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

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?

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

Vibrations, potential energy and motion

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

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?

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

Examples of stretching frequencies and correlations with bond strengths (bond order) C-CC=C Bond strength* Bond order cm cm cm -1 *In kJ/mol

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

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?

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”?

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

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

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

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

Stretching Vibrations of a CH 2 Group SymmetricAntisymmetric

Bending Vibrations of a CH 2 Group In plane

Bending Vibrations of a CH 2 Group Out of plane

Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved 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

Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved 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

Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp C—H sp 2 C—H sp 3 C—H sp 2 C—O1200 sp 3 C—O Infrared Absorption Frequencies

Structural unitFrequency, cm -1 Stretching vibrations (multiple bonds) Infrared Absorption Frequencies C C —CN —CC—

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

Structural unitFrequency, cm -1 Bending vibrations of alkenes Infrared Absorption Frequencies CH 2 RCH R2CR2CR2CR2C CHR' cis-RCH CHR' trans-RCH CHR' R2CR2CR2CR2C

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

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

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

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

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