Basic concepts of organic spectroscopy

Spectroscopy, UV, IR and NMR. Probelms and Examples

1. BASIC CONCEPTS OF ORGANIC SPECTROSCOPY Dr. Basavarajaiah S. M. M . Sc., Ph.D . Coordinator PG Department of Chemistry Vijaya college Bengaluru-56000.

2. Spectroscopy Spectroscopy is a general term referring to the interactions of various types of electromagnetic radiation with matter. Exactly how the radiation interacts with matter is directly dependent on the energy of the radiation.

3. THE ELECTROMAGNETIC SPECTRUM Important: As the wavelength gets shorter, the energy of the radiation increases.

4. Electromagnetic radiation displays the properties of both particles and waves The particle component is called a photon The energy (E) component of a photon is proportional to the frequency . Where h is Planck’s constant and n is the frequency in Hertz (cycles per second) E = h ν The term “photon” is implied to mean a small, massless particle that contains a small wave-packet of EM radiation/light – we will use this terminology in the cour se

5. Spectroscopy The higher energy ultraviolet and visible wavelengths affect the energy levels of the outer electrons. Radio waves are used in nuclear magnetic Resonance and affect the spin of nuclei in a magnetic field. Infrared radiation is absorbed by matter resulting in rotation and/or vibration of molecules.

6. Ultraviolet radiation stimulates molecular vibrations and electronic transitions. Absorption spectroscopy from 160 nm to 780 nm. Measurement absorption or transmittance. Identification of inorganic and organic species. UV-Vis Spectroscopy

7. UV/Vis Spectroscopy Visible (380-780 nanometers ). Ultraviolet (UV) (10 – 380 nanometers). Below about 200 nm, air absorbs the UV light and instruments must be operated under a vacuum

8. Types of Electronic Transitions:

9. 1. Bathochromic Shift or Red shift : A shift of an absorption maximum towards longer wavelength ( λ ) or lower energy (E). 2 . Hypsochromic Shift or Blue Shift : A shift of an absorption maximum towards shorter wavelength ( λ ) or higher energy (E). 3 . Hyperchromic Effect : An effect that results in increased absorption intensity ( ε ). 4 . Hypochromic Effect : An effect that results in decreased absorption intensity ( ε ).

10. Wavelengths Absorbed by Functional Groups Again, demonstrates the moieties contributing to absorbance from 200-800 nm, because pi electron functions and atoms having no bonding valence shell electron pairs.

11. Influence of conjugation on UV absorption

12. UV Spectra of 1, 3-Butadiene

13. UV Spectra of Isoprene

14. UV Spectra of Benzene and Styrene

15. UV Spectra of Naphthalene, Anthracene and Tetracene

16. UV Spectra of Lycopene ( Polyene )

17. λ max = 455 nm λ max = 471 nm

18. Comparison of UV spectra of Acetone and Methyl vinyl ketone

19. INFRARED SPECTROSCOPY The IR region has lower energy than visible radiation and higher energy than microwave.

20. The Major Regions of the IR Spectrum

21. IR ABSORPTION BY MOLECULES Molecules with covalent bonds may absorb IR radiation Absorption is quantized Molecules move to a higher energy state IR radiation is sufficient enough to cause rotation and vibration Radiation between 1 and 100 µm will cause excitation to higher vibrational states Radiation higher than 100 µm will cause excitation to higher rotational states

22. IR ABSORPTION BY MOLECULES Absorption spectrum is composed of broad vibrational Absorption bands Molecules absorb radiation when a bond in the molecule vibrates at the same frequency as the incident radiant energy Molecules vibrate at higher amplitude after absorption A molecule must have a change in dipole moment during vibration in order to absorb IR radiation

23. IR ABSORPTION BY MOLECULES Absorption frequency depends on - Masses of atoms in the bonds - Geometry of the molecule - Strength of bond - Other contributing factors

24. DIPOLE MOMENT (µ) µ = Q x r Q = charge and r = distance between charges - Asymmetrical distribution of electrons in a bond renders the bond polar - A result of electronegativity difference - µ changes upon vibration due to changes in r - Change in µ with time is necessary for a molecule to absorb IR radiation

25. DIPOLE MOMENT (µ) The repetitive changes in µ makes it possible for polar molecules to absorb IR radiation Symmetrical molecules do not absorb IR radiation since they do not have dipole moment (O 2 , F 2 , H 2 , Cl 2 ) Diatomic molecules with dipole moment are IR-active ( HCl , HF, CO, HI) Molecules with more than two atoms may or may not be IR active depending on whether they have permanent net dipole moment

26. Excitation depends on atomic mass and how tightly they are bound Hooke’s Law for 2 masses connected by a spring C—H Bond: Reduced Mass = (12+1)/(12x1) = 13/12 = 1.08 C—C Bond: Reduced Mass = (12+12)/(12x12) = 24/144 = 0.167 k = constant f (force constant) = bond strength m-term= µ = reduced mass Frequency Determination in IR

27. PRINCIPAL MODES OF VIBRATION Stretching Change in bond length resulting from change in interatomic distance (r) Two stretching modes - Symmetrical and asymmetrical stretching - Symmetrical stretching is IR-inactive (no change in µ) H H C H H C asymmetric symmetric

28. Bending - Change in bond angle or change in the position of a group of atoms with respect to the rest of the molecule Bending Modes - Scissoring and Rocking - In-plane bending modes (atoms remain in the same plane) - Wagging and Twisting Out-of-plane ( oop ) bending modes (atoms move out of plane) scissor H H C C H H C C H H C C H H C C rock twist wag in plane out of plane

29. To locate a point in three-dimensional space requires three coordinates . To locate a molecule containing N atoms in three dimensions, 3N coordinates are required. The molecule is said to have 3N degrees of freedom. To describe the motion of such a molecule, translational, rotational, and vibrational motions must be considered. In a nonlinear molecule: 3 of these degrees are rotational and 3 are translational and the remaining correspond to fundamental vibrations; In a linear molecule: ( Linear molecules cannot rotate about the bond axis) 2 degrees are rotational and 3 are translational. The net number of fundamental vibrations: Theoretical Vibrational Normal modes

30. Vibrational modes of H 2 O (3 atoms – non linear) Vibrational modes (degrees of freedom) = 3 x 3 - 6= 3 These normal modes of vibration: are a symmetric stretch, and asymmetric stretch, and a scissoring (bending) mode.

31. Fundamental Vibrational modes (degrees of freedom) = 3 x 3 – 5 = 4 These normal modes of vibration: The asymmetrical stretch of CO 2 gives a strong band in the IR at 2350 cm –1 (may noticed in samples due to presence of CO 2 in the atmosphere). The two scissoring or bending vibrations are equivalent and therefore, have the same frequency and are said to be degenerate , appearing in an IR spectrum at 666 cm -1. Fundamental Vibrational modes of CO 2 (3 atoms – Linear)

32. n -pentane CH 3 CH 2 CH 2 CH 2 CH 3 3000 cm -1 1470 &1375 cm -1 2850-2960 cm -1 sat’d C-H

33. cyclohexane no 1375 cm -1 no –CH 3

34. 1-decene C=C 1640-1680 unsat’d C-H 3020-3080 cm -1 910-920 & 990-1000 RCH=CH 2

35. ethylbenzene 690-710, 730-770 mono- 1500 & 1600 Benzene ring 3000-3100 cm -1 Unsat’d C-H

36. o -xylene 735-770 ortho

37. styrene no sat’d C-H 910-920 & 990-1000 RCH=CH 2 mono 1640 C=C

38. 1-butanol CH 3 CH 2 CH 2 CH 2 -OH C-O 1 o 3200-3640 (b) O-H

39. 2-butanol C-O 2 o O-H

40. tert -butyl alcohol C-O 3 o O-H

41. methyl n -propyl ether no O--H C-O ether

42. 2-butanone  C=O ~1700 (s)

43. IR Spectra of Benzoic acid

44. IR Spectra of Methyl benzoate

45. NMR SPECTROSCOPY

46. Nuclear magnetic resonance spectrometry (NMR) is based on the absorption of electromagnetic radiation in the radio-frequency region of the spectrum resulting in changes in the orientation of spinning nuclei in a magnetic field. NMR SPECTROSCOPY NMR Energies 0.1 J/ mol IR Energies 6000 to 42,000 J/ mol UV/Vis Energies >100,000 J/ mol

47. Introduction NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1 H 13 C 15 N 19 F 31 P

48. The nuclei of some atoms have a property called “SPIN” . NUCLEAR SPIN Each spin-active nucleus has a number of spins defined by its spin quantum number, I . ….. we don’t know if they actually do spin!

49. Nuclear Spin Energy Levels B o +1/2 -1/2 In a strong magnetic field (B o ) the two spin states differ in energy. aligned unaligned N S

50. Nuclear Spin Energy Levels B o +1/2 -1/2 In a strong magnetic field (B o ) the two spin states differ in energy. aligned unaligned N S

51. B o D E + 1/2 - 1/2 = kB o = h n degenerate at B o = 0 increasing magnetic field strength THE ENERGY SEPARATION DEPENDS ON B o

52. Resonance Frequencies of Selected Nuclei Nuclie Percentage Abundance Applied field in Tesla Precessional frequency in MHz 1 H 99.98 1.0 42.6 2 H 0.0156 1.0 6.5 13 C 1.108 1.0 10.7 19 F 100 1.0 40.0

53. The Larmor Equation!!! g n = 2 p B o g is a constant which is different for each atomic nucleus (H, C, N, etc ) D E = kB o = h n can be transformed into gyromagnetic ratio g strength of the magnetic field frequency of the incoming radiation that will cause a transition

54. The strength of the NMR signal depends on the Population Difference of the two spin states resonance induced emission excess population Radiation induces both upward and downward transitions. For a net positive signal there must be an excess of spins in the lower state. Saturation = equal populations = no signal POPULATION AND SIGNAL STRENGTH

55. The NMR Spectrometer

56. Effect of Electronegativity of Adjacent A toms on Chemical Shift ( δ ) V alues

57. Shielding and Deshielding Effects for ( i ) Methane (ii) CH 3 −Cl (iii) CH 2 Cl 2 (iv) CHCl 3

58. Spin-Spin Splitting

59. NMR S pectrum of Benzene (C 6 H 6 )

60. Examples for Spin-spin coupling in 1 H NMR

61. NMR Spectrum of Dichloroacetaldheyde

62. Applications of NMR Spectroscopy

63. Spectroscopy Learning Websites http :// www.rsc.org/learn-chemistry/collections/spectroscopy . http:// www.rsc.org/learn-chemistry/resource/res00001041/spectroscopy-videos . http:// www.spectroscopyonline.com https:// www.khanacademy.org/science/organic-chemistry/spectroscopy . http:// chem.sci.ubu.ac.th/e-learning .