Infrared Spectroscopy. Provides information about the vibraions of functional groups in a molecule

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1 Infrared Spectroscopy Provides information about the vibraions of functional groups in a molecule 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 intensity of the signal, I What structural features of a molecule do! and I depend on?

3 IR: Masses, Atoms and Springs 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 F = force, restoring back to equilibrium position k = characteristic stretching constant x = displacement from the equilibrium position

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: What Does the Frequency,!, Depend On? " = 1 2# k m r m r = m 1 m 2 m 1 + m 2! = frequency k = spring strength (bond stiffness) m r = reduced mass (mass of largest reduced mass (mass of largest atom)

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) Bond strength* Bond order! C-C cm -1 C=C cm -1 C C cm -1 *In kj/mol

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

10 What does the absorption intensity depend on? 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

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 Dipole generated

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 Infrared Spectroscopy region of infrared that is most useful lies between µm m ( cm -1 ) depends on transitions between vibrational energy states stretching bending

16 Stretching Vibrations of a CH 2 Group Symmetric Antisymmetric

17 Bending Vibrations of a CH 2 Group In plane In plane

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

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

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

21 Infrared Absorption Frequencies Structural unit Frequency, cm -1 Stretching vibrations (single bonds) sp C H sp 2 C H sp 3 C H sp 2 C O 1200 sp 3 C O

22 Infrared Absorption Frequencies Structural unit Frequency, cm -1 Stretching vibrations (multiple bonds) C C C C C N

23 Infrared Absorption Frequencies Structural unit Frequency, cm -1 Stretching vibrations (carbonyl groups) Aldehydes and ketones Carboxylic acids Acid anhydrides and Esters Amides C O

24 Infrared Absorption Frequencies Structural unit Bending vibrations of alkenes Frequency, cm -1 RCH R 2 C CH 2 CH cis-rch trans-rch CHR' CHR' R 2 C CHR'

25 Infrared Absorption Frequencies Structural unit Frequency, cm -1 Bending vibrations of derivatives of benzene Monosubstituted and Ortho-disubstituted Meta-disubstituted and Para-disubstituted

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

27 Infrared Absorption Frequencies: functional groups Structural unit Stretching vibrations (single bonds) Frequency, cm -1 O H H (alcohols) O H H (carboxylic acids) N H

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

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

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