UV-vis Spectroscopy - CHE-10063

Electronic Spectroscopy Electronic spectroscopy involves measuring the absorption and emission of EMR caused by electrons moving between orbitals. Absorptions are broad: – Time scales: absorption of a photon is quick (10-18 s) – Quicker than molecular vibration or rotation – Orbi...

Electronic Spectroscopy Electronic spectroscopy involves measuring the absorption and emission of EMR caused by electrons moving between orbitals. Absorptions are broad: – Time scales: absorption of a photon is quick (10-18 s) – Quicker than molecular vibration or rotation – Orbital energy depends on molecular geometry – A range of energies required for excitation of electron as molecules are vibrating and rotating – Broad Absorption UV-visible Spectroscopy: Chromophores A chromophore is a group of atoms within a molecule that is responsible for its colour. Coloured compounds arise because visible light is absorbed by the electrons in the chromophore, which are then promoted to a higher energy molecular orbital. By comparing chromophores, we can find out about the energy of light that is being absorbed. Coloured Molecules LYCOPENE Tomatoes λmax = 469 nm -CAROTENE -CAROTENE Carrots Carrots and andMangoes Mangoes λλmax = 452 nm max = 452 nm  More on this later Conjugated Systems in semester! Organic molecules that are coloured contain delocalised electrons spread over a number of atoms are said to be conjugated. β-carotene  More on this later Conjugated Systems in semester! The electrons are delocalised through the molecular orbital along the length of the conjugated system. The more atoms that are spanned by the delocalised electrons, the smaller the energy gap between the delocalised orbital and the next unoccupied orbital. Exciting the electrons will require less energy (longer wavelegths). If this falls within the visible range, then the compound are coloured. – If they require higher energy – possibly UV Conjugated Systems Larger chromophores absorb lower energy (higher wavelength) Number of C=C Main colour Colour Compound in conjugated absorbed compound system appears Vitamin A 5 Violet Yellow β-carotene 11 Blue Orange Lycopene 11 Green Red Complimentary Colours The colours we see are NOT absorbed by the molecule. If the chromophore absorbs light of one colour, then the complementary colour is observed. Main colour Colour compound Compound absorbed appears Vitamin A Violet Yellow β-carotene Blue Orange Lycopene Green Red  More on this in Transition Metal Complexes SEM2! Conjugation is not the only cause of colour. In transition metal complexes, electrons moving between d-orbitals on the metal or between orbitals on the ligands and the metals lead to absorption or emission of light. In these cases, the chromophore is comprised of the metal and the donor atoms on the ligands. Transition Metal Complexes UV-vis Spectroscopy UV-vis Spectrophotometer: – Light (ultraviolet and/or visible) passes through a sample – Intensity of light of each wavelength measured – Absorption or transmission of light plotted It I0 D S E A T M E P C L T E O R UV-vis Spectroscopy: practical considerations Sample holder is called a cuvette (plastic, glass or quartz) Samples are usually in solution using a solvent that does not absorb UV or visible light UV visible 4 3.5 Quartz Cuvette 3 Glasscuvette absorbance 2.5 2 1.5 1 0.5 0 250 350 450 550 650 wavelength/nm  Consider these UV-vis Lab Measurements tips for your practical module! Record calculations for concentration Note down solvent, and what was used as a blank Note down instrument settings For spectra and Beer-Lambert plots: Label axes Use appropriate units Use legend if multiple spectra on one graph Insert into lab diary / report Spectra Spectrum (plural: spectra): the graphical representation of the signal of interest (e.g. absorbance) as a function of wavelength (or frequency). 0.5 0.4 Absorbance 0.3 0.2 0.1 0 200 250 300 350 400 Wavelength/nm Describing Spectra Absorption (A) bands may be broad and are described in terms of max and  max is the wavelength corresponding to the maximum absorption  is the molar absorption max 0.5 coefficient at that wavelength 0.4 (mol-1 dm3 cm-1) Absorbance 0.3 0.2 0.1 0 200 250 300 350 400 Wavelength/nm Molar Absorption Coefficient () The absorption of light is related to how many molecules there are present in solution. The Molar Absorption Coefficient is corrected for the number of molecules. It is constant at a specified wavelength (max) This allows values of  to be compared easily. Maximum value for  is ~105 dm3 mol-1 cm-1 Found in chlorophyll and some sunscreens  It Beer-Lambert Law I0 D S E A T M E P C L T E O R ( ) Io 𝐼𝑡 𝐼𝑡 𝑇= %𝑇= × 100 light intensity (I) 𝐼𝑜 𝐼𝑜 Io It l cuvette path length (cm) It at a fixed wavelength () l Sample depth Beer-Lambert Law The intensity of the light falls exponentially with increasing sample concentration, when it goes through an absorbing medium. Io It(i) Io light intensity (I) l It(i) Io It(ii) l It(ii) (i) (ii) Sample concentration Beer-Lambert Law The relationship between the transmittance and the sample path length (l) and sample concentration (c) is: 𝐼𝑡 − 𝜀 𝑐𝑙 𝐼𝑜 −1 𝜀𝑐𝑙 =𝑇 =1 0 𝑜𝑟 =𝑇 =1 0 𝐼𝑜 𝐼𝑡 molar absorption cl coefficient (dm3 mol-1 cm-1) path length (cm) concentration (mol dm-3)  Beer-Lambert Law 𝐼0 −1 𝜀 𝑐𝑙 =𝑇 =10 𝐼𝑡 ( ) 𝐼𝑜 log =𝜀 𝑐𝑙= 𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝐼𝑡  UV-vis calculations This is the spectrum a 4.6 x 10-4 mol dm-3 solution of DTMB, taken using a 1 cm cuvette What is the value of max? What is the transmittance (T) at max? Estimate the molar absorption coefficient () at max Beer-Lambert Plot A graph of A against c is a Beer-Lambert Plot The gradient (slope) of a Beer-Lambert Plot is the molar absorption coefficient and the path length (l) It has a theoretical y-intercept of zero Beer-Lambert Plot gradient Absorbance Concentration Beer-Lambert Plot y-axis is absorption (no units) x-axis is concentration (units: mol dm-3) Best fit line (trendline) used, individual data points plotted Trendline has equation and R2 value added if Excel is used There is a y-intercept! (theoretically it should be zero) Beer-Lambert Plot mx+b Absorbance Concentration / mol dm-3 UV-vis calculations What is the molar absorption coefficient for the Fe complex depicted in the Beer-Lambert plot below?