Qualitative applications of UV Visible spectrophotometry - Instrumental Methods of Analysis B. Pharma 7th Semester

Qualitative applications of UV Visible spectrophotometry

Objectives

After this session, students will be able to

• Identify possible electronic transitions in UV spectroscopy

• Enlist the types of shifts observed in UV spectroscopy

• Identify the significance of Absorption maxima

• Explain the solvent effects

Electronic transitions in UV spectroscopy

UV spectrum of Isoprene

Concept of Chromophore and Auxochrome

• Chromophore is defined as any isolated covalently bonded group that shows a characteristic absorption in the ultraviolet or visible region (200-800 nm).

Chromophores can be divided into two groups

• a) Chromophores which contain πelectrons and which undergo π→π* transitions.

• Ethylenes and acetylenes are the example of such chromophores.

• b) Chromophores which contain both πand nonbonding electrons. They undergo two types of transitions; π→π* and n→π*

• Carbonyl, nitriles, azo compounds, nitro compounds etc. are the example of such chromophores.

Auxochromes

• An auxochrome can be defined as any group which does not itself act as a chromophore but whose presence brings about a shift of the absorption band towards the longer wavelength of the spectrum.

• –OH,-OR,-NH₂,-NHR, -SH etc. are the examples of auxochromic groups.

Chromophore characteristics

Chromophore	Example	Excitation	λmax (nm)	Solvent
C = C	Ethene	π → π*	171	Hexanes
C = O	Ethanal	π → π* n → π*	180 290	Hexane
N = O	Nitromethane	π → π* n → π*	200 275	Hexane

Terminology for Absorption Shifts

Nature of the Shift	Descriptive Term
To Greater Absorbance	Hyperchromic
To Lesser Absorbance	Hypochromic
To Longer Wavelength	Bathchromic or Red Shift
To Shorter Wavelength	Hypsochromic or Blue Shift

n àp* and p àp* Transitions

• Most applications of absorption spectroscopy are based upon transitions for n or p electrons to the p* excited state

• 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 p orbitals.

• The molar absorptivities for peaks associated with excitation to the n, p* state are generally low and ordinarily range from 10 and 100 L cm^-1 mol ^-1;

• Values for p àp* transitions, on the other hand, normally fall in the range between 1000 and 10,000.

Effect of Conjugation of Chromophores

• p 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 p* 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 displaced to a longer wavelength by 20 nm compared with the corresponding peak for an unconjugated diene.

Absorption Involving d and f Electrons

• Most transition-metal ions absorb in the ultraviolet or visible region of the spectrum.

• For the lanthanide and actinide series, the absorption process results from electronic transition of 4f and 5f electrons

• For elements of the first and second transition-metal series, the 3d and 4d electrons are responsible.

Absorption by Lanthanide and Actinide Ions

• The ions of most lanthanide and actinide elements absorb in the ultraviolet and visible regions.

• Their spectra consist of narrow, well-defined, and characteristic absorption peaks.

• The transitions responsible for absorption by elements of the lanthanide series involve the various energy levels of 4f electrons, while those of 5f electrons of the actinide series

Absorption by Elements of the First and Second Transition-Metal Series

• The ions and complexes of the first two transition series tend to absorb visible radiation in one if not all of their oxidation states.

• The absorption bands are often broad and are strongly influenced by chemical environmental factors.

• The spectral characteristics of transition metals involve electronic transitions among the various energy levels of d orbitals.

Charge-Transfer Absorption

• Species that exhibit charge-transfer absorption are of particular importance because their molar absorptivities are very large (e_max> 10,000).

• These complexes provide a highly sensitive means for detecting and determining absorbing species.

• Complexes exhibit charge transfer absorption are called charge-transfer complexes.

• In order for a complex to exhibit a charge-transfer spectrum, it is necessary for one of its components to have electron-donor characteristics and for the other component to have electron-acceptor properties.

• Absorption of radiation then involves transfer of an electron from the donor to an orbital that is largely associated with the acceptor.

APPLICATION OF ABSORPTION MEASREMENT TO QUALITATIVE ANALYSIS

• Methods of Plotting Spectral Data: Several different types of spectral plots are encountered in qualitative molecular spectroscopy. The ordinate is most commonly percent transmittance, absorbance, log absorbance, or molar absorptivity. The abscissa is usually wavelength or wavenumber, although frequency is occasionally employed.

Solvent effects

• 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.

• The same solvent must be used when comparing absorption spectra for identification purposes.