Infrared Spectrometry

1. Infrared Spectrometry • Wavelengths • 0.78 mm to 1000 mm • 3 regions • Near-, mid-, and far • Theory • Sources • Instrumentation

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

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

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

5. Theory • Harmonic oscillator derived • Vibrational Frequency • F=ma • a=d2y/dt2 • md2y/dt2=-ky • Substitute y=Acos2pnmt • m goes to reduced mass

6. 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

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

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

9. Theory • Coupling can influence wavelength of absorption peak • Common atom in stretching mode • Common bond • Bending • Bending and stretching modes • Similar frequencies

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 dipole • Raman 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. 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 • Same information can be used to assign symmetry

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 .

14. C2v SALC

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

16. Td group

17. Td SALC

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

19. Detectors • Thermocouple • Bolometer • Ni, Pt resistance thermometer (thermistor) highly • Pyroelectric fast and • Photoconducting • PbS, HgCdTe

20. Instruments

21. Instruments