Theory of Ultraviolet-Visible (UV-Vis) Spectroscopy
Ultraviolet and visible radiation interacts with matter which causes electronic transitions (promotion of electrons from the ground state to a high energy state). The ultraviolet region falls in the range between 190-380 nm, the visible region fall between 380-750 nm.
The following electronic transitions are possible:
π- π* (pi to pi star transition)
n - π* (n to pi star transition)
σ - σ * (sigma to sigma star transition)
n - σ * (n to sigma star transition)
and are shown in the below hypothetical energy diagram
The σ to σ* transition requires an absorption of a photon with a wavelength which does not fall in the UV-vis range (see table 2 below). Thus, only π to π* and n to π* transitions occur in the UV-vis region are observed.
UV-Vis analysis of Tetraphenylcyclopentadienone
The UV-vis spectrum of tetraphenyclopentadienone is given below and should be similar to the one you obtained from lab. The "n" electrons (or the nonbonding electrons) are the ones located on the oxygen of the carbonyl group of tetraphenyclopentadienone. Thus, the n to π* transition corresponds to the excitation of an electron from one of the unshared pair to the π* orbital.
Reference data:
1. Solvents used in UV-Vis spectroscopy (near UV)
Solvent | lower limit (nm) |
Acetonitrile | 190 |
Chloroform | 240 |
Cyclohexane | 205 |
95% Ethanol | 205 |
n-Hexane | 195 |
Methanol | 205 |
Water | 190 |
2. Reference list
a. Common functional groups
Compound | λ(nm) | Intensity/ε | transition with lowest energy | Comments |
CH4 | 122 | intense | σ-σ*(C-H) | |
CH3CH3 | 130 | intense | σ-σ* (C-C) | |
CH3OH | 183 | 200 | n-σ* (C-O) | |
CH3SH | 235 | 180 | n-σ* (C-S) | |
CH3NH2 | 210 | 800 | n-σ* (C-N) | |
CH3Cl | 173 | 200 | n-σ* (C-Cl) | |
CH3I | 258 | 380 | n-σ* (C-I) | |
CH2=CH2 | 165 | 16000 | π-π* (C=C) | |
CH3COCH3 | 187 | 950 | π-π* (C=O) | |
273 | 14 | n-π* (C=O) | ||
CH3CSCH3 | 460 | weak | n-π* (C=S) | |
CH3N=NCH3 | 347 | 15 | n-π* (N=N) |
b. Aromatic rings
Compound | Peak 1 (in nm/ε) | Peak 2 (in nm//ε) | Peak 3 (in nm/ε) | Color |
Benzene | 184/60000 | 204/7900 | 256/200 | colorless liquid |
Naphthalene | 221/133000 | 286/9300 | 312/289 | white solid |
Anthracene | 256/180000 | 375/9000 | pale yellow | |
Naphthacene | 278/170000 | 474/10000 | yellow |
c. Compounds used in the laboratories
Compound | Solvent | λ(nm) | ε(cm-1*mol-1*L) | Source | Color observed |
Tetraphenylcyclopentadienone | Methanol | 500 | 1120 | Sadtler 17187 | dark purple |
331 | 6460 | ||||
258 | 24500 | ||||
Tetraphenylnaphthalene | Methanol | 293 | 11900 | Sadtler 18865 | white |
240 | 56500 | ||||
217 | 54900 | ||||
Jacobsen Catalyst | CHCl3 | 288 | 27600 (sh) | (1) | brown |
325 | 17200 | ||||
362 | 9000 (sh) | ||||
436 | 5400 | ||||
496 | 2200 |
(1) Kunkely, H., Vogler, A., Inorg. Chem.Commun. 2001, 4, 692.
Since this is an absorption technique the observer will precieve the complementary color that has not been absorbed. A spectrum is divided into colors: blue, green, yellow, and red.