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Electronic Absorption Spectroscopy of Organic Compounds

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Presentation on theme: "Electronic Absorption Spectroscopy of Organic Compounds"— Presentation transcript:

1 Electronic Absorption Spectroscopy of Organic Compounds
W. R. Murphy, Jr. Department of Chemistry and Biochemistry Seton Hall University

2 Course Topics UV absorption spectroscopy Chiroptic Spectroscopy
Basic absorption theory Experimental concerns Chromophores Spectral interpretation Chiroptic Spectroscopy ORD, CD Effects of inorganic ions (as time permits)

3 Electric and magnetic field components of plane polarized light
Note that refractive index reflect changes in speed of light in different media Light travels in z-direction Electric and magnetic fields travel at 90° to each other at speed of light in particular medium c (= 3 × 1010 cm s-1) in a vacuum

4 Characterization of Radiation
Note inverse relationship between energy and wavelength. Easy to get confused with the way scientists talk about changes (blue shift, red shift, etc)

5 Wavelength and Energy Units
1 cm = 108 Å = 107 nm = 104  =107 m (millimicrons) N.B. 1 nm = 1 m (old unit) Energy 1 cm-1 = cal mol-1 of particles =  1016 erg molecule-1 = 1.24  10-4 eV molecule-1 E (kcal mol-1)  (Å) =  105 E(kJ mol-1) = 1.19  105/(nm) 297 nm = 400 kJ Much of the data on organic molecules is recorded in millimicrons, but not to worry, same as nm. Radiation has lots of units. Must be careful because different types of spectroscopy use different units. IR uses energy units, while absorption spectroscopy uses wavelength. Usually due to instrumental considerations (hard to make monochromators linear in energy). Easy to work in energy now since computers can readily transform data, but is rarely done due to investigator comfort (why do we still use kcal?)

6 Absorption Spectroscopy
Provide information about presence and absence of unsaturated functional groups Useful adjunct to IR Needed for chiroptic techniques Determination of concentration, especially in chromatography For structure proof, usually not critical data, but essential for further studies NMR, MS not good for purity

7 Importance of UV data Particularly useful for
Polyenes with or without heteroatoms Benzenoid and nonbenzenoid aromatics Molecules with heteroatoms containing n electrons Chiroptic tool to investigate optically pure molecules with chromophores Practically, UV absorption is measured after NMR and MS analysis

8 UV Spectral Nomenclature

9 UV and Visible Spectroscopy
Vacuum UV or soft X-rays nm Quartz, O2 and CO2 absorb strongly in this region N2 purge good down to 180 nm Quartz region 200 – 350 nm Source is D2 lamp Visible region 350 – 800 nm Source is tungsten lamp Lots of talk now about deep uv, soft x-ray for microlithography. Not much useful chemical information here. Quartz (normal UV) and visible region of best use

10 All organic compounds absorb UV-light
C-C and C-H bonds; isolated functional groups like C=C absorb in vacuum UV; therefore not readily accessible Important chromophores are R2C=O, -O(R)C=O, -NH(R)C=O and polyunsaturated compounds

11 Spectral measurement usually dissolve 1 mg in up to 100 mL of solvent for samples of D molecular weight data usually presented as A vs (nm) for publication, y axis is usually transformed to  or log10 to make spectrum independent of sample concentration

12 Preparation of samples
Concentration must be such that the absorbance lies between 0.2 and 0.7 for maximum accuracy Conjugated dienes have   8,000-20,000, so c  4  10-5 M n* of a carbonyl have   , so c  10-2 M Successive dilutions of more concentrated samples necessary to locate all possible transitions

13 UV cut-offs for common solvents

14 Solvent choices Important features to consider are solubility of sample and UV cutoff of solvent Filtration to remove particulates is useful to reduce scattered light Solvent purity is very important

15 Chromophores Structures within the molecule that contain the electrons being moved by the photon of light Only those absorbing above 200 nm are useful n* in ketones at ca 300 nm is only isolated chromophore of interest all other chromophores are conjugated systems of some sort

16 Types of organic transitions (Chromophores)
* Sat’d hydrocarbons Vacuum UV n* Sat’d hydrocarbons with heteroatoms Possibly quartz UV * Olefins UV n* Olefins with heteroatoms Organic molecules have a number of possible transitions, but practically, transitions involving the sigma symmetry orbitals are not observed

17 Modes of electronic excitation
Remember, we don’t use the sigma transitions since they are in the vacuum UV

18 Simple lone pair system
Presence of a lone pair containing heteroatom adds an additional transition

19 Simple olefin Olefins have two types of transitions, and the energies are now accessible to the quartz UV region

20 Simple chromophores

21 Examples of n* and  * transitions
Both 1,3-butadiene and 2-butenal have similar pi to pi* bands at 218 nm but 2-butenal has low intensity peak at 316 nm due to n to pi* transition

22 Molecular orbitals for common transitions
Molecular orbital diagram for 2-butenal Shows n  * on right Shows   * on left Both peaks are broad due to multiple vibrational sublevels in ground and excited states

23 Energy level diagram for a carbonyl
Typical example of a system with both n and pi orbitals. This group allows a great deal of information to be inferred due to the number of potential transitions.

24 Beer’s Law Io = Intensity of incident light
I = Intensity of transmitted light  = molar extinction coefficient l = path length of cell c = concentration of sample

25 Transition Energies Electronic transitions are quantized, so sharp bands are expected In reality, absorption lines are broadened into bands due to other types of transitions occurring in the same molecules For electronic transitions, this means vibrational transitions and coupling to solvent Quantum mechanics says that to a first approximation, transitions are narrow. This works for atoms (consider AA and related techniques), but polyatomic systems have vibrations. These get coupled to the electronic transitions, yielding broadened transitions.

26 Actual transition with vibrational levels
Note that presence of vibrational levels in the excited state gives rise to broadened absorption bands. Vibrational levels in the ground state are important for fluorescence and phosphorescence

27 Spectrum for energy level diagram shown on previous slide
Spectrum resulting from energy level diagram on previous slide. Note that the nomenclature indicates the starting vibrational energy level going to the final vibrational level (nomenclature is a little casual here, but is effective).

28 Vibrational fine structure
Rigid molecules such as benzene and fused benzene ring structures often display vibrational fine structure Example is benzene in heptane Usually only observed in gas phase, but rigid molecules do display this Real spectra showing multiple transitions of benzene. We’ll go into the details later, but note the fine structure on the low energy band. Heptane is pretty nonpolar, facilitating the fine structure

29 Benzene (note use of m in this older data)
Solvent effect. Note the drastic change in the spectrum shifting from a nonpolar to a polar solvent.

30 Pyridine Substitution of the heteroatom deresolves the fine structure relative to benzene. Extra vibrations and some degree of structural flexibility is at play here.

31 Mesityl oxide Extra flexibility

32 Intensities of transitions
Strictly speaking, one should work with integrated band intensities However, overlap of bands prevents clean isolation of transitions (hence the popularity of fluorescence in photophysical studies) Therefore, intensities are used

33 Selection Rules After resonance condition is met, the electromagnetic radiation must be able to electrical work on the molecule For this to happen, transition in the molecule must be accom-panied by a change in the electrical center of the molecule Selection rules address the requirements for transitions between states in molecules Selection rules are derived from the evaluation of the properties of the transition moment integral (beyond scope of this course

34 Selection Rule Terminology
Transitions that are possible according to the rules are termed “allowed” Such transitions are correspond-ingly intense Transitions that are not possible are termed “forbidden” and are weak Transitions may be “allowed” by some rules and “forbidden” by others

35 Common Selection Rules
Spin-forbidden transitions Transitions involving a change in the spin state of the molecule are forbidden Strongly obeyed Relaxed by effects that make spin a poor quantum number (heavy atoms) Symmetry-forbidden transitions Transitions between states of the same parity are forbidden Particularly important for centro-symmetric molecules (ethene) Relaxed by coupling of electronic transitions to vibrational transitions (vibronic coupling)

36 Intensities P is the transition probability; ranges from 0 to 1
a is the target area of the absorbing system (the chromophore) chromophores are typically 10 Å long, so a transition of P = 1 will have an  of 105

37 Intensities, con’t. this intensity is actually observed, and has been exceeded by very long chromophoric systems Generally, fully allowed systems have  > 10,000 and those with low transition probabilities will have  < 1000 Generally, the longer the chromophore, the longer wavelength is the absorption maximum and the more intense the absorption

38 Intensities - Important forbidden transitions
near 300 nm in ketones  ca In benzene and aromatics band around 260 nm and equivalent in more complex systems  > 100 Prediction of intensities is a very deep subject, covered in Physical Methods next year

39 Fundamentals of spectral interpretation
Examining orbital diagrams for simple conjugated systems is helpful (lots of good programs available to do these calculations) Wavelength and intensity of bands are both useful for assignments

40 Solvent effects Franck-Condon Principle
nuclei are stationary during electronic transitions Electrons of solvent can move in concert with electrons involved in transition Since most transitions result in an excited state that is more polar than the ground state, there is a red shift ( nm) upon increasing solvent polarity (hexane to ethanol)

41 Solvent effects Hydrocarbons  water * n*
Weak bathochromic or red shift n* Hypsochromic or blue shift (strongly affected by hydrogen bonding solvents) Solvent effects due to stabilization or destabilization of ground or excited states, changing the energy gap

42 Solvent effects, con’t n* in ketones is the exception
there is a blue shift this is due to diminished ability of solvent to hydrogen bond to lone pairs on oxygen example - acetone in hexane, max = 279 nm ( = 15) in water, max = nm

43 Band assignments: n*
 < 2000 Strong blue shift observed in high dielectric or hydrogen-bonding solvents n* often disappear in acidic media due to protonation of n electrons Blue shifts occur upon attachment of an electron-donating group Absorption band corresponding to the n* is missing in the hydrocarbon analog (consider H2C=O vs H2C=CH2 Usually, but not always, n* is the lowest energy singlet transition * transitions are considerably more intense

44 Searching for chromophores
No easy way to identify a chromophore too many factors affect spectrum range of structures is too great Use other techniques to help IR - good for functional groups NMR - best for C-H

45 Identifying chromophores
complexity of spectrum compounds with only one (or a few) bands below 300 nm probably contains only two or three conjugated units extent to which it encroaches on visible region absorption stretching into the visible region shows presence of a long or polycyclic aromatic chromophore

46 Identifying chromophores
Intensity of bands - particularly the principle maximum and longest wavelength maximum Simple conjugated chromophores such as dienes and  unsaturated ketones have  values from 10,000 to 20,000 Longer conjugated systems have principle maxima with correspondingly longer max and larger 

47 Identifying chromophores
Low intensity bands in the nm (with  ca ) are result of ketones Absorption bands with  ,000 almost always show the presence of aromatic systems Substituted aromatics also show strong bands with  > 10,000, but bands with  < 10,000 are also present

48 Next steps in spectral interpretation
Look for model systems Many have been investigated and tabulated, so hit the literature Major references Organic Electronic Spectral Data, Wiley, New York, Vol 1-21 ( ) Sadtler Handbook of Ultraviolet Spectra, Heyden, London

49 Substructure identification

50 Substituted acyclic dienes
max shifts Presence of substituents Length of conjugation

51 Conjugated dienes Strong UV absorber
max affected by geometry and substitution pattern S-trans  217 nm S-cis  253 nm Replacement of hydrogen with alkyl or polar groups red shift these base values Extending conjugation also red shifts max

52 Conjugated Polyenes

53 Diene example

54 Energy levels for butadiene

55 Distinguishing between polyenes

56 Diene Examples 1

57 Diene Examples 2

58 Effects of Ring Strain

59 Molecular orbitals for common transitions
Molecular orbital diagram for 2-butenal Shows n  * on right Shows   * on left Both peaks are broad due to multiple vibrational sublevels in ground and excited states

60 Orbital Diagram for Carbonyl Group
n* bands are weak due to unfavorable orientation of n electrons relative to the * orbitals

61 Rules for calculation of * max for conjugated carbonyls

62 Distinguishing between enones

63 Selected References Harris, D. C., Bertolucci, M. D., Symmetry and Spectroscopy, Dover, 1978. Pasto, D. J., Johnson, C. R., Organic Structure Determination, Prentice-Hall, 1969. Drago, R. S., Physical Methods for Chemists, Surfside Publishing, 1992. Nakanishi, K., Berova, N., Woody, R. W., Circular Dichroism, VCH Publishers, 1994 Williams, D. H., Fleming, I., Spectroscopic methods in organic chemistry, McGraw-Hill, 1987.

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