Chapter 2 Ultraviolet and Visible Absorption Spectroscopy (UV-Vis)

• At room temperature, most of the atoms, molecules and electrons are in the lowest energy orbital called ground state. 

• The electron of atom (molecule) at ground state can absorb proton and transit to higher energy orbital called excited state.

• Atom or molecule can absorb the radiation only when the energy of proton is equal to the energy difference of the two orbitals

2.1 Basic principles of UV-vis

• Ultraviolet-visible spectroscopy corresponds to excitations of outer shell electron between the energy levels that correspond to the molecular orbital of the systems. 

• The band spectrum of molecule due to vibrational and rotational levels

Comparing: Atomic spectrum

2.2 Molecular orbitals and electronic transition

• While two atoms form chemical bond, their atomic orbital combine together to form molecular orbital.

• Bonding orbital and antibonding orbital Bonding orbital energy level is always low

er than that of the original atomic orbital Antibonding orbital energy - higher , orbitals and electrons

Types of Electronic Transitions

1. Transitions involving , , and n electrons

2. Transitions involving charge-transfer electrons

3. Transitions involving d and f electrons (not covered in this Unit)

Absorbing species containing , , and n electrons

• Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy. The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band.

* Transitions

• An electron in a bonding orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane (which has only C-H bonds, and can only undergo * transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to * transitions are not seen in typical UV-Vis. spectra (200 - 700 nm)

n* Transitions

• Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n* transitions. These transitions usually need less energy than * transitions. They can be initiated by light whose wavelength is in the range 150 - 250 nm. The number of organic functional groups with n* peaks in the UV region is small.

n* and * Transitions

• Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the electrons.

n* and * Transitions(continue)

• Molar absorbtivities from n* transitions are relatively low, and range from 10 to100 L mol-1 cm-1 . * transitions normally give molar absorbtivities between 1000 and 10,000 L mol-1 cm-1 .

The corresponding absorption band

• R band (group type,德文 Radikalartig) originated from n* transition. The maximum absorption wavelength > 270 nm, єmax < 100

Example:

Acetone

λ max 279 nm, єmax =15

The corresponding absorption band

• K band (conjugation band, 德文 Konjuierte) form * transition. High єmax ( > 104)

Example:

Dienes

Acetophenone

The corresponding absorption band

• B band (Benzene band, Benzenoid bands) from the * transition of Benzene. Broad band with fine structure between 230 – 270 nm. This band can be used to identify aromatic compound.

The corresponding absorption band

• E band (Ethylenic bands) also from * transition of ethylenic band in benzene

E1 band and E2 band

Solvent effect

• The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species.

• Peaks resulting from n* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation of the lone pair, which lowers the energy of the n orbital.

Solvent effect (cont)

• The reverse (i.e. red shift) is seen for * transitions. This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced - resulting in a small red shift.

• This effect also influences n* transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.

Choice of SolventSolvent Minimum

Wavele-ngth (nm)

Solvent Minimum Wavelength

(nm)

Solvent Minimum Wavelength

(nm)

Acetonitrile 190 water 191 cyclohexane 195

hexane 195 methanol 201 ethanol 204

ether 215 methylene chloride

220 Chloroform 237

carbon tetrachloride

257

2.3 UV spectra and molecular structure

• The absorbing groups in a molecule are called chromophores

• A molecule containing a chromophore is called a chromogen

• An auxochrome does not itself absorb radiation, but can enhance the absorption

• Bathochromic shift – red shift• Hypsochromic shift – blue shift• Hyperchromism – an increase in absorption• Hypochromism – a decrease in absorption

2.3 UV spectra and molecular structure

•

• Chromophore max Transition• Alkanes ~ 150 to

• Alkenes ~ 175 to

• Alkynes ~ 170• Carbonyls ~ 188• alcohols, ethers ~ 185 to

• Amines ~ 195• sulfur compounds ~ 195• Carbonyls ~ 285 to

Woodward-Fieser Rules for Dienes

•    Homoannular Heteroannular(cisoid) (transoid)

• Parent =253 nm =214 nm• Increments for:• Double bond extending conjugation 30 30• Alkyl substituent or ring residue 5 5• Exocyclic double bond 5 5

Woodward-Fieser Rules for Dienes

•    Homoannular Heteroannular(cisoid) (transoid)

• Parent =253 nm =214 nm• Increments for:• -OC(O)CH3   0   0• -OR 6 6• Cl, -Br 5 5• -NR2 60 60• -SR 30 30

Woodward's Rules for Conjugated Carbonyl Compounds

- C = C – C = C – C = O

|

R

Woodward's Rules for Conjugated Carbonyl Compounds

• Base values:•  X = RSix-membered ring or acyclic parent enone =215 nm Five-membered ring parent enone =202 nmAcyclic dienone =245 nm

•  X = H =208 nm•  X = OH, OR =193 nm• Increments for:Double bond extending conjugation 30Exocyclic double bond 5Endocyclic double bond in a 5- or 7-membered ring for X = OH, OR

 5Homocyclic diene component 39

Woodward's Rules for Conjugated Carbonyl Compounds

Alkyl substituent or ring residue 10

12

or higher 18

•  Polar groupings:-OH 35

30

50

-OC(O)CH3   6

-OCH3 35

30

17

31

Woodward's Rules for Conjugated Carbonyl Compounds

-Cl 1512-Br 3025

-NR2 95

• Solvent correction*:*Solvent shifts for various solvents:

Solvent max shift (nm)water + 8chloroform - 1ether - 7cyclohexane - 11dioxane - 5hexane - 11

Woodward's Rules for Aromatic Compounds

• 1. Absorption for Mono-Substituted Benzene Derivatives

E K B R (>30000) (~10000) (~300) (~50)

Electronic Donating Substituents

 none 184 204 254

-R 189 208 262

-OH 211 270

-OR 217 269

-NH2 230 280

 

Woodward's Rules for Aromatic Compounds

  E K B R

Electronic Withdrawing Substituents

-F 204 254

-Cl 210 257

-Br 210 257

-I 207 258

-NH3+ 203 254

Woodward's Rules for Aromatic Compounds

  E K B R

-Conjugating Substituents

-C=CH2 248 282

-CCH 202 248 278

-C6H5 250

-CHO 242 280 328

-C(O)R 238 276 320

 -CO2H 226 272

-CN 224 271

-NO2 252 280 330

Woodward's Rules for Aromatic Compounds

• The adsorption band would have red shift and disappearance of B band fine structure with Mono-Substitution (F is an exception) and Di-Substituted Benzene

UV-vis Spectrophotometer

• Single-Beam UV-Vis Spectrophotometer

• Single-Beam spectrophotometers are often sufficient for making quantitative absorption measurements in the UV-Vis spectral region.

• Single-beam spectrophotometers can utilize a fixed wavelength light source or a continuous source.

Single-Beam UV-Vis Spectrophotometer

The simplest instruments use a single-wavelength light source, such as a light-emitting diode (LED), a sample container, and a photodiode detector.

Instruments with a continuous source have a dispersing element and aperture or slit to select a single wavelength before the light passes through the sample cell.

Dual-Beam uv-vis Spectrophotometer

• In single-beam Uv-vis absorption spectroscopy, obtaining a spectrum requires manually measuring the transmittance of the sample and solvent at each wavelength. The double-beam design greatly simplifies this process by measuring the transmittance of the sample and solvent simultaneously.

Instrumentation

• The dual-beam design greatly simplifies this process by simultaneously measuring P and Po of the sample and reference cells, respectively. Most spectrometers use a mirrored rotating chopper wheel to alternately direct the light beam through the sample and reference cells. The detection electronics or software program can then manipulate the P and Po values as the wavelength scans to produce the spectrum of absorbance or transmittance as a function of wavelength.

Array-Detector Spectrophotometer

• Array-detector spectrophotometers allow rapid recording of absorption spectra.

• Dispersing the source light after it passes through a sample allows the use of an array detector to simultaneously record the transmitted light power at multiple wavelengths.

• There are a large number of applications where absorbance spectra must be recorded very quickly. Some examples include HPLC detection, process monitoring, and measurement of reaction kinetics.

Instrumentation

• These spectrometers use photodiode arrays (PDAs) or charge-coupled devices (CCDs) as the detector. The spectral range of these array detectors is typically 200 to 1000 nm. The light source is a continuum source such as a tungsten lamp.

• All wavelengths pass through the sample. The light is dispersed by a diffraction grating after the sample and the separated wavelengths fall on different pixels of the array detector.

Instrumentation

• The resolution depends on the grating, spectrometer design, and pixel size, and is usually fixed for a given instrument.

• Besides allowing rapid spectral recording, these instruments are relatively small and robust. Portable spectrometers have been developed that use optical fibers to deliver light to and from a sample.

• These instruments use only a single light beam, so a reference spectrum is recorded and stored in memory to produce transmittance or absorbance spectra after recording the sample spectrum.

UV spectra analysis procedure

• It’s impossible to conclude the molecular structure directly from it’s UV spectrum

• UV spectra can be applied to identify the types, numbers and position of chromophores and auxochrome; saturated and unsaturated compounds;

Identification of organic compounds with UV

• If no absorption peaks between 200 ~ 400 nm were detected, there isn’t any conjugate double bond and C=O group. Most probably it’s a saturated compound

• If there is a weak peak (ε=10~100) between 270 ~ 350 nm, and no other peaks detected over 200 nm. It may contain >C=O, >C=C-O- or >C=C-N< etc. The weak peak is from n* transition

Identification of organic compounds with UV

• If there are a lot of peaks in UV region, some of them are even within the visible region, the compound may have long conjugation bonds

• When max is over 250 nm, ε is between 1000 ~ 10000, the compound may contain aromatic structure

• ε between 10000 ~ 20000 for the long wave absorption peak maybe conjugated diene or carbonyl compounds