, and the corresponding electrons are  electrons.

Types of Absorbing Electrons The double bond in a molecule contains two types of molecular orbitals: a sigma () orbital and a pi () molecular orbital. Pi orbitals are formed by the parallel overlap of atomic p orbitals. In addition to  and  electrons, many compounds contain nonbonding electrons. These unshared electrons are designated by the symbol n.

Four types of transitions are possible: Energy levels The energies for the various types of molecular orbitals differ significantly. The energy level of a nonbonding electron lies between the energy levels of the bonding and the antibonding  and  orbitals. Electronic transitions among certain of the energy levels can be brought about by the absorption of radiation. Four types of transitions are possible:  *, n *, n *, and  *.

Types of Electronic transitions  *, n * n *  *. Organic Molecules

Energy levels for Electronic transitions in a Molecule * Antibonding * Antibonding ∏ n *  * n *  * n Nonbonding  Bonding Energy  Bonding

 * Transition An electron in a bonding  orbital of a molecule is excited to the corresponding antibonding orbital by the absorption of radiation. The energy required to induce a  * transition is large. Methane can undergo only  * transitions, exhibits an absorption maximum at 125 nm. Absorption maxima due to  * transitions are never observed in the ordinarily accessible ultraviolet region.

n * Transitions Saturated compounds containing atoms with unshared electrons are capable of n * transitions. These transitions require less energy than the  * type and can be brought about by radiation in the region of between 150 and 250 nm, with most absorption peaks appearing below 200 nm. The molar absorptivities are low to intermediate in magnitude and range between 100 and 3000 L cm-1 mol -1.

n * and  * Transitions Most applications of absorption spectroscopy are based upon transitions for n or  electrons to the * excited state because the energies required for these processes bring the absorption peaks into an experimentally convenient spectral region (200 to 700 nm). Both transitions require the presence of an unsaturated functional group to provide the  orbitals. The molar absorptivities for peaks associated with excitation to the n, * state are generally low and ordinarily range from 10 and 100 L cm-1 mol -1; values for  * transitions, on the other hand, normally fall in the range between 1000 and 10,000.

Types of Electronic transitions Charge Transfer Complexes d d transitions f f Charge Transfer Complexes Inorganic Molecules

d-d or f-f transitions Inorganic Molecules

Charge Transfer Complexes Many a times a given compound that is transparent in the UV region starts absorbing after interacting with another species. This happens if one of the species has an electron donor group and the interacting species has an electron acceptor group. When the two species bind to each other, the resulting species is intensely coloured. This is due to the formation of a complex between the two species. Such a complex is called charge transfer complex. For example, the blood red color of the complex ion, thiocyanate iron (III) ion, Fe (SCN)2+ is due to the formation of a charge transfer complex.

Effect of Conjugation of Chromophores  electrons are considered to be further delocalized by conjugation; the orbitals involve four (or more) atomic centers. The effect of this delocalization is to lower the energy level of the * orbital and give it less antibonding character. Absorption maxima are shifted to longer wavelengths as a consequence. Conjugation of chromophores, has a profound effect on spectral properties. 1,3-butadiene, CH2=CHCH=CH2, has a strong absorption band that is displaced to a longer wavelength by 20 nm compared with the corresponding peak for an unconjugated diene.

Factors affecting molecular absorption pH of the solution Nature of the solvent Temperature Effect of substituents

Presence of Electrolytes The presence of small amounts of colourless electrolytes which do not react chemically with the coloured components does not affect the light absorption as a rule. However, large amounts of electrolytes may affect the absorption spectrum qualitatively as well as quantitatively. This is due to the physical interaction between the ions of the electrolyte and the coloured ions or molecules. This interaction results in a deformation of the later, thereby causing a change in its light absorption property.

Effect of pH The chromate ion has a single λmax at 375 nm whereas dichromate ion has two peaks in the spectra; λmax at 350 and 450 nm. The position of equilibrium depends on the pH of the solution and yellow colour of solution changes to orange on increasing the concentration of hydrogen ions. Therefore, the results of the determination of chromate ion concentration will depend on the pH. Thus, it is imperative that substances, whose colour is influenced by change in hydrogen ion concentration, must be studied under the condition of same pH. Cr2072- + H20 2HCrO4 2H+ +2CrO42- (Orange) (Yellow)

Temperature The temperature is not considered as an important factor since ordinarily the measurements are made at a constant temperature. However, changes in temperature sometimes may shift ionic equilibrium and the absorptivity. For example, the colour of acidic ferric chloride solution changes from yellow to reddish brown on heating due to change in λ max and absorptivity.

Solvents: In choosing a solvent, consideration must be given not only to its transparence, but also to its possible effects upon the absorbing system. Polar solvents such as water, alcohols, esters, and ketones tend to obliterate spectral fine structure arising from vibrational effects; spectra that approach those of the gas phase are more likely to be observed in nonpolar solvents such as hydrocarbons. In addition, the positions of absorption maxima are influenced by the nature of the solvent. Clearly, the same solvent must be used when comparing absorption spectra for identification purposes.

Polar solvents should be used to dissolved polar compounds Effect of solvents Polar solvents should be used to dissolved polar compounds Non-polar solvents should be used to dissolved non-polar compounds Polar and nonpolar Vibration occure

Effect of substituents, The presence of small amounts of colourless electrolytes which do not react chemically with the coloured components does not affect the light absorption as a rule. However, large amounts of electrolytes may affect the absorption spectrum qualitatively as well as quantitatively. This is due to the physical interaction between the ions of the electrolyte and the coloured ions or molecules. This interaction results in a deformation of the later, thereby causing a change in its light absorption property.

Application of UV spectroscopy Simultaneous Determination Sometimes we come across a situation wherein the analyte contains two species which have overlapping spectra. In order to determine these species we need to find two wavelengths where molar absorptivity of two species is different. In such a case, measurements are made on the solution of the analytes at two different wavelengths.

measurements are made on the solution of the analytes at two different wavelengths. This gives a set of simultaneous equations which could be solved for the concentrations of the individual constituents. For best results it is desirable to select two such wavelengths where the ratio of molar absorptivities is largest.

Origin of UV spectrum molecules, the electronic, vibrational as well as the rotational energies are each of the vibrational energy level has a number of rotational energy levels in it. When a photon of a given wavelength interacts with the molecule it may cause a transition amongst the electronic energy levels if its energy matches with the difference in the energies of these levels. In the course of such transitions, for the sample in gaseous or vapour phase, the spectrum consists of a number of closely spaced lines (Fig 2.1(a)), constituting what is called a band spectrum. However, in the solution phase, the absorbing species are surrounded by solvent molecules and undergo constant collisions with them. These collisions and the interactions among the absorbing species and the solvent molecules cause the energies of the quantum states to spread out. As a consequence quantized. A given electronic energy level has a number of vibrational energy levels in it and sequence, the sample absorbs photons spread over a range of wavelength. Thereby, the spectrum acquires the shape of a smooth and continuous absorption peak in the solution phase. A typical UV-VIS spectrum in the solution phase is depicted in Fig. 2.1 (b). The absorption of radiation in the UV-VIS region of the spectrum causes the transitions amongst the electronic energy levels.