Chemistry 1B-AL, Fall 2016 Topics Spectroscopy

Transcription

1 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy Topics 1920 Spectroscopy SPECTROSCOPY: short wavelength regions ESCA (photoelectron) and UV Fall 2016 handout 1 2 alert spectroscopy handout 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 (MTu 2829 November): inquiry exercise Galen Gorski (UCSC graduate student EarthSci, ISEE) 3 4 clicker questions worksheet 11, sections III.3 spectroscopic principles (Chem 1M/1N exps. #6, #9 and #11) 5 6 1

2 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy spectroscopic excitations ( E = h = hc/ ; = c ) spectroscopic excitations ( E = h = hc/ ; = c ) (nm) (sec 1 ) radiation technique molecular excitation (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) l increases far uv vacuum UV excitation of electrons near uv 300 excitation of and UVVIS nonbonding (n) electrons visible infrared IR vibrational excitations (IR) energy decreases 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 7 8 spectroscopic excitations: ESCA ESCA (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) Electron Spectroscopy for Chemical Analysis l increases 9 10 ESCA photoelectric effect for inner shells ESCA (and photoelectron effect) xrays emitted photoelectron 0 velocity of each electron measured involved in bonding 2p 2s h absorbed energy emitted electrons from different energy levels characteristic of atom type 1s O 1s 2 2s 2 2p 4 11 binding energy ( ) 1 mv 2 2 X ray electron h 12 2

3 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy ESCA (binding energy is like work function for inner electrons) spectroscopic excitations: ESCA Why O1s higher binding energy than C1s? (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) l increases binding energy ( ) h X ray mv electron vacuum UV vacuum UV (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) far uv vacuum UV excitation of electrons l increases excitedstate 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 : * = sp 3 on C 1s on C 2 4 : * = sp 2 on sp 2 on * = p on p on 17 energy energies of orbitals in double bond * * [sp 2 on sp 2 on ] [p on p on ] [p on p on ] [sp 2 on sp 2 on ] 18 3

4 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy s s* and s * excitations are in the far UV regrion vacuum UV energy * * * (weak) * [sp 2 on sp 2 on ] [p on p on ] [p on p on ] [sp 2 on sp 2 on ] 19 l increases (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) far uv vacuum UV excitation of of electrons 20 spectroscopic excitations (UVVIS) near UV transitions l increases (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) far uv vacuum UV excitation of electrons near uv 300 excitation of and UVVIS nonbonding (n) electrons visible energy * * * [sp 2 on sp 2 on ] [p on p on ] [p on p on ] [sp 2 on sp 2 on ] relative energy on lone pair nonbonding electrons summary * * n * 347nm uvvis energy { n: nonbonded e s (higher E than in some molecules; lower E than in others) weak 23 adapted from:

5 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy spectroscopic excitations (UVVIS) UVVIS spectrometers (Chem 1M/1N exps. #9 and #11) (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) far uv vacuum UV excitation of electrons near uv visible UVVIS excitation of of and nonbonding (n) (n) electrons continue in next class: absorptions in the visible region why do objects appear colored?? (previously) what molecules absorb visible light?? Done for now!! W 910 this video on short wavelength spectroscopic techniques was extracted from a full lecture length movie produced for a previous UCSC CEM1B flip class 29 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

6 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy 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. electronic transition OMO LUMO LUMO OMO Br 2 (g), I 2 (g), NO 2 closely spaced OMO and LUMO due to dorbital m.o.s or open shells (unpaired e s) transition metal complex ions octahedral complex t 2g e g (lonepair) n * * in molecules with conjugated pisystems ighest Occupied Molecular Orbital Lowest Unoccupied Molecular Orbital rhodopsin, the molecule most important to seeing color absorbs photon of energy hn =E LUMO E OMO spectroscopic excitations diatomic molecules (vibrating and rotating) (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) far uv vacuum UV excitation of electrons near uv visible UVVIS excitation of and nonbonding (n) electrons infrared IR vibrational excitations (IR) vibrations vibrational frequencies of homonuclear diatomic molecules and ions new unit wave number 1 cm c cm 10 1 E hc c cm sec larger higher energy photon larger higher energy vibration 1 35 wave number 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 b.o. low mass vibrational frequency follows bond order greater b.o. î greater frequency 36 6

7 10 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy vibrational motion in molecules ( 2 O) vibrational motions in molecules (benzene) wave number 1 1 cm cm 1 E hc c cm sec breathing (stretching) mode asymmetric stretching mode Chubby Checkers twisting mode American Bandstand ORIGINALLY FROM: Movies provided courtesy: timro@hydrogen.cchem.berkeley.edu 38 IR spectrometer infrared vibrational spectrocopy (fig ) photons at infrared wavelengths excite the vibrational motion of atoms in a molecule group frequencies IR spectra 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 NO O NO C=O C ~3200 cm 1 C=O ~1700 cm 1 O ~3600 cm 1 C = O C O note energy to excite bond vibration: E CC > E C=C > E CC

8 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy spectroscopic excitations radiowave (nm) (sec 1 ) radiation technique molecular excitation (nm) (sec 1 ) radiation technique molecular excitation xrays ESCA breaking of bonds (xray damage) xrays ESCA breaking of bonds (xray damage) far uv vacuum UV excitation of electrons far uv vacuum UV excitation of electrons near uv visible UVVIS excitation of and nonbonding (n) electrons near uv visible UVVIS excitation of and nonbonding (n) electrons infrared IR vibrational excitations (IR) infrared IR vibrational excitations (IR) microwave microwave ESR rotations of molecules and flipping unpaired electron spins in external magnetic field 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 NMR (MRI) spectrometers 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 identifying equivalent and nonequivalent 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 ) not responsible for spinspin coupling (pp ) (two environments, Cl 2 C and C 3 2 peaks two chemical shifts ) will get plenty in ochem

9 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy vocabulary: fluorescence vocabulary: radiationless decay (nonradiative decay) fluorescence emission of radiation (almost) directly from the excited state excited photon E in =h in ground ( out in ) ( out in ) photon Eout =h out 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 photon E in =h in excited ground radiationless decay thermal energy time ~ to 10 9 sec (fluorescence stops soon after exciting light is turned off) phosphorescence chemiluminescence phosphorescence slow return to ground state by emission of photon from intermediate state excited chemiluminescence light given off when chemical reaction leaves products in excited states and then the product fluoresces photon E in =h in ground ( out < in ) ( out > in ) intermediate state photon E out =h out A B C* C light molecule C in excited state time ~ 10 3 to 10 sec and longer (phosphorescence continues after exciting light is turned off) chemiluminescence: fireflies chemiluminescence: fireflies

10 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy chemiluminescence: light sticks bioluminescence Bioluminescence: Explanation for Glowing Seas Suggested 55 According to the study, here is how the lightgenerating 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 vacuolewhich holds an abundance of protons. These electrical impulses open socalled voltagesensitive proton channels that connect the vacuole to tiny pockets dotting the vacuole membrane, known as scintillons. Once opened, the voltagesensitive 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 colored transition metal complexes glazes end of lectures on spectroscopy Ni(N 3 ) 6 Br 2 CoCl O NiSO O dorbitals and ligand Interaction (octahedral field) absorption of visible light in octahedral transition metal complexes e g 2O Ni(N 3 ) 6 Cl 2 [Ni(N 3 ) 6 ] 2 (aq) 2Cl (aq) 3d Ni 2 [Ar]3d 8 energy dorbitals pointing directly at axis are affected most by electrostatic interaction dorbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction 59 ibchem.com/ib/ibfiles/periodicity/per_ppt/crystal_field_theory.ppt [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 3dorbitals into two levels in [Ni(N 3 ) 6 ] 2 visible light causes electronic transitions between the two levels resulting in colored transition metal complexes t 2g 60 10

11 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy ß carotene; conjugated double bonds (figure 14.56, 14.57) rhodopsin (11cis retinal opsin) intradiscal (lumen) 11cis retinal opsin (protein) interdiscal (cytoplasmic) how do we see color??? AA! that s what its all about signal amplification in visual excitation cascade cytoplasmic tail FROM: Advanced (don t Fret) photonî10 6 Na Visual transduction cascade, 11

12 Chemistry 1BAL, Fall 2016 Topics 1920 Spectroscopy amplification summary 1 photon 1 Rh* ~200 T* 1 T* 1 PDE 1 PDET*GDP many cgmp many GMP closes 200 Na channels 106 Na ions