Electronic Absorption Spectroscopy of Organic Compounds W. R. Murphy, Jr. Department of Chemistry...

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.