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.