Topics Spectroscopy. Fall 2016

Transcription

1 Topics Spectroscopy Fall

2 SPECTROSCOPY: short wavelength regions ESCA (photoelectron) and UV handout 2

3 alert approach for spectroscopy material not straight from text chapter must FOLLOW videos, lectures, handout and worksheet (WA) W is from SAMPLE FINAL QUESTIONS on spectroscopy for discussion group (M-Tu November): inquiry exercise Galen Gorski (UCSC graduate student EarthSci, ISEE) 3

4 spectroscopy handout 4

5 clicker questions worksheet 11, sections I-II.3 5

6 spectroscopic principles (Chem 1M/1N exps. #6, #9 and #11) 6

7 spectroscopic excitations ( E = hυ = hc/λ ; λυ = c ) λ (nm) υ (sec -1 ) radiation technique molecular excitation 7

8 spectroscopic excitations ( E = hυ = hc/λ ; λυ = c ) λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) increases far uv vacuum UV excitation of σ electrons near uv visible UV-VIS excitation of π and non-bonding (n) electrons infra-red IR vibrational excitations (IR) microwave microwave ESR rotations of molecules and flipping unpaired electron spins in external magnetic field energy decreases radiowave NMR (MRI) flipping of nuclear spins in an external magnetic field 8

9 spectroscopic excitations: ESCA λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) increases 9

10 ESCA Electron Spectroscopy for Chemical Analysis 10

11 ESCA photoelectric effect for inner shells x-rays emitted photoelectron 0 characteristic of atom type involved in bonding 2s 1s 2p hν absorbed energy O 1s 2 2s 2 2p 4 11

12 ESCA (and photoelectron effect) velocity of each electron measured emitted electrons from different energy levels binding energy ( Φ) 2 = hν X ray mv electron

13 ESCA (binding energy is like work function for inner electrons) Why O1s higher binding energy than C1s? binding energy ( Φ) h 1 mv 2 2 = ν X ray electron 13

14 spectroscopic excitations: ESCA λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) increases 14

15 vacuum UV λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σ electrons increases 15

16 vacuum UV 16

17 excited-state orbitals in polyatomic molecules destructive interference leads to antibonding orbitals which are not usually occupied in the ground state of molecules but which may become occupied upon excitation of electrons by light types of antibonding orbitals: C 4 : σ* = sp 3 on C 1s on C 2 4 : σ* = sp 2 on C A sp 2 on C B π* = p π on C A p π on C B 17

18 energies of orbitals in double bond C A C B σ* - C A + - C B + [sp 2 on C A - sp 2 on C B ] energy π* π σ + - C A - C A C A C B C B - C B + - [p π on C A - p π on C B ] [p π on C A + p π on C B ] [sp 2 on C A + sp 2 on C B ] 18

19 * and π* excitations are in the far UV regrion σ* - C A + - C B + [sp 2 on C A - sp 2 on C B ] energy π* π σ σ π* (weak) σ σ* + - C A - C A C A C B C B - C B + - [p π on C A - p π on C B ] [p π on C A + p π on C B ] [sp 2 on C A + sp 2 on C B ] 19

20 vacuum UV λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σof σ electrons increases 20

21 spectroscopic excitations (UV-VIS) λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σ electrons increases near uv visible UV-VIS excitation of π and non-bonding (n) electrons 21

22 near UV transitions C A C B σ* - C A + - C B + [sp 2 on C A - sp 2 on C B ] energy π* π σ π π* + - C A - C A C A C B C B - C B + - [p π on C A - p π on C B ] [p π on C A + p π on C B ] [sp 2 on C A + sp 2 on C B ] 22

23 relative energy on lone pair non-bonding electrons σ* π* n π* 347nm energy π σ { n: non-bonded e s (higher E than π in some molecules; lower E than π in others) 23

24 summary uv-vis weak adapted from: 24

25 spectroscopic excitations (UV-VIS) λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σ electrons near uv visible UV-VIS excitation of of ππ and non-bonding (n) (n) electrons 25

26 UV-VIS spectrometers (Chem 1M/1N exps. #9 and #11) 26

27 continue in next class: absorptions in the visible region why do objects appear colored?? (previously) what molecules absorb visible light?? 27

28 Done for now!! 28

29 this video on short wavelength spectroscopic techniques was extracted from a full lecture length movie produced for a previous UCSC CEM1B flip class 29

30 W 9-10 this week WebAssign W9 spectroscopy problems from sample final (conceptual) due Tuesday November 29 next week WebAssign W10 kinetics calculations (last WebAssign) due Sunday December 4 30

31 why do objects appear colored? why do objects appear colored?? low energy electronic absorptions in the visible region of electromagnetic spectrum result in the reflection (transmission) of wavelengths of the complementary color. LUMO electronic transition OMO LUMO OMO ighest Occupied Molecular Orbital Lowest Unoccupied Molecular Orbital absorbs photon of energy h =E LUMO -E OMO 31

32 Br 2 (g), I 2 (g), NO 2 closely spaced OMO and LUMO due to d-orbital m.o.s or open shells (unpaired e s) transition metal complex ions octahedral complex t 2g e g (lone-pair) n π* π π* in molecules with conjugated pi-systems rhodopsin, the molecule most important to seeing color 32

33 spectroscopic excitations λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σ electrons near uv visible UV-VIS excitation of π and non-bonding (n) electrons infra-red IR vibrational excitations (IR) 33

34 diatomic molecules (vibrating and rotating) 34

35 vibrations new unit wave number 1 ν ν = = [ ν] = cm λ c E = hc ν cm c = cm sec larger ν higher energy photon larger ν higher energy vibration -1 35

36 vibrational frequencies of homonuclear diatomic molecules and ions larger ν wave number higher energy photon vibrational frequency ( mass of atoms (lighter higher ) ) depends on strength of bond (tighter spring higher ) ν ν ν Molecule Bond Order Vibrational frequency (cm -1 ) Li C N N O O F b.o. ν low mass vibrational frequency follows bond order greater b.o. greater frequency 36

37 vibrational motion in molecules ( 2 O) E = hc ν wave number 1 ν = [ ν] = cm λ cm c = cm sec -1 ORIGINALLY FROM: 37

38 vibrational motions in molecules (benzene) breathing (stretching) mode asymmetric stretching mode Chubby Checkers twisting mode American Bandstand Movies provided courtesy: 38

39 IR spectrometer 39

40 infrared vibrational spectrocopy (fig ) photons at infrared wavelengths excite the vibrational motion of atoms in a molecule 40

41 group frequencies different types of bonds require different energy photons for vibrational excitation a given bond type will have a similar absorption energy in various molecules Bond Characteristic Frequency (approximate) ~ υ (cm -1 ) [E = hcυ] ~ λ (nm) C C C = C C C C O C = O C O note energy to excite bond vibration: E C C > E C=C > E C-C 41

42 IR spectra NO O C ~3200 cm -1 NO C=O C=O ~1700 cm -1 O- ~3600 cm -1 42

43 spectroscopic excitations λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σ electrons near uv visible UV-VIS excitation of π and non-bonding (n) electrons infra-red IR vibrational excitations (IR) microwave microwave ESR rotations of molecules and flipping unpaired electron spins in external magnetic field 43

44 radiowave λ (nm) υ (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) far uv vacuum UV excitation of σ electrons near uv visible UV-VIS excitation of π and non-bonding (n) electrons infra-red IR vibrational excitations (IR) microwave microwave ESR rotations of molecules and flipping unpaired electron spins in external magnetic field radiowave NMR (MRI) flipping of nuclear spins in an external magnetic field 44

45 NMR (MRI) spectrometers 45

46 NMR- WY? protons (hydrogen nuclei), like electrons, behave as if they were tiny magnets electron ~650 x stronger magnet in an external magnetic field, spin up and spin down will have different energies in NMR spectroscopy, photons in the radiowave region have the correct energy to cause a hydrogen nucleus to flip its spin 46

47 identifying equivalent and non-equivalent protons to flip hydrogen atoms (nuclei) in different chemical environments requires slightly different energies (chemical shift) (all equivalent, 2 ClC 1 peak one chemical shift ) (two environments, Cl 2 C and C 3 2 peaks two chemical shifts ) 47

48 not responsible for spin-spin coupling (pp ) will get plenty in o-chem 48

49 vocabulary: fluorescence fluorescence- emission of radiation (almost) directly from the excited state excited photon photon E in =hυ in Eout =hυ out ground ( υ out υ in ) ( λ out λ in ) time ~ to 10-9 sec (fluorescence stops soon after exciting light is turned off) 49

50 vocabulary: radiationless decay (nonradiative decay) radiationless decay- transition from a higher to a lower energy state with a loss of energy in the form of heat rather than emission of a photon excited photon radiationless decay E in =hυ in thermal energy ground 50

51 phosphorescence phosphorescence- slow return to ground state by emission of photon from intermediate state excited photon E in =hυ in intermediate state photon ground ( υ out < υ in ) ( λ out > λ in ) E out =hυ out time ~ 10-3 to 10 sec and longer (phosphorescence continues after exciting light is turned off) 51

52 chemiluminescence chemiluminescence- light given off when chemical reaction leaves products in excited states and then the product fluoresces A + B C* C + light molecule C in excited state 52

53 chemiluminescence: fireflies 53

54 chemiluminescence: fireflies 54

55 chemiluminescence: light sticks 55

56 bioluminescence Bioluminescence: Explanation for Glowing Seas Suggested According to the study, here is how the light-generating process in dinoflagellates may work: As dinoflagellates float, mechanical stimulation generated by the movement of surrounding water sends electrical impulses around an internal compartment within the organism, called a vacuole--which holds an abundance of protons. These electrical impulses open so-called voltage-sensitive proton channels that connect the vacuole to tiny pockets dotting the vacuole membrane, known as scintillons. Once opened, the voltage-sensitive proton channels may funnel protons from the vacuole into the scintillons. Protons entering the scintillons then activate luciferase- -a protein, which produces flashes of light, that is stored in scintillons. Flashes of light produced by resulting luciferase activation would be most visible during blooms of dinoflagellates. 56

57 end of lectures on spectroscopy 57

58 colored transition metal complexes- glazes Ni(N 3 ) 6 Br 2 CoCl O NiSO O 58

59 d-orbitals and ligand Interaction (octahedral field) 2 O Ni(N 3 ) 6 Cl 2 [Ni(N 3 ) 6 ] 2+ (aq) + 2Cl (aq) d-orbitals pointing directly at axis are affected most by electrostatic interaction 3d Ni 2+ [Ar]3d 8 energy d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction ibchem.com/ib/ibfiles/periodicity/per_ppt/crystal_field_theory.ppt 59

60 absorption of visible light in octahedral transition metal complexes e g t 2g [Ni(N 3 ) 6 ] 2+ 3d orbitals all have same energy in Ni 2+ (g) presence of 6N 3 cause splitting of the energies of the 3d-orbitals into two levels in [Ni(N 3 ) 6 ] 2+ visible light causes electronic transitions between the two levels resulting in colored transition metal complexes 60

61 ß- carotene; conjugated double bonds (figure 14.56, 14.57) 61

62 rhodopsin (11-cis retinal + opsin) intradiscal (lumen) 11-cis retinal + opsin (protein) interdiscal (cytoplasmic) 62

63 how do we see color??? 63

64 AA! that s what its all about 64

65 Advanced (don t Fret) FROM: Visual transduction cascade, 1 photon 10 6 Na + 65

66 signal amplification in visual excitation cascade cytoplasmic tail 66

67 amplification 1 photon 1 Rh* ~200 T* 1 T* 1 PDE 1 PDE-T*-GDP many cgmp many GMP closes 200 Na + channels 10 6 Na + ions 67

68 summary 68