1. Provides information about the vibraions of functional groups in a molecule Infrared Spectroscopy Therefore, the functional groups present in a molecule can be deduced from an IR spectrum

2. Two important parameters in all IR spectra: The frequency of the signal, The frequency of the signal, The intensity of the signal, I What structural features of a molecule do and I depend on?

3. IR: Masses, Atoms and Springs F = force, restoring back to equilibrium position k = characteristic stretching constant x = displacement from the equilibrium position A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds). As a first approximation use Hooke’s Law F = -kx

4. Vibrations, potential energy and motion

5. The functional group concept of organic chemistry IR spectroscopy can identify functional groups

6. IR Stretching Frequencies of two bonded atoms: = frequency = frequency k = spring strength (bond stiffness) m r = reduced mass (mass of largest atom) What Does the Frequency,, Depend On?

7. IR Stretching Frequencies: What Do they Depend On? Directly on the strength of the bonding between the two atoms ( ~ k) Inversely on the reduced mass of the two atoms (v ~ 1/m) Expect: will increase with increasing bond strength (bond order) and decreasing mass

8. Examples of stretching frequencies and correlations with bond strengths (bond order) C-CC=C Bond strength* 350 600 840 Bond order 1 2 3 2200 cm -1 1600 cm -1 1000 cm -1 *In kJ/mol

9. Quantum mechanics: The frequency  depends on the energy gap between vibrational levels E = h  = hc/  (cm -1 ) The natural frequency (8.67 x 10 13 s -1 ) is absorbed selectively Only the natural frequency will be absorbed

10. The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency can be transferred to the atoms involved in the vibration The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon What does the absorption intensity depend on?

11. Dipole Moment Must Change during for a vibration to be “IR active”! In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole moment changes. Which of the vibrations are “IR” active”?

12. What is the intensity of an IR signal of: O 2 or N 2 or H 2 ? Ans: In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of the molecule O 2, N 2 and H 2 DO NOT ABSORB IR LIGHT! The are not “Greenhouse” gases

13. Does O=C=O absorb IR light? Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light No dipole generated Dipolegenerated

14. Which should have a higher stretching frequency, CO, CO +, or CO - ? Why? Ans: The higher the bond order, the higher the frequency for the IR stretch. Bond order: CO = 3, CO + = 5/2, CO - = 5/2 CO will have the higher stretching frequency CO + and CO - will have similar, lower frequencies

15. region of infrared that is most useful lies between 2.5-16  m (4000-625 cm -1 ) depends on transitions between vibrational energy states stretchingbending Infrared Spectroscopy

16. Stretching Vibrations of a CH 2 Group SymmetricAntisymmetric

17. Bending Vibrations of a CH 2 Group In plane

18. Bending Vibrations of a CH 2 Group Out of plane

19. Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.200035003000250010001500500 Wave number, cm -1 Infrared Spectrum of Hexane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 C—H stretching bending bending bending

20. Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.200035003000250010001500500 Wave number, cm -1 Infrared Spectrum of 1-Hexene H 2 C=CHCH 2 CH 2 CH 2 CH 3 H—C C=C—H C=C H 2 C=C

21. Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp C—H3310-3320 sp 2 C—H3000-3100 sp 3 C—H2850-2950 sp 2 C—O1200 sp 3 C—O1025-1200 Infrared Absorption Frequencies

22. Structural unitFrequency, cm -1 Stretching vibrations (multiple bonds) Infrared Absorption Frequencies C C 1620-1680—CN —CC— 2100-2200 2240-2280

23. Structural unitFrequency, cm -1 Stretching vibrations (carbonyl groups) Aldehydes and ketones1710-1750 Carboxylic acids1700-1725 Acid anhydrides1800-1850 and 1740-1790 Esters1730-1750 Amides1680-1700 Infrared Absorption Frequencies C O

24. Structural unitFrequency, cm -1 Bending vibrations of alkenes Infrared Absorption Frequencies CH 2 RCH R2CR2CR2CR2C CHR' cis-RCH CHR' trans-RCH CHR' R2CR2CR2CR2C910-990 890 665-730 960-980 790-840

25. Structural unitFrequency, cm -1 Bending vibrations of derivatives of benzene Monosubstituted730-770 and 690-710 Ortho-disubstituted735-770 Meta-disubstituted750-810 and 680-730 Para-disubstituted790-840 Infrared Absorption Frequencies

26. Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.200035003000250010001500500 Wave number, cm -1 Infrared Spectrum of tert-butylbenzene H—C Ar—H Monsubstituted benzene C 6 H 5 C(CH 3 ) 3

27. Structural unitFrequency, cm -1 Stretching vibrations (single bonds) O—H (alcohols)3200-3600 O—H (carboxylic acids) 3000-3100 N—H3350-3500 Infrared Absorption Frequencies: functional groups

28. Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.200035003000250010001500500 Wave number, cm -1 Infrared Spectrum of 2-Hexanol H—C O—H OH CH 3 CH 2 CH 2 CH 2 CHCH 3

29. Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.200035003000250010001500500 Wave number, cm -1 Infrared Spectrum of 2-Hexanone H—C C=O O CH 3 CH 2 CH 2 CH 2 CCH 3