波譜分析課件 chapter 2 UV-vis.ppt

1、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 excit

2、ed 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

3、 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 a

4、nd antibonding orbital Bonding orbital energy level is always lower than that of the original atomic orbital Antibonding orbital energy - higher s , p orbitals and h electrons,Types of Electronic Transitions,Transitions involving p, s, and n electrons Transitions involving charge-transfer electrons

5、Transitions involving d and f electrons (not covered in this Unit),Absorbing species containing p, s, 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

6、. 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.,s - s* Transitions,An electron in a

7、 bonding s 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 s - s* transitions) shows an absorbance maximum at 125 nm. Absorption maxima due to s - s* transitions are not seen in typical UV

8、-Vis. spectra (200 - 700 nm),n - s* Transitions,Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n - s* transitions. These transitions usually need less energy than s - s * transitions. They can be initiated by light whose wavelength is in the range 150 - 2

9、50 nm. The number of organic functional groups with n - s* peaks in the UV region is small.,n - p* and p - p* Transitions,Most absorption spectroscopy of organic compounds is based on transitions of n or p electrons to the p* excited state. This is because the absorption peaks for these transitions

10、fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the p electrons.,n - p* and p - p* Transitions(continue),Molar absorbtivities from n - p* transitions are relatively low, and range from 10 to100 L mol-1

11、 cm-1 . p - p* 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 - p* transition. The maximum absorption wavelength 270 nm, max 100 Example: Acetone max 279 nm, max =15,The corr

12、esponding absorption band,K band (conjugation band, 德文Konjuierte) form p - p* transition. High max ( 104) Example: Dienes Acetophenone,The corresponding absorption band,B band (Benzene band, Benzenoid bands) from the p - p* transition of Benzene. Broad band with fine structure between 230 270 nm. Th

13、is band can be used to identify aromatic compound.,The corresponding absorption band,E band (Ethylenic bands) also from p - p* 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

14、species. Peaks resulting from n -p* 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 p - p* tra

15、nsitions. 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 red

16、uced - resulting in a small red shift. This effect also influences n -p* transitions but is overshadowed by the blue shift resulting from solvation of lone pairs.,Choice of Solvent,2.3 UV spectra and molecular structure,The absorbing groups in a molecule are called chromophores A molecule containing

17、 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,Chromoph

18、orelmaxTransition Alkanes 150s to s* Alkenes 175p to p* Alkynes 170 Carbonyls 188 alcohols, ethers 185h to s* Amines 195 sulfur compounds 195 Carbonyls 285h to p*,Woodward-Fieser Rules for Dienes,Homoannular Heteroannular (cisoid) (transoid) Parentl=253 nm l=214 nm Increments for: Double bond extend

19、ing conjugation 3030 Alkyl substituent or ring residue 5 5 Exocyclic double bond 5 5,Woodward-Fieser Rules for Dienes,Homoannular Heteroannular (cisoid) (transoid) Parentl=253 nm l=214 nm Increments for: -OC(O)CH3 00 -OR 6 6 Cl, -Br 5 5 -NR2 60 60 -SR 30 30,Woodwards Rules for Conjugated Carbonyl Co

20、mpounds,d g b a - C = C C = C C = O | R,Woodwards Rules for Conjugated Carbonyl Compounds,Base values: X = R Six-membered ring or acyclic parent enonel=215 nm Five-membered ring parent enonel=202 nm Acyclic dienonel=245 nm X = Hl=208 nm X = OH, ORl=193 nm Increments for: Double bond extending conjug

21、ation30 Exocyclic double bond5 Endocyclic double bond in a 5- or 7-membered ring for X = OH, OR 5 Homocyclic diene component39,Woodwards Rules for Conjugated Carbonyl Compounds,Alkyl substituent or ring residuea 10 b 12 g or higher 18 Polar groupings: -OHa35 b30 d50 -OC(O)CH3a,b,g,d6 -OCH3a35 b30 g1

22、7 d31,Woodwards Rules for Conjugated Carbonyl Compounds,-Cla15 b,g,d12 -Brb30 a,g,d25 -NR2b95 Solvent correction*: *Solvent shifts for various solvents: Solventlmax shift (nm) water+ 8 chloroform- 1 ether- 7 cyclohexane- 11 dioxane- 5 hexane- 11,Woodwards Rules for Aromatic Compounds,1. Absorption f

23、or Mono-Substituted Benzene Derivatives E KBR (e30000) (e10000) (e300) (e50) Electronic Donating Substituents none184204254 -R189208262 -OH211270 -OR217269 -NH2230280,Woodwards Rules for Aromatic Compounds,E KBR Electronic Withdrawing Substituents -F204254 -Cl210257 -Br210257 -I207258 -NH3+203254,Wo

24、odwards Rules for Aromatic Compounds,E KBR p-Conjugating Substituents -C=CH2248282 -CCH202248278 -C6H5250 -CHO242280328 -C(O)R238276320 -CO2H226272 -CN224271 -NO2252280330,Woodwards Rules for Aromatic Compounds,The adsorption band would have red shift and disappearance of B band fine structure with

25、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

26、 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 ele

27、ment 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.

28、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 spectrometer

29、s 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 w

30、avelength.,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. Th

31、ere 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 dete

32、ctor. 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 pix

33、els 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 s