MOLECULAR SPECTROSCOPY AND PHOTOCHEMISTRY

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1 20 CHAPTER MOLECULAR SPECTROSCOPY AND PHOTOCHEMISTRY 20.1 Introduction to Molecular Spectroscopy 20.2 Experimental Methods in Molecular Spectroscopy 20.3 Rotational and Vibrational Spectroscopy 20.4 Nuclear Magnetic Resonance Spectroscopy 20.5 Electronic Spectroscopy and Excited State Relaxation Processes 20.7 Photosynthesis 941 Fluorescence microscope image of the mouse cerebral cortex. Three different dyes were used to selectively image structural proteins called neurofilaments, a small protein called General GFAP Chemistry that a component II of intermediate filaments, and cellular 2 nuclei.

2 20.4 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 966 Measures the energies and intensities of transition between nuclear spin states of 1 H, 13 C, nuclei Degeneracy of these energies lifted with applied external magnetic field Transition between these levels corresponding to radio frequency region ( MHz) Resonance frequency observed for a particular nucleus gives information around the nucleus Structure of the molecule Magnetic resonance imaging (MRI) technique in medicine Electron (spin 1/2) Magnitude of the electron spin angular momentum, s: s s( s1) ( h/ 2 ) Projection of the electron spin angular momentum, s, along the laboratory z-axis: s m, ( m ) z s s m s (spin "up", ), m (spin "down", ) s 2

3 967 Nuclear spin Magnitude of the nuclear spin angular momentum, I I I( I 1) Projection of the nuclear spin angular momentum, I, along the laboratory z-axis: I m, ( m ) z I I 1 2 Spin 1/2 nuclei: 1 H, 13 C, 15 N, 19 F, 29 Si, and 31 P Spin 0 nuclei: 12 C, 16 O Magnetic dipole moment of a nucleus i vs. nuclear spin I g I i i N i 967 N 27 1 eh / 4mP J T : Nuclear magneton g i : nuclear g-factor of nucleus i ( 1 H: g = 5.586, 13 C: g =1.405) Energy, E, of a magnetic dipole moment,, in an external magnetic field, B 0, oriented along the z-axis E B g I B ( g I B ) m, where m 1 z 0 N z 0 N z 0 I I 2 E g B h Zeeman effect N N 0 0

4 967 Nobel Prize in Physics (1902) "in recognition of the extraordinary service they rendered by their researches into the influence of magnetism upon radiation phenomena" Hendrik A. Lorentz (DEU, ) Pieter Zeeman (DEU, ) NMR Spectrometer Sample in the tube (5 mm diameter) in a magnetic field Radio frequency input radiation by the 1 st set of coil Radiofrequency output radiation by the 2 nd set of coil Setting the frequency and scanning the magnetic field until the sample comes into resonance. 968 Fig Schematic of an FT NMR spectrometer. 50

5 968 Fig (a) Energy level splitting diagram for a proton in an external magnetic field. The vertical line shows the absorption transition for a 7.05 T field. (b) Proton NMR spectrum obtained by scanning the magnetic field in a 300 MHz NMR spectrometer. Fourier transform NMR (FT-NMR) 969 (1) Excitation of spins in thermal equilibrium by a short burst of broad band radiofrequency radiation (2) Emit radiation as they return to thermal equilibrium (3) Transient decay of this emission is recorded (4) Transient is analyzed using a Fourier transform algorithm producing a spectrum showing resonance frequencies which differ due to shielding and spin-spin splitting.

6 Analogy with a struck piano string 969 Fig (a) A pure sine wave oscillating at 440 Hz (concert A) and its Fourier transform. (b) A struck piano string (concert E = 330 Hz), its transient decay and Fourier transform showing the relative intensities of the harmonics. Intensities of NMR spectra depend on the population difference between the spin states Fractional population difference: N / N 1exp( / k T) For NMR, N / N 1 exp( g B / k T) B N 0 B NMR spectrometers identified by proton resonance frequency. 969 Ex. 300 MHz instrument has 7.05 T (tesla) magnet.

7 Chemical Shift 970 Local magnetic field felt by a nucleus is slightly different from the external magnetic field B (1 ) B local 0 : shielding constant External magnetic field induces current that shields the nucleus Very sensitive to the nature of the bonds formed with neighbors Frequency shifts are very small ( is very small) 100 ~ 1000 Hz in a 300 MHz spectrometer Scale based on the difference between the resonance freq. s of the sample and the reference material, TMS (tetramethylsilane) Chemical shift, : s r 10 r (1 ) (1 ) 6 s : resonance freq. of sample r : resonance freq. of reference s r 6 6 or 10 ( r s) 10 (1 r ) 970 Fig H NMR spectrum of methyl acetate showing resonances from two different methyl groups, one attached to the carbonyl carbon atom and one attached to the oxygen atom of the ester group.

8 970 TMS protons are well shielded TMS proton resonances at very high frequencies upfield Other protons are less well shielded downfield Equivalent protons (CH 3 ) L (at = 2.0) vs. (CH 3 ) R (at = 3.6) When the methyl group is free to rotate Protons in (CH 3 ) R are deshielded compared with protons in (CH 3 ) L (Due to the high electronegativity of O) downfield Same heights of two peaks Same number of protons in each group 971 Fig Ranges of 1 H NMR chemical shifts for protons in different functional groups.

9 Spin-spin splitting Fine structure for a particular resonance due to the magnetic fields of neighboring nuclei multiplet structure 971 Fig The NMR spectrum of 1,1-dichloroethane showing a doublet of peaks due to the splitting of the methyl(ch 3 ) protons by the proton of the adjacent methane proton and a quartet of peaks due to the splitting of the methine proton by the adjacent methyl protons. 972 J ab : spin-spin splitting constant Fig Spin-spin coupling between a single proton H a and groups of one, two and three equivalent protons H b on the adjacent carbon atom.

10 EXAMPLE H spectrum of diethyl ketone, (C 2 H 5 ) 2 CO. 972 Combination of a triplet and a quartet is a characteristic of an ethyl group: ~ Triplet for methyl protons due to methylene protons ~ Quartet for methylene protons due to methyl protons 20.5 ELECTRONIC SPECTOSCOPY AND EXCITED STATE RELAXATION PROCESSES 973 Electronic spectroscopy Transitions between electronic states (UV or visible range) Electronic emission spectroscopy (fluorescence) ~ Study of dynamics of energy and electron transfer processes (femtosecond processes, 1 fs = s) Relaxation of excited electronic states by ~ Emission of fluorescence or phosphorescence ~ Dissipation of energy as heat by nonradiative processes

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