Slide 1 Group 5 Chapter 9 Ultraviolet and visible absorption spectrocopy Content UV /Vis spectral region The UV/Vis spectrum Electronic transition of organic compound Chromophore groups Solvent effect[.]
Group Chapter Ultraviolet and visible absorption spectrocopy Content UV /Vis spectral region The UV/Vis spectrum Electronic transition of organic compound Chromophore groups Solvent effects: Solvatochromism Fieser – Woodward rules Instrumentation in the UV/Visible UV/Vis Spectrophotometer Quantitative analysis: law of molecules obsorption Methods in quantitative analysis UV /Vis spectral region UV /Vis spectral region Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm The UV/Vis spectrum Transmittance (T) The transmittance (T) is a fraction of incident rations transmitted by the solution T = I / I0 I: Transmitted light I0: Incident light T is often expressed as percentage %T = I / Io x 100 The UV/Vis spectrum Absorbance: (A) Is the logarithmic ratio of intensity of incident light to that of transmitted light A = log Io/ I A = - log 10 T It is also known as optical density (O.D.) Electronic transition of organic compound Electronic transition of organic compound • There are several types of electronic transitions available to a molecule including: – σ to σ* (alkanes) – σ to π * (carbonyl compounds) – π to π* (alkenes, carbonyl compounds, alkynes, azo compounds) – ή to σ * (oxygen, nitrogen, sulfur, and halogen compounds) – ή to π * (carbonyl compounds) Example Chromophore groups • Chromophore: A covalently unsaturated group responsible for electronic absorption Or Any group of atoms that absorbs light whether or not a color is thereby produced Eg C=C, C=O, NO2, amine • There are two types of chromophore – Independent chromophore: single chromophore is sufficient to import color to the compound eg Azo group – Dependent chromophore: When more then one chromophore is required to produce color Eg acetone having kentone group is colorless where as diacetyl having two kentone group is yellow Chromophore groups Conjugated chromophore system Azo Compound ... percentage %T = I / Io x 10 0 The UV/Vis spectrum Absorbance: (A) Is the logarithmic ratio of intensity of incident light to that of transmitted light A = log Io/ I A = - log 10 T It is also known