THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth UV/Vis (Electronic) Spectroscopy

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1 THERY and INTERPRETATIN of RGANIC SPECTRA H. D. Roth UV/Vis (Electronic) Spectroscopy Electrons are raised from σ, π, n levels to n, π, σ levels. All transitions are strictly quantized ΔE = hν E Spectral Range σ* π* n π σ } } anti-bonding non-bonding bonding n-π* π-π* n-σ* σ-σ* nm Visible (conjugated π-systems) nm UV (near) nm Vacuum UV This technique can be used quantitatively; in a typical application the eluent of an HPLC chromatograph is detected by UV Lambert Beer Law A = ε x c x b = log I 0 /I I 0 /I ε intensity of the incident/ transmitted light molar absorptivity or extinction coeffient (a characteristic property of substances) may be solvent dependent (hydrogen bonding solvents) general range of ε: c concentration (mol l 1 ) b pathlength of the cell (usually1 cm; sometimes1 mm) 1

2 For quantitative analysis: Case 1: measure A, know ε and b calculate c; Case 2: measure A, know c and b calculate ε. Some solvents (cut off, nm) cyclohexane 190 ethanol (95%) 198 hexane 187 methanol 198 CCl4 245 water 197 CHCl3 223 dioxane 215 CH2Cl2 215 isooctane 195 Beware of impurities (and sexist phrases): ("one man's signal is another's impurity") Chromophore a functional group that absorbs UV Bathochromic shift, a shift to longer wavelength (lower energies) Hypsochromic shift, a shift to shorter wavelength (higher energies) Auxochrome a group that causes a bathochronic shift (it shifts absorption to a more accessible region) hypsochromic shift bathochromic Spectra of systems with more than one chromophore are additive, unless the chromophores interact (charge transfer spectra, vide infra)

3 Electronic Transitions 1. σ σ* transitions are typical for alkanes; they require high energies, λ max <150 nm 2. n-σ* transitions are typical for compounds containing one hetero atom (, N, halogen), thus have occupied n-orbitals: (CH3)3N λmax 199nm ε 3950 Absorption Spectra of Haloalkanes CH3Cl λmax 173 nm ε 200 C3H7Br 208 nm 300 CH3I 259 nm 400 CH3H 177 nm 200 C5H11SH 224 nm 126 Comparing the spectra of three alkyl halides and two alkyl chalcogenides, we note that there is a shift to lower energies (by nm) when going to the higher halogen or the higher chalcogen. Rule (of thumb): the λmax of the next higher halogen or chalcogen is shifted to lower energies (bathochromic) by nm. 3. n-π* transitions are typical for ketones π π C C σ The non-bonding (n-) orbital is orthogonal to the π*orbital transition is "forbidden"; it has low probability and low extinction coefficient, ε. 3

4 4. π-π* transitions are typical for alkenes and conjugated systems Ψ 2 π* α β ethene Ψ 1 ΔE = 2 β π α + β λ max 165nm ε 10,000 Ψ 4 Ψ 3 Ψ 2 π* α β ΔE = 1.24 β π α + β butadiene λ max 217nm ε 4,600 Ψ 1 Conjugated Double Bonds ethene λmax 165 ε 10,000 butadiene ,000 hexatriene ,000 octatetraene 304 3,300 Rule (of thumb): each double bond shifts the λmax to lower energies (bathochromic) by approximately 50 nm. However, you should be consider also that the configuration and structure affects the absorption band of a diene in a major way. λ max ε 239 4, , ,000 4

5 The structures of some naturally occurring anti-oxidants containing extensive conjugated systems are shown below. Role of Alkyl Groups butadiene λmax 217 ε 21,000 isoprene ,000 2,5-dimethylhexadiene ,000 Rule (of thumb): each alkyl group shifts the λmax to lower energies (bathochromic) by 5 nm. The substituent effects on λmax are additive. 5

6 The Woodward Rules, e.g. for polyenes, cyclic unsaturated systems, unsaturated ketones, allow one to predict the λmax of an unknown compound based on characteristic increments for substituents in each position. β R C C β H 3 C α Calc. λmax : 215 base 10 α subst. 12 β subst. 237 nm (vs. 232 nm observed) Benzene and Annulated Aromatic Systems Increasing the conjugation (the number of annulated benzene rings in an aromatic compound) shifts the λmax by 50 nm to lower energies. This is demonstrated in the figure for benzene (blue), naphthalene (yellow), anthracene (green), and tetracene (red). Rule (of thumb): each annulated benzene ring shifts the λmax to lower energies (bathochromic) by 50 nm. 6

7 Just as is the case for polyenes, the structure also affects the absorption band of aromatic π systems. For example, naphthalene (colorless) absorbs at lower wavelength, than 1,6-methano-cyclodecapentaene (yellow), or azulene (purple). colorless yellow purple 7

8 So far, we have classified electronic transitions by the type of orbitals involved, e.g., n-π* or π-π*. For example acetophenone has three bands: λ max ε transition H 3 C ,600 1,600 π-π* π-π* n-π* Another method of classification uses the type of chromophore, i. e., B enzenoid R adical like E thylenic K onjugated λ max transition type 245 nm π-π* K 435 nm n-π* R Banded Spectra Many typical electronic spectra show only broad, featureless bands. In special cases, however, the spectra have some fine structure: they are banded. The spectra of the aromatic molecules (vide supra) are excellent examples. This feature is due to vibrationally excited levels in the product of the spectroscopic transition. In our case, the π-π * transition of an electron from HM to LUM generates an excited state with two singly occupied orbitals. Although the ground state has only one vibrational level populated (vibrational relaxation is fast), the excited state may be populated into several vibrationally excited states with decreasing probabilities. The vibrational spacing can be determined from the separation of the lines in the spectrum. 8

9 Some excited states decay by emitting the energy difference in form of a photon. Depending on the nature of the excited state this process is called fluorescence or phosphorescence; the general term is luminescence. Emission occurs from the vibrational ground level of the excited state (vibrational relaxation is fast) to several vibrationally excited levels of the ground state. These considerations explain why luminescence always occurs at wavelengths slightly longer than the excitation wavelength. 9

10 Charge Transfer Spectra We consider the electronic spectra of an electron donor and an electron acceptor in a polar solvent. Upon irradiation either donor or acceptor gives rise to an excited state (D* or A*) which can be quenched by electron transfer (ET) generating a pair of radical ions (A D + ). Because ET can occur before the two reacting molecules are in contact the two ions are separated by a few solvent molecules (solvent separated radical ion pair, SSRIP). A A* A* + D A + D+ D D* D* + A A + D+ Some donors and acceptors may interact without light energy to form charge transfer complexes with characteristic charge transfer (CT) spectra. A mixture of a donor, D (red band) and an acceptor, A (blue band), show a new band in the visible region (green band). D + A Aδ.. Dδ+ Irradiation of such a mixture at wavelengths where only D or A absorb generates SSRIPs; irradiating the donor-acceptor (CT) complexes at the characteristic charge transfer band also gives rise to radical ion pairs; 10

11 [Aδ.. Dδ+] [Aδ.. Dδ+] however, these ion pairs are different from the SSRIPs obtained upon irradiation of either A or D, because no solvent molecules separate the two ions ( intimate or contact radical ion pairs, CRIP). [A solvent D+] vs [Aδ.. Dδ+] solvent separated radical ion pairs SSRIP contact radical ion pairs CRIP The concentration of the charge transfer complex (green spectrum) is determined by [A], [D] and Keq (an intrinsic constant for each pair) Keq = [Aδ.. Dδ+] or [Aδ.. Dδ+] = Keq [A] [D] [A] [D] The UV spectra of solutions of tetrafluorobenzoquinone (3.5x10 3 M) in acetonitrile (a) and benzene (b) are an interesting example. The benzene solution shows a tenfold increase in ε, an effect much too large for a simple solvent effect. If this were a CT spectrum, a better donor [e.g., 1,3,5- trimethylbenzene (c)] should result in a red-shifted spectrum, as observed. 11

12 Summary Irradiation of acceptor (top), donor (bottom) or a CT complex (left) generates either SSRIPs (top, bottom) or CRIPs (left). Some CRIPSs may diffuse apart to form SSRIPs, and these, in turn, may diffuse apart to form free radical ions (not coordinated with a counter ion). Time-Resolved Spectroscopy The application of time- resolved spectroscopy has become an important tool for the study of short-lived reactive intermediates. Modern laser spectroscopy allows the study reactions on timescales of ms, µs, ns, ps and even fs. The example below covers the range of nano- to microseconds; 12

13 open circles: irradiation of p-methoxy styrene (D) forms the radical cation. filled circles: irradiation of chloranil (A) in the presence of D forms both ions. If the reactive intermediate (diphenylamine radical cation, 695 nm) is stabilized by incorporation into a zeolite, the conversion to diphenylaminyl radical (460 nm) by deprotonation can be studied over a period of several hours. 13