1. Electronic Spectroscopy Ultraviolet and visible

2. Where in the spectrum are these transitions?

3. Why should we learn this stuff
Why should we learn this stuff? After all, nobody solves structures with UV any longer!
Many organic molecules have chromophores that absorb UV
UV absorbance is about 1000 x easier to detect per mole than NMR
Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology.
Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors
One wavelength may not be the best for all compound in a mixture.
Affects quantitative interpretation of HPLC peak heights

4. What are the nature of these absorptions?
Example:   * transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W):
h 170nm photon
LUMO g antibonding molecular orbital
HOMO u bonding molecular orbital

5. The UV Absorption process
  * and   * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis.
n  * and   * transitions: non-bonding electrons (lone pairs), wavelength (max) in the nm region.
n  * and   * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = nm.
Any of these require that incoming photons match in energy the gap corrresponding to a transition from ground to excited state.
Energies correspond to a 1-photon of 300 nm light are ca. 95 kcal/mol

6. How Do UV spectrometers work?
Rotates, to achieve scan
Matched quartz cuvettes
Sample in solution at ca M.
System protects PM tube from stray light
D2 lamp-UV
Tungsten lamp-Vis
Double Beam makes it a difference technique
Two photomultiplier inputs, differential voltage drives amplifier.

7. Experimental details What compounds show UV spectra?
Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers
Just heteroatoms are not enough but C=O are reliable
Transition metal complexes, inorganics
Solvent must be UV grade (great sensitivity to impurities with double bonds)
The NIST databases have UV spectra for many compounds

8. An Electronic Spectrum
Absorbance
Wavelength, , generally in nanometers (nm)
0.0
400
800
1.0
200
maxwith certain extinction  UV
Visible

9. Solvents for UV (showing high energy cutoffs)
Water 205
CH3CN 210
C6H
Ether 210
EtOH 210
Hexane 210
MeOH 210
Dioxane 220
THF 220
CH2Cl2 235
CHCl3 245
CCl4 265
benzene 280
Acetone 300
Various buffers for HPLC, check before using.

10. Organic compounds (many of them) have UV spectra
One thing is clear
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.

11. 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.

12. The Quantitative Picture
(power in) P
(power out)
Transmittance:
T = P/P0
Absorbance:
A = -log10 T = log10 P0/P
B(path through sample)
The Beer-Lambert Law (a.k.a. Beer’s Law):
A = ebc
Where the absorbance A has no units, since A = log10 P0 / P
e 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)

13. Interpretation of UV-Visible Spectra
Transition metal complexes; d, f electrons.
Lanthanide complexes – sharp lines caused by “screening” of the f electrons by other orbitals
One advantage of this is the use of holmium oxide filters (sharp lines) for wavelength calibration of UV spectrometers.
See Shriver et al. Inorganic Chemistry, 2nd Ed. Ch. 14

14. UV of Benzene in heptane
Group
K band ()
B band()
R band
Alkyl
208(7800)
260(220) --
-OH
211(6200)
270(1450)
-O-
236(9400)
287(2600)
-OCH3
217(6400)
269(1500)
NH2
230(8600)
280(1400) -F
204(6200)
254(900)
-Cl
210(7500)
257(170)
-Br -I
207(7000)
258/285(610/180)
-NH3+
203(7500)
254(160)
-C=CH2
248(15000)
282(740)
-CCH
248(17000)
278(6500
-C6H6
250(14000)
-C(=O)H
242(14000)
328(55)
-C(=O)R
238(13000)
276(800)
320(40)
-CO2H
226(9800)
272(850)
-CO2-
224(8700)
268(800)
-CN
224(13000)
271(1000)
-NO2
252(10000)
280(1000)
330(140)
Benzenoid aromatics
UV of Benzene in heptane
From Crewes, Rodriguez, Jaspars, Organic Structure Analysis