Topic 2 - Index of slides on IR (link to copies of slides: http://www.cpp.edu/~psbeauchamp/pdf_videos/lecture_2_IR.pdf) 1. Topic 2 Slide Index 23. Real IR versus simulated IR used in our problems 2. Common features of electromagnetic radiation 24. Example how touseIRwith molecular formula 3. Different ways of measuring light energy 25. Example how to use IR with molecular formula 4. Overview of relative energies of UV-Vis, IR and NMR 26. Possible structures for slides 22 and 23 5. Excitation and Emission 27. Example IR functional groups: alkane, mono alkene, 6. IR vibrations, coupling to eletromagnetic radiation gem alkene 7. Ball and spring model of bonds (Hook'sLaw,masses 28. Example IR FGs: cisalkene, trans alkene, tri alkene and force constants 29. Example IR FGs: terminal alkyne, internal alkyne, 8. Vibrations (stretching and bending), 2 atoms symmetrical alkyne 9. Vibrations (stretching and bending), 3 atoms 30. Example IR FGs: mono aromatic, ortho aromatic, 10. Asymmetric and symmetric stretching groupings meta aromatic 11. Complex groupings of atoms 31. Example IR FGs: alcohol, para phenol, mono aromatic 12. Overtones, combination and difference bands, ether functional group region & fingerprint region 32. Example IR FGs: 1o amine, 2o amine, 1o aromatic 13. IR data presentation (wave numbers, , cm-1) amine 14. Functional groups we use and chart where they absorb 33. Example IR FGs: ketone, unsaturated ketone, mono 15. Detailed IR data information (part a) aromatic ketone 16. Detailed IR data information (part b) 34. Example IR FGs: aldehyde, aromatic aldehyde, 17. Special FG features (ROH,RCO2H),(acyl,alkoxy C-O), carboxylic acid (amine,amide N-H) 35. Example IR FGs: aromaticacid,simpleester, 18. Special FG features (C=O bonds),(triple bonds) aromatic ester 19. IR flowchart to decide what functional groups are 36. Example IR FGs: 1o amide, 2o amide, 3o amide present (slides 18, 19 and 20) 37. Example IR FGs: simple nitrile, para aromatic nitrile, 20. Carbonyl (C=O) patterns acid chloride 21. Triple bonds and sp2 C-H bending (alkenes and 38. Example IR FGs: anhydride, thiol, sulfide aromatics) 39. Example IR FGs: simple nitro, mono aromatic nitro 22. Other non-carbonyl functional groups 1 Slide 1 Common Features of Electromagnetic Radiation All of the spectroscopic techniques in this workbook, except mass spectrometry, involve the interaction of electromagnetic radiation with matter. The responses of matter to electromagnetic radiation allow us to probe beyond our senses into the world of molecules and atoms. The following figureillustrates the usual spectroscopic divisions of the electromagnetic spectrum. Even though every designated region is a term in common everyday usage, you may be unfamiliar with these terms as methods to study the structure of molecules. E = h = h c (1/) = h c High Energy Low Energy frequency (Hz = sec-1) = 1018 1016 1015 1014 1013 1012 1011 1010 109 108 107 wave length (m) = 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 101 wave number (cm-1) = 107 106 105 104 103 102 101 1 10-1 10-2 10-3 x-rays UV Vis IR microwaves radiowaves VIBGYOR V = violet higher energy I = indigo lower energy higher frequency B = blue lower frequency higher wave number G = green lower wave number Y =yellow longer wave length shorter wave length O = ornage R = red = frequency = # / sec = Hz (NMR) Two constants used in the calculations of = wave length = nm (UV/Vis) the energy in electromagnetic radiation. 8 = (1/) = wave number The speed of light = c = 3.0 x 10 m/s -1 -34 = # / distance = cm (IR) Plank's constant = h = 6.62 x 10 j-s 2 Slide 2 Light is a form of energy that can produce changes in molecules, which can provide clues about how the atoms are connected together (their structure). Three common ways of viewing this energy are by frequency (z, NMR), wave length (, nm, UV-Vis) and wave number (, cm-1, IR). Two constants used in the calculations of The units of hertz (Hz) are used in NMR. the energy in electromagnetic radiation. units of frequency (#/second) 8 The speed of light = c = 3.0 x 10 m/s = c Plank's constant = h = 6.62 x 10-34 j-s 14 red = 3.8 x 10 Hz (lower energy) visible 14 E = h = h c (1/) = h c c = () () violet = 7.5 x 10 Hz (higher energy) light These units (nm) are used in UV-Vis. These units (cm-1) are used in IR. units of wavelength (nm = 10-9 m) units of wave number (#/cm) = c = 1 = c -1 red = 800 nm (lower energy) visible red = 12,000 cm (lower energy) visible light -1 violet = 400 nm (higher energy) violet = 25,000 cm (higher energy) light Visible light Infrared Ultra violet violet indigo blue green yellow orange red (UV) (IR) This is where our detectors work = eyes! = 400 nm = 800 nm = 7.5 x 1014 / sec = 3.5 x 1014 / sec = 25,000 / cm = 12,000 / cm 3 Slide 3 UV-Vis IR Microwaves NMR 130 kcal (40-200 kcal/mole) (1-10 kcal/mole) ( 1 kcal/mole) (0.00001 kcal/mole) 120 kcal 110 kcal 100 kcal E = h The variables are , and . 90 kcal Changes in these values will E = h (c / ) = h c (1 / ) change the energy of 80 kcal electromagnetic radiation. E = h = hc(1/) = hc E = h c 70 kcal (typical units = nm) h = 6.62 x 10-34 joul-sec These two 60 kcal c = 3.0 x 1010 cm/sec = 3.0 x 108 m/sec are constants. 50 kcal 40 kcal 30 kcal (typical units = Hz) 20 kcal The difference (typical units = cm-1) in energies is too 10 kcal 10 kcal small to see on 1 kcal this graph. 0 kcal 0 kcal 0 kcal 0 kcal 0 kcal Energy changes Energy causes Energy causes Energy causes location of electrons higher energy higher rotation changes in spin in bonds (bonding vibrations of molecules or of nuclear to antibonding (stretching and changes in spin particles (e.g. orbitals). bending of bonds). of electrons in a protons and magnetic field. neutrons in a magnetic field) Other Ways of Looking at Energy ( = frequency, =wavenumber, =wavelength) Parameter X-rays UV Vis IR Microwave Radiowaves E (kcal/mole) 24,000 200 50 10 3x10-3 6x10-6 E (kj/mole) 100,000 800 200 40 1.2x10-2 2x10-5 E (eV/photon) 1,200 8 2 0.4 1x10-4 3x10-7 (Hz) 3x1017 2x1015 5x1014 1x1014 3x1010 3x108 (cm-1) 1x107 50,000 17,000 3,000 1 0.01 (nm) 1 200 600 3,000 1x107 1x109 4 Slide 4 A* E2 E2 E2 (excited (excited (excited state) state) state) emission excitation E (relaxation) E IR and UV-Vis 1H and 13C NMR detected here (absorbs a photon) detected here (releases a photon) A A E1 E1 E1 (ground (ground (ground state) state) state) A = molecule of interest in ground state A* = molecule of interest in excited state E2 > E1 = energy states of the molecule E = (E2 - E1) 5 Slide 5 Infrared Spectroscopy Vibrational changes between atoms in molecules are excited when IR light is passed through a sample. When the electrical component of the oscillating frequency of electromagnetic radiation matches the electrical dipole oscillation of thevibrating atoms, efficient transfer of energy occurs from thephoton to the vibrating bond. In effect, the bond receives a synchronous push from the light wave (think of you and a swing). The two frequencies are said to be coupled. Bonds without an electrical dipole moment cannot efficiently couple to the electrical vector of the oscillating photon. Symmetrically substituted bonds are said to be IR ininactive and are either very weak or do not show an absorption band at allll. Common inactive patterns include symmetrically substituted alkene and alkyne bonds. - - - - - - - - - - - - - - - - - - - - - - - - - vibrating dipole of a bond + + + + + + + + + + + + + + + + + + + + + + + + + matching frequency from the electrical component of electromagnetic radiation causes absorption and E = h excitation electromagnetic radiation (UV, Vis, IR, microwaves, radiowaves) magnetic component detector electrical component source 6 Slide 6 A Ball and Spring Model of a Bond Atoms in molecules can be approximated as balls of various relative masses attached through bonds, roughly considered to be springs with relative force constants (Hook's Law). Hook's Law A bond isviewed as a spring and the atoms are viewed as balls. The vibrations = (1/2)k/reduced m x m A B between the two atoms can be estimated = 1 2 = reduced using Hook's law (depends on the masses m1 + m2 mass and the force constant). k = force constant This means that different vibrations will occur depending on what atoms are connected together and how strong the bonds are connecting them. For example, organic atoms can be attached in single bonds in a variety of ways. Each bond has characteristic vibrational frequencies dependent on reduced masses and force constants present in the IR spectrum. C H C D N H O H S H C C C N (sp, sp2, sp3) C F C Cl C Br C I C O C S Additionally, even when the same type of atom is bonded to carbon, there can be variable strength bonds, i.e. single, double and triple bonds (changes the force constant). triple double single C C C C C C bond strengths E = h = h c (1/) = h c CN CN CN triple > double > single CO CO CO 7 Slide 7 Two Types of Vibrations are Common in IR Spectroscopy - Stretching and Bending of Bonds Two atoms present a very simplepictureof vibration and bending. Inastretching vibration the two atoms stretch away from one another or contract towards one another.