Molecular Spectroscopy

Transcription

1 Molecular Spectroscopy Types of transitions: 1) Electronic (UV-Vis-Near IR) 2) Vibrational (IR) 3) Rotational (microwave)

2 Electronic Absorption Spectra π π* Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.

3 Atomic Orbitals (AO) Combine to give Molecular Orbitals (MO) Bond Order = (# bonding e - - # antibonding e - ) / 2 Total Spin Quantum Number (S) S = Σs i Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.

4 Are you getting the concept? Calculate the bond order for CO and predict its stability. Assuming that the same set of molecular orbitals applies, suggest a diatomic that would be unstable.

5 Are you getting the concept?

6 Polyatomic Electronic Transitions s orbitals σ p orbitals π non-bonding orbitals - n Ingle and Crouch, Spectrochemical Analysis

7 Common Chromophores: Alkenes π* π π* E λ max ~165 nm ε ~15,000 π

8 Common Chromophores: Carbonyl Compounds n π* 290 nm ε = 10 forbidden π π* 170 nm ε = 100 allowed π* n π E

9 Possible transitions

10 Frank Condon Principle The electronic transition is fast (10-15 s) with respect to nuclear motions. Transitions where the position and momentum of the nuclei don t change are favored. J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

11 Vibronic transitions where the wavefunctions line up are favored. Ingle and Crouch, Spectrochemical Analysis

12 What do the intensity patterns tell us about the allowed transitions? J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

13 Series of bands with the same v (absorption) or v (emission). Bands are separated by v (for absorption) or v (for emission). Progressions J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

14 Progressions J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

15 Sequences V v is constant. Series of bands separated by (v -v ). More commonly observed in emission experiments. V J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

16 Shape of the electronic spectrum is determined by vibrational and rotational structure. Ingle and Crouch, Spectrochemical Analysis

17 UV-vis Absorption (Extinction) Spectroscopy Single-Beam or Double-Beam Fixed λ or Dispersive Common: Source Tungsten Halogen Lamp ( nm) Sample Liquid In Cuvette Dispersion Spectrograph w/ Diffraction Grating Detector CCD Beer s Law: A = εbc

18 Assumptions Ingle and Crouch, Spectrochemical Analysis

19 Apparent Deviations from Beer s Law Non-Zero Intercept Improper blank measurement or correction. Instrumental drift. Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

20 Apparent Deviations from Beer s Law Non-Linear Calibration Plot Chemical Equilibrium if multiple chemical forms of analyte exist and only one absorbs Other Chemical Effects solute/solvent interactions, solute/solute interactions, H bonding at high concentrations Using Polychromatic Radiation non-optimum wavelengths are still transmitted and detected Stray Light causes measured transmittance to be larger than it should be Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

21 Absorbed/Emitted Colors Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

22 Chromophores Often transitions are localized in specific bonds or functional groups within a molecule. Group will have a characteristic λ max and ε. Molecular structure or environment can influence λ max and ε. Shift to longer λ bathochromic (red) shift. Shift to shorter λ hypsochromic (blue) shift. Increase in ε hyperchromic effect. Decrease in ε hypochromic effect. What effect does conjugation usually have? hyperchromic effect / bathochromic shift

23 Characteristic Electronic Transitions L mol -1 cm -1 Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

24 Characteristic Electronic Transitions L mol -1 cm -1 Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

25 Auxophore Does not absorb Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore (e.g. -OH, -Br, -NH 2 ). Solvent Effects Hypsochromic shift in n π* transitions as solvent polarity increases. Solvation stabilizes the nonbonding pair. Bathochromic shift in π π* transitions as solvent polarity increases. Solvation stabilizes π*, which is often more polar than π.

26 Conjugated Alkenes Woodward-Fieser Rules Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

27 1,3-butadiene antibonding π π* bonding

28 Conjugated Dienes

29 UV Absorption of Conjugated Alkenes ε units = L mole -1 cm -1 E π* π λmax ε 15,000 21,000 35,000 Increasing conjugation gives: longer wavelength absorption more intense absorption

30 β-carotene 11 double bonds λ max = 460 nm (ε = 139,000)

31 Fieser-Kuhn Rules Systems with More than 4 Double Bonds λ max (nm) = M + n( n) 16.5R endo 10R exo n = number of conjugated double bonds M = number of alkyl or alkyl like substituents on the conjugated system R endo = number of rings with endocyclic double bonds in the conjugated system R exo = number of rings with exocyclic double bonds

32 Are you getting the concept? Calculate the absorption maximum for lycopene: CH 3 CH 3 CH 3 CH 3 H 3 C CH 3 CH 3 CH 3 CH 3 CH 3

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34 Substituted Benzenes π 6 π 4 π 5 π 2 π 3 π 1 Symmetry and selection rules limit actual transitions: ~185 nm (ε~60,000), primary band or E band ~204 nm (ε~8,000), second primary band or K band ~256 nm (ε~200), secondary band or B band

35 Substituted Benzenes Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

36 Polyaromatics

37 Substituent Effects on Aromatic Absorption 256 nm band is sensitive to electron density of aromatic ring ε units = L mole -1 cm -1 O OH OCH 3 OC CH nm (ε 1,450) 287 nm (ε 2,600) 258 nm (ε 250) PHENOL ANISOLE PHENYL ACETATE Electron density Red = highest Green = moderate

38 ph Effects on Aromatic Absorption ε units = L mole -1 cm -1 base OH O 270 nm (ε 1,450) 287 nm (ε 2,600) NH 2 NH nm (ε 1,430) 254 nm (ε 160) Phenoxide ion electrostatic potential map Anilinium ion electrostatic potential map

39 Aromatic Carbonyl Compounds Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

40 Are you getting the concept? Predict the absorption λ max for cholesta-1,4-dien-3-one and the enol of 1,2-cyclopentanedione. ~ ~ HO O O

41 Common Solvents Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds