Structure Determination in Conjugated Systems: UV Spectroscopy (P

CH437 CLASS 18 ULTRAVIOLET-VISIBLE SPECTROSCOPY 1

Synopsis. Introduction: energy transitions and absorption spectra. Selection rules. Beer-Lambert Law. Solvents for UV-visible spectroscopy. Chromophores.

Introduction

Energy transitions in UV-visible spectroscopy are electronic, involving 125-650 kJ/mol of energy (much higher than those needed for vibrational transitions in IR spectroscopy or for nuclear spin transitions in NMR spectroscopy). The range of energies above roughly correspond to absorbed wavelengths of 190-800 nm: that is, in the ultraviolet-visible region of the electromagnetic spectrum. The major electronic transitions in UV-visible absorption spectroscopy are HOMO  LUMO transitions, as shown below.

A common molecular orbital energy level diagram for organic molecules is shown below, where the most common HOMOLUMO transitions are *, n* and *, but * and n* are also possible.

1 These are most commonly associated with the following organic molecules:

 Alkanes

 C=O

 Alkenes, alkynes, aromatics, C=O, azo compounds, etc n  Oxygen, nitrogen, sulfur and halogen compounds n  C=O

In practice, for molecules there is a series of vibrational energy levels between each of the electronic energy levels: molecules can undergo vibrational and electronic transition at the same time, as illustrated below.

 

 LUMO E

 

 HOMO

2 It can be seen that there is a considerable number of transition possibilities, over a range of wavelengths. This gives the characteristically broad bands (covering several nm) seen in typical UV-visible spectra, as illustrated in the examples below. If the absorbing substance of reasonable polarity is dissolved in a non- protic apolar solvent, it is often possible to see vibrational fine structure, as in example (a), since there is minimal interaction between the solute and solvent. However in polar protic solvents, the interactions (e.g. hydrogen bonded and dipole-dipole interactions) are considerable and very little fine structure is seen, the band being an envelope of the spectrum in (a), but with max at a somewhat different value.

Selection Rules

Not all possible transitions are actually allowed. There are selection rules that quantum theory imposes on electronic transitions (as with vibrational and nuclear spin transitions) to determine which are allowed and which are forbidden. In practice, certain forbidden transitions are observed, but the absorption intensity is always low compared with those of allowed transitions. The selection rules are based upon orbital symmetry: this symmetry is broken by vibrational components of the transition, thus allowing “forbidden” transitions to occur (although with low probability). The most important forbidden transition is n*.

3 The Beer-Lambert Law

This law relates the intensity of light absorption (the absorbance) to the concentration of absorbing substance

Absorbance is defined as: A = log (Io/I) = cl, as illustrated below.

The corresponding expression for Amax (the value of A at max) is

Amax = log(Io/I) = maxcl

 is the molar absorptivity, a constant that is characteristic of the particular substance (in specified solvent and at specified temperature) at a particular wavelength and hence of the particular -system present. max is the corresponding constant for max. For conjugated dienes, max ~10,000 – 25,000 (l/mol/cm) and consequently conjugated substances can be identified by their

(allowed)   *  max values and max values. Similarly, max values for (the forbidden) n* transitions are between 0 and 1000. Also, max is useful in analytical chemistry and biochemistry for the determination of concentrations of known substances.

Solvents for UV-Visible spectroscopy

The important spectroscopic constants max and max, associated with a molecule that absorbs UV-visible light are solvent-dependent, so that when quoting these 4 constants the solvent should always be stated. A polar solvent forms hydrogen bonds more readily with the ground state (e.g. the HOMO) of polar molecules, thus stabilizing this state with respect to the excited state (e.g. the LUMO) as for carbonyl n* transitions:

This shifts max to lower values (toward the UV). An example of this is found in the UV-visible spectra of carbonyl compounds, as typified by solvent shifts for the n* transition of acetone, below.

Solvent H2O CH3OH C2H5OH CHCl3 C6H14

max/nm 265 270 272 277 279

For less polar compounds, polar solvents may form hydrogen bonds more easily with the LUMO than with the HOMO (as for * transitions). In which case, the opposite effect occurs, max is shifted to higher wavelengths in polar solvents.

A second important feature of solvent is its own absorption in the UV- visible region: a good solvent should not absorb in the same region as the compound under investigation.

Usually solvents that do not contain conjugated systems are best for UV-visible spectroscopy, since their max values will be low wavelength (well below 200 nm if possible – well away from the absorption regions of most molecules, particularly conjugated or aromatic molecules). The table below shows some UV

5 spectroscopy solvents and their “cutoff points” – minimum regions of transparency.

Solvent Cutoff /nm Solvent Cutoff /nm

Acetonitrile 190 n-Hexane 201 Chloroform 240 Methanol 205 Cyclohexane 195 Isooctane 195 1,4-Dioxane 215 Water 190 95% Ethanol 205 Trimethyl 210 phosphate

Chromophores, Molecular Orbitals and Transitions

A chemical entity (group of atoms) that absorbs (UV-visible) radiation is known as a chromophore. The exact energy (and hence max) of absorption depends on the molecular orbital energies, which in turn depend upon the structure of the chromophore. It is usually very difficult to give accurate prediction purely from theory how absorption changes as the chromophore structure changes. Instead, empirical rules (Class 20) are much more conveniently used for certain kinds of conjugated chromophores. For now, the major chromophores will be considered in terms of MO orbital transitions, as outlined below. * Transitions

6 _ + C _ + C  C*-C

_ _ C + C  C-C

These transitions are the only ones possible in alkanes, where there is lack of both –bonding and atoms with non-bonded electrons. They are of such high energy that they require absorption of UV radiation of very short wavelength: shorter than those that are readily accessible using ordinary spectrometers. n* Transitions

7 _ + C _ + N  C*-N

_ C N n +

_ _ C + N  C-N

These are also rather high energy transitions, but most of them are within the range of typical spectrometers, although below the cutoff point of most solvents. They are found in alcohols, ethers, amines and sulfur compounds and have typical ranges of 175-200 nm (alcohols and ethers) and 200-220 nm (thiols and sulfides).

* Transitions

8 _ + C C * + _

+ C C  _

With unsaturated compounds (alkenes, alkynes, aromatics and carbonyl compounds), * transitions become possible. Likewise, these transitions are also of rather high energy (max typically ~170-175 nm for non-conjugated alkenes and alkynes), but their max values are sensitive to conjugation and to substitution, as will be seen in Classes 19 and 20. n* Transitions

_ + C O * + _

+ n(p ) C O y _

+ C C  _

9 Unsaturated molecules that contain oxygen, nitrogen or sulfur may undergo n* as well as * transitions. The most important of these molecules are carbonyl compounds, whose transitions are also rather sensitive to substitution (see Class 19). Typical carbonyl compounds undergo n* transitions around

280-290 nm with very low intensity (max ~ 15), being forbidden transitions. They also have * transitions at about 190 nm (max ~ 1000). A list of absorptions of simple isolated chromophores is given below

It will be seen that many of these chromophores absorb in the region 160-210 nm. However, attachment of substituent groups in place of hydrogen in a chromophore structure changes the position and intensity of the absorption band due to the principal (or basic) chromophore. These substituents are known as auxochromes and have four different effects on absorptions:

Bathochromic shift (red shift) – shift to longer wavelength (lower energy). Hypsochromic shift (blue shift) – shift to shorter wavelength (higher energy). Hyperchromic shift – increase in intensity of absorption. Hypochromic shift – decrease in intensity.