ASR UV-Vis Spectroscopy Tutorial

2025 JC2 H3 Chemistry Tutorial ©2025/ASRJC/CHEM 2 C O Ultraviolet/Visible Spectroscopy 1 Explain the underlying principles of UV/VIS spectroscopy. Details of instrumentation are not required. When a photon of UV light is absorbed by a molecule, an electron gains the photon’s energy and is promoted to a higher energy orbital. The lowest energy electronic transition is that between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Absorption in the UV/VIS region corresponds to →  *, n→  * and n→* transitions. 2 Organic molecules, such as propanone, (CH3)2CO, absorb energy in the uv/visible region of the spectrum as a result of electronic transitions. (a) Draw an energy level diagram to show the electronic transiti ons which cause absorptions in propanone. p* n p (b) Diphenylmethanone, shown below, also absorbs in the uv/visible region of the spectrum. Predict where, relative to the absorptions shown by propanone, diphenylmethanone will absorb energy. Explain your answer. Conjugation of π bonds from the two benzene rings reduces the energy gap between the π and the π* orbitals and between n and the π* orbitals , and hence shifts the absorption towards longer wavelength for diphenylmethanone as compared with propanone. 3 (a) A solution of guanine of concentration 2 x 10–4 mol dm–3 in a 1.00 cm path length cell has an absorbance of 1.56 at 275 nm. What is the molar extinction coefficient of guanine at this wavelength? Using Beer’s Law, A275nm = 275nmlc 275nm = A275nm ÷ lc = 1.56 ÷ (1 cm × 2 x 10-4 mol dm-3) = 7800 dm3 mol-1 cm-1 (b) A solution of quinone (molar extinction coefficient of quinone = 24 000 dm3 mol-1 cm-1) had an absorbance of 0.574 in a 1.0 cm cell at 242 nm. What is the concentration of the solution? Using Beer’s Law, A242nm = 242nmlc [Quinone] = A242nm ÷ (242nm × l) = 0.574 ÷ 24000 = 2.39 x 10-5 mol dm-3

2025 JC2 H3 Chemistry Tutorial 2025/ASRJC/CHEM 3 4 The amount of losartan (C22H22KN6ClO) in a table t may be estimated by UV spectroscopy. The molar extinction coefficient for losartan sho uld be taken as 1980 dm 3 mol-1 cm-1 at a wavelength of 254 nm. The absorbance A of a solution is given by the expression A = cl. losartan (a) Explain why losartan absorbs UV radiation. Losartan absorbs UV radiation due to the following elec tronic transitions between MOs with energy gap that coincides with energy of UV radiation : • Losartan shows n→* transitions since it contains atoms like Cl and O with lone pair of electrons in saturated bonds. • Losartan shows n→* transitions since it contains atoms like N with lone pairs of electrons that form unsaturated bonds with C or N. • Losartan shows  →* transitions as it has unsaturated N=N, C=N and C=C bonds. Furthermore, the presence of conjugated  bonds reduce