Applications of UVVis Spectroscopy Lecture 26 1 Absorbing

Applications of UV-Vis Spectroscopy Lecture 26 1

Absorbing Molecular Species When an incident beam of radiation having a suitable wavelength hits a molecule, absorption of a photon takes place and the molecule becomes excited. Excited molecules will lose excitation energy as heat or photons (luminescence). Absorption of UV-Vis radiation is capable of affecting the excitation of bonding electrons and other valence electrons. Therefore, excitation of electrons in chemical bonds (p and s) or nonbonding electrons (n) is the result of absorption of UV-Vis radiation of a suitable wavelength. Absorption will thus be dependent on the availability of p and s bonds or n electrons that can absorb incident radiation. 2

Molecules Containing p, s, and n Electrons 1. Molecules with s Bonds Only Let us start with a molecule like CH 4 and then expand our discussion to more complex molecules: All bonds in methane are s bonds and the only transition possible is the s-s* transition. However, the s-s* transition requires very high energy which occurs in vacuum UV. 3

It is not wise to think of doing UV measurements on molecular species in the vacuum UV region (125 -185 nm) for five important reasons: • • • 4 The high energy required can cause rupture of the s bonds and breakdown of the molecule Air components absorb in vacuum UV which limits the application of the method Working in vacuum UV requires special training and precautions which limit wide application of the method. Special sources and detectors, other than those described earlier, must be used All solvents contain s bonds

2. Molecules with n Electrons in the valence shell that are not used up in chemical bonds are referred to as nonbonding electrons (n electrons). Consider a molecule like ammonia: The line on nitrogen is a symbol for two nonbonding electrons. Now, the type of transitions observed in this molecule can be listed as: s-s* n-s* We have seen earlier that the s-s* transition is not useful in practical UV-Vis spectroscopy but the other transition (n-s*) is of lower energy and should be further discussed. 5

The absorption wavelength for a n-s *transition occurs at about 185 nm where, unfortunately, most solvents absorb. For example, the most important solvent is, undoubtedly, water which has two pairs of nonbonding electrons that will strongly absorb as a result of the ns *transitions; which precludes the use of this transition for studies in aqueous and other solvents with nonbonding electrons. In summary, it is also impractical to think of using UV-Vis absorption spectroscopy to determine analytes based on a n-s * transition. 6

3. Molecules with p Bonds Absorption of radiation by an alkene, containing a double bond, can result in s-s* or p-p* transitions. We have seen that a s-s* transition is not useful but on the other hand, the p-p* turned out to be very useful since it requires reasonable energy and has good absorptivity. A molecule having s, p, and n electrons can show all types of transitions possible in UV-Vis spectroscopy. For example, an aldehyde molecule shows all these transitions since it contains s, p, and two pairs of n electrons. Two transitions are possible for the n electrons: n-s* n-p* 7

We have seen that a n-s * is not very useful due to absorbance from solvents and other frequently used additives which have n electrons. The n-p * transition requires very little energy and seem to be potentially useful. However, unfortunately, the absorptivity of this transition is very small which precludes its use for sensitive quantitative analysis. 8

Effect of Solvent Polarity on Absorption Wavelength The molar absorptivity for a n-p* transition is rather small (10 -100 L mol-1 cm-1) and the energy required for transition is affected by solvent polarity. In presence of a polar solvent, nonbonding electrons will interact with protic solvents to form hydrogen bonds. The solvation of n electrons is the result; which lowers the energy of the orbitals holding the n electrons. Partial stabilization of the polar p* is also observed but to a much lower extent than the n electrons. 9

A net increase in energy required for a n-p* transition is thus observed in protic solvents; like water or alcohols. Therefore, an increase in energy will reflect a decrease in transition wavelength, or what is called hypsochromic shift or blue shift. 10

On the other hand, a p-p* transition is affected in an opposite manner with solvent polarity. In presence of a polar solvent, the more polar p* orbital will be more stabilized than the p orbital leading to a net decrease in the transition energy. This results in an increase in transition wavelength or what is called a bathochromic shift (red shift). 11

Spectral nomenclature of shifts 12

Conclusions on Electronic Transitions A s-s* and a n-s* are not useful for reasons discussed earlier. The n-p* transition requires low energy but the molar absorptivity is also low and transition energy will increase in presence of polar solvents. The n-p* transition is seldom used in quantitative UV-Vis spectroscopy. The most frequently used transition is the p-p* transition for the following reasons: 1. The molar absorptivity for the p-p* transition is high allowing sensitive determinations. 2. The energy required is moderate, far less than dissociation energy. 3. In presence of the most convenient solvent (water), the energy required for a p-p* transition is usually smaller. It is therefore primitive that an analyte to be determined by UVVis absorption spectroscopy be of unsaturated nature (or!!). 13

Organic Chromophores Molecules having unsaturated bonds or free nonbonding electrons that can absorb radiation of relatively low energy are called chromophores. Examples include alkenes, alkynes, ketones, aldehydes, phenyl and other aromatic species, etc. Auxochromes An auxochrome is a functional group that does not absorb by itself, but its presence in a molecule can increase absorption and usually results in a red-shift. 14

UV-Visible Absorption Chromophores 15

The effects of substitution Auxochrome function group Auxochrome is a functional group that does not absorb in UV region but has the effect of shifting chromophore peaks to longer wavelength as well As 16 increasing their intensity.

Effect of Conjugation of Chromophores As conjugation is increased in a molecule, more delocalization (stability) of the p electrons results. The effect of this delocalization is to decrease the p* molecular orbital. The result is a decrease in transition energy from p-p* and thus a red or bathochromic shift. The molar absorptivity will increase in this case and better quantitative analysis will be achieved. In cases of introduction of more unconjugated double bonds, the molar absorptivity will increase as well depending on the number of the double bonds. For example, at 185 nm, 1 -hexene has a molar absorptivity of about 10, 000 L mol-1 cm-1 but hexa-1, 4 -diene has a molar absorptivity of twice as much as 1 -hexene. However, when the double bonds are conjugated as in hexa-1, 3 -diene the molar absorptivity is about 21, 000 L mol-1 cm-1. 17

Rule of thumb for conjugation If greater then one single bond apart - e are relatively additive (hyperchromic shift) - l constant CH 3 CH 2 CH=CH 2 lmax= 184 emax = ~10, 000 CH 2=CHCH 2 CH=CH 2 lmax=185 emax = ~20, 000 If conjugated - shifts to higher l’s (red shift) H 2 C=CHCH=CH 2 18 lmax=217 emax = ~21, 000

Effect of Aromaticity of Chromophores On the other hand, aromaticity results in extraordinarily high degree of delocalization of electrons and thus stabilization of the p*. If we assume a molar absorptivity of about 10, 000 L mol-1 cm-1 for each double bond, we expect the sum of the three double bonds in benzene to be just above 30, 000 L mol-1 cm-1 (at 185 nm) but actually the value is about 60, 000 L mol-1 cm-1 due to increased delocalization as a result of aromaticity. It is therefore advantageous to use UV-Vis absorption spectroscopy for determination of compounds having aromatic character. 19

Absorption by Inorganic Groups Inorganic groups containing double bonds absorb in the UV-Vis region. The most transitions are a result of n-p* transitions as in nitrate (313 nm), carbonate (217 nm), nitrite (280 and 360 nm) and azide (230 nm) 20