1 Spectroscopy  Atomic emission spectra  UV/Vis spectra  Infrared (IR)

2 Spectroscopy  Atomic emission spectrum. Recall.

3 Spectroscopy  An atomic light source may be obtained by burning an element over a flame (flame test). When this light hits a prism, the different wavelengths of light are refracted at different angles.

4 Spectroscopy  Atomic emission spectrum. Recall.  c = and E = h

5 Spectroscopy Absorption and Emission spectra Evidences to support the atomic model. 1. All atoms have a unique emission/absorption spectrum. This allows the identification of elements. 2. The spectra of the pure element is ALWAYS the same regardless of where is the spectrum obtained. This suggest that the separation of the energy levels for each atom is different but it is constant for that element. 3. Not all lines appear equally separated. It suggests that not all energy levels are equally separated. 4. The transition n=7 to n=1 releases more energy than the transition n=5 to n=1. Therefore the lines associated with these transitions will have different frequencies and wavelengths.

6 Spectroscopy Energy n=1 n=5 n=4 n=3 n=2 n=6 n=7 UV Vis IR

7 Ultraviolet/Visible or UV/Vis Spectroscopy It examines transitions in electronic energy levels. It is used in Lambert Beer’s law. wavelength (nm) absorbanceabsorbance The peaks correspond with energy transitions between MOs. Peaks at about 480 nm and 600 nm allows to calculate the energy transitions using c = and E = h

8 This compound absorbs in the UV section and it is colorless. Any colored molecule can be used in UV/Vis spectroscopy experiments, as they will absorb light from the visual spectrum. wavelength (nm) absorbanceabsorbance Ultraviolet/Visible or UV/Vis Spectroscopy

9 Recall Lambert Beer’s law A = a b c where: A is absorbance (amount of light absorbed by the sample) a: molar absorptivity (M -1 cm -1 ). Constant and specific to the atom, ion or molecule and to the wavelength. b: path length of sample (cm). Length from front to back of the cuvette that is used. Larger cuvettes absorb more as the light will encounter more particles. c: concentration (M). When concentration increases, absorbance increases as the light will encounter more particles. Ultraviolet/Visible or UV/Vis Spectroscopy

10 IR Spectroscopy  All covalent bonds in molecules experience vibrations within the bond. See animationSee animation  This vibrational frequency falls in the IR section of the spectrum.  Vibrational frequencies depend on the mass of the atoms and the strength of the bonds.  Recall frequency is related to wavelength. c =  Wavenumbers are the reciprocal of the wavelengths measured in cm -1.  IR spectra is often plotted with wavenumbers along the top and wavelengths along the bottom, both in the x axis.

11 IR Spectroscopy Bond typeRange of wavelengths (  m) Range of wavenumbers (cm -1 ) - C - H3.38 – = C - H3.23 – C = C5.95 – O - H2.74 – N - H2.94 – C – O7.69 – C = O5.56 – Molecules that have the same functional groups such as alcohols (O-H) or carboxylic acids (-COOH) can be identified through IR since they will have peaks within the same range. Every compound have a unique IR spectrum that allows its identification.

12 IR Spectroscopy O ║ Acetone CH 3 – C – CH 3

13 IR Spectroscopy

Visit the webcast below for a better understanding of this important technique. italServices/swf/ap- webcasts/chemistry/ap_chem_pe s.html italServices/swf/ap- webcasts/chemistry/ap_chem_pe s.html 14