授課教師: Professor 吳逸謨 教授 Warning: Copyrighted by textbook publisher. Do not use outside class. Principles of Instrumental Analysis Section III – Molecular Spectroscopy Chapter 13 Introduction to Ultraviolet-Visible Absorption Spectrometry + Chapter 14 Applications of Ultraviolet-Visible Absorption Spectrometry

Reference: p. 423, Visible and UV spectra, in “Organic Chemistry” textbook – Solomons, 3rd Ed] 1 Molecular Spectroscopy Ultraviolet–visible (UV-Vis) absorption vs. fluorescence Spectroscopy

Ultraviolet–visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region.

This means it uses light in the visible and adjacent (near-UV and near- infrared [NIR]) ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved.

In this region of the electromagnetic spectrum, molecules undergo electronic transitions.

Fluorescence spectroscopy is based on molecular emission: The “UV-Vis” technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while UV-Vis absorption measures transitions from the ground state to the excited state.[1]

2 (2007/3) UV-Vis Spec 儀器 -實驗課

3 FIGURE 6-3 Regions of the electromagnetic spectrum. (For UV-Vis, λ = 100~500 nm, 500~1000 nm)

4 Ch6 An Introduction to Spectrometric Methods P.135 Apendix: Chap. 7E - Radiation Transducers p. 191 Read the texts in Chap. 7

7E-2 Photon Transducers for optical spectroscopy

– Barrier-Layer Photovoltaic Cells – Vacuum Phototubes

– Photomultiplier tubes (PMT), picture on p. 195 – Silicon Photodiodes (Fig. 7-32) [semiconductor type] [A silicon photodiode transducer consists of a reverse- biased p-n junction.]

(The latter two types of transducers are more commonly used in UV-Vis.) – We will discuss more details later. 5 Ultraviolet–visible absorption Molecules containing π-electrons or non-bonding electrons (n-electrons) [will be explained next page] can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals.[ UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds, and biological macromolecules.

Spectroscopic analysis is commonly carried out in solutions but solids and gases may also be studied.

1. Solutions of transition metal ions can be colored (i.e., absorb visible light) because d electrons within the metal atoms can be excited from one electronic state to another.

2. Organic compounds, especially those with a high degree of conjugation, also absorb light in the UV or visible regions of the electromagnetic spectrum.

The solvents for these determinations are often water for water-soluble compounds, or ethanol for organic-soluble compounds. (Organic solvents may have significant UV absorption; not all solvents are suitable for use in UV spectroscopy.)

3. While charge transfer complexes also give rise to colours, the colours are often6 too intense to be used for quantitative measurement [Note: pi electrons] The Greek letter π in their name refers to p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis. P orbitals often engage in this sort of bonding. D-orbitals also engage in pi bonding, and form part of the basis for metal-metal multiple bonding.

Electrons in pi bonds are sometimes referred to as pi electrons. -Molecular fragments joined by a pi bond cannot rotate about that bond without breaking the pi bond, because rotation involves destroying the parallel orientation of the constituent p orbitals.

A typical double bond consists of one sigma bond and one pi bond; for example, the C=C double bond in ethylene (CH2=CH2). A typical triple bond, for example in acetylene (CHCH), consists of one sigma bond and two pi bonds in two mutually perpendicular planes containing the bond axis.

Two pi bonds (π) are the maximum that can exist between a given pair of atoms. 7 Note: non-bonding orbitals/electrons

A non-bonding orbital, also known as non-bonding molecular orbital (NBMO), is a molecular orbital whose occupation by electrons neither increases nor decreases the bond order between the involved atoms.

A non-bonding orbital with electrons would commonly be a HOMO (highest occupied molecular orbital).

Electrons in molecular non-bonding orbitals can undergo electron transitions such as n→σ* or n→π* transitions [where * indicates “anti-bonding”].

For example, n→π* transitions can be seen in ultraviolet-visible spectroscopy of compounds with carbonyl groups (C=O), although absorbance is fairly weak.[2]

The following molecular electronic transitions exist: σ → σ* π → π* n → σ* n → π* aromatic π → aromatic π*

* : σ → σ Electrons occupying a HOMO of a sigma bond can get excited to the 8 LUMO of that bond. Sigma bonds (σ bond) : - Ref. to your organic chem.

Sigma bonds (σ bond) are the strongest type of covalent bonds due to the direct overlap of orbitals, and the electrons in these bonds are sometimes referred to as sigma electrons.[3]

Sigma bonds are the strongest type of covalent bonds due to the direct overlap of orbitals, and the electrons in these bonds are sometimes referred to as “sigma electrons”.

Typically, a single bond is a sigma bond, while a multiple bond is composed of one sigma bond together with pi or other bonds.

A double bond has one sigma plus one pi bond, and a triple bond has one sigma plus two pi bonds.

9 When the molecule is in the ground state, both electrons are paired in the lower-energy bonding orbital – this is the Highest Occupied Molecular Orbital (HOMO).

The antibonding σ* orbital, in turn, is the Lowest Unoccupied Molecular Orbital (LUMO).

This is referred to as a σ - σ* transition. ΔE for this electronic transition is 258 kcal/mol, corresponding to light with a wavelength of 111 nm (in UV range !). 10 σ  σ* transition absorption is usually very strong (but in UV region)! Where UV-vis spectroscopy becomes useful to most organic and biological chemists is in the study of molecules with conjugated pi systems. [Conjugation: double-single-double-single, etc.]

In these groups, the energy gap for π -π* transitions is smaller than for isolated double bonds, and thus the wavelength absorbed is longer. (in Visible region)

Molecules or parts of molecules that absorb light strongly in the UV-Vis region are called chromophores11 . Introduction p.336 • Molecular absorption spectroscopy is based on the measurement of the transmittance (T), or the absorbance A of solutions contained in transparent cells having a path length of b centimeters. • Ordinarily, the concentration (c) of an absorbing analyte is linearly related to absorbance (A) as given by Beer's law: P A = −log T = log 0 = εbc (13-1) P All of the variables in this equation are defined in Table 13-1 (next page).

12 TABLE 13-1 Important Terms and Symbols for Absorption Measurements

13 Ch13 An Introduction to Ultraviolet-Visible Molecular Absorption Spectrometry P.337 13A. MEASUREMENT OF TRANSMITTANCE AND ABSORBANCE p.336 • Transmittance and absorbance, as defined in Table 13-1, cannot normally be measured in the laboratory because the analyte solution must be held in a transparent container, or cell. • As shown in Figure 13-1 (next page), reflection occurs at the two air-wall interfaces as well as at the two wall-solution interfaces.

• The resulting beam attenuation (decay) is substantial, as we demonstrated in Example 6-2 (Chap 6), where it was shown that about 8.5% of a beam of yellow light is lost by reflection in passing through a glass cell containing water.

• In addition, attenuation of a beam may occur as a result of scattering by large molecules and sometimes from absorption by the container walls. 14 FIGURE 13-1 Reflection and scattering losses with a solution contained in a typical glass cell. Losses by reflection can occur at all the boundaries that separate the different materials. In this example, the light passes through the air-glass, glass-solution, solution-glass, and glass-air interfaces. 15 Ch13 An Introduction to Ultraviolet-Visible Molecular Absorption Spectrometry P.337 13A. MEASUREMENT OF TRANSMITTANCE AND ABSORBANCE • To compensate for these effects, the power of the beam transmitted by the analyte solution is usually compared with the power of the beam transmitted by an identical cell containing only solvent. • An experimental transmittance and absorbance that closely approximate the true transmittance and absorbance are then obtained with the equations P P (13-2) T = solution ≈ Psolvent P0 P P (13-3) A = log solvent ≈ log 0 Psolution P

The terms P0 and P, as used in the remainder of this book, refer to the power of radiation after it has passed through cells containing the solvent and the analyte solutions, respectively.

16 13B BEER’S LAW Equation 13-1 represents Beer's law (A = εbc). This relationship can be rationalized as follows.

• Consider the block of absorbing matter (solid, liquid, or gas) shown in Figure 13-2 (below).

FIGURE 13-2 Radiation of initial radiant power P0 is attenuated to transmitted power P by a solution containing c moles per liter of absorbing solution with a path length of b centimeters. 17 Cont’d – differential derivation of Beer’s law • A beam of parallel monochromatic radiation with power P0 strikes the block perpendicular to a surface. After passing through a length b of the material, which contains n absorbing atoms, ions, or molecules, its power is decreased to P as a result of absorption.

• Consider now a cross section of the block having an area S and an infinitesimal thickness dx.

• Within this section there are dn absorbing particles; we can imagine a surface associated with each particle at which photon capture will occur. ……………… [details see textsbook]

log[Po/P]= anb/2.303 [i.e. ln(Po/P)= anb] (13-7) 18 ………. Details of derivation – see textsbook

And c in moles per liter is given by

n 1000 cm3 / L 1000n c = mol × = mol / L 6.02×1023 V cm3 6.02×1023V

P anb log 0 = Combining this relationship with Equation 13-7 P 2.303V yields P 6.02×1023 abc log 0 = P 2.303×1000

Finally, the constants in this equation can be collected into a single term ε to give P log 0 = εbc = A P (13-8) which is Beer’s law. 19 13D. Instrumentation (UV-Vis) p. 348~351 • 13D-1 Instrument component

• Sources (for UV-Vis Absorption Spectroscopy):

- Deuterium (D) and Hydrogen (H) Lamps [Note as next page]. - Tungsten Lamps (or Tungsten-Halogen Lamps). - Light-emitting Diodes (LED). - Xenon-Arc Lamps. (Usually UV-Vis would have two sources: Deuterium and Tungsten lamps)

• 13D-2 Types of Instruments - Double-beam Instruments - Single-beam Instruments 20 Note: A deuterium arc lamp (or simply deuterium lamp) is a low-pressure gas- discharge light source often used in spectroscopy when a continuous spectrum in the ultraviolet region is needed.

Hydrogen and deuterium lamps for UV vary in the gases that are utilized in the discharge..

Deuterium lamps also generate a higher intensity radiation compared to hydrogen Deuterium is used rather than hydrogen because of its lamps. greater intensity of UV emission in the molecular band.

The arc created excites the molecular deuterium contained within the bulb to a higher energy state. The deuterium then emits light as it transitions back to its initial state. This continuous cycle is the origin of the continuous ultraviolet radiation. This process is not the same as the process of decay of atomic excited states (atomic emission), where electrons are excited and then emit radiation. Instead, a molecular emission process, where radiative decay of excited states in molecular deuterium21 (D2), causes the effect. Note: Tungsten Halogen Lamps are similar in construction to conventional gas filled tungsten filament lamps except for a small trace of halogen (normally bromine) in the fill gas.

Tungsten Halogen (usually bromine-Br2) Lamps are ideal light sources for spectrophotometers as they provide broad band spectral radiation ranging from the ultraviolet, through the visible and into the infrared out to five microns.

Some radiation output can also be obtained at 320 and 340 nanometers.

22 FIGURE 13-11 (a) A deuterium (D2-heavy hydrogen) lamp of the type used in spectrophotometers and (b) its spectrum.

The plot is of irradiance Eλ (proportional to radiant power) versus wavelength. Note that the maximum intensity occurs at ~225 nm.

Typically, instruments switch from deuterium lamp to tungsten-halogen lamp at ~350 nm (for better intensity at higher wavelength). 23 Ch13 An Introduction to Ultraviolet-Visible Molecular Absorption Spectrometry P.349 FIGURE 13-12 (a) A tungsten lamp of the type used in spectroscopy and its spectrum (b).

Intensity of the tungsten (halogen) source is usually quite low at wavelengths shorter than about 350 nm.

Note that the intensity reaches a maximum in the near-IR region of the spectrum (~1200 nm in this case).

24 Ch13 An Introduction to Ultraviolet-Visible Molecular Absorption Spectrometry P.350 FIGURE 13-13 Instrmental designs for UV-visible photometers or spectrophotometers. In (a), a single-beam instrument is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector.

25 FIGURE 13-13 In (b), a double-beam-in-space instrument is shown. Here, radiation from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cells before striking two matched photodetectors.

26 double-beam UV/Vis instrument

FIGURE above shows that in addition to two beams (ref and sample), there are also two sources (D2 lamp and Tungsten lamp). 27 Chap. 14 UV-Vis Applications - Class on Sections 14-A to 14-C only. [Other sections are not covered due to time limit.]

• MOLAR ABSORPTIVITIES • ABSORBING SPECIES • Instrumentation • Solvents • Spectra interpretations • Applications 28 14A THE MAGNITUDE OF MOLAR ABSORPTIVITIES • Empirically, molar absorptivities (ε values) that range from zero up to a maximum on the order of 105 L mol-1 cm-1 (100,000 L mol-1 cm-1) are observed in UV-visible molecular absorption spectrometry.

• For any particular absorption maximum, the magnitude of ε depends on the capture cross section (Section 13B, Equation 13-5) of the species and the probability for an energy-absorbing transition to occur. The relationship between ε and these variables has been shown to be 19 ε = 8.7 ×10 PA • where P is the transition probability and A is the cross section target area in square centimeters per molecule.

• The area (A) for typical organic molecules has been estimated from electron diffraction and X-ray studies to be about 10-15 cm2/molecule; transition probabilities vary from zero to one. • For quantum mechanically allowed transitions, values of P range from 0.1 to 1 (not 0 to 1), which leads to strong 4 5 -1 -1 29 absorption bands (εmax = 10 to 10 L mol cm ). 14B ABSORBING SPECIES – in UV-Vis

• The absorption of ultraviolet or visible radiation by an atomic or molecular species M can be considered to be a two- step process. The first step involves electronic excitation as shown by the equation: + ν → ∗ M h M • The product of the absorption of the photon hν by species M is an electronically excited species symbolized by M*. The lifetime of the excited species is brief (10-8 to 10-9 s).

• Any of several relaxation processes can lead to de-excitation of M*. The most common type of relaxation involves conversion of the excitation energy to heat as shown by M*→M + heat • Relaxation may also occur by a photochemical process such as decomposition of M* to form new species.

•  Alternatively, relaxation may involve re-emission of 30 fluorescence or phosphorescence.

Cont’d

 It is important to note that the lifetime of M* is usually so very short that its concentration at any instant is ordinarily negligible.

• The absorption of ultraviolet or visible radiation generally results from excitation of bonding electrons (i.e., between atoms, such as π or σ electrons. Some absorption can also be from “non-bonding electrons”, n. These will be explained later).

• Because of this, the wavelengths of absorption bands can be correlated with the types of bonds in the species under study. Molecular absorption spectroscopy is, therefore, valuable for identifying functional groups in a molecule. • [i.e., UV-Vis is useful for some limited qualitative analysis.]

• More important, however, are the applications of ultraviolet and visible absorption spectroscopy to the quantitative

determination of compounds containing absorbing groups31 . Broad UV/Vis peaks  Why UV-Vis peaks appear as bands (many closely spaced lines), rather than discrete lines? • As noted in Section 6C, absorption of ultraviolet and visible radiation by molecules generally occurs in one or more electronic absorption bands, each of which is made up of many closely packed but discrete lines.

• Each line arises from the transition of an electron from the ground state to one of the many vibrational and rotational energy states associated with each excited electronic energy state.

 Because there are so many of these vibrational and rotational states and because their energies differ only slightly, many closely spaced lines are contained in the typical band. (refer to Fig. next page) 32 FIGURE 6-21 Energy-level diagrams for (a) a sodium atom showing the source of a line spectrum and (b) a simple molecule showing the source of a band spectrum. [補充From Chap. 6] 33 Ch6 An Introduction to Spectrometric Methods P.151 Figure 14-1 (next page) – discussion on UV absorption in gas, condensed state, and solution, etc. – 1,2,4,5-tetrazine

• As can be seen in Figure 14-1a (next page), the visible absorption spectrum for 1,2,4,5-tetrazine vapor shows the fine structure that is due to the numerous rotational and vibrational levels associated with the excited electronic states of this aromatic molecule.

• In the gaseous state, the individual tetrazine molecules are sufficiently separated from one another to vibrate and rotate freely, and the many individual absorption lines appear as a result of the large number of vibrational and rotational energy states.

NH2

三聚氰胺（Melamine， C 化學式： ） 1,2,4,5-tetrazine C3H6N6 N N

C C N 34 H2N NH2 FIGURE 14-1 Ultraviolet absorption spectra for 1,2,4,5-tetrazine. In (a), the spectrum is shown in the gas phase, where many lines due to electronic, vibrational, and rotational transitions can be observed, but the vibrational and rotational structure has been lost.

(b) In nonpolar solvent

In a polar solvent (water) (c) the strong intermolecular forces cause the electronic peaks to blend, giving only a single smooth absorption band.

35 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.368 In the condensed state or in solution, however, the tetrazine molecules have little freedom to rotate, so lines due to differences in rotational energy levels disappear.

Furthermore, when polar solvent molecules surround the tetrazine molecules, energies of the various vibrational levels are modified in a nonuniform way, and the energy of a given state in a sample of solute molecules appears as a single, broad peak.

This effect is more pronounced in polar solvents, such as water, than in nonpolar hydrocarbon media. This solvent effect is illustrated in Figure 14- 1b and c.

36 Note: Discussion of Ultraviolet–visible absorption in relation to luminescence (opposite to “absorption”), including fluorescence spectroscopy and phosphorescence spectroscopy), is now delayed to Chap. 15.

Possible physical process following absorption of a photon by a molecule. This graph demonstrates processes of absorption and luminescence

37 14B-1 Absorption by Organic Compounds p. 368

• All organic compounds are capable of absorbing electromagnetic radiation because all contain valence electrons that can be excited to higher energy levels.

• The excitation energies associated with electrons forming most single bonds are sufficiently high that absorption occurs in the so-called vacuum ultraviolet region (λ < 185 nm), where components of the atmosphere also absorb radiation strongly.

•  Such transitions involve the excitation of non-bonding n electrons to σ* orbitals. The molar absorptivities of n→σ* transitions are low to intermediate and usually range between 100 and 3000 Lmol-1cm-1.

[Note: n-electron = non-bonding electron, “σ bond” – overlap of sp3 and 1s orbitals – ref. see p. 38, “Organic Chemistry” textbook by Solomons/3rd Ed]

• Because of experimental difficulties associated with the vacuum ultraviolet region, most spectrophotometric investigations of organic compounds have involved longer wavelengths than 185 nm. (i.e., range with λ > 185 nm). [We already discussed this earlier.]

38 Note: n→π* and ππ* transitions • Most applications of absorption spectroscopy to organic compounds are based on transitions from n or π electrons to the π* excited state because the energies required for these processes bring the absorption bands into the ultraviolet-visible region (from 200 to 700 nm).

• Both n→π* and ππ* transitions require the presence of an unsaturated functional group to provide the π orbitals.

[Note: π bonds or π orbitals in sp2 carbon – see “Organic Chemistry” textsbook – Solomons/3rd Ed]

• Molecules containing such functional groups and capable of absorbing ultraviolet-visible radiation are called chromophores [color-producer or color-bearer].

Note: [chromophore = a molecular group capable of selective light absorption resulting in coloration of compounds].

39 TABLE 14-1 Absorption Characteristics of Some Common Chromophores – 帶色團. Transition types involved in chromophores of molecules.

40 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.369 Some Theoretical principles in UV-Vis absorption

Different molecules have different chromophores. For the same chromophores, they absorb at same wavelength.

Different molecules absorb radiation of different wavelengths.

An absorption spectrum will show a number of absorption bands corresponding to structural groups within the molecule.

For example, the absorption that is observed in the UV region for the carbonyl group (C=O) in acetone is of the same wavelength as the absorption from the carbonyl group in diethyl ketone.

This serves as basis of qualitative analysis by UV/Vis.

41 Broad bands in UV absorption and difficulty in UV/Vis spectroscopy as a qualitative analysis tool

• The electronic spectra of organic molecules containing chromophores are usually complex, because the superposition of vibrational transitions on the electronic transitions leads to an intricate combination of overlapping lines.

• The result is a broad band of absorption that often appears to be continuous. The complex nature of the spectra makes detailed theoretical analysis difficult or impossible.

• Nevertheless, qualitative or semiquantitative statements concerning the types of electronic transitions responsible for a given absorption spectrum can be deduced from molecular orbital considerations.

42 Table 14-1 – Chromophores and Transition Types

• Table 14-1 (previous page) lists the common organic chromophores and the approximate position (λmax) and peak intensity (εmax) wavelengths at which they absorb.

• The data for position (λmax) and peak intensity (εmax) can serve as only a rough guide for identification purposes, because both are influenced by solvent effects as well as other structural details of the molecule.

• In addition, conjugation (joining together) between two or more chromophores tends to cause shifts in absorption maxima to longer wavelengths.

• Finally, vibrational effects broaden absorption peaks in the ultraviolet and visible regions, which often makes precise determination of an absorption maximum difficult. 43 Molar absorptivities of different transitions • The molar absorptivities for n→π* transitions are normally low and usually range from 10 to 100 Lmol-1cm-1.

• On the other hand, values of molar absorptivities for ππ* transitions generally range between 1000 and 15,000 Lmol-1cm-1. [stronger!]

• Typical absorption spectra are shown in Figure 14-2.

44 Note: A = εbc, where ε is a constant of proportionality, called the absorbtivity. ππ* transitions only

FIGURE 14-2 Absorption spectra for typical organic compounds.

n→π* transitions only

absorbtivities from n → π∗ transitions normally is very low. 45 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.370 Organic compounds • Saturated organic compounds containing such heteroatoms as oxygen-O, nitrogen-N, sulfur-S, or halogens (F, Cl, Br, etc) have nonbonding (n) electrons that can be excited by radiation in the range of 170 to 250 nm (at high UV frequencies). [Table 14-2 lists a few examples of such compounds.]

Organic compounds in water or alcohol: • Some of these compounds, such as alcohols and ethers, are common solvents, so their absorption in this region prevents measuring absorption of analytes dissolved in these compounds at wavelengths shorter than 180 to 200 nm.

46 TABLE 14-2 Absorption by Organic compounds Containing Heteroatoms with Nonbonding (n) Electrons (in UV range)

n  σ∗ transitions 47 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.370 The absorption of UV or visible radiation corresponds to the excitation of outer electrons.

There are three types of electronic transition which can be considered; 1.Transitions involving π, σ, and n electrons; 2.Transitions involving charge-transfer electrons; 3.Transitions involving d and f electrons (not covered in this Unit, but will be explained later).

Possible electronic transitions of π, σ, and n electrons are;

σ  σ∗ has the highest energy. Thus, at very high frequencies.

n  π∗ has the lowest energy. Thus, at low frequencies.

48 1.Transitions involving π, σ, and n electrons σ σ∗ Transitions An electron in a bonding σ orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo σ  σ* transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to σ  σ* transitions are not seen in typical UV-Vis spectra (lower than 200 nm). n  σ* Transitions Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n  σ* transitions. These transitions usually need less energy than σ → σ * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n  σ* peaks in the UV region is small. n  π* and π  π* Transitions Most absorption spectroscopy of organic compounds is based on transitions of n or π electrons to the π* excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide49 the π electrons. Molar absorbtivities Molar absorbtivities from n → π∗ transitions are relatively low, and range from 10 to100 L mol-1 cm-1 .

π→ π∗ transitions normally give molar absorbtivities between 1000 and 10,000 L mol-1 cm-1 (much higher than n → π∗ transitions).

For either of these two types, these transitions need an unsaturated group in the molecule to provide the π electrons

Solvent effects: The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species. Peaks resulting from n → π∗ transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital.

Often (but not always), the reverse (i.e. red shift) is seen for π → π∗ transitions.

Molar absorbtivities from charge-transfer absorption are large (greater that 10,000 L mol-1 cm-1) [actually too intense to be measured!]. 50 14B-2 Absorption by Inorganic Species p.370

• A number of inorganic anions exhibit ultraviolet absorption bands that are a result of exciting nonbonding (n) electrons.

• Examples include nitrate (313 nm), carbonate (217 nm), nitrite (360 and 280 nm), azido (230 nm), and trithiocarbonate (500 nm) ions.

Not just organic compounds absorb UV/Vis light; these examples demonstrate inorganic Species can also absorb UV/Vis light.

51 first two transition series - ions and complexes • In general, the ions and complexes of elements in the first two transition series absorb broad bands of visible radiation in at least one of their oxidation states and are, as a result, colored (see, for example, Figure 14-3).

• Here, absorption involves transitions between filled and unfilled d-orbitals with energies that depend on the ligands bonded to the metal ions.

• The energy differences between these d-orbitals (and thus the position of the corresponding absorption maximum) depend on the position of the element in the periodic table, its oxidation state, and the nature of the ligand bonded to it.

52 53 FIGURE 14-3 Absorption spectra of aqueous solutions of transition metal ions.

54 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.370 Sharp absorption peaks - 4f and 5f electrons (of rare earth metals). • Absorption spectra of ions of the lanthanide and actinide transitions series (sharp peaks) differ substantially from those shown in Figure 14-3 (broad peaks for d-orbital in transition-metal ions). • The electrons responsible for absorption by these elements (4f and 5f, respectively, - with high atomic numbers) are shielded from external influences by electrons that occupy orbitals with larger principal quantum numbers. • As a result, the bands tend to be narrow and relatively unaffected by the species bonded by the outer (see ). electrons Figure 14-4 55 FIGURE 14-4 Absorption spectra of aqueous solutions of rare earth ions (lanthanide series). Ho (Z=67) Er (Z=67) Pr (Z=59) Sm (Z=62)

Note: lanthanide series (La, Z=57-72) and actinide (Ac, Z=89-103) series.

– Note that they all have sharp UV/Vis peaks!  4f and 5f electrons are shielded from external influence by higher quantum-number electrons.56 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.371 14B-3 Charge-Transfer Absorption p.371 • For quantitative purposes, charge-transfer absorption is particularly important because molar absorptivities are unusually large (ε > 10,000), which leads to high sensitivity.

• Many inorganic and organic complexes exhibit this type of absorption and are therefore called charge-transfer complexes.

• A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor (donor-acceptor pairs).

• When this product absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor.

• The excited state is thus the product of a kind of internal oxidation-reduction process. This behavior differs from that of an organic chromophore, in which the excited electron is in a molecular orbital shared by two or more atoms. 57 Schemes for charge-transfer complex

For a complex to demonstrate charge-transfer behaviour, one of its components must have electron donating properties and another component must be able to accept electrons.  [see the red-blue pair demonstrated as following:]

58 charge-transfer complexes Familiar examples of charge-transfer complexes include: - phenolic complex of iron(III), - 1,10-phenanthroline complex of iron(II), - iodide complex of molecular iodine, and

- the hexacyanoferate(II)-hexacyanoferrate(III) complex responsible for the color of Prussian blue. Prussian blue: [Fe4[Fe(CN)6]3]

-The red color of the iron(III) thiocyanate complex is a further example of charge-transfer absorption.

Three spectra of charge-transfer complexes are shown in Figure 14-5. -Generally, the absorptivity (e) for charge-transfer complexes is quite high (and broad)! 59 FIGURE 14-5 Absorption spectra of aqueous charge-transfer complexes. [Absorption is very intense, but very broad!]

a/ iron(III) thiocyanate complex b/ phenolic complex of iron(III) c/ starch-iodine

In inorganic chemistry, most charge- transfer complexes involve electron transfer between metal atoms and ligands.

starch-iodine complex has strong blue color!

60 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.371 Complexes between organic compounds • Organic compounds form many interesting charge transfer complexes. • An example is quinhydrone (which is a 1:1 complex of quinone and hydroquinone – see graphs on bottom), which exhibits strong absorption in the visible region. • Other examples include iodine complexes with amines, aromatics, and sulfides. [Iodine also form complexes with polymers.] Note: Natural polymers that afford such complexes with iodine species are starch (amylose and amylopectin), chitosan, glycogen, silk, wool, albumin, cellulose, xylan, and natural rubber; iodine-starch being the oldest iodine-natural polymer complex. By contrast, numerous synthetic polymers are prone to make complexes, including poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), nylons, poly(Schiff base)s, polyaniline, unsaturated polyhydrocarbons (carbon nanotubes, fullerenes C60/C70, polyacetylene; iodine-PVA being the oldest iodine-synthetic polymer complex.

61 14-C Applications of UV-Vis p. 372 -UV-Vis absorption spectroscopy useful for identifying functional groups and quantitative determination of compounds containing absorbing groups. -Can also be used for qualitative analysis of organic compounds containing conjugated bonds (dienes, ketones. -The wavelengths of absorption peaks can be correlated with the types of bonds in a given molecule and are valuable in determining the functional groups within a molecule.

Woodward-Fieser rules will be discussed later. 62 14C QUALITATIVE APPLICATIONS OF UV-VIS ABSORPTION SPECTROSCOPY p. 372 • Spectrophotometric measurements with ultraviolet radiation are useful for detecting chromophoric groups, such as those shown in Table 14-1 (p. 369-textbook – shown earlier).

• Because large parts of even the most complex organic molecules are transparent to radiation longer than 180 nm, the appearance of one or more peaks in the region from 200 to 400 nm is clear indication of the presence of unsaturated groups or of atoms such as sulfur or halogens. 63 TABLE 14-1 Absorption Characteristics of Some common Chromophores – shown again here!

Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.369 Cont’d – UV as qualitative analysis • Often, the identity of the absorbing groups can be determined by comparing the spectrum of an analyte with those of simple molecules containing various chromophoric groups.

• Usually, however, ultraviolet spectra do not have enough fine structure to permit an analyte to be identified unambiguously.

• Thus, ultraviolet qualitative data must be supplemented with other physical or chemical evidence such as infrared, nuclear magnetic resonance, and mass spectra as well as solubility and melting- and boiling-point information. 65 14C-1 Solvents p. 372 • Ultraviolet spectra for qualitative analysis are usually measured using dilute solutions of the analyte.

• For volatile compounds, however, gas-phase spectra are often more useful than liquid-phase or solution spectra (for example, compare Figure 14-1-a and b).

• Gas-phase spectra can often be obtained by allowing a drop or two of the pure liquid to evaporate and equilibrate with the atmosphere in a stoppered cuvette (吸收器, with a plug).

66 Considerations on choosing a solvent (for UV-vis) • In choosing a solvent, consideration must be given not only to its transparency, but also to its possible effects on the absorbing system.

• Quite generally, polar solvents such as water, alcohols, esters, and ketones tend to obliterate spectral fine structure arising from vibrational effects.

• Spectra similar to gas-phase spectra (see Figure 14-6) are more likely to be observed in nonpolar solvents such as hydrocarbons.

• In addition, the positions of absorption maxima are influenced by the nature of the solvent.

• As a rule, the same solvent must be used when comparing absorption spectra for identification purposes. 67 FIGUER 14-6 Effect of solvent on the absorption spectrum of acetaldehyde.

Top: non-polar solvent.

Bottom: polar solvents – water or alcohol

68 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.372 Cont’d – solvents for UV spectroscopy

• Table 14-3 (next page) lists some common solvents and the approximate wavelength below which they cannot be used because of absorption. These wavelengths, called the cutoff wavelengths, depend strongly on the purity of the solvent.

• Common solvents for ultraviolet spectrophotometry include water, 95% ethanol, cyclohexane, and 1,4- dioxane.

• For the visible region, any colorless solvent is suitable.

69 Table 14-3. Solvents for the UV and Visible regions (correction)

70 Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.372 14C-3. Detection of functional groups by UV-Vis p. 373 UV-Vis is useful for detection of some functional groups. e.g., weak absorption at 280-290 nm – presence of carbonyl group. e.g., weak absorption at 260 nm – presence of an aromatic ring group.

Note: The π−π* transition in aromatic hydrocarbons is characterized with three absorption bands from high to low frequencies: [e.g. benzene]:

1. Strong absorption at 184 nm (beyond UV detector limit) [E1 band].

2. A weaker (medium) band (E2 band) at 204 nm 3. A still weaker band (called B band) at 256 nm (aromatic rings).

[More information – see Table 14-4 next page] 71 E and B bands will be explained in next page. Note: UV-Vis absorption of aromatic hydrocarbons: Definitions of E and B bands (as well as R- and K-bands)

The following bands are defined: the R-band from the German radikalartig or radical-like, the K-band from the German Konjugierte or conjugated,

B-band from benzoic, and the E-band from ethylenic (system devised by A. Burawoy in 1930).[3]

For example, the absorption spectrum for ethane shows a σ → σ* transition at 135 nm and that of water a n → σ* transition at 167 nm with an extinction coefficient of 7,000. (These are “E-band”.)

Benzene has three aromatic π → π* transitions; two E-bands (E1 and E2) at 180 and 200 nm, and one B-band at 255 nm with extinction coefficients respectively 60,000, 8,000 and 215.

These absorptions are not narrow bands but are generally broad because the electronic transitions are superimposed on the other molecular energy states.

72 TABLE 14-4 Absorption Characteristics of Aromatic Compounds

Ch14 Applications of Ultraviolet-Visible Molecular Absorption Spectrometry P.373 Applications of UV/Vis spectroscopy

1. UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds (see next page), and biological macromolecules. Beer–Lambert law for quantitative analysis 2. A UV/Vis spectrophotometer may be used as a detector for HPLC.

3. The wavelengths of absorption peaks can be correlated with the types of bonds in a given molecule and are valuable in determining the functional groups within a molecule. [for qualitative analysis] The Woodward-Fieser rules, for instance, are a set of empirical observations used to predict λmax, the wavelength of the most intense UV/Vis absorption, for conjugated organic compounds such as dienes and ketones.

4. UV-Vis spectroscopy is also used in the semiconductor industry to measure the thickness and optical properties of thin films on a wafer.

74 Woodward-Fieser rules

I) CONJUGATED DIENE CORRELATIONS (Example):

(i) Base value for homoannular diene = 253 nmase value (ii) Base value for heteroannular diene = 214 nm (iii) Alkyl substituent or Ring residue attached to the parent diene = 5nmreteroannularkylubstituent or Ring residue attached to iene5 n iv) Double bond extending conjugation = 30 nm v) Exocyclic double bonds = 5 nm vi) Polar groups: a) -OAc = 0 nm b) -OAlkyl = 6 nm c) -Cl, -Br = 5 nm

e.g.:

Base value = 214 nm [hetero-annular diene – see left] Ring residue = 3 x 5 = 15 nm

Exocyclic double bond = 1 x 5 = 5 nm

λmax = 214 + 15 + 5 = 234 nm [The wavelength at max. UV-Vis abs.) 75 Postnote: Conjugated compounds - Definition

Conjugation is the overlap of one p-orbital (π) with another across a sigma bond (σ), in larger atoms, d-orbitals can be involved.

A conjugated system has a region of overlapping p-orbitals, bridging the interjacent single bonds. They allow a delocalization of pi electrons across all the adjacent aligned p-orbitals.[2] The pi electrons do not belong to a single bond or atom, but rather to a group of atoms.

The largest conjugated systems are found in graphene, graphite, conductive polymers, and carbon nanotubes. Some examples are shown below: 6 π-electrons

Chemical structure of beta-carotene. The eleven conjugated double bonds 76 that form the chromophore of the molecule are highlighted in red. Anthocyanidine_svg - pigments in plants Conjugated compounds

They are water-soluble pigments that may appear red, purple, or blue depending on the pH. (they are “chromophores”

Pigments in Nature

Many science textbooks incompletely state that autumn coloration (including red) is the result of breakdown of green chlorophyll, which unmasks the already-present orange, yellow, and red pigments (carotenoids, xanthophylls, and anthocyanins, respectively). While this is indeed the case for the carotenoids and xanthophylls (orange and yellow pigments), anthocyanins are not synthesized until the plant has begun 77 breaking down the chlorophyll. Chlorophyll (a and b) is an extremely important biomolecule, critical in photosynthesis, which allows plants to absorb energy from light. Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion.

Conversely, it is a poor absorber of green and near-green portions of the spectrum, hence the green color of chlorophyll-containing tissues.

78 實驗課UV-Vis 儀器 規格 (specifications)

79 UV-Vis 1700 硬體部份 spec * 波長範圍：190.0~1100.0nm • * 光學系統：雙光束 [double beams] • * 光束大小：10*1mm • * 波長帶寬：1nm • * 波長顯示單位：0.1nm • * 波長準確性：±0.3nm • * 波長再現性：±0.1nm • * 波長掃描速度：波長移動速度約6000nm/min • * 光源轉換：有3種選擇，自動轉換 • * 迷光：低於0.05%

* 記錄範圍： • 吸收值:-0.5-3.99 Abs • 穿透率:-0.0-300% • * 測光準確性：±0.002Abs (at 0.5Abs) • * 基準線穩定性：0.001Abs (at 1100~200nm) • * 雜訊P-P：0.002Abs or less • * 基準線校正：自動電腦校正 • * 檢出器：Silicon photodiode [semiconductor-type Detector] • * * 單光器：Aberration corrected concave blazed holographic grating • * RS-232C and Centronic port 標準 80 End of Chaps. 13+14 UV-Vis Absorption spectroscopy Principles and applications

期中考: 化學分析 Chap. 1 + 7 儀器分析 Atomic: Chaps. 8+9, X-ray (Chap. 12)

Reviews of past lectures to be covered in mid-exam (open + closed).

Next: Chap. 15 – Photoluminescence Spectroscopy - Fluorescence and Phosphorescence.

Chap. 16 – IR Fundamental, IR spectroscopy Chap. 17 – IR Applications 81