11-1 infrared spectrometry wavelengths 0.78 m to 1000 m §3 regions ànear-, mid-, and far theory...
TRANSCRIPT
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