# Uvs can be very non specific its hard to interpret

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Uvs can be very non-specific Its hard to interpret except at a cursory level, and to say that the spectrum is consistent with the structure Each band can be a superposition of many transitions Generally we don’t assign the particular transitions.

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An Example--Pulegone Frequently plotted as log of molar extinction So at 240 nm, pulegone has a molar extinction of 7.24 x 10 3 Antilog of 3.86 O
Can we calculate UVs? Electronic Spectra Wavelength r Absorptivity (l/mol-cm) 220 230 240 250 260 270 280 290 300 0 10049 20097 30146 40194 50243 nacindolA Electronic Spectra Wa Molar Absorptivity (l/mol-cm) 220 230 240 250 260 270 280 290 300 0 10394 20789 31183 41578 51972 Na Semi-empirical (MOPAC) at AM1, then ZINDO for config. interaction level 14 Bandwidth set to 3200 cm -1

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The orbitals involved 200 210 220 230 240 250 260 270 280 290 300 0 11097 22195 33292 44390 55487 Showing atoms whose MO’s contribute most to the bands
The Quantitative Picture Transmittance: T = P/P 0 B(path through sample) P 0 (power in) P (power out) Absorbance: A = -log 10 T = log 10 P 0 /P The Beer-Lambert Law (a.k.a. Beer’s Law): A = bc Where the absorbance A has no units, since A = log 10 P 0 / P is the molar absorbtivity with units of L mol -1 cm -1 b is the path length of the sample in cm c is the concentration of the compound in solution, expressed in mol L -1 (or M , molarity)

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Beer-Lambert Law Linear absorbance with increased concentration--directly proportional Makes UV useful for quantitative analysis and in HPLC detectors Above a certain concentration the linearity curves down, loses direct proportionality--Due to molecular associations at higher concentrations. Must demonstrate linearity in validating response in an analytical procedure.
Polyenes, and Unsaturated Carbonyl groups; an Empirical triumph R.B. Woodward, L.F. Fieser and others Predict max for π  * in extended conjugation systems to within ca. 2-3 nm. Homoannular, base 253 nm heteroannular, base 214 nm Acyclic, base 217 nm Attached group increment, nm Extend conjugation +30 Addn exocyclic DB +5 Alkyl +5 O-Acyl 0 S-alkyl +30 O-alkyl +6 NR2 +60 Cl, Br +5

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Similar for Enones x O O X=H 207 X=R 215 X=OH 193 X=OR 193 215 202 227 239 Base Values, add these increments… Extnd C=C +30 Add exocyclic C=C +5 Homoannular diene +39 alkyl +10 +12 +18 +18 OH +35 +30 +50 OAcyl +6 +6 +6 +6 O-alkyl +35 +30 +17 +31 NR 2 S-alkyl Cl/Br +15/+25 +12/+30  With solvent correction of….. Water +8 EtOH 0 CHCl 3 -1 Dioxane -5 Et2O -7 Hydrcrbn -11
Some Worked Examples O Base value 217 2 x alkyl subst. 10 exo DB 5 total 232 Obs. 237 Base value 214 3 x alkyl subst. 30 exo DB 5 total 234 Obs. 235 Base value 215 2 ß alkyl subst. 24 total 239 Obs. 237

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Distinguish Isomers! HO 2 C HO 2 C Base value 214 4 x alkyl subst. 20 exo DB 5 total 239 Obs. 238 Base value 253 4 x alkyl subst. 20 total 273 Obs. 273
Generally, extending conjugation leads to red shift “particle in a box” QM theory; bigger box Substituents attached to a chromophore that cause a red shift are called “auxochromes” Strain has an effect… max 253 239 256 248

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Interpretation of UV-Visible Spectra Transition metal complexes; d, f electrons.
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