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.