1. 11-1 Infrared Spectrometry Wavelengths  0.78  m to 1000  m §3 regions àNear-, mid-, and far Theory Sources Instrumentation

2. 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

3. 11-3 Theory Polar molecules IR active §H 2 O, HCl, NO àMost molecules will absorb IR §Homonuclear species IR inactive àO 2, N 2, Cl 2 Vibrations §Stretching àSymmetric and asymmetric §Bending àRocking àScissoring àWagging àTwisting

4. 11-4 Theory Only some modes IR active Model based on Hooke’s law §F=-ky àF=force, k=constant, y=displacement distance §Change in energy related to F

5. 11-5 Theory Harmonic oscillator derived Vibrational Frequency §F=ma §a=d 2 y/dt 2 §md 2 y/dt 2 =-ky  Substitute y=Acos2  m t §m goes to reduced mass

6. 11-6 Theory Quantum treatment §h is Planck constant  is vibrational quantum number àInteger > 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

7. 11-7 Theory Calculate stretching frequency of C=O §Calculate mass in kg àm c =2e-26 kg àm o =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

8. 11-8 Theory Electron repulsion Bond breaking Vibrational modes §Depends upon number of atoms and degrees of freedom à3N total Constraints due to §Translational and rotational motion of molecule §Motion of atoms relative to each other àNon linear 3N-6 àLinear 3N-5

9. 11-9 Theory Coupling can influence wavelength of absorption peak §Common atom in stretching mode §Common bond àBending àBending and stretching modes §Similar frequencies

10. 11-10 Vibrational spectroscopy and group Molecules with inversion cannot be both IR and Raman active §For CO 2, symmetric stretch is IR inactive àNo net change of dipole àRaman 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. 11-11 Vibrational spectroscopy Consider cis (C 2v ) and trans (D 2h ) PdCl 2 (NH 3 ) 2 §Both have Pd-Cl stretch §For C 2v, all 1 is symmetric àA 1 §Asymmetric mode  C 2 and  v are -1 *B 2 group Same information can be used to assign symmetry

12. 11-12

13. 11-13 Symmetry and vibration a 1 vibration generates a changing dipole moment in the z-direction b 1 vibration generates a changing dipole moment in the x-direction b 2 vibration generates a changing dipole moment in the y-direction a 2 vibration does not generate a changing dipole moment in any direction (no ‘x’, ‘y’ or ‘z’ in the a2 row). Thus, a 1, b 1 and b 2 vibrations give rise to changing dipole moments and are IR active However, a 2 vibrations do not give rise to changing dipole moments and are IR inactive C 2v EC2C2 σ v (xz)σ v (yz) A1A1 1111zx 2, y 2, z 2 A2A2 11 RzRz xy B1B1 11 x, R y xz B2B2 1 1y, R x yz.

14. 11-14 C 2v SALC

15. 11-15 Symmetry and vibration Which bonds are IR active in CCl 4 ? §Symmetry is T d §From table, which bonds are dipole active in x, y, or z àt 2 is active in x,y, and z àWhat do these bonds look like? §xz, yz, xy, x 2, y 2, z 2 are Raman active àFrom table, a 1 and t 2 are Raman active

16. 11-16 Td group TdTd E 8C38C3 3C23C2 6  d 6S46S4 A1A1 11111 x 2 + y 2 + z 2 (= r 2 ) A2A2 111 E 2 200 ( x 2 - y 2, 3 z 2 - r 2 ) T1T1 30 1 ( I x, I y, I z ) T2T2 301 ( x, y, z )( xy, yz, zx )

17. 11-17 T d SALC

18. 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 CO 2 laser §stimulated emission lines 9-11 µm

19. 11-19 Detectors Thermocouple Bolometer §Ni, Pt resistance thermometer (thermistor) highly Pyroelectric fast and Photoconducting §PbS, HgCdTe

20. 11-20 Instruments

21. 11-21 Instruments