Molecular Spectroscopy - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Molecular Spectroscopy - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Molecular Spectroscopy docsity.com Spectroscopy = methods based on the interaction of electromagnetic radiation (EM) and matter Electromagnetic Radiation = form of energy with both wave and particle properties EM moves through space as a wave Most interactions of EM with matter are best understood in terms of electric vector docsity.com Normally η increases with ν and this is referred to as “Normal Dispersion” When absorption occurs, the nature of the interaction changes η is a measure of the extent to which the electric vector interacts with the medium & slows down For a given frequency and medium, a larger η means more interaction with electric field & the medium is said to have greater polarizability i.e., is more able to follow the electric vector docsity.com A Typical Dispersion Curve Three trends can be seen 1) an overall decrease in η as ν increases This reflects the inability of the electric fields in the material to keep up with the oscillations of the EM (ν) (η ) docsity.com A Typical Dispersion Curve 2) regions where η increases with ν prior to an absorbtion band This is referred to as normal dispersion (ν) (η ) docsity.com Refractive indices (η) for various substances at 589 nm (the sodium D line) Substance η --------------- ------------ air 1.00027 water, 20 oC 1.33336 NaCl crystal 1.544 benzene 1.501 quartz (fused) 1.46 glass (crown) 1.52 ethyl alcohol 1.36 carbon disulfide 1.63 docsity.com Refraction = change in velocity of EM as it goes from one medium to another Normal to surface Medium 1 (air) Velocity larger η = 1.00 Medium 2 (glass) Velocity smaller η = 1.50 Incident ray Ф1 Ф2 Refracted ray Original direction Ray bent toward normal docsity.com Equation for Refraction sin Ф1 ν1 η2 if medium 1---------- = ----- = ------ = η2sin Ф2 ν2 η1 is air η1 = 1.0 Magnitude of the direction change (i.e., size of the angle depends on wavelength (shown in equation as ν) this is how a prism works Direction of bending depends on relative values of η for each medium. Going from low η to higher, the ray bends toward the normal. Going from higher η to lower the ray bends away from the normal. docsity.com Wave Equation y = A sin (ωt + α) Where A = amplitude ω = angular frequency α = phase angle t = time For a collection of waves the resulting position y at a given t can be calculated by y = A1 sin (ω1t + α1) + A2 sin (ω2t + α2) + … docsity.com Interference - amplitude of the resulting wave depends on phase difference α1 - α2 Constructive Interference ⇒waves add Destructive Interference ⇒ waves cancel docsity.com At α1 - α2 = 0o adding of waves gives Maximum Constructive Interference 0o 180o 360o 540o 720o 900o Wave 1 Wave 2 Resultant wave Phase angle difference between Wave 1 & Wave 2 is zero α1 - α2 = 0o A m pl itu de docsity.com Fourier Analysis – mathematical process of resolving a combination of waves of various frequencies into their individual frequencies. This requires a Fourier integral and is important in all Fourier Transform (FT) methods like FTIR and FT NMR. Requires complex mathematics and a computer to figure out amplitudes of various component frequencies. docsity.com Diffraction = EM going past an edge or through a slit (2 edges) tends to spread The combination of diffraction effects & interference effects are important in spectroscopy for 1)diffraction gratings 2) slit width considerations docsity.com Scattering = EM interacts with matter and changes direction, usually without changing energy This can be described using both the wave or particle nature of light: 1) Wave – EM induces oscillations in electrical charge of matter ⇒ resulting in oscillating dipoles which in turn radiate secondary waves in all directions = scattered radiation 2) Particle (or Quantum) – EM interacts with matter to form a virtual state (lifetime 10-14 s) which reemits in all directions. Raman effect = when some molecules return to a different state ⇒ change in frequency docsity.com