Chemistry Chapter 7: Spectroscopy

Spectroscopy

• The science that studies the interaction of radiant energy with matter is called spectroscopy.
• Spectrophotometry studies the absorption and/or emission of electromagnetic radiation by matter.
• Electromagnetic radiation has both particle and wave properties.
• Nuclear magnetic resonance (NMR) is an example of magnetic resonance spectroscopy.

Analytical Techniques

• Volumetric and gravimetric analysis are considered classical techniques.
• The method that relies on measuring a physical property related to the concentration of the analyte is known as instrumental analysis.
• One limitation of volumetric and gravimetric analysis is that they can't achieve a high degree of sensitivity.
• UV-Vis absorption spectroscopy is crucial in understanding how substances react to light.

Electromagnetic Radiation

• EMR has two fields: electric and magnetic, which are perpendicular to the direction of radiation propagation.
• The energy of a photon is given by E = hν = hc/λ, where h is Planck's constant.

Absorption and Emission

• When energy is absorbed, electrons can be elevated to excited electronic states.
• In the relaxation process, M (molecule) loses energy either as heat or radiation.
• The absorption of a molecule is the sum of absorption of all its chromophores.

Chromophores and Auxochromes

• Chromophores are unsaturated groups responsible for π → π* and n → π* electronic transitions.
• Auxochromes do not absorb radiation longer than 200 nm by themselves.
• Auxochromes can increase the extent of electronic interaction with nearby chromophores.

UV-Vis Spectroscopy

• The visible region of the electromagnetic spectrum extends from about 400 to 800 nm.
• The far ultraviolet region extends from 10 to 200 nm.
• The color wheel shows that colors directly opposite each other are said to be complementary colors.
• An absorption spectrum is a plot of the absorbance as a function of wavelength λ.
• Each substance has its characteristic absorption spectrum which depends on its molecular structure.

Wavelength and Absorption

• The more the number of double bonds, the higher the wavelength of maximum absorption.
• The shift of λmax to a longer wavelength is known as the bathochromic shift.
• The increase in the intensity of absorption is referred to as the hyperchromic effect.
• A decrease in the intensity of absorption is called the hypochromic effect.

Other Key Concepts

• Absorption of radiation is a process where chemical species gain certain quanta of EMR.
• The wavelength of a wave is the linear distance between two successive maxima or minima.
• In an alkaline medium, phenol exists predominantly as the phenolate form.
• An isosbestic point is a specific wavelength at which two chemical species have the same absorbance.
• An increase in temperature may cause a hyperchromic effect on some ions.
• The UV spectrum of aniline in acid medium shows a bathochromic shift.
• Dilution of dichromate solution changes its color from orange at 440 nm to yellow at 390 nm.