Basic Concepts of UV & IR Spectroscopy

Introduction to Spectroscopy,
Introduction to UV, electronic transitions, terminology, chromophore, Auxochrome, Examples and Applications.
Introduction to IR, Fundamental vibrations, Types of Vibrations, Factors affecting the vibrational freaquencies, Group frequencies, examples and applications.

1. BASIC CONCEPTS OF UV-Visible and IR SPECTROSCOPY Dr. Basavarajaiah S. M. M. Sc., Ph.D . Coordinator PG Department of Chemistry Vijaya college Bengaluru-560004. [email protected]

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

8. Electronic transitions The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition which can be considered; Transitions involving  σ , π , and  n  electrons. Transitions involving charge-transfer electrons. Transitions involving  d  and  f  electrons. CLASSIFICATION OF ELECTRONIC TRANSITIONS

9. Possible Electronic Transitions of  π , σ , and  n electrons are;

10. Auxochromes

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

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

13. Influence of conjugation on UV absorption

14. UV Spectra of 1, 3-Butadiene

15. UV Spectra of Isoprene

16. UV Spectra of Benzene and Styrene

17. UV Spectra of Naphthalene, Anthracene and Tetracene

18. UV Spectra of Lycopene (Polyene)

19. λ max = 455 nm λ max = 471 nm

20. Comparison of UV Spectra of Acetone and Methyl vinyl ketone

21. Which of the following alkenes would have the largest λ max ?

22. Which molecule absorbs at the longest wavelength, 1,3-hexadiene or 1,4-hexadiene? Why the λ max for the diene (I) is observed at lower nm than (II).

23. INFRARED SPECTROSCOPY

24. INTRODUCTION Infra-red spectrum is an important record which gives sufficient information about the structure of a compound. IR electromagnetic radiation is just less energetic than visible light. The infrared spectral regions are as follows. The absorption of Infra-red radiations (quantized) causes the various bands in a molecule to stretch and bend with respect to one another.

25. The frequency of IR radiation is commonly expressed in wave numbers. Wavenumber ( ῡ ): The number of waves per centimeter, cm -1 (read reciprocal centimeters). Expressed in wavenumbers, the vibrational IR extends from 4000 cm -1 to 670 cm -1 . Convert a wavenumber ( ῡ ) to a frequency ( υ ) by multiplying it by the speed of light. The main reason chemists prefer to use wave numbers as units is that they are directly proportional to energy. Wavenumber ( ῡ ) = 1/  F requency ( υ ) = c/ 

26. USES OF THE INFRARED SPECTRUM Fingerprint region:   The 1500-600 cm -1  range of an infrared spectrum, called the fingerprint region because (like a human fingerprint) this region of the spectrum is almost unique for any given compound. Functional group region: The functional group region runs from 4000 cm -1 to 1500 cm -1 .

27. 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

28. Ethane, C 2 H 6  has eight atoms (N=8) and is a nonlinear molecule so of the 3N=24 degrees of freedom, three are translational and three are rotational. The remaining 18 degrees of freedom are internal (vibrational). This is consistent with: 3N−6=3(8)−6=18 Carbon Dioxide, CO 2  has three atoms (N=3) and is a linear molecule so of the 3N=9 degrees of freedom, three are translational and two are rotational. The remaining 4 degrees of freedom are vibrational. This is consistent with: 3N−5=3(3)−5=4

29. THE MODES OF VIBRATION : STRETCHING AND BENDING The simplest types, or modes, of vibrational motion in a molecule that are infrared active-those, that give rise to absorptions-are the stretching and bending modes. Stretching vibration involves a continuous change in the inter-atomic distance along the axis of the bond between two atoms. These are two types; Symmetric and Asymmetric Stretching . Bending vibrations are characterized by a change in the angle between two bonds and are of four types: Scissoring, Rocking, Wagging and Twisting .

30. Stretching frequencies are higher than corresponding bending frequencies. Symmetric and Asymmetric Stretching Vibrations Symmetrical stretching: The atoms of a molecule either move away or towards the central atom, but in the same direction . Asymmetric stretching: One atom approach towards the central while other departs from it.

31. BENDING VIBRATIONS Scissoring  is the movement of two atoms toward and away from each other. Rocking  is like the motion of a pendulum on a clock, but an atom is the pendulum and there are two instead of one. Wagging  is like the motion in which you make a "V" sign with your fingers and bend them back and forth from your wrist. Twisting  is a motion as if the atoms were walking on a treadmill.

32. Examples:

33. 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.

34. Fundamental Vibrational modes = 3 x 3-5 = 4. Fundamental Vibrational modes of CO 2 (3 atoms –Linear)

35. Let us now consider how bond strength and the masses of the bonded atoms affect the infrared absorption frequency. The natural frequency of vibration of a bond is given by the equation (Hooke’s law). VIBRATIONAL FREQUENCY

36. A new expression is obtained by inserting the actual values of π and c: Examples: Note: Vibrational frequency is directly proportional to force constant ( K ) (Bond strength) and inversely proportional to reduced mass ( μ ).

37. In general, triple bonds are stronger than double or single bonds between the same two atoms and have higher frequencies of vibration (Higher wavenumbers): The C-H stretch occurs at about 3000 cm -1 . As the atom bonded to carbon increases in mass, the reduced mass ( μ ) increases, and the frequency of vibration decreases (wavenumbers get smaller):

38. Bending motions occur at lower energy (lower frequency) than the typical stretching motions because of the lower value for the bending force constant K. Hybridization affects the force constant K, also. Bonds are stronger in the order sp>sp 2 >sp 3 , and the observed frequencies of C-H vibration illustrate this nicely.

39. GROUP FREQUENCIES

40. Factors affecting group frequencies The value of vibrational frequency of a bond calculated by Hooke’s Law is not always equal to their observed value. The force constant is changed with the electronic and steric effects caused by other groups present in the surroundings. Following are some important factors affecting the vibrational frequency of a bond. Effect of Bond Order Bond order affects the position of absorption bands. Higher the bond order larger is the band frequency. A C-C triple bond is stronger than a C=C bond, so a C-C triple bond has higher stretching frequency than does a C=C bond.

41. Similarly, a C=O bond stretches at a higher frequency than does a C-O bond and a C-N triple bond stretches at a higher frequency than does a C=N bond which in turn stretches at a higher frequency than does a C-N bond.

42. Electronic Effects: Changes in the absorption frequencies for a particular group take place when the substituent's in the neighbourhood of that particular group are changed. The frequency shifts are due to the electronic effects which include Inductive effect, Mesomeric effect, Field effects etc. Under the influence of these effects, the force constant or the bond strength changes and its absorption frequency shifts from the normal value. The introduction of alkyl group causes +I effect which results in the lengthening or the weakening of the bond and hence the force constant is lowered and wavenumber of absorption decreases. Wavenumber of ν C=O Formaldehyde (HCHO) 1750 cm -1 Acetaldehyde (CH 3 CHO) 1745 cm -1 Acetone (CH 3 COCH 3 ) 1715 cm -1 Note: Aldehydes absorb at higher wavenumber than ketones

43. The introduction of an electronegative atom or group causes –I effect which results in the bond order to increase. Thus, the force constant increases and hence the wavenumber of absorption rises. Wavenumber of ν C=O Acetone (CH 3 COCH 3 ) 1715 cm -1 Chloroacetone (ClCH 2 COCH 3 ) 1725 cm -1 Dichloroacetone (Cl 2 CHCOCH 3 ) 1740 cm -1 Conjugation lowers the absorption frequency of C=O stretching whether the conjugation due to α , β- unsaturation or due to an aromatic ring. ν C=O 1706 cm -1 1693 cm -1 Note: -I effect is dominated by mesomeric effect.

44. The electron pair on nitrogen atom in amide is more labile and participates more in conjugation, hence the amide absorbs less frequency than the esters. The lone pair of electrons participates more in conjugation in compound I as compared to that compound III. Thus, in compound I, ν(C=O) absorption occurs at lower wave number compared to that in compound III. In compounds II and IV, inductive effect dominates over mesomeric effect and hence absorption takes place at comparatively higher frequencies.

45. Hydrogen Bonding The presence of hydrogen bonding changes the position and shape of an infrared absorption band. Frequencies of both stretching as well as bending vibrations are changed because of hydrogen bonding. The X-H stretching bands move to lower frequency usually with increased intensity and band widening. The X-H bending vibration usually shifts to higher frequencies. Stronger the hydrogen bonding, greater is the absorption shift towards lower wavenumber from the normal values. The two types of hydrogen bonding (intramolecular and intermolecular) can be differentiated by the use of infrared spectroscopy.

46. The extent of inter-molecular hydrogen bonding depends upon the concentration of the solution and hence the position and the shape of an absorption band also depend on the concentration of the solution. The more concentrated the solution, the more likely it is for the OH-containing molecules to form intermolecular hydrogen bonds. It is easier to stretch an O-H bond if it is hydrogen bonded, because the hydrogen is attracted to the oxygen of neighbouring molecule. Therefore, the O-H stretching of a concentrated (hydrogen bonded) solution of an alcohol occurs at about 3550 cm -1 , whereas the O-H stretching band of a dilute solution (with little or no hydrogen bonding) appears at 3650 cm -1 . Additionally, hydrogen-bonded OH groups also have broader absorption bands whereas the absorption bands of non-hydrogen–bonded OH groups are sharper.

47. Field effect: In ortho substitution, inductive effect, mesomeric effect along with steric effect is considered. In ortho substituted compounds, the lone pairs of electrons on two atoms influence each other through space interactions and change the vibrational frequencies of both the groups. This effect is called field effect. The non-bonding electrons present on oxygen atom and halogen cause electrostatic repulsions. This causes a change in the C=O hybridization and which in turn makes it to go out of plane of the double bond. Thus, the conjugation is diminished and absorption occurs at a higher wavenumber. Thus, for such ortho substituted compounds, cis absorbs (field effect) at a higher frequency as compared to the trans isomer.

48. Bond angles Smaller ring requires the use of more p-character to make the internal C-C bonds for the requisite small angles. This gives more s character to the C=O sigma bond which causes the strengthening and stiffening of the exocyclic double bond. The force constant K is then increased and the absorption frequency increases.

49. Complementarity of IR and Raman spectroscopy For the infra-red spectrum to occur, the molecule must show a change in the dipole moment. For the Raman spectra, there must be a polarstability of the molecule. As these two requirements are somewhat different, lines may be formed in one of the spectra or in both. The symmetrical stretching of the molecule which are usually missing in the infra-red appear prominently in Raman spectra. On the other hand, asymmetric vibrations show opposite behavior. Thus, we say that vibrational modes which are inactive in Infra-red are somewhat active in Raman spectra.

50. For carbon dioxide, the bending and antisymmetric modes are infrared active, while the symmetric stretch mode is Raman active. This behaviour is typical of all centrosymmetric molecules. Modes that are infrared active are Raman inactive and vice versa. This is the Rule of Mutual Exclusion, which states that no normal mode can be both infrared and Raman active in a molecule that possesses a centre of symmetry. Rule of Mutual Exclusion:

51. 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

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

53. 1-decene C=C 1640-1680 unsat’d C-H 3020-3080 cm -1 910-920 & 990-1000 RCH=CH 2

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

55. o -xylene 735-770 ortho

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

57. 1-butanol CH 3 CH 2 CH 2 CH 2 -OH C-O 1 o 3200-3640 (b) O-H

58. 2-butanol C-O 2 o O-H

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

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

61. 2-butanone  C=O ~1700 (s)

62. IR Spectra of Benzoic acid

63. IR Spectra of Methyl benzoate

64. Applications of IR spectroscopy Identification of organic compounds Structure determination Qualitative analysis of functional group Distinction between two types of hydrogen bonding Quantitative analysis Study of chemical reaction Study of Keto-Enol tautomerism Study of complex molecules Detection of impurity in a compound.

65. 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