Infrared spectrometry
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Infrared Spectrometry. Wavelengths 0.78 m m to 1000 m m 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

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Infrared Spectrometry

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Infrared spectrometry

Infrared Spectrometry

  • Wavelengths

    • 0.78 mm to 1000 mm

    • 3 regions

      • Near-, mid-, and far

  • Theory

  • Sources

  • Instrumentation


Ir energy

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

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


Theory1

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


Theory2

Theory

  • Harmonic oscillator derived

  • Vibrational Frequency

    • F=ma

    • a=d2y/dt2

    • md2y/dt2=-ky

      • Substitute y=Acos2pnmt

    • m goes to reduced mass


Theory3

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


Theory4

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


Theory5

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


Theory6

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

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


Vibrational spectroscopy

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


Symmetry and vibration

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

.


C 2v salc

C2v SALC


Symmetry and vibration1

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 group

Td group


T d salc

Td SALC


Instruments

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

Detectors

  • Thermocouple

  • Bolometer

    • Ni, Pt resistance thermometer (thermistor) highly

  • Pyroelectric fast and

  • Photoconducting

    • PbS, HgCdTe


Instruments1

Instruments


Instruments2

Instruments


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