1 SPECTROSCOPY PHAM VAN HUNG, PhD INTRODUCTION • The study how the chemical compound interacts with different wavelenghts in a given region of electromagnetic radiation is called spectroscopy or spectrochemical analysis. • The collection of measurements signals (absorbance) of the compound as a function of electromagnetic radiation is called a spectrum. Spectroscopy Utilises the Absorption and Emission of electromagnetic radiation by atoms Absorption: Low energy electrons absorb energy to move to higher energy level Emission: Excited electrons return to lower energy states Absorption vs. Emission Ground State 1st 2nd 3rd Energy is absorbed as electrons jump to higher energy levels Energy is emitted by electrons returning to lower energy levels Excited States Spectroscopic Techniques • UV-Visible Spectroscopy (UV-Vis). • Infrared Spectroscopy (IR) • Atomic Absorption Spectroscopy (AAS). • Colorimetry. UV radiation and Electronic Excitations • The difference in energy between molecular bonding, non- bonding and anti-bonding orbitals ranges from 125-650 kJ/mole • This energy corresponds to electromagnetic radiation in the ultraviolet (UV) region, 100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum • For comparison, recall the electromagnetic spectrum: • Using IR we observed vibrational transitions with energies of 8-40 kJ/mol at wavelengths of 2500-15,000 nm UVX-rays IR γ-rays RadioMicrowave Visible 2 X-ray: core electron excitation UV: valance electronic excitation IR: molecular vibrations Radio waves: Nuclear spin states (in a magnetic field) Electronic Excitation by UV/Vis Spectroscopy : 3-D structure AnaylysisX-raysX-ray Crystallography Elemental Analysis X-raysX-Ray Spectroscopy Structure determinationRadio wavesFT-NMR Functional Group Analysis/quantIR/UVRaman Functional Group AnalysisIR/MicrowaveFT-IR Quantitative analysis Beer’s LawUV-vis regionAtomic Absorption Quantitative analysis/Beer’s LawUV-vis regionUV-vis Spectroscopic Techniques and Common Uses Different Spectroscopies • UV/Vis – electronic states of valence e/d-orbital transitions for solvated transition metals • Fluorescence – emission of UV/vis by certain molecules • FT-IR – vibrational transitions of molecules • FT-NMR – nuclear spin transitions • X-Ray Spectroscopy – electronic transitions of core electrons Dispersion of Polymagnetic Light with a Prism Polychromatic Ray Infrared Red Orange Yellow Green Blue Violet Ultraviolet monochromatic Ray SLIT PRISM Polychromatic Ray Monochromatic Ray • Prism - Spray out the spectrum and choose the certain wavelength (λ) that you want by slit. • In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation • If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed Electronic Excitation The absorption of light energy by organic compounds in the visible and ultraviolet region involves the promotion of electrons in σ, π, and n-orbitals from the ground state to higher energy states. This is also called energy transition. These higher energy states are molecular orbitals called antibonding. Energy σ * π * n π σ σ →σ* π→π* n →σ* n →π* Antibonding Antibonding N onbonding Bonding Bonding Electronic Molecular Energy Levels Energy σ * π * n π σ σ →σ* π→π* n →σ* n →π * Antibonding Antibonding N onbonding Bonding Bonding • For any bond (pair of electrons) in a molecule, the molecular orbitals are a mixture of the two contributing atomic orbitals; for every bonding orbital “created” from this mixing (s, p), there is a corresponding anti-bonding orbital of symmetrically higher energy (s*, p*). • The lowest energy bonding orbitals are typically the s; likewise, the corresponding anti- bonding s* orbital is of the highest energy. • p-orbitals are of somewhat higher energy, and their complementary anti-bonding orbital somewhat lower in energy than s*. • Unshared pairs lie at the energy of the original atomic orbital, most often this energy is higher than p or s (since no bond is formed, there is no benefit in energy). • The higher energy transitions (σ→σ*) occur a shorter wavelength and the low energy transitions (π→π*, n →π*) occur at longer wavelength. 3 Observed electronic transitions From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy: Energy σ∗ π σ π∗ n σ σ π n n σ ∗ π ∗ π ∗ σ ∗ π ∗ alkanes carbonyls unsaturated cmpds. O, N, S, halogens carbonyls Observed electronic transitions - Routine organic UV spectra are typically collected from 200-700 nm - This limits the transitions that can be observed: σ σ π n n σ ∗ π ∗ π ∗ σ ∗ π ∗ alkanes carbonyls unsaturated cmpds. O, N, S, halogens carbonyls 150 nm 170 nm 180 nm √ - if conjugated! 190 nm 300 nm √ UV 210 nm Double Bonds 233 nm Conjugated Diene 268 nm Conjugated Triene 315 nm Conjugated Tetraene Observed electronic transitions - Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N - A functional group capable of having characteristic electronic transitions is called a chromophore (color loving). - Chromophore is a functional group which absorbs a characteristic ultraviolet or visible region. CC CC CO CO H σ −> σ∗ 135 nm π −> π∗ 165 nm n −> σ∗ 183 nm weak π −> π∗ 150 nm n −> σ∗ 188 nm n −> π∗ 279 nm weak λ A 180 nm 279 nm CO Spectrum Spectrum Glass cell filled with concentration of solution (C) I I Light 0 Transmittance is defined as the ratio of the electromagnetic radiation’s power exiting the sample, I, to that incident on the sample from the source, I 0 , I I 0 T = An alternative method for expressing the attenuation of electromagnetic radiation is absorbance, A, which is defined as A = - Log T = - Log = Log I 0 I I I 0 Transmittance and Absorbance 4 Beer – Lambert Law • There is a logarithmic dependence between the transmission, T, of light through a substance and the product of the absorption coefficient of the substance, α, and the distance the light travels through the material, ℓ. • The absorption coefficient can, in turn, be written as a product of either a molar absorptivity (extinction coefficient) of the absorber, ε, and the molar concentration c of absorbing species in the material. • The molar absorptivity give, in effect, the probability that the analyte will absorb a photon of given energy. As a result, value for ε depend on the wavelength of electromagnetic radiation. Compound x has a unique e at different wavelengths. • Unit of ε : L*cm -1 *M -1 Steps in Developing a Spectrometric Analytical Method 1. Run the sample for spectrum 2. Obtain a monochromatic wavelength for the maximum absorption wavelength. 3. Calculate the concentration of your sample using Beer Lambert Equation: A = ε lc Wavelength (nm) Absorbance 0.0 2.0 200 250 300 350 400 450 Spectrophotometer An instrument which can measure the absorbance of a sample at any wavelength. Light Lens Slit Monochromator Sample Detector Quantitative Analysis Slits Instrument to measures the intensity of fluorescent light emitted by a sample exposed to UV light under specific conditions. Emit fluorescent light as energy decreases Ground state Sample 90 ° C Detector UV Light Source Monochromator Monochromator Antibonding Antibonding Nonbonding Bonding Bonding Energy σ π σ π σ −>σ π −>π ' ' ' ' ' n-> n σ n->π ' Electron's molecular energy levels Fluorometer The optics of the light source in UV-visible spectroscopy allow either visible [approx. 400nm (blue end) to 750nm (red end) ] or ultraviolet (below 400nm) to be directed at the sample under analysis (common range: 200 – 800 nm). UV/Vis Spectrophotometer 5 Cuvette UV Spectrophotometer Quartz (crystalline silica) Visible Spectrophotometer Glass, Plastic Light Sources UV Spectrophotometer 1. Deuterium (200-400 nm) Visible Spectrophotometer 1. Tungsten Lamp (350-2500 nm) UV Spectrometer Application Protein Amino Acids (aromatic) Pantothenic Acid Glucose Determination Enzyme Activity (Hexokinase) Visible Spectrometer Application Niacin Pyridoxine Vitamin B12 Metal Determination (Fe) Fat-quality Determination (TBA) Enzyme Activity (glucose oxidase) Flurometric Application Thiamin (365 nm, 435 nm) Riboflavin Vitamin A Vitamin C Standard Practice • Prepare standards of known concentration • Measure absorbance at λmax of solution at different concentration • Plot A vs. concentration • Obtain slope • Use slope (and intercept) to determine the concentration of the analyte in the unknown 6 Typical Beer’s Law Plot y = 0.02x 0 0.2 0.4 0.6 0.8 1 1.2 0.0 20.0 40.0 60.0 concentration (uM) A R 2 = 0.995 Characteristics of Beer’s Law Plots • One wavelength • Good plots have a range of absorbances from 0.010 to 1.000 • Absorbances over 1.000 are not that valid and should be avoided Chromophoric Structure Group Structure nm Carbonyl > C = O 280 Azo -N = N- 262 Nitro -N=O 270 Thioketone -C =S 330 Nitrite -NO2 230 Conjugated Diene -C=C-C=C- 233 Conjugated Triene -C=C-C=C-C=C- 268 Conjugated Tetraene -C=C-C=C-C=C-C=C- 315 Benzene 261 Practice Examples 1. Calculate the Molar Extinction Coefficient E at 351 nm for aquocobalamin in 0.1 M phosphate buffer. pH = 7.0 from the following data which were obtained in 1 Cm cell. Solution C x 10 5 M Io I A 2.23 100 27 B 1.90 100 32 2. The molar extinction coefficient (E) of compound riboflavin is 3 x 10 3 Liter/Cm x Mole. If the absorbance reading (A) at 350 nm is 0.9 using a cell of 1 Cm, what is the concentration of compound riboflavin in sample? 3. The concentration of compound Y was 2 x 10 -4 moles/liter and the absorption of the solution at 300 nm using 1 Cm quartz cell was 0.4. What is the molar extinction coefficient of compound Y? 4. Calculate the molar extinction coefficient E at 351 nm for aquocobalamin in 0.1 M phosphate buffer. pH =7.0 from the following data which were obtained in 1 Cm cell. Solution C x 10 5 M I0 I A 2.0 100 30 Spectroscopy Homework 1. A substance absorbs at 600 nm and 4000 nm. What type of energy transition most likely accounts for each of these absorption processes? 2. Complete the following table. [X](M) Absorbance Transmittance(%) E(L/mole-cm) L(cm) 30 2000 1.00 0.5 2500 1.00 2.5 x 10 -3 0.2 1.00 4.0 x 10 -5 50 5000 2.0 x 10 -4 150 [X](M) = Concentration in Mole/L 7 3. The molar absorptivity of a pigment (molecular weight 300) is 30,000 at 550 nm. What is the absorptivity in L/g-cm. 4. The iron complex of o-phenanthroline (Molecular weight 236) has molar absorptivity of 10,000 at 525 nm. If the absorbance of 0.01 is the lowest detectable signal, what concentration in part per million can be detected in a 1-cm cell? . > C = O 28 0 Azo -N = N- 26 2 Nitro -N=O 27 0 Thioketone -C =S 330 Nitrite -NO2 23 0 Conjugated Diene -C=C-C=C- 23 3 Conjugated Triene -C=C-C=C-C=C- 26 8 Conjugated. unknown 6 Typical Beer’s Law Plot y = 0.02x 0 0 .2 0.4 0.6 0.8 1 1 .2 0.0 20 .0 40.0 60.0 concentration (uM) A R 2 = 0.995 Characteristics of Beer’s Law Plots •