11-1

Infrared Spectrometry

• Wavelengths 0.78 m to 1000 m 3 regions

Near-, mid-, and far

• Theory• Sources• Instrumentation

11-2

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

11-3

Theory

• Polar molecules IR active H2O, HCl, NO

Most molecules will absorb IR

Homonuclear species IR inactive O2, N2, Cl2

• Vibrations Stretching

Symmetric and asymmetric

Bending Rocking Scissoring Wagging Twisting

11-4

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

11-5

Theory

• Harmonic oscillator derived

• Vibrational Frequency F=ma a=d2y/dt2

md2y/dt2=-ky Substitute

y=Acos2mt

m goes to reduced mass

2

2

1kyE

km 2

1

21

21

mm

mm

11-6

Theory

• Quantum treatment h is Planck constant is vibrational quantum number

Integer > 0

• Solve for

• Express in wavenumbers In cm-1, k in N/m, c in m/s, in kg

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

mhkh

E

)()(2

1

22

1

khhEhEhE mmm 22

3

2

110 ,)(,)(

km 2

1

k

Ek

c1235

2

1 .

11-7

Theory

• Calculate stretching frequency of C=O Calculate mass in kg

mc=2e-26 kg

mo=2.7e-26 kg

=2.7x2x1e-26/(2.7+2)=1.1E-26 kg

Experimental value 1600 cm-1 to 1800 cm-1

• Actual system is anharmonic Selection rules +2 and 3 are observed

13612611

311235

cmE

E

EE .

..

11-8

Theory• Electron repulsion• Bond breaking• Vibrational modes

Depends upon number of atoms and degrees of freedom 3N total

• Constraints due to Translational and

rotational motion of molecule

Motion of atoms relative to each other Non linear 3N-6 Linear 3N-5

11-9

Theory

• Coupling can influence wavelength of absorption peak Common atom in

stretching mode Common bond

Bending Bending and

stretching modes

Similar frequencies

11-10

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 dipoleRaman active

• 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

11-11

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 v are -1

* B2 group• Same information can be

used to assign symmetry

11-12

11-13

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 E C2 σv (xz) σv (yz)    

             

A1 1 1 1 1 z x2, y2, z2

A2 1 1 -1 -1 Rz xy

B1 1 -1 1 -1 x, Ry xz

B2 1 -1 -1 1 y, Rx yz

.

11-14

C2v SALC

11-15

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

11-16

Td group

TdE 8C3 3C2

6    d

6S4    

A1 1 1 1 1 1  x2+y2+z2(=r2

)

A2 1 1 1 -1 -1    

E 2 -1 2 0 0  (x2-y2, 3z2-r2)

T1 3 0 -1 -1 1 (Ix, Iy, Iz)  

T2 3 0 -1 1 -1 (x, y, z) (xy, yz, zx)

11-17

Td SALC

11-18

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

11-19

Detectors

• Thermocouple • Bolometer

Ni, Pt resistance thermometer (thermistor) highly

• Pyroelectric fast and• Photoconducting

PbS, HgCdTe

11-20

Instruments

11-21

Instruments