UV/vis spectroscopy

Ultraviolet UV and visible (UV/vis) spectroscopy uses radiation from the electromagnetic spectrum whose wavelength is between 100 and 800 nm (energy between 286 and 36 Kcal/mol) and its effect on organic matter, as indicated above, is to produce electronic transitions between the atomic and/or molecular orbitals of the substance.

The typical appearance of a UV spectrum is as shown in the figure:

The absorption maxima are due to the presence of chromophores in the molecule, in this case there are two absorptions at 190 and 270 nm, but to characterize these absorptions in addition to the maximum wavelength for each absorption we must remember the Lambert-Beer law, according to which:
Depending on the type of bond we consider as chromophore the electronic excitation that can be observed is:

Absorbance = ε·l·c

Where:

ε = Molar extinction coefficient, is a constant related to the area of incidence of the chromophore and the probability of its absorption.

l = path in cm of radiation through the sample
c = concentration of the sample in moles/litre

We will consider that when ε is less than 10000 that absorption is due to an electronic transition forbidden by the selection rules.

Electronic transitions

Depending on the type of bond we consider as chromophore the electronic excitation that can be observed in UV and visible spectroscopy is:

Single bond:

fig-2

Single bonding with unshared electron pairs:

Double bond:

Carbonyl group:

Dienes:

 

It is necessary to clarify that although in these figures only the transition with the longest wavelength is underlined, this does not imply that the other transitions do not occur. Moreover, and even that these transitions, although of shorter wavelength, are of greater absorption than the one indicated.

Chromophores in UV and visible spectroscopy

When radiation strikes a substance, not all of it is affected by it. Thus, the atom or set of atoms that absorb radiation was called a chromophore.

In addition, in molecules, there are also atoms or groups of atoms that do not absorb radiation, but cause some of the absorption characteristics of the chromophore to change; such groups are called auxochromes.

According to the above excitation modes we will have the following simple chromophores in UV and visible spectroscopy:


Table 1
: Transition values for different chromophores in UV/vis
BondTransition
max, nm)
σ electrons involved
C-C, C-Hσ → σ*
(150)
-O-n → σ*
(185)
-N-n → σ*
(195)
n electrons involved
-S-n → σ*
(195)
C=On → π*
(290)
C=On → σ*
(190)
π electrons involved
C=Cπ → π*
(190)

Examples:

Table 2: Absorption values of different chromophores in UV/vis
Substanceλmax, nm
(ε)
Chromophore C=C
Ethylene170
(15800)
t-2-Hexene184
(10000)
Cyclohexene182
(7600)
1,3-Butadiene214
(20000)
Chromophore C≡C
1-Octyne185 (2000)
222 (126)
Chromophore C=O
Acetaldehyde277 (8) en H2O
290 (16) en hexano
Acetone279 (15)
Acetic acid204 (60)
Chromophore C=NOH
Acetoxime190 (5000)
Chromophore NO2
Nitromethane271 (19)
Chromophore S=O
Cyclohexyl methyl sulfoxide210 (1500)

Bathochromic and hyperchromic effect

It should be borne in mind that obtaining a UV spectrum involves first dissolving the substance in a suitable solvent, which would also absorb in the UV, so in practice UV and visible spectroscopy is limited to wavelengths above 200-220 nm.

Because of this, as we can imagine, there are not many functional groups that we can determine with UV and visible spectroscopy, and it should be noted that all of them must have at least one double bond.

The existence of a second double bond conjugated with the previous one or the presence of an auxochrome group increases the λmax of the absorption (bathochromic effect) (also the absorbance and ε, (hyperchromic effect).

In case of any circumstance a decrease in λmax would be an ipsochromic effect, or a decrease in absorbance (hypochromic effect).

Table 2 below shows the characteristics of some chromophores in UV spectroscopy:


Table 2
: UV/vis absorption of different functional groups
Functional
group
λmax, nm
(ε)
Acetylenes170-175
(4500)
Diacetylenes225-235
(200)
Enynes220-225
(~10000)
Allenes175-185
(~10000)
Cumulenes (butatriene)241
(20300)
Nitriles~340
(120)
Nitro derivatives~210 (~16000)
270-280 (~200)
Nitrates260-270
(150)
Nitrites~350
(~150)
Azo derivatives350
(bajo)
Diazo derivatives~400
(~3)
Sulfoxides210-215
(~1600)
Sulfones< 208
Vinylsulfones~210
(~300)

Aromatic hydrocarbons

A separate case are the aromatic hydrocarbons which have two characteristic absorptions. These are called E-band (ethylene) and B-band (benzenoid).

Consequently, they are modified by the presence of conjugated double bonds (E → K (conjugation)) and the presence of elements with unshared electron pairs: R-band (radicalary).

The Table 3 summarizes the characteristic values of these bands.


Table 3
: UV/vis absorption of the characteristic bands of aromatic hydrocarbons.
Band
(transition)
λmax, nm
(ε)
E
(π → π*)
180-220
(2000-6000)
K
(π → π*)
220-250
(10000-30000)
B
(π → π*)
250-290
(100-1000)
R
(n → π*)
275-330
(10-100)

For example:

 

Table 3: UV/vis absorption of the characteristic bands for different compounds.
 

Compound

Band λmax, nm, (ε)
EKBR
Benzene184
(47000)
204
(7400)*
254
(204)
t-Butylbenzene208
(7800)
257
(170)
Styrene244
(12000)
282
(450)
Acetophenone240
(13000)
278
(1100)
319
(50)

 

  • A single one at 198nm (8000)

Although it seems to be of limited use for structural determination, UV and visible spectroscopy is very useful for the study of conjugated dienic systems, in natural products, in α,β-unsaturated carbonyl compounds, in the study of quinonic products and in the study of aromatic and heterocyclic products, having established empirical formulas that allow the determination of the λmax as a function of the structure.

Some application examples are shown in the following examples:

fig-