Lectures Spectroscopy. Fall 2012

Transcription

1 Lectures Spectroscopy Fall spectroscopic principles (Chem 1M/1N exps. #6 and #11) 4 1

2 spectroscopic excitations ( E = h = hc/ ; = c ) (nm) (sec -1 ) radiation technique molecular excitation 5 spectroscopic excitations (nm) (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 electrons near uv visible UV-VIS excitation of 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 6 2

3 spectroscopic excitations: ESCA (nm) (sec -1 ) radiation technique molecular excitation x-rays ESCA excitation of inner shells breaking of bonds (x-ray damage) 7 ESCA Electron Spectroscopy for Chemical Analysis 8 3

4 ESCA photoelectric effect for inner shells x-rays emitted photoelectron 0 involved in bonding 2s 1s 2p h absorbed energy O 1s 2 2s 2 2p 4 9 ESCA (and photoelectron effect) velocity of each electron measured emitted electrons from different energy levels binding energy ( ) h X ray mv electron

5 ESCA (binding energy is like work function for inner electrons) Why O1s higher binding energy than C1s? binding energy ( ) h X ray mv electron 11 vacuum UV (nm) (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 electrons 12 5

6 vacuum UV 13 energies of orbitals in double bond * - C A + - C B + C A C B [sp 2 on C A -sp 2 on C B ] energy * * * + - C A - C A C A C B - C B - C B [p on C A -p on C B ] [p on C A + p on C B ] [sp 2 on C A + sp 2 on C B ] 14 6

7 spectroscopic excitations (UV-VIS) (nm) (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 electrons near uv visible UV-VIS excitation of and non-bonding (n) electrons 15 near UV transitions C A C B * - C A + - C B + [sp 2 on C A -sp 2 on C B ] energy * * + - C A - C A C A C B - C B - C B [p on C A -p on C B ] [p on C A + p on C B ] [sp 2 on C A + sp 2 on C B ] 16 7

8 UV-VIS spectrometers (Chem 1M/1N exps. #6 and #11) 17 why are objects colored? why are objects 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 18 8

9 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 * * in molecules with conjugated pi-systems rhodopsin, the molecule most important to seeing color 19 spectroscopic excitations (nm) (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 electrons near uv visible UV-VIS excitation of and non-bonding (n) electrons infra-red IR vibrational excitations (IR) 20 9

10 diatomic molecules (vibrating and rotating) 21 vibrational frequencies of homonuclear diatomic molecules and ions 1 wave number cm cm E hc c cm sec larger higher energy photon vibrational frequency ( ) depends on mass of atoms (lighter ï higher ) strength of bond (tighter spring ï higher ) Molecule Bond Order Vibrational frequency (cm 1 ) Li C N N O O F low mass 22 10

11 vibrational motion in molecules ( 2 O) E hc 1 wave number cm cm c cm sec ORIGINALLY FROM: 23 vibrational motions in molecules (benzene) breathing (stretching) mode asymmetric stretching mode Chubby Checkers twisting mode American Bandstand Movies provided courtesy: timro@hydrogen.cchem.berkeley.edu 24 11

12 IR spectrometer 25 infrared vibrational spectrocopy (fig ) photons at infrared wavelengths excite the vibrational motion of atoms in a molecule 26 12

13 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) ~ (nm) υ (cm -1 ) [E hcυ] ~ 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 27 IR spectra NO O C ~3200 cm -1 NO C=O C=O ~1700 cm -1 O- ~3600 cm

14 spectroscopic excitations (nm) (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 electrons near uv visible UV-VIS excitation of 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 29 radiowave (nm) (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 electrons near uv visible UV-VIS excitation of 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 30 14

15 NMR (MRI) spectrometers 31 NMR- WY? protons (hydrogen nuclei), like electrons, behave as if they were tiny magnets 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 32 15

16 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 ) 33 not responsible for spin-spin coupling (pp ) will get plenty in o-chem 34 16

17 vocabulary: fluorescence fluorescence- emission of radiation (almost) directly from the excited state excited photon photon E in =h in ground ( out in ) ( out in ) Eout =h out time ~ to 10-9 sec (fluorescence stops soon after exciting light is turned off) 35 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 =h in thermal energy ground 36 17

18 phosphorescence phosphorescence- slow return to ground state by emission of photon from intermediate state excited photon E in =h in ground ( out < in ) ( out > in ) intermediate state photon E out =h out time ~ 10-3 to 10 sec and longer (phosphorescence continues after exciting light is turned off) 37 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 38 18

19 chemiluminescence: fireflies 39 chemiluminescence: fireflies 40 19

20 chemiluminescence: light sticks 41 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

21 end of lectures on spectroscopy 43 colored transition metal complexes- glazes Ni(N 3 ) 6 Br 2 CoCl O NiSO O

22 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 45 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 46 22

23 ß- carotene; conjugated double bonds (figure 14.56, 14.57) 47 rhodopsin (11-cis retinal + opsin) intradiscal (lumen) 11-cis retinal + opsin (protein) interdiscal (cytoplasmic) 48 23

24 how do we see color??? 49 AA! that s what its all about 50 24

25 FROM: 51 signal amplification in visual excitation cascade 52 25

26 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 53 26