Electronic Absorption Spectroscopy of Organic
Compounds
W. R. Murphy, Jr.
Department of Chemistry and Biochemistry
Seton Hall University
Course Topics
• UV absorption spectroscopy– Basic absorption theory– Experimental concerns– Chromophores– Spectral interpretation
• Chiroptic Spectroscopy– ORD, CD
• Effects of inorganic ions (as time permits)
Electric and magnetic field components of plane polarized light
• 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
Characterization of Radiation
υhcλ
hchυ)moleculeΔE(erg
λ(cm)
1υ
λ(cm)
)secc(cm)υ(sec
molecule
sec erg106.626h
E
hcλor
λ
hcE
energyor υ,υλ,
1
11-
27
Wavelength and Energy Units
• Wavelength– 1 cm = 108 Å = 107 nm = 104 =107 m
(millimicrons)
– N.B. 1 nm = 1 m (old unit)
• Energy– 1 cm-1 = 2.858 cal mol-1 of particles
= 1.986 1016 erg molecule-1 = 1.24 10-4 eV molecule-1
E (kcal mol-1) (Å) = 2.858 105
– E(kJ mol-1) = 1.19 105/(nm)297 nm = 400 kJ
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
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
UV Spectral Nomenclature
UV and Visible Spectroscopy
• Vacuum UV or soft X-rays– 100 - 200 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
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
Spectral measurement
• usually dissolve 1 mg in up to 100 mL of solvent for samples of 100-200 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
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 10-100, so c 10-2 M
• Successive dilutions of more concentrated samples necessary to locate all possible transitions
UV cut-offs for common solvents
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
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
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
•UV
Modes of electronic excitation
Simple lone pair system
Simple olefin
Simple chromophores
Examples of n* and * transitions
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
Energy level diagram for a carbonyl
Beer’s Law
lcAI
I 010log
• Io = Intensity of incident light
• I = Intensity of transmitted light = molar extinction coefficient• l = path length of cell• c = concentration of sample
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
Actual transition with vibrational levels
Spectrum for energy level diagram shown on
previous slide
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
Benzene (note use of m in this older data)
Pyridine
Mesityl oxide
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
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
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
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)
Intensities
a P201087.0• 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
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
Intensities - Important forbidden transitions
• n* – near 300 nm in ketones ca 10 - 100
• 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
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
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 (10 - 20 nm) upon increasing solvent polarity (hexane to ethanol)
Solvent effects
Hydrocarbons water*
– 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
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 = 264.5 nm
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
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
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
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
Identifying chromophores
• Low intensity bands in the 270 - 350 nm (with ca 10 - 100) are result of ketones
• Absorption bands with 1000 - 10,000 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
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 (1960-85)
– Sadtler Handbook of Ultraviolet Spectra, Heyden, London
Substructure identification
Substituted acyclic dienes
max shifts– Presence of substituents
– Length of conjugation
Conjugated dienes
• Strong UV absorbermax 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
Conjugated Polyenes
Diene example
Energy levels for butadiene
Distinguishing between polyenes
Diene Examples 1
Diene Examples 2
Effects of Ring Strain
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
Orbital Diagram for Carbonyl Group
• n* bands are weak due to unfavorable orientation of n electrons relative to the * orbitals
Rules for calculation of * max for conjugated carbonyls
Distinguishing between enones
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.