The stereochemistry of the complex. The splitting for tetrahedral and for square planar complexes will be different from that for an octahedral complex

For example the chromium(III) ion, Cr³⁺, has fi ve 3d orbitals with the same energy. However, when the ion is surrounded by six water ligands, the fi ve d orbitals are split into three orbitals of lower energy and two orbitals of higher energy.

When white light falls on the complex, energy is absorbed and electrons in the lower energy d orbitals are excited to a higher energy d orbital. The wavelength of energy absorbed, depends on the difference in the energy of the split orbitals. The remaining wavelengths of the light combine to give the colour that is observed (the complementary colour).

Colourful transition metal compounds

A.8.1

Describe the effect of different ligands on the splitting of the d orbitals in transition metal complexes. © IBO 2007

A.8.2

Describe the factors that affect the colour of transition metal complexes. © IBO 2007

lower energy orbitals higher energy

orbitals Cr³⁺ [Ar]3d³

3d

Figure 1.7.8 When the Cr³⁺ ion is surrounded by six water ligands, the five d orbitals are split into three orbitals of lower energy and two of higher energy.

For example, if a complex ion absorbs light of a wavelength of 450 nm in the blue range of visible light spectrum, then the ion appears yellow, its complementary colour in the opposite segment of the colour wheel (see fi gure 1.7.6) . Similarly, for the hexaaquacopper(II) ion to appear blue it must absorb light with a wavelength between 580 and 595 nm.

Figure 1.7.9 When white light is shone on [Ti(H₂O)₆]³⁺, it absorbs strongly in the yellow-green region, causing the solution to appear purple.

yellow-green

light absorbed white light

energy

solution

appears purple Absorption of light by [Ti(H₂0)₆]³⁺

Because the energies of the d orbitals are determined by the presence of ligands, it follows that different ligands will produce compounds of different colours. One of the most well-known examples of this is the copper tetrammine complex [Cu(NH₃)₄(H₂O)₂]²⁺, which is royal blue in colour whereas the

hexaaquo complex [Cu(H₂O)₆]²⁺ is a sky blue in colour. The reaction in which the water ligands are displaced by ammonia ligands is:

[Cu(H₂O)₆]²⁺(aq) + 4NH3(aq) → [Cu(NH3)₄(H₂O)₂]²⁺(aq) + 4H2O(l) hexaaquacopper(II) ion tetraamminediaquacopper(II) ion

sky blue royal blue

The presence of ammonia ligands changes the energy of the d orbitals suffi ciently to produce the more ‘purplish’ royal blue colour.

Another common example of the replacing of ligands to form a different complex involves the addition of 1.0 mol dm^–3 ammonia to the [Ni(H₂O)₆]²⁺

complex. The ammonia ligands replace all six water ligands:

[Ni(H₂O)₆]²⁺(aq) + 6NH3(aq) → [Ni(NH3)₆]²⁺(aq) + 6H2O(l) hexaaquanickel(II) ion hexaamminenickel(II) ion green blue

While many organic compounds are colourless, there are substantial numbers of coloured organic compounds. Indeed organic dyes such as indigo, saffron and the exotic Tyrian purple have been used for centuries. The bright orange of β-carotene (found in carrots and other orange vegetables) is familiar to most people, as is the green colour of chlorophyll, the light-capturing molecule found in leaves. When observing the molecular structures of some of these compounds in fi gures 1.7.11 and 1.7.12, the most obvious feature that they have in common is the large number of carbon–carbon double bonds. These are often conjugated (alternating with single bonds) and the pi electrons are delocalized within the structure. It is this part of the structure that allows the Figure 1.7.10 The colour of the copper ion solution

depends on the ligands surrounding the copper ion.

These four test tubes contain [Cu(H₂O)₆]²⁺, [CuCl₄]^2–, [Cu(NH₃)₄(H₂O)₂]²⁺ and [Cu(EDTA)]^2–.

Absorption of UV and visible light by organic molecules

A.8.3

State that organic molecules containing a double bond absorb UV radiation. © IBO 2007

A.8.4

Describe the effect of the conjugation of double bonds in organic molecules on the wavelength of the absorbed light. © IBO 2007

CHAPTER 1 MODERN ANALYTICAL CHEMISTRY Chlorophyll is a substance with a vital

role in the food chain. In this role the absorption of visible light is fundamental (see fi gure 1.7.11). There are two forms of chlorophyll which differ by a single functional group. In chlorophyll a the R group is a methyl group, –CH₃, while in chlorophyll b, the R group is an aldehyde functional group, –CHO. The two forms of chlorophyll have maximum absorption at slightly different wavelengths, adding greater light-capturing power to

chloroplasts in which these pigments are found. Plants can obtain all their energy requirements from the blue and red parts of the spectrum. In the large region between 500 nm and 600 nm very little light is absorbed. This is the green region of the spectrum, and the refl ection of this green light makes the chlorophyll (and consequently leaves) appear green.

O

O OH

HO

OH CH₃

OH CO₂H

kermesic acid from the insect Coccus cacti

O O

H

CH₃ CH₃

CH₃ CH₃

O O H

crocetin from saffron

β-carotene from carrots

Figure 1.7.12 The conjugation of double bonds in these molecules allows them to absorb light in the UV and visible regions of the electromagnetic spectrum.

This feature makes these compounds strongly coloured: kermesic acid is crimson, crocetin is yellow and ␤-carotene is orange.

N N

R

N N

O O O

O O

Mg

400 500 600 700

Chlorophyll a: R = –CH3

Chlorophyll b: R = –CHO

absorbance

a b

b a

wavelength (nm)

Figure 1.7.11 Visible spectrum and structure of chlorophyll. This molecule absorbs strongly at 420 nm (violet) and 660 nm (red) giving it a green colour.

CHEM COMPLEMENT

The royal colour

The colour purple has been a symbol of royalty for centuries.

As early as 1600 BC in the city of Tyre, the ancient

Phoenicians used a purple-red dye known as Tyrian purple to colour cloth. This dye was extracted from the hypobranchial gland of the Murex brandaris mollusc. About 12 000 shellfish needed to be crushed to extract 1.5 g of the pure dye. As a result the purple dye was so expensive that only royalty could afford clothes dyed with it.

Although Tyrian purple is often translated as ‘scarlet’, it appears that the method of extraction could have caused variation in the colour. Another dye extracted from a related

sea snail Hexaplex trunculus could produce a purple-blue colour when processed in the shade, or a sky-blue indigo colour when processed in sunlight. Sea snails in other parts of the world such as the tropical eastern Pacific have been found to produce a similar substance that makes a purple dye in the sunlight. The snails are predatory and use the secretion as part of this behaviour.

The structure of the dye indigo is very similar to Tyrian purple, the only difference being the replacement of two hydrogen atoms with bromine atoms.

H

N

N

H O

O

a

b

H

H

indigo

H

N

N

H O

O

Br

Br

Tyrian purple

Figure 1.7.13 The shell of a Murex sea snail with the molecular structures of (a) indigo and (b) Tyrian purple.

Absorbance of high energy UV light causes molecular electrons to be excited to higher energy molecular orbitals. Although molecular orbital theory is beyond this course, it is important to realize that absorption of this energy promotes electrons from the highest occupied molecular energy level (HOMO) to the lowest unoccupied molecular energy level (LUMO). When a double bond or a triple bond occurs in a molecule, the only radiation with enough energy to achieve this effect is the highest energy UV radiation. A group such as a carbon–carbon double bond that is able to absorb UV or visible radiation is known as a chromophore. The wavelength of the light absorbed by just one carbon–carbon double or triple bond in a molecule is below 200 nm. For

example, ethene absorbs UV radiation at 171 nm and 1-hexyne (with one triple bond) absorbs at 180 nm. This absorption is below the range of UV–visible

CHAPTER 1 MODERN ANALYTICAL CHEMISTRY absorbed light (a bathochromic shift) to a longer wavelength and UV light of

wavelength greater than 200 nm is absorbed. This is because the difference in energy between the HOMO and LUMO molecular energy levels decreases as the degree of conjugation increases.

Phenolphthalein is part of another group of organic compounds that are valued for their colour: acid–base indicators. These are weak acids that are two different colours in their acid and conjugate base form. The

structure of phenolphthalein is shown in fi gure 1.7.15 in its acid and base forms. Notice that the acid form of phenolphthalein has less conjugation than the base form. This change in structure when the H⁺ ion is donated is enough to increase the wavelength of light that is absorbed by

phenolphthalein into the visible region and to make the compound coloured. Retinol (vitamin A) is another well-known coloured organic compound.

Figure 1.7.15 Retinol is yellow while phenolphthalein is colourless in its acid form, but pink in its basic form.

OH

CH3 CH3

H₃C CH₃

CH3 retinol

O

HO OH

O

O⁻

HO O

O

+ H3O⁺ + H2O

phenolphthalein

(colourless) (pink)

A group of pharmaceutical preparations that have become vital to our existence, especially in the southern hemisphere where the hole in the ozone layer increases the risk of sunburn, is sunscreens. The function of sunscreens is to prevent UV radiation from penetrating our skin. UV radiation is harmful to skin cells and can cause mutations that lead to skin cancer. Sunscreens can act in two ways: they may absorb most of the UV radiation or they may refl ect most of UV radiation. Those that refl ect the UV rays contain TiO₂ or ZnO,

log ε

λ (nm) 5.0

4.0

3.0

2.0

200 300 400 500

Figure 1.7.14 The UV–visible absorption spectra of naphthalene (2 rings), anthracene (3 rings) and tetracene (4 rings). As the degree of conjugation increases the wavelength of absorbed light increases.

While naphthalene and anthracene are colourless, tetracene is orange.

which are both white compounds and may physically block the passage of UV radiation into the skin. The compounds that are used to absorb the UV radiation contain conjugated carbon–carbon double bonds. Octyl

methoxycinnamate is a compound that absorbs UV radiation of wavelengths 290–320 nm (UVB radiation), while butyl methoxydibenzoylmethane (‘parsol’) absorbs UV radiation of wavelengths 320–400 nm (UVA radiation).

O

O

O

octyl methoxycinnamate

O O

O

butyl methoxydibenzoylmethane

Figure 1.7.16 These two molecules are used to absorb UV radiation in sunscreens.

The structure of a molecule gives a good indication of whether it will absorb UV or visible radiation. The larger the number of conjugated double bonds, or delocalized bonds, such as in a benzene ring, the greater the chance that visible light will be absorbed and the molecule will be coloured. The presence of just one double bond will result in UV absorption, most likely below 200 nm. If

two double bonds are present, but they are isolated (i.e. not conjugated) then the absorption will be at a higher wavelength, but still not in the visible region. Compare the absorbance of 1-pentene with that of isoprene: 1-pentene with one carbon–

carbon double bond absorbs at just 178 nm, isoprene has two conjugated carbon–carbon double bonds, and its absorbance is at the short end of the UV range (just within the range of a UV–

visible spectrometer). The compounds in fi gure 1.7.12 all contain considerable conjugation of double bonds and delocalization of electrons. These compounds absorb visible light and appear coloured. Our uses for them over the centuries have been based on these colours.

A.8.5

Predict whether or not a particular molecule will absorb UV or visible radiation. © IBO 2007

Figure 1.7.17 Isoprene is a colourless liquid that is used to make

1.0 0.8 0.6 0.4 0.2 0

200 220 240 260 280 300 320 340

λ (nm)

abundance

1.2

H2C C

C CH₂ CH3

H isoprene λ_max = 222 nm

CHAPTER 1 MODERN ANALYTICAL CHEMISTRY 1 Compare the wavelengths of UV and visible radiation.

2 Explain why electron transitions occur when UV light is absorbed, whereas when IR light is absorbed only molecular vibrations occur.

3 Consider the visible spectrum of chlorophylls a and b given in fi gure 1.7.11.

State the four wavelengths of light at which chlorophyll a and chlorophyll b absorb light most strongly.

4 Explain why transition metal complexes are coloured and so absorb visible light.

5 List the factors that affect the colour of transition metal complexes.

6 Explain how UV–visible spectrometry may be used for:

a qualitative analysis b quantitative analysis.

7 The protein content of a biological sample may be determined using a reagent to convert the protein into a coloured complex.

The absorbance of the complex is then measured using a UV–visible

spectrophotometer set at an appropriate wavelength. Using standard protein solutions, the calibration curve shown at right was obtained. A sample containing protein was diluted 100-fold. The diluted sample was found to have an absorbance of 0.40.

a Determine the protein content of the sample in ␮g cm^–3.

b Briefl y describe how an ‘appropriate wavelength’ for this analysis would be determined.

8 Consider the following molecules.

O

O OH

OH HO

CH3

CO2H

OH

C C

H H

H H

H C C C

H

C OCH3

H

I II III

a Order these molecules from shortest maximum wavelength of light absorbed to longest maximum wavelength of light absorbed.

b Explain whether you would expect any of these molecules to be coloured.

Section 1.7 Exercises

absorbance

protein content (μg cm^–3)

50 100 150 200 250 300

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

0 0.0

9 The UV–visible absorption spectra for three compounds is shown in the fi gure to the right. All of the compounds have the general formula R(CH=CH)nR, where n is the number of carbon–

carbon double bonds in the molecule.

a State which compound has the greatest value for n.

b Explain the relationship between the number of double bonds and the maximum wavelength at which an organic compound absorbs radiation in the UV–visible region.

Recall from section 1.4 that ¹H NMR spectroscopy involves the application of a strong magnetic fi eld to a sample and irradiation with radio waves. The sample absorbs energy and hydrogen nuclei are excited. As these hydrogen nuclei relax from their excited state they emit radio frequency energy that is picked up by a radio receiver and passed on to be recorded as a spectrum.

In section 1.4, the environment of different hydrogen nuclei was discussed as infl uencing the signal produced by those nuclei. The location of these NMR signals needs to be compared to a reference signal, just as IR and UV–visible absorption data needs to be compared to the absorption of the solvent.

The reference standard used in NMR is a compound called tetramethylsilane (TMS), (CH₃)₄Si. It has been chosen as the reference because of the following properties:

• All 12 hydrogen atoms in TMS are equivalent.

• It is chemically unreactive.

• It is easily removed from the sample after the measurements have been taken.

• It produces a single, sharp NMR signal that does not interfere with other resonances.

It is important to realize that the magnets used in different NMR

spectrophotometers are not identical. So resonance frequencies of identical protons may vary from one instrument to another. An NMR spectrophotometer is calibrated by reporting other signals in terms of how far they have shifted relative to the reference signal. In doing this, the scale is divided by the frequency of the radiation used (it may be 100 MHz or even 500 MHz, depending on the spectrometer) and this value is then multiplied by 10⁶ (otherwise the value is too small). This produces the quantity known as chemical shift, which has the symbol δ and units ppm (parts per million). Note that NMR spectrophotometers are very sensitive and very precise. The proton resonances that they record all fall within a 12 ppm range of each other. The

abundance

λ (nm) 200

R(CH=CH)_nR

III

II

I

300 380

1.8 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

^HL

A.9.1

Explain the use of tetramethylsilane (TMS) as the reference standard.

© IBO 2007

Figure 1.8.1 Tetramethylsilane.

C

Si C C

C H

H H

H

H H

H H H

H H

H

CHAPTER 1 MODERN ANALYTICAL CHEMISTRY TABLE 1.8.1 CHARACTERISTIC ¹H NUCLEI NMR CHEMICAL SHIFTS

(These values can also be found in table 18 of the IB Data Booklet. © IBO 2007)

Type of ¹H nucleus (proton) Chemical shift (ppm)

R C H

H

H

0.9–1.0

R C R

H

H

1.3–1.4

C R

R H R

1.4–1.6

C

H H C O

O

R H

2.0–2.5

R C C

O H

H H

2.2–2.7

C

H H

H

2.5–3.5

C

H H

Halogen R

3.5–4.4

C

H H

H O

R

3.3–3.7

R C

O

O H

9.0–13.0

H O

R 4.0–12.0

C R

H

C H

H

4.5–6.0

H

6.9–9.0

R H

O

C

9.4–10.0

The different electronegativities of the surrounding atoms infl uence the

chemical shift of ¹H nuclei. In the case of tetramethylsilane, TMS, silicon is less electronegative than carbon, so the electron density (and hence shielding) around the methyl hydrogens is greater than in the carbon derivative,

2,2-dimethylpropane. This causes a downward shift for the resonance of those

1H nuclei and this resonance is defi ned as zero. Elements that are more electronegative than carbon reduce the electron density around the carbon atom, they deshield the hydrogen atoms that are bonded to that carbon atom.

The proton shifts for a number of methyl derivatives is given in table 1.8.2 and the deshielding effect is shown in fi gure 1.8.2.

Another example of the effect of increasingly electronegative neighbours can be seen in the comparison of halogenoalkanes. The more electronegative the neighbouring halogen, the greater the chemical shift, and the more halogen atoms that are bonded to the carbon, the greater the chemical shift.

TABLE 1.8.2 THE EFFECT OF ELECTRONEGATIVITY ON CHEMICAL SHIFT FOR METHYL PROTONS

In document Pearson Options (Page 61-70)