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Infrared Spectrometry

- Wavelengths
- 0.78 mm to 1000 mm
- 3 regions
- Near-, mid-, and far

- Theory
- Sources
- Instrumentation

IR energy

- Changes are vibrational or rotational
- Data in wavenumbers
- cm-1

- IR not energetic
- Promotion to differences in vibrational and rotational states
- For IR absorption molecule must under change in dipole moment

Theory

- Polar molecules IR active
- H2O, HCl, NO
- Most molecules will absorb IR

- Homonuclear species IR inactive
- O2, N2, Cl2

- H2O, HCl, NO
- Vibrations
- Stretching
- Symmetric and asymmetric

- Bending
- Rocking
- Scissoring
- Wagging
- Twisting

- Stretching

Theory

- Only some modes IR active
- Model based on Hooke’s law
- F=-ky
- F=force, k=constant, y=displacement distance

- Change in energy related to F

- F=-ky

Theory

- Harmonic oscillator derived
- Vibrational Frequency
- F=ma
- a=d2y/dt2
- md2y/dt2=-ky
- Substitute y=Acos2pnmt

- m goes to reduced mass

Theory

- Quantum treatment
- h is Planck constant
- n is vibrational quantum number
- Integer > 0

- Solve for n
- Express in wavenumbers
- In cm-1, k in N/m, c in m/s, m in kg

- K 3-8E2 for single bonds
- 1e3 double, 1.5e3 triple

Theory

- Calculate stretching frequency of C=O
- Calculate mass in kg
- mc=2e-26 kg
- mo=2.7e-26 kg
- m=2.7x2x1e-26/(2.7+2)=1.1E-26 kg

- Experimental value 1600 cm-1 to 1800 cm-1

- Calculate mass in kg
- Actual system is anharmonic
- Selection rules Dn+2 and 3 are observed

Theory

- Electron repulsion
- Bond breaking
- Vibrational modes
- Depends upon number of atoms and degrees of freedom
- 3N total

- Depends upon number of atoms and degrees of freedom
- Constraints due to
- Translational and rotational motion of molecule
- Motion of atoms relative to each other
- Non linear 3N-6
- Linear 3N-5

Theory

- Coupling can influence wavelength of absorption peak
- Common atom in stretching mode
- Common bond
- Bending
- Bending and stretching modes

- Similar frequencies

Vibrational spectroscopy and group

- Molecules with inversion cannot be both IR and Raman active
- For CO2, symmetric stretch is IR inactive
- No net change of dipole
- Raman active

- For CO2, symmetric stretch is IR inactive
- A vibrational mode is IR active if it is symmetric with electric dipole vector
- Causes change in dipole

- Mode is Raman active if it has component of molecular polarizability

Vibrational spectroscopy

- Consider cis (C2v) and trans (D2h) PdCl2(NH3)2
- Both have Pd-Cl stretch
- For C2v, all 1 is symmetric
- A1

- Asymmetric mode
- C2 and sv are -1
- B2 group

- C2 and sv are -1

- Same information can be used to assign symmetry

Symmetry and vibration

- a1 vibration generates a changing dipole moment in the z-direction
- b1 vibration generates a changing dipole moment in the x-direction
- b2 vibration generates a changing dipole moment in the y-direction
- a2 vibration does not generate a changing dipole moment in any direction (no ‘x’, ‘y’ or ‘z’ in the a2 row).
- Thus, a1, b1 and b2 vibrations give rise to changing dipole moments and are IR active
- However, a2 vibrations do not give rise to changing dipole moments and are IR inactive

.

C2v SALC

Symmetry and vibration

- Which bonds are IR active in CCl4?
- Symmetry is Td
- From table, which bonds are dipole active in x, y, or z
- t2 is active in x,y, and z
- What do these bonds look like?

- xz, yz, xy, x2, y2, z2 are Raman active
- From table, a1 and t2 are Raman active

Td SALC

Instruments

- Nernst Glower
- heated rare earth oxide rod (~1500 K) 1-10 µm

- Globar
- heated SiC rod (~1500 K) 1-10 µm

- W filament lamp
- 1100 K 0.78-2.5 µm

- Hg arc lamp plasma
- >50 µm

- CO2 laser
- stimulated emission lines 9-11 µm

Detectors

- Thermocouple
- Bolometer
- Ni, Pt resistance thermometer (thermistor) highly

- Pyroelectric fast and
- Photoconducting
- PbS, HgCdTe

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