Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 -...

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Introduction to Infrared Introduction to Infrared Spectrometry Spectrometry Chap 16 Chap 16
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Transcript of Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 -...

Page 1: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Introduction to Infrared Introduction to Infrared SpectrometrySpectrometry

Chap 16Chap 16

Page 2: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Infrared Spectral RegionsTable 16-1

Most used 4000 - 670 2.5 – 15

Page 3: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Spectral data may be plotted with

ordinate as:

• absorbance (A)

• percent transmittance (%T)

abscissa as:

• wavenumber (cm-1)

• often called “frequency”

• wavelength (μm)

ordinate

abscissa

Page 4: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

IR Spectrum IR Spectrum of a thin polystyrene filmof a thin polystyrene film

Fig. 16-1

Page 5: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Dipole Changes During Vibrations and RotationsDipole Changes During Vibrations and Rotations

• Energy of IR photon insufficient to cause electronic excitation

• But can cause vibrational or rotational excitation

• To absorb an IR photon, molecule must undergo a net change in dipole moment (gross selection rule)

• Electric field of molecule (i.e., dipole moment) interacts with

electric field of IR photon

• Both dynamic fields

Page 6: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Dipole Changes During Vibrations and RotationsDipole Changes During Vibrations and Rotations

• Magnitude of dipole moment determined by:

(i) charge (δ+ or δ-)

(ii) separation of charge (r)

• Vibration or rotation causes varying separation:

• Absorption causes increase in vibrational amplitude or rotational frequency

Page 7: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Molecules with permanent dipole moments (µ) are IR active

IR active IR inactive

Also: all homonuclear diatomics,CH4 SF6 C6H6 etc.

Page 8: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Types of Molecular VibrationsTypes of Molecular Vibrations

• Stretching ⇒ change of bond length

Fig 16-2 (a)

Page 9: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

• Bending ⇒ change of bond angle

Fig 16-2 (b)

Page 10: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Classical Vibrational MotionClassical Vibrational Motion

• Harmonic oscillator model

• Force required to displace mass, m:

F = -ky

where k ≡ force constant

• Potential energy

dE = -F dy = ky dy

• Integrating: E = ½ ky2

Page 11: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Vibrational Frequency

• Natural frequency of the classical oscillator:

mk

m

21

• In terms of the reduced mass, μ, of two atoms:

km 2

1

where21

11mm

Page 12: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Quantum Mechanical Treatment of VibrationsQuantum Mechanical Treatment of Vibrations

• Required to include quantized nature of E

• From solving the wave equations of QM:

1) v(for2

1 v

... 2, 1, 0,v

molecule diatomic for22

1v

khhE

khE

resvib

vib

Selection rule for vib. transitions

Page 13: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Quantum Mechanical Treatment of VibrationsQuantum Mechanical Treatment of Vibrations

Interatomic distance, r →

hvres1) v(for

2

kh

hE resvib

2

21krE

• Plot of potential energy:

• where level spacings:

• All vib levels spacedequally for HO only

Page 14: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Anharmonic Oscillator (AHO)Anharmonic Oscillator (AHO)

Problems with Harmonic Oscillator (HO) ModelProblems with Harmonic Oscillator (HO) Model

• Real vib levels coalesce as v levels increaseReal vib levels coalesce as v levels increase

• Does not allow for dissociation of bond

• Repulsion is steeper at small r

• Appears as if atoms can pass througheach other during vibrational amplitude

Solution:

Page 15: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Potential Energy Curve of Harmonic OscillatorPotential Energy Curve of Harmonic Oscillator

Fig. 16-3 (b)

Page 16: Introduction to Infrared Spectrometry Chap 16. Infrared Spectral Regions Table 16-1 Most used 4000 - 670 2.5 – 15.

Anharmonic Oscillator (AHO)Anharmonic Oscillator (AHO)

Three consequences:

(1) Harmonic at low v levels

(2) ΔE becomes smaller at high v levels

(3) Selections rule fails: Δv = ±1 and ±2...

• referred to as overtones