Transcription
Factors affecting the frequency of infrared peaks1. Resonance, symmetry and conjugation
Effect of resonance, symmetry and conjugation on infraredfrequenciesOO NNO -OO-OCCO -O NR2NR2CCO -OWhat about the effect of conjugation? ODo the facts support this interaction?O-
O-OCCOO -Carbonyl frequency: 1720-1680 (acid) 1700 cm-1Ether frequency 1000 to 1400 cm-1 1200 cm-1Average of the two fundamentals 1450 cm-1
1560 cm-1O-OCCOO -1410 cm-1(1560 1410)/2 1485 cm-1
Resonance100% 1500Wavenumbers, cm-1Figure IR-42. Sodium benzoate, KBr pellet:1000500
Sodium benzoate: (1550, 1400 cm-1 ); Average : 1475 cm-1Sodium acetate: (1560, 1410 cm-1 ); Average : 1485 cm-1
ORO-NR2RHow important is resonance in amides?NR2
CH3CH3NCH3CO
ORO-NR2RNR2 How important is resonance in amides?OInCH3HNCH3the barrier to rotation of the twoCH3 groups is approximately 18kcal/mol(A C-C bond is worth about 60 kcal/mol)
Table 4. The effect of conjugation on carbonyl frequencies.Non-conjugatedCompoundFrequency ConjugatedCompoundcm -1Frequencycm -1Frequencycm ne1717methyl vinylketone1700,1681acetophenone1685propanoic acid1715propenoic acid1702benzoic acid1688ethyl propionate1740ethyl acrylate1727ethyl benzoate1718butanoic nhydride1844,1767phthalicanhydride1852,1762
Other effects of conjugation on carbonyl frequencies100% 500Wavenumbers, cm-1Figure IR-45. Ethyl vinyl ketone, neat liquid: CH3CH2COCH CH2
100% 500Wavenumbers, cm-1Figure IR-44. 3-Nonen-2-one, 95%; neat liquid, thin film:OWhy the extra carbonyl peaks?CH3CH CH(CH2)4CH3
OOFactors affecting the intensities :Extent of interaction (dipolemoment change)Concentration of each conformer OO-
Overtones in terminal olefins
What’sthis?1804 cm-1
1796 cm-1
Factors affecting the frequency of infrared peaks1. Resonance and conjugation2. Ring strain:A: on carbonyl frequencies
1720 cm-11750 cm-11775 cm-1cyclopropanone1800 cm-1
Table. The Effect of Ring Strain on the Carbonyl Frequencies ofSome Cyclic MoleculesRing Size ketone: cm-1lactone:cm-1lactam:cm-13cyclopropanone: 18004cyclobutanone:1775 -propiolactone: 18405cyclopentanone:1751 -butyrolactone: 1750 -butyrolactam: 16906cyclohexanone:1715 -valerolactone: 1740 -valerolactam: 16687cycloheptanone: 1702 caprolactone: 1730 caprolactam: 1658
Factors affecting the frequency of infrared peaks1. Resonance and conjugation2. Ring strain:A: on carbonyl frequenciesB. on C-H stretching frequencies
What is the hybridization of a C-H bond in cyclopropane?
CH2OH
CH2OH
Factors affecting the frequency of infrared peaks1. Resonance and conjugation2. Ring strain:A: on carbonyl frequenciesB. on C-H stretching frequencies3. HalogensA: on carbonyl frequenciesB. on C-H stretching frequencies
30111760
1754,1783 cm-1
Factors affecting the frequency of infrared peaks1. Resonance and conjugation2. Ring strain:A: on carbonyl frequenciesB. on C-H stretching frequencies3. HalogensA: on carbonyl frequenciesB. on C-H stretching frequencies4. Chirality
Why the big differences between D-serine and DL-serine?
dlddl
Some Examples of ConglomeratesAsparagineThreonineGlutamic AcidSerine AnhydrideN-Acetylproline
Factors affecting the frequency of infrared peaks1. Resonance and conjugation2. Ring strain:A: on carbonyl frequenciesB. on C-H stretching frequencies3. HalogensA: on carbonyl frequenciesB. on C-H stretching frequencies4. Chirality5. Phase: solid, liquid and gas (fundamentals in the gas phase areshifted to higher frequencies) ie. solvent or solute interactionslead to weakening of force constants; effects of H-bonding.
Effects of H-bonding120Vapor phase spectrum(135 C)% Transmittance10080liquid , cm-1Figure IR-25. The liquid and vapor spectra of phenol.OH
Gas Phase FT-IR spectrum of 2,4-pentanedione, Aldrich Chemical Co.
Effect of PhaseLiquid film
Effect of PhaseCCl4 solution
Effect of PhaseKBr
Effects of H-bonding20001000cm-1
FT IR
ATR: attenuated total reflectance; when a beam of electromagneticradiation is reflected off an object, it actually penetrates ofthe order of one wavelength.In IR, this is of the order of microns which is sufficient toobtain a spectrum of the material provided it is polydispersed.The depth of penetration is wavelength dependent, it istherefore necessary to compensate for this dependency.
11109Single Beam8765432400030002000Wavenumbers (cm-1)1000Infrared spectrum of ATR and of the background CO2 and H2Ovapor.
Figure IR-42. Sodium benzoate, KBr pellet: 1550. 1400. Resonance. Sodium benzoate: (1550, 1400 cm -1 ); Average : 1475 cm-1 Sodium acetate: (1560, 1410 cm-1 ); Average : 1485 cm-1. R NR2 O R O-NR2 How important is resonance in amides? N CH3 CH3 C O CH3. R NR2 O R O-NR2 How important is resonance in amides? In the barrier to rotation of the two . CH. 3 . groups is approximately 18 kcal/mol
