Ultraviolet visible spectroscopy
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This document discusses ultraviolet-visible (UV-Vis) spectroscopy. It defines key terms like chromophore and provides background on Beer's law and how UV-Vis spectra are measured. Empirical rules are presented for calculating absorption wavelengths for conjugated systems like dienes, carbonyl compounds, and substituted benzene derivatives based on structural features. Common solvents used in UV-Vis spectroscopy and their minimum usable wavelengths are also listed.
Ultraviolet visible spectroscopy
- 2. Background Information UV/Vis Spectroscopy Terminology The following definitions are useful in a discussion of UV/Vis spectroscopy. chromophore Any group of atoms that absorbs light whether or not a color is thereby produced. auxochrome A group which extends the conjugation of a chromophore by sharing of nonbonding electrons. bathochromic shift The shift of absorption to a longer wavelength. hypsochromic shiftThe shift of absorption to a shorter wavelength. hyperchromic effect An increase in absorption intensity. hypochromic effect A decrease in absorption intensity.
- 3. UV/Vis Spectroscopy Laws of Light Absorption Beer-Lambert Law The ultraviolet spectra of compounds are usually obtained by passing light of a given wavelength (monochromatic light) through a dilute solution of the substance in a non-absorbing solvent. The intensity of the absorption band is measured by the percent of the incident light that passes through the sample: % Transmittance = (I / I0) * 100% where: I = intensity of transmitted light I0 = intensity of incident light Because light absorption is a function of the concentration of the absorbing molecules, a more precise way of reporting intensity of absorption is by use of the Beer-Lambert Law: Absorbance = -log(I / I0) = εcl where: ε = molar absorptivity c = molar concentration of solute l = length of sample cell (cm)
- 4. UV/Vis Spectroscopy Measurement of the Spectrum The UV spectrum is usually taken on a very dilute solution (1 mg in 100 ml of solvent). A portion of this solution is transferred to a silica cell. A matched cell containing pure solvent is prepared, and each cell is placed in the appropriate place in the spectrometer. This is so arranged that two equal beams of light are passed, one through the solution of the sample, one through the pure solvent. The intensities of the transmitted light are then compared over the whole wavelength range of the instrument. The spectrum is plotted automatically as a log10(I0/I) ordinate and λ abscissa. For publication and comparisons these are often converted to an ε vs. λ or log(ε) vs. λ plot. The λ unit is almost always in nanometers (nm). UV/Vis Spectroscopy Choice of Solvent The table below gives a list of common solvents and the minimum wavelength from which they may be used in a 1 cm cell. Solvent Minimum Wavelength (nm) acetonitrile 190 water 191 cyclohexane 195 hexane 195 methanol 201 ethanol 204
- 5. ether 215 methylene chloride 220 chloroform 237 carbon tetrachloride 257 اﻻﻣﺘﺼﺎص ﻣﻮﺟﺔ ﻃﻮل ﻟﺤﺴﺎب اﻟﻨﻈﺮﻳﺔ اﻟﻘﻮاﻋﺪ Empirical Rules for Caluclating Uv/Vis Absorptions -------------------------------------------------------------------- UV/Vis Spectroscopy Woodward-Fieser Rules for Dienes Woodward-Fieser Rules for Dienes Homoannular (cisoid) Heteroannular (transoid) Parent λ=253 nm λ=214 nm =217 (acyclic) Increments for: Double bond extending conjugation 30 30 Alkyl substituent or ring residue 5 5 Exocyclic double bond 5 5 Polar groupings: -OC(O)CH3 0 0
- 6. -OR 6 6 -Cl, -Br 5 5 -NR2 60 60 -SR 30 30 Example 1: Transoid: 217 nm Alkyl groups or ring residues: 3 x 5 = 15 nm Calculated: 232 nm Observed: 234 nm Example 2: Cisoid: 253 nm Alkyl groups or ring residues: 2 x 5 = 10 nm Calculated: 263 nm Observed: 256
- 7. nm Example 3: Transoid: 214 nm Alkyl groups or ring residues: 3 x 5 = 15 nm Exocyclic double bond: 5 nm Calculated: 234 nm Observed: 235 nm Example 4: Cisoid: 253 nm Alkyl groups or ring residues: 4 x 5 = 20 nm Exocyclic double bond: 5 nm Calculated: 278 nm Observed: 275
- 8. nm UV/Vis Spectroscopy Woodward's Rules for Conjugated Carbonyl Compounds Woodward's Rules for Conjugated Carbonyl Compounds Base values: X = R Six-membered ring or acyclic parent enone λ=215 nm Five-membered ring parent enone λ=202 nm Acyclic dienone λ=245 nm X = H λ=208 nm X = OH, OR λ=193 nm Increments for: Double bond extending conjugation 30 Exocyclic double bond 5 Endocyclic double bond in a 5- or 7- membered ring for X = OH, OR 5 Homocyclic diene component 39 Alkyl substituent or ring residue α 10 β 12 γ or higher 18 Polar groupings: -OH α 35 β 30 δ 50 -OC(O)CH3 α,β,γ,δ 6
- 9. -OCH3 α 35 β 30 γ 17 δ 31 -Cl α 15 β,γ,δ 12 -Br β 30 α,γ,δ 25 -NR2 β 95 Solvent correction* : variable λmax (calc'd) total * Solvent shifts for various solvents: Solvent λmax shift (nm) water + 8 chloroform - 1 ether - 7 cyclohexane - 11 dioxane - 5 hexane - 11 Example 1: Acyclic enone: 215 nm α-Alkyl groups or ring residues: 10 nm β-Alkyl groups or ring residues: 2 x 12 = 24 nm
- 10. Calculated: 249 nm Observed: 249 nm Example 2: Five-membered ring parent enone: 202 nm β-Alkyl groups or ring residues: 2 x 12 = 24 nm Exocyclic double bond: 5 nm Calculated: 231 nm Observed: 226 nm Example 3: Six-membered ring or alicyclic parent enone: 202 nm Extended conjugation: 30 nm Homocyclic diene component: 39 nm δ-Alkyl groups or ring residues: 2 x 12 = 18 nm Calculated: 302 nm Observed: 300 nm Example 4:
- 11. Five-membered ring parent enone: 202 nm α-Br: 25 nm β-Alkyl groups or ring residues: 2 x 12 = 24 nm Exocyclic double bond: 5 nm Calculated: 256 nm Observed: 251 nm Example 5: Carboxylic acid: 193 nm α-Alkyl groups or ring residues: 10 nm β-Alkyl groups or ring residues: 12 nm Calculated: 215 nm Observed: 217 nm Example 6: Ester: 193
- 12. nm α-Alkyl groups or ring residues: 10 nm β-Alkyl groups or ring residues: 12 nm Endocyclic double bond in 7-membered ring: 5 nm Calculated: 220 nm Observed: 222 nm Example 7: Aldehyde: 208 nm α-Alkyl groups or ring residues: 10 nm β-Alkyl groups or ring residues: 2 x 12 = 24 nm Calculated: 242 nm Observed: 242 nm Example 8:
- 13. Aldehyde: 208 nm Extended conjugation: 30 nm Homodiene component: 39 nm α-Alkyl groups or ring residues: 10 nm δ-Alkyl groups or ring residues: 18 nm Calculated: 304 nm Observed: 302 nm UV/Vis Spectroscopy Absorption for Mono-Substituted Benzene Derivatives Absorption for Mono-Substituted Benzene Derivatives Substituent E K B R (ε>30000) (ε~10000) (ε~300) (ε~50) Electronic Donating Substituents none 184 204 254 -R 189 208 262 -OH 211 270 -OR 217 269 -NH2 230 280
- 14. Electronic Withdrawing Substituents -F 204 254 -Cl 210 257 -Br 210 257 -I 207 258 -NH3 + 203 254 π-Conjugating Substituents -C=CH2 248 282 -CCH 202 248 278 -C6H5 250 -CHO 242 280 328 -C(O)R 238 276 320 -CO2H 226 272 -CN 224 271 -NO2 252 280 330 Absorption for Di-Substituted Benzene Derivatives Absorption for Di-Substituted Benzene Derivatives R R' Orientation K B λmax εmax λmax εmax -OH -OH ortho 214 6000 278 2630 -OR -CHO ortho 253 11000 319 4000 -NH2 -NO2 ortho 229 16000 275 5000 -OH -OH meta 277 2200 -OR -CHO meta 252 8300 314 2800 -NH2 -NO2 meta 235 16000 373 1500 -OH -OH para 225 5100 293 2700
- 15. -OR -CHO para 277 14800 -NH2 -NO2 para 229 5000 375 16000 -Ph -Ph meta 251 44000 -Ph -Ph para 280 25000 In disubstituted benzenes, two situations are important: • When electronically complementary groups, such as amines and nitro, are situated para to each other, there is a pronounced shift to longer wavelength in the main absorption band. • Alternatively, when two groups are situated ortho or meta to each other or when the para disposed groups are not electronically complementary, then the observed spectrum is usually closer to that of the separate, noninteracting chromophores.