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Vibrational Spectroscopy

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Published in: Chemistry | Physics
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Presentration On Vibrational Spectroscopy

Akhilesh K / Lucknow

4 years of teaching experience

Qualification: M.Sc (NIT Rourkela - 2019)

Teaches: All Subjects, English, Mathematics, Science, Chemistry, Physics, Algebra, IIT JEE Mains, AIPMT, NEET

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  1. kdooso.goads
  2. Vibrational Spectra Molecules are not Static Vibration of bonds occurs in the liquid, solid and gaseous phase Vibrating —Energy Frequency (and the appropriate frequencies for molecular vibrations are in the Infrared region of the electromagnetic spectrum Vibrations form therefore, a fundamental basis for spectroscopy in chemistry--the bonds are what makes the chemistry work in structure and function For Organic Chemistry the most important uses of these vibrations is for analysis of: 'functional groups •structural identity, "fingerprinting
  3. What Kind of vibrations are These? Bonds can Bend Stretch wag These can number into the hundreds. Some are symmetrical, some antisymmetrical and many are coupled across the molecule Can be calculated. One easy approximation is: 12 -=5.3 10 k k is the "force constant", like the Hookes Law restoring force for a spring. Known and tabulated for different vibrations The "reduced mass' where ml, m2 are the masses on either side of vibration
  4. The Fundamentals antisymmetric symmetric R H scissoring wagging R rocking twisting stretching H -plane bending out-of-plane bending These oscillating electric dipoles match in frequency the incoming e-field oscillations of IR light. All the simple possibilities. For n atoms in a molecule; Linear: 3n — 5 modes Non-linear: 3n —6 modes Example for a methylene,given n=3 While useful, this oversimplifies, since molecular orbital picture requires that atoms can t vibrate without affecting the rest of the molecule.
  5. A Functional Group Chart 40 2 2 2000 1 12 800 rou O-H str NH str COO-H =C-H str csp3-H -c=c phenyl c-o C-N
  6. Regions of Frequencies S ectral Re ion Near -to visible- IR (NIR) Combination bands Mid Infrared Fundmental bands for organic molecules Far IR Inorganics organometallics Wavenumber cm 1 Fre uenc Hz 3.8 x 1014 to 1.2 X 1014 1.2 x 1014 to 6.0 x 1012 6.0 x 1012 to 3.0 X 1011 12800 to 4000 4000 to 200 200 to 10 Wavelen th X m 0.78 to 2.5 2.5 to 50 50 to 1000
  7. Looking at a Spectrum Divide the spectrum in to two regions: 4000 cm-I —1600 cm I most of the stretching bands, specific functional groups (specific atom pairs). This is the 'functional group region. 1600 cm-I 400 cm-I Many band of mixed origin. Some prominent bands are reliable. This is the "fingerprint "region. Use for comparison with literature spectra. Wavenumber is
  8. What kinds of Bonds Absorb in which Regions? Bending is easier than stretching-- happens at lower energy (lower wavenumber) Bond Order is reflected in ordering-- triple>double>single (energy) with single bonds easier than double easier than triple Heavier atoms move slower than lighter ones The k in the frequency equation is in mDyne/Å of displacement Single bond str 3-6 mD/Å Double bond str, 10-12 mD/Å Triple Bond 15-18 mD/Å
  9. Effects of conjugation O Lowers to 1715 cm I O Similar, to 1715 cm I Weakens DB character Raises to 1770 cm I Strengthens DB character (inductive over resonance)
  10. Degrees Of Freedom: Translation, Rotation, and Vibration Consider a single Ar atom moving in 3-D space: - Moving motion is referred to as Translation - To analyze the translation of an Ar, we need to know position (x, y, z) and momentum (px, py, p) Where it is Where it is headed - Each coordinate-momentum pair [for example, is referred to as a Degree of Freedom - An Ar atom moving through 3-D space has three DFs N argon atoms possesses 3N DFs: All translational DFs
  11. Translation C02 Rotation Vibration Symmetric stretch Antisymmetric stretch Bends 10
  12. Atoms Degrees of freedom 3 x 1=3 Ar 3 Ar Ar Ar Ar Ar Ar Molecules Center of mass z Center of mass z HN03
  13. Center of Mass (Balanced Point) - A point mass that can represent the molecule - Motion of the center of mass requires 3 DFs to describe it - In general, regardless of its size or complexity, a molecule has 3 translational DFs - Thus, (3N — 3) DFs for the internal motions of rotation and vibration
  14. DFs N atomic Linear Molecule 2 DFs Rotation Vibration Rotational and vibrational 3N — 5 3N - 6 N atomic Non-Linear Molecule 3 DFs
  15. Av = +1 (absorption) Av = --1 (emission) Vibrational Spectroscopy Vibrationa/ selection rule Av=+l j=?o Aj j=?o
  16. TABLE 8.4 Vibrational band positions characteristic of molecular bond types C—H C—H 1295-1310 c=c c=o 1700-1850 3200-3650 Bond 0--.---1.-1 Group CH2, CH3 Benzene -CHE c=c Type of vibration Stretch Stretch Stretch Bend Bend, Bend, Bend, out-of-plane in-plane out-of-plane (cm— ) 2900-3000 3300 3030 1465 960—970 690 H -O—H H Stretch Stretch A (11m) 3.4-3.3 3.0 3.3 6.8 10.4—10.3 7.7-7.6 14.5 5.9—5.4 3.1-2.7
  17. The Vibrations of CO The stretching modes are not independent, and if one CO group is excited the other begins to vibrate. The symmetric and antisymmetric stretches are independent, and one can be excited without affecting the other: they are normal modes. The two perpendicular bending motions are also normal modes.
  18. The Normal Modes of Water The three normal modes of 1-420. The mode is predominantly bending, and occurs at lower wavenumber than the other two. -4 (3652 cm l) 2 (1595 cm-I) e 3 56
  19. Absorption and Emission Spectroscopy Absorption Emission 3p Stationary AE states E Spectra 3p Upward transition 41 3s Electron hc AE Dark line 3p Downward transition 3s uu Bright line
  20. Electronic Transitions in Molecules Molecular Orbital (MO) Theory for C2H4 molecule, UV or Visible spectral region Ground state Light Excited state
  21. Absorption (b) Absorptio
  22. Fate of Excited Electronic States Excitation 0 2003 Thomson-Brooks/Cole Excited singlet Excited triplet Phosphorescence Nonradiative energy loss Fluorescence Ground state Bond length
  23. Introduction to Absorption Reflection and Scattering Losses Reflection losses at interfaces Incident beam, PO Scattering losses in solution Emergent beam, P Reflection losses at interfaces
  24. LAMBERT-BEER LAW Power of radiation after passing through the solvent Power of radiation solution solvent A = -logT after through the sample solution = —log A a b abc = kc absorptivity pathlength concentraton
  25. Beer' s law and mixtures ' Each analyte present in the solution absorbs light! The magnitude of the absorption depends on its € — Al+A2+... +A total — €1 bcl+€2bc2+.. .+€nbcn total , If€l — €2 = then simultaneous determination is impossible Need nX' s where € s are different to solve the mixture
  26. Molecular Transitions for UV-Visible Absorptions What electrons can we use for these transitions? coo c (a) o orbital (b) orbital (c) o* orbital (d) orbital Antibonding Antibonding Nonbonding Bonding Bonding
  27. Spectral nomenclature of shifts hypsochromic hyperchromic hypochromic 400 blue red bathochromic 800 X (nm)
  28. Introduction to Emission ' Luminescence: emission of photons from electronically excited states of atoms, molecules, and ions. ' Fluorescence: Average lifetime from < 10 10 to 10 7 sec from singlet states. Phosphorescence: Average lifetime from 10 5 to > 10+3 sec from triplet excited states.
  29. Importance of Emission Spectroscopy ' Sensitivity to local electrical environment — polarity, hydrophobicity Track (bio-)chemical reactions Measure local friction (microviscosity) Track solvation dynamics Measure distances using molecular rulers: fluorescence resonance energy transfer (FRET)
  30. Photophysics: Jablonski Diagram excited Yibrational states (excited rotational states not shown) A = photon absorption F = fluorescence (emission) P = phosphorescence $ = singlet state T = triplet state •c IC = internal conyersion ISC = intersystem crossing Isc p so electronic ground state Kasha's rule, Internal Conversion, Intersystem crossing, fluorescence, phosphorescence
  31. Principles ' Interaction of photons with molecules results in promotion of valence electrons from ground state orbitals to high energy levels. The molecules are said to be in excited state. ' Molecules in excited state do not remain there long but spontaneously relax to more stable ground state.
  32. The relaxation process is brought about by collisional energy transfer to solvent or other molecules in the solution. ' Some excited molecules however return to the ground state by emitting the excess energy as light. This process is called fluorescence.
  33. Light Amplification by Stimulated Emission of Radiation Common Laser System Configurations • Z. E700D Carbon Dioxide Laser A on-lon ser Semiconductor Maiman•s Pulsed Ruby Laser Laser Helium-Neon Laser Ti:Sapphire Mode Locked Laser Lasers Figure 2
  34. In a laser Amplifying medium Pumping process optical feedback (cavity) Three key elements in a laser 'Pumping process prepares amplifying medium in suitable state 'Optical power increases on each pass through amplifying medium 'If gain exceeds loss, device will oscillate, generating a coherentoutput

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