UV-VIS SPECTROSCOPYGeneral info: Spectrometry is based on the absorption of photons by the analyte. Spectrometric method: the sample solution absorbs electromagnetic radiation from an appropriate source and the amount absorbed is related to the concentration of the analyte in the solution. Eg: copper solution is blue because it absorbs the yellow color from white light and transmits the remaining blue light. The higher[cu], the more yellow light is absorbed and the deeper the resulting blue color of the solution. The electromagnetic spectrum- Review A form of radiant energy that is propagated as a transverse wave Vibrate perpendicular to the direction of propagation - a wave motion to the radiation λ = wavelength in cm ѵ = frequency in s-1 or Hz c = velocity of light (3x1010 cm/s) The wavelength varies from a few Å to several meters. ṽ in cm-1 c v .Continue… The relationship between wavelength and frequency: . λ for UV & Vis – nanometer λ for IR – micrometer. commonly used as wave number. greater energy UV from the sun burns you!!!! .Continue… Electromagnetic radiation possesses a certain amount of energy called photon. 62 10 Joule sec h = Plank’s constant. Note: Shorter wavelength. related to the v or by: E hv hc E energy of the photon in ergs 34 6 . 5 μm 2.5-15 μm The most analytically region .78-2.Working ranges of the UV-Vis and IR spectra UV Vis Near IR Mid IR 200-380 nm 380-780 nm 0. The molecules may then absorb a discrete amount of energy and be raised to a higher vibrational energy level. and the energy of this vibration occurs at definite quantized levels. the energy of rotation being a definite energy levels. all involve raising the molecule to a higher internal energy level. hv . Three type of internal energy are quantized. UV-VIS spectroscopy principal/theory . Electron transition: the electron of a molecule may be raised to higher electron energy. the increase in energy being equal to the energy of the absorbed radiation. Vibrational transition: the atoms or groups of atoms within a molecule vibrate relative to each other. so the molecule may absorb radiation and be raised to a higher rotational energy level. Rotational transition: the molecule rotates about various axes.How does matter absorb radiation Three basic processes by which a molecule can absorb radiation. Energy level diagram illustrating energy changes associated with absorption of electromagnetic radiation A = pure rotational changer (far IR) B = rotational + vibrational changes (near IR) C = rotational + vibrational + electronic transition (Vis + UV) Eo = electronic ground state E1 = first electronic exited state . iii. and halogens. These are the most readily excited and are responsible for a majority of electronic spectra in the UV-VIS regions. Paired non bonding outer – shell electron (n electrons) such as those on N. S. bonds and functional groups within the molecule Kinds of transitions: Electrons in a molecule can be classified into 4 different types: i. Electron in orbitals. These are less tightly held then electrons and can be excited by visible or UV radiation iv. in = or ≡ bonds. Closed shell electron – not involved in bonding ii. * and n electrons responsible for UV-VIS spectrum .Electronic Spectra and Molecule Structure The electronic transition in the UV-VIS regions are due to the absorption of radiation by specific types of: Groups. O. Covalent single bond electron. state energy levels (either * or * ). Therefore. The most common transition: n n * or * * above 200 nm occur at very short λ . these correspond to excited.Other electron remain/occupate in orbitals A molecule also possesses unoccupied orbitals called antibonding orbitals. absorption of radiation results in an electronic transition to antibonding orbitals. HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital Example. molecule H2 . Example of transition in ketone (acetone) * and n * * n * * is higher than The probability of * n Molar absorptivities. * (1000-100.000) * (less than 1000) n . amine (-NH2). . Example: hydroxyl (OH).ABSORPTION BY ISOLATED CHROMOPHORES The absorbing groups (the electrons responsible for the absorption) in a molecules are called chromophores A molecule containing a chromophore – chromogen The present of auxochrome enhance the absorption by a chromophore or shift the λ of absorption. halogens (X) posses n electrons – interact with electrons in the chromophores (n ) conjugation. Eg.max shifted to shorter wavelength. It may caused by removal of conjugation.Spectra changes can be classed as: Bathochromic shift (red shift) – max shifted to shorter wavelength. Hypsochromic shift (Blue shift) . It may caused by removal of conjugation. The conjugation of the lone pair electrons on the N atom on aniline with the bond system of the benzene ring is removed on protonation Neutral aniline absorbs at max= 230 nm = 8600 But in acid solution (protonated) max = 203 nm = 7500 . Spectra changes can be classed as: continue…. Hyperchromic effect – increased absorption intensity ( ↑) Hypochromic effect – decreased absorption intensity ( ↓) Note: max = the wavelength of the absorption maximum 1% 1cm = Absorption [ log ( I I ) ] of a 1% solution in a cell with 1 cm pathlength.used when MW of a compound is not known 10 0 . BEER’S LAW-ABSORPTION LAW Fraction of radiant energy transmitted decays exponentially with pathlength. Incident radiation Transmitted radiation I0 Pathlength c I b k concentration Transmittance in exponential form: I T 10kb I0 log T log is a constant (eq 1) Putting in logarithmic form. I kb I0 (Eq 2 . I a is combined constant of k and k’ (Eq 5) 10abc I0 I log T log abc I0 1 I0 Omit the –ve sign. describes the dependence of T on the pathlength and concentration.BEER’S LAW-ABSORPTION LAW…continue Similar law holds for the dependence of T on the concentration. T I 10k 'c I0 c k ' is a new constant (Eq 4) (Eq 3) log T log I k ' c I0 Combining (Eq 1) and (Eq 3). and rearrange… A log T log log abc T I T Hence. A abc Conc in g/L Pathlength in cm Absorbance Absorptivity . the absorbance varies with wavelength in direct proportion to a (b & c are held constant).BEER’S LAW-ABSORPTION LAW…continue Note: in absorption spectrum. (cm 1mol 1L) Thus A bc . The product of the absorptivity and the molecular weight of the absorbing species is called the molar absorptivity. Example Amines. If this final solution exhibit an absorbance of 0. A 1 ml aliquot of this is diluted to 250 ml for measurement.00mL 9.00 cm cell.25 104 cm 1mol 1L 1.454 at 359 nm using a 1.1155 g) is dissolved in water and dilute to 100 mL. An unknown amine (0.63 105 mol / L (3. RNH2 react with picric acid to form amine picrates.65 105 mol / L)(0.08 104 mol in original flask 0.00cm c c 3. what is the formula weight of the amine? What is a probable formula? Solution: A bc 0.1155g 1 127 gmol 9. which absorb strongly at 359 nm (ε=1.08 104 mol .454 1.25 x 104 ).250L) 100mL 1. Portion of the solution is transferred to a cuvett quartz of 1 cm (Styrene.Measurement of the UV Spectrum: Using a dilute solution: 1 mg of compound with MW (100-400 g/mol) is dissolved in 100 mL volumetric flask. MW = 104 g/mol max=250 nm . Hexane is used as a solvent. however electrons of the atom and solvent molecules may reorganise Transition state is more polar than ground state due to dipole-dipole interaction with solvents. Most commonly used – 95% ethanol Solvent Effect Electronic Transition. longer wavelength) .Choice of solvent Solvent consideration: cheap. good solvent transparent down to about 210 nm. π →π*: atom do not move. thus lower the energy of the excited state. Eg: Styrene in ethanol (max= 270 nm) Styrene in hexane (max= 250 nm) about 20 nm shift (red shift. Electronic transition. n → π* (the weak transition of the O lone pair in ketones .